Student Study Guide for
Grotzinger, Jordan, Press, and Siever' UNDERSTANDING EARTH Fifth Edition
Peter L. Kresan Unive...
141 downloads
1992 Views
8MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
Student Study Guide for
Grotzinger, Jordan, Press, and Siever' UNDERSTANDING EARTH Fifth Edition
Peter L. Kresan University of Arizona, Tucson (retired)
Reed Mencke, Ph.D. University of Arizona, Tucson (retired)
IS W. H. Freeman and Company New York
CHAPTER
1
Brief Preview of the Study Guide for Understanding Earth
W
e k n o w from personal e x p e r i e n c e that studying g e o l o g y can be very r e w a r d i n g . G e o l o g y allows you to look at the Earth a r o u n d you and u n d e r s t a n d h o w it c a m e to look the w a y it d o e s . G e o l o g y offers scientifically s o u n d e x p l a n a t i o n s for g e o logic disasters such as e a r t h q u a k e s and v o l c a n o e s . We also k n o w that m a s t e r i n g the ideas of g e o l o g y can be challenging. T h e m a i n goal of this Study G u i d e is to h e l p you m e e t that challenge. We think you will find the organization of this g u i d e b o t h practical a n d helpful. S u c c e s s in g e o l o g y revolves a r o u n d lecture, as do the study aids in this g u i d e . Specific g e o l o g y study strategies are w o v e n t h r o u g h o u t the g u i d e , i n c l u d i n g h o w to take great notes a n d h o w to prep a r e for e x a m s . Study aids are laid out step by step (see the flow chart on p a g e 3) so y o u k n o w w h a t to do before lecture, during lecture, after lecture, and d u r i n g e x a m p r e p a r a t i o n . A final a p p e n d i x , H o w to S t u d y G e o l o g y , brings all these aids together into a short c o u r s e on effective study strategies. H e r e ' s h o w to use these materials.
Before Lecture: Preview T h e key to taking g o o d notes is to arrive in class with an o v e r v i e w of w h a t will be c o v e r e d a l r e a d y i n m i n d . T h a t w a y y o u will a l r e a d y k n o w w h a t g e o l o g i c a l p r o c e s s e s will b e e x p l a i n e d a n d w h a t key q u e s t i o n s t h e l e c t u r e will a n s w e r . In l e c t u r e y o u c a n actively listen for a n s w e r s to those questions. Listening actively with specific q u e s t i o n s in m i n d is guaranteed to result in u n d e r s t a n d i n g m o r e of w h a t is said and including m o r e of the key points in y o u r n o t e s . T h e m e t h o d you use to gain an overview is called C h a p t e r Preview. To m a k e p r e v i e w ing easier for you, we begin e a c h c h a p t e r of the S t u d y G u i d e with the three or four key q u e s tions the c h a p t e r (and its c o r r e s p o n d i n g lecture) will cover. Brief a n s w e r s d e s i g n e d to help you start thinking about the material are supplied as well. Working with these questions and a n s w e r s ahead of t i m e will bring you to lecture ready to listen and take g o o d notes. 1
Contents Part I: How to Study Geology 1 Brief Preview of the Study G u i d e 2 Meet the A u t h o r s
1
4
3 How to Be Successful in G e o l o g y
9
Part II: Chapter-by-Chapter Study Resources 1 The Earth S y s t e m
19
2 Plate Tectonics: T h e Unifying T h e o r y 3 Earth Materials: M i n e r a l s and R o c k s 4 Igneous R o c k s : Solids from M e l t s
26 35
50
5 Sedimentation: R o c k s F o r m e d by Surface P r o c e s s e s
64
6 Metamorphism: Modification of R o c k s by T e m p e r a t u r e and P r e s s u r e 7 Deformation: Modification of R o c k s by Folding and Fracturing 8 Clocks in R o c k s : T i m i n g the G e o l o g i c R e c o r d 9 Early History of the Terrestrial Planets 10 Evolution of the C o n t i n e n t s
12 Volcanoes
94
106
113
11 Geobiology: Life Interacts with the Earth
13 Earthquakes
74
84
124
133 144
14 Exploring Earth's Interior 15 T h e Climate S y s t e m
155
166
16 Weathering, E r o s i o n , and M a s s Wasting: Interface B e t w e e n C l i m a t e and Tectonics 17 T h e H y d r o l o g i c C y c l e and G r o u n d w a t e r
191
18 Stream Transport from M o u n t a i n s to O c e a n s 19 W i n d s and Deserts
201
211
2 0 Coastlines and O c e a n B a s i n s 21 Glaciers: T h e W o r k of Ice
217 226
2 2 L a n d s c a p e s : Tectonic and C l i m a t e Interaction 23 The H u m a n Impact on Earth's Environment
Appendix A: Eight-Day Study Plan Appendix B: Final Exam Prep
234 244
253
255
Answers to Practice Exercises and Review Questions
257
During Lecture: Take an Excellent Set of Notes W i t h the basic q u e s t i o n s in m i n d , you have a h e a d start on taking g o o d notes. But note taking is a skill, and as is true with j u s t a b o u t everything else in life, y o u get better with practice. We p r o v i d e a N o t e - T a k i n g Checklist: specific actions y o u can take to get m o r e of the content into y o u r n o t e s in a u s a b l e form. For m a n y chapters we p r o v i d e additional note-taking tips that are specific to the chapter, for e x a m p l e , s u g g e s t i o n s a b o u t w h a t to listen for or figures y o u n e e d to review carefully before a particular lecture.
After Lecture After e a c h lecture we suggest that y o u s c h e d u l e t w o separate sessions: a brief note review and i m p r o v e m e n t session and an Intensive S t u d y S e s s i o n (lasting at least an h o u r ) d u r i n g w h i c h y o u a c h i e v e m a s t e r y of the l e c t u r e ' s content. Have you checked your notes?
N o t e taking i s not confined j u s t t o lecture. N o t e s can almost
a l w a y s be i m p r o v e d after lecture. G o o d students k n o w this and take the time after lecture to revise their n o t e s . T h e s e s t u d e n t s also pull additional p o i n t s o n every m i d t e r m a s t h e i r " p a y check." Specific suggestions in every chapter of the Study G u i d e will help you i m p r o v e y o u r lecture notes. Often these suggestions focus on a d d i n g visual material from the text or s u m m a r i z i n g w h a t w a s covered in a form that will be easy to r e m e m b e r . Intensive S t u d y S e s s i o n .
T h e p u r p o s e o f the intensive study session i s t o m a s t e r the
m a t e r i a l t a u g h t i n l e c t u r e . L e a r n i n g and c o g n i t i v e p s y c h o l o g y t e a c h e s u s that r e a d i n g p a s sively is an inefficient w a y to learn. You learn best if you spend y o u r study time a n s w e r i n g q u e s t i o n s . W i t h this m e t h o d y o u read portions of the text as y o u n e e d t h e m to find a n s w e r s to q u e s t i o n s . I t ' s a lot like t h e w a y we l e a r n a n e w software p a c k a g e on t h e c o m p u t e r . We m a y not r e a d the software m a n u a l first. Instead we load a p r o g r a m a n d start u s i n g it. T h e n w e g o t o t h e m a n u a l (or o n l i n e h e l p ) w h e n w e n e e d t o f i n d out i f w e c a n d o s o m e t h i n g w i t h t h e p r o g r a m , o r t o see h o w t o d o it. A p p l y i n g this t o the t e x t b o o k , y o u will l e a r n g e o l o g y m o s t efficiently b y w o r k i n g b a c k w a r d from q u e s t i o n s , a n d for e a c h c h a p t e r w e s u g g e s t q u e s t i o n s y o u c a n u s e this way. A n d often w e s u g g e s t o t h e r g u i d i n g q u e s t i o n s f r o m Understanding Earth or the W e b site.
S i m p l y stated:
Do a n s w e r q u e s t i o n s , d o n ' t " j u s t
r e a d . " You will learn a n d r e m e m b e r m o r e s c i e n c e m u c h faster.
Exam Prep D o i n g well on college e x a m s is mostly about organization. Effective study r e q u i r e s a s y s t e m a t i c , orderly review. M o s t of y o u r review time (about 70 p e r c e n t ) should be spent a n s w e r ing review q u e s t i o n s . F o r e a c h c h a p t e r of the text the Study G u i d e i n c l u d e s a C h a p t e r S u m m a r y , Practice Exercises, R e v i e w Q u e s t i o n s , and W e b activities. U s i n g t h e s e features is the fastest w a y to learn and the best w a y to r e m e m b e r the information y o u h a v e learned. S t u d y tips for preparing for m i d t e r m and final e x a m s , helpful hints that will i m p r o v e y o u r testtaking skills on m u l t i p l e - c h o i c e tests, and information on h o w to use y o u r p e r s o n a l l e a r n i n g style to y o u r a d v a n t a g e d u r i n g e x a m s m a y also be found in this section.
CHAPTER
2
Meet the Authors of Understanding Earth: How to Use Your Geology Textbook
A
s you begin y o u r study of geology, first take a m i n u t e or t w o to get a c q u a i n t e d with y o u r text. First of all, w h y do you think it w a s w r i t t e n ? H e r e is w h a t y o u r text a u t h o r s h a v e to say about w h y they w r o t e the b o o k :
Geology fascinates and excites us. We wrote U n d e r s t a n d i n g Earth to help you discover for yourselves how interesting geology is in its own right and how important an understanding of geology has become for making decisions of public policy. What can we do to protect people and property from natural disasters such as volcanoes, earthquakes, and landslides? How can we use the resources of Earth—coal and oil, minerals, water, and air—in ways that minimize damage to the environment? In the end, understanding Earth helps us understand how to preserve life on Earth. T h e y add that p e o p l e t e n d to enjoy w h a t they do well. T h e y d e s i g n e d their text to h e l p y o u do well in y o u r g e o l o g y c o u r s e . M a n y aids to learning are built into the text. L e t ' s s p e n d a m i n u t e or t w o talking about these aids and h o w you can use t h e m .
Clues and Tools: "What's Important?" O n e of the t o u g h e s t things about taking an introductory course is that y o u typically h a v e very little k n o w l e d g e of the subject matter. N o t only do you have a lot to learn, but you h a v e a lot to learn about how to learn it. W h e r e should you focus y o u r attention? W h a t skills and c o n cepts should receive the bulk of y o u r study e n e r g y ? A i d s to h e l p y o u see w h a t material is i m p o r t a n t are built into every p a g e of Understanding Earth. But you h a v e to k n o w w h e r e to look for these aids and how to use them. H e r e is a short list of learning aids in y o u r text and a few preliminary t h o u g h t s about h o w to m a x i m i z e their usefulness. 4
Meet the Authors of Understanding Earth: How to Use Your Geology Textbook
/ Chapter Outline T h e a u t h o r s begin e a c h c h a p t e r with an outline of w h a t will be c o v e r e d . To use this tool you n e e d to l o o k actively for clues. L o o k at the outline for C h a p t e r 1. E a c h item in the outline is a clue to w h a t will be covered in the chapter. Try t u r n i n g e a c h item in the outline into a q u e s t i o n . E x a m p l e : A s k yourself, " W h a t is the scientific m e t h o d ? " as you r e a d that section. Pay particular attention to material in the outline y o u find intriguing, surprising, or p u z z l i n g in any way. U s e y o u r surprise o r p u z z l e m e n t t o motivate y o u r s e l f w h e n y o u read that section. E x a m p l e : C o n s i d e r the item " E a r t h as a S y s t e m of Interacting C o m p o n e n t s . " H e r e there are t w o p o s s i b i l ities. You m a y be p u z z l e d a b o u t w h a t c o m p o n e n t s the a u t h o r s are referring to. In that case, activate y o u r curiosity. F r a m e y o u r surprise in a q u e s t i o n such as " W h a t in t h e w o r l d are t h e a u t h o r s talking a b o u t ? " o r " W h a t are E a r t h ' s interacting c o m p o n e n t s ? " R e a d w i t h t h e q u e s t i o n in m i n d . / Chapter Summary At the e n d of each c h a p t e r a brief s u m m a r y e m p h a s i z e s the m o s t important ideas of the chapter. T h i s is a very useful r e s o u r c e w h e n you are studying for an e x a m .
Time Saver Tip Before r e a d i n g a c h a p t e r of the text, read t h e C h a p t e r S u m m a r y first, referring as you do so to the C h a p t e r O u t l i n e . T h e outline and s u m m a r y will provide an overview, or organizer, that will greatly accelerate your reading and understanding of material. Hint: Also use the C h a p t e r P r e v i e w questions in this Study G u i d e . T h e s e questions are specially devised to h e l p you focus on the m o s t important material.
/ Key Terms within the text are printed in bold. You can speed up your reading time by being vigilant for key terms. W h e n you see a boldfaced t e r m you k n o w it is the most important concept in that paragraph. Focus on understanding the concept. / Photographs G e o l o g y is a very visual science. Pictures in the text are essential. U s e t h e m as a "virtual field t r i p " to h e l p you learn w h a t particular r o c k s and formations l o o k like. P e r h a p s you h a v e h e a r d of or even visited s o m e of the p h o t o sites. E x a m p l e : See Figure 1.4 in C h a p t e r 1. P a y particular attention to the p h o t o g r a p h s that are paired with s c h e m a t i c figures or d i a g r a m s and refer to t h e m as you read.
Study Tip H a v i n g a t o u g h t i m e g e t t i n g y o u r s e l f started in a study s e s s i o n ? U s e the art to motivate y o u . Start a c h a p t e r s t u d y session by s c a n n i n g t h e i m a g e s a n d c a p t i o n s t o f i n d m a t e r i a l that interests y o u . B e g i n y o u r r e a d i n g with this m a t e r i a l . T h i s a p p r o a c h w o r k s like starting a fire. You g e n e r a t e a small s p a r k of interest and t h e n fan it by b r i n g i n g in n e w m a t e r i a l .
/ Figures (Flowcharts) F i g u r e s are even m o r e important than p h o t o g r a p h s . F l o w c h a r t s such as the r o c k cycle in F i g u r e 3.26 present key c o n c e p t s . Pay careful attention to the a r r o w s in flow m o d e l s and ask yourself w h a t drives the p r o c e s s and w h a t p r o d u c t s are formed. If you are a visual learner, you m a y even want to study the figures a n d sketches before y o u start to read. T h e n read text on an a s - n e e d e d basis, to clarify the figure. / Figure Stories In g e o l o g y the story of a particular feature or area is often instructive. In e a c h c h a p t e r of Understanding Earth, you will find a "figure story," an illustrated vignette that explains an i m p o r t a n t g e o l o g i c p r o c e s s or principle. F i g u r e stories contain
5
Meet the Authors of Understanding Earth: How to Use Your Geology Textbook
/ Chapter Outline T h e a u t h o r s b e g i n e a c h c h a p t e r w i t h an outline of w h a t will be c o v e r e d . To u s e this tool you n e e d to look actively for clues. L o o k at the outline for C h a p t e r 1. E a c h item in the outline is a clue to w h a t will be c o v e r e d in the chapter. Try turning e a c h i t e m in the outline into a q u e s t i o n . E x a m p l e : A s k yourself, " W h a t is the scientific m e t h o d ? " as y o u r e a d that section. P a y particular attention to material in the o u t l i n e you find intriguing, surprising, or p u z z l i n g in any way. U s e y o u r surprise o r p u z z l e m e n t t o m o t i v a t e yourself w h e n you r e a d that section. E x a m p l e : C o n s i d e r the item " E a r t h as a S y s t e m of Interacting C o m p o n e n t s . " H e r e there are t w o p o s s i b i l ities. You m a y be p u z z l e d a b o u t w h a t c o m p o n e n t s the a u t h o r s are referring to. In that c a s e , activate y o u r curiosity. F r a m e y o u r surprise in a q u e s t i o n such as " W h a t in the w o r l d are t h e a u t h o r s talking a b o u t ? " o r " W h a t are E a r t h ' s interacting c o m p o n e n t s ? " R e a d w i t h the q u e s t i o n in m i n d . / Chapter Summary At the end of e a c h c h a p t e r a brief s u m m a r y e m p h a s i z e s the m o s t i m p o r t a n t ideas of the chapter. T h i s is a very useful r e s o u r c e w h e n you are studying for an e x a m .
Time Saver Tip Before r e a d i n g a c h a p t e r of the text, read the C h a p t e r S u m m a r y first, referring as you do so to the C h a p t e r O u t l i n e . T h e outline and s u m m a r y will p r o v i d e an overview, or organizer, that will greatly accelerate your reading and understanding of material. Hint: Also use the C h a p t e r P r e v i e w q u e s t i o n s in this Study G u i d e . T h e s e q u e s t i o n s are specially d e v i s e d to h e l p you focus on the m o s t important material.
/ Key Terms within the text are printed in bold. You can speed up your reading time by being vigilant for key terms. W h e n you see a boldfaced t e r m you know it is the most important concept in that paragraph. Focus on understanding the concept. / Photographs G e o l o g y is a very visual science. Pictures in the text are essential. U s e t h e m as a "virtual field t r i p " to h e l p you learn what particular r o c k s and formations l o o k like. P e r h a p s you h a v e heard of or even visited s o m e of the p h o t o sites. E x a m p l e : See F i g u r e 1.4 in C h a p t e r 1. Pay particular attention to the p h o t o g r a p h s that are paired with s c h e m a t i c figures or d i a g r a m s a n d refer to t h e m as you read.
Study Tip H a v i n g a t o u g h t i m e g e t t i n g y o u r s e l f started in a study s e s s i o n ? U s e the art to m o t i v a t e y o u . Start a c h a p t e r s t u d y s e s s i o n by s c a n n i n g t h e i m a g e s a n d c a p t i o n s t o f i n d m a t e r i a l that interests y o u . B e g i n y o u r r e a d i n g with this m a t e r i a l . T h i s a p p r o a c h w o r k s like starting a fire. You g e n e r a t e a small s p a r k of interest a n d t h e n fan it by b r i n g i n g in n e w m a t e r i a l .
/ Figures (Flowcharts) Figures are even m o r e important than p h o t o g r a p h s . F l o w c h a r t s such as the r o c k cycle in F i g u r e 3.26 present key c o n c e p t s . Pay careful attention to the a r r o w s in flow m o d e l s and ask yourself w h a t drives the p r o c e s s and w h a t p r o d u c t s are formed. If you are a visual learner, you m a y even want to study the figures and sketches before you start to read. T h e n read text on an a s - n e e d e d basis, to clarify the figure. Figure Stories In g e o l o g y the story of a particular feature or area is often instructive. In e a c h c h a p t e r of Understanding Earth, you will find a "figure story," an illustrated vignette that e x p l a i n s an i m p o r t a n t g e o l o g i c p r o c e s s or principle. Figure stories contain
5
s o m e data, affording you the opportunity to see h o w g e o l o g i s t s interpreted the d a t a a n d the kind of thinking they did to reach a conclusion. W h e n r e a d i n g figure stories ask yourself how t h e story illustrates and d e v e l o p s w h a t w a s said in the text. Try to understand the story, then test yourself by s u m m a r i z ing (out loud if you are an auditory learner) w h a t y o u learned. Kinesthetic learners m a y find the figure stories particularly helpful b e c a u s e e a c h p r o v i d e s a real-life e x a m p l e , similar to a lab or field experience. Visual learners will a p p r e c i a t e h o w the carefully c h o s e n illustrations clarify the text. / C o l o r is u s e d in the figures to provide you with clues about h o w e a c h p r o c e s s w o r k s and w h a t is involved. L o o k at Key Figure 1.11 in C h a p t e r 1. W h a t color is used to describe p r o c e s s e s that utilize heat? W h a t color depicts w a t e r ? C o o l i n g ? S u c h c u e s can support learning at a subliminal level of a w a r e n e s s , particularly if you are a visual l e a r n e r — s o be sure to pay close attention to the colors in the figures! / C l u e s to rock texture are also cleverly built into the text. L o o k at F i g u r e 3.23 on p a g e 6 4 . T h e i g n e o u s r o c k is s h o w n in s h a d e s of black with lots of d o t s and s p e c k s of varying size. T h i s m a k e s sense given that igneous rock is classified on the basis of the size of its crystals. W h y are s e d i m e n t s and s e d i m e n t a r y rock depicted a s horizontal stripes? B e c a u s e s e d i m e n t s are f o r m e d of earthy materials like sand and m u d , and they are originally laid d o w n in horizontal sheets and layers. W h y is m e t a m o r p h i c r o c k d e p i c t e d with a series of distorted and folded b a r s ? B e c a u s e m e t a m o r p h i c r o c k is created w h e n other k i n d s of r o c k are subjected to high pressures and t e m p e r a t u r e s that m a y distort the r o c k into folds and its crystals into wavy, foliated b a n d s . T h u s the figure s p e a k s to us about w h a t the r o c k l o o k s like as well as the p r o c e s s e s that p r o d u c e it. Pay close attention to these and other visual learning clues.
Questions and Exercises to Help You Learn /
E x e r c i s e s (end of chapter)
/
T h o u g h t Q u e s t i o n s (end of chapter)
P s y c h o l o g i s t s h a v e f i r m l y e s t a b l i s h e d t h e i m p o r t a n c e o f actively l e a r n i n g b y a s k i n g ourselves q u e s t i o n s a b o u t t h e n e w i n f o r m a t i o n w e are a t t e m p t i n g t o learn and t h e n s e e k i n g t h e a n s w e r s t o t h e s e q u e s t i o n s . W h e n w e f o r m u l a t e i n q u i r i e s before w e start r e a d i n g n e w m a t e rial, we t e n d to r e a d in a m o r e active m a n n e r s i n c e we are r e a d i n g in s e a r c h of specific a n s w e r s . T h i s is far m o r e efficient than t r y i n g to r e a d a b o u t t h e i n f o r m a t i o n y o u a r e a t t e m p t i n g to learn in a p a s s i v e m a n n e r (passive r e a d i n g is o k a y for m a g a z i n e s s u c h as Sports Illustrated b u t not for a c o l l e g e s c i e n c e t e x t b o o k ) . T h i n k of t h e e n d - o f - c h a p t e r s u m m a r i e s a n d r e v i e w q u e s t i o n s a s y o u r o w n p e r s o n a l tutor. W h a t d o e s a g o o d tutor d o ? S h e o r he a s k s y o u q u e s t i o n s to get y o u t h i n k i n g a b o u t t h e m a t e r i a l . L i k e a g o o d tutor, t h e q u e s t i o n s at t h e e n d of e a c h c h a p t e r clue y o u in to w h a t is i m p o r t a n t and e n c o u r a g e y o u to r e a l l y master the material. You can use both the E x e r c i s e s and T h o u g h t Q u e s t i o n s at the end of e a c h c h a p t e r of Understanding Earth in three different w a y s . First, use t h e m to preview the chapter. S k i m quickly to get an overview of w h a t you will be expected to k n o w after you have r e a d the c h a p ter. S e c o n d , seek out a n s w e r s . Active learning w o r k s better than passively r e a d i n g the text. So organize y o u r study t i m e a r o u n d finding an a n s w e r to o n e or m o r e exercises or q u e s t i o n s using y o u r text as you w o u l d a reference b o o k . Third, use the questions to r e v i e w for e x a m s . Q u i z yourself by trying to a n s w e r the questions w i t h o u t using the text as y o u r r e f e r e n c e . Go b a c k to y o u r annotated text to c h e c k the accuracy of y o u r a n s w e r s . T h e best w a y to learn from a text is to u s e it as a reference to a n s w e r questions.
Meet the Authors of Understanding Earth: How to Use Your Geology Textbook
7
Text-Marking Strategy for Understanding Earth You can m a k e the 30 p e r c e n t of y o u r study t i m e you spend r e a d i n g the text even m o r e efficient by skillful text m a r k i n g . M a r k i n g y o u r text as y o u r e a d is a vital part of learning the material and m a y be particularly i m p o r t a n t if you are a kinesthetic learner. T h e action of m a r k i n g will aid y o u r k i n e s t h e t i c m e m o r y . If y o u are a visual learner, you will t e n d to r e m e m b e r m a r k e d sections d u r i n g e x a m s . G o o d m a r k i n g involves both t h i n k i n g a n d p l a n ning. First, y o u think a b o u t w h a t is i m p o r t a n t . T h e n you plan for e x a m review, d e c i d i n g which of the m a r k i n g t e c h n i q u e s in the following table will best h e l p you w h e n you return t o the c h a p t e r d u r i n g e x a m review. O n l y then s h o u l d you m a r k . T h e a u t h o r s h a v e a l r e a d y m a r k e d k e y t e r m s in b o l d letters. You will w a n t to add u n d e r l i n e s , n u m b e r s , a n d a n n o t a t i o n s in the m a r g i n . H e r e are s o m e brief s u g g e s t i o n s for h o w and w h e n to use e a c h k i n d of m a r k .
Useful markings for geology
Text-marking tips
Underlining Underline i m p o r t a n t points. Underline to highlight t h e k e y i d e a s . Do pay attention to the k e y t e r m s that are in boldfaced t y p e in the text.
1. Read before you mark. To avoid overusing underlining, set y o u r pencil d o w n on the table as you begin to read a p a r a g r a p h . D o n ' t let yourself pick up t h e pencil until y o u have read the entire p a r a g r a p h . S t o p and think. T h e n underline w h a t y o u c o n s i d e r to be the key point(s). 2. Be selective. U n d e r l i n e brief but meaningful p h r a s e s or key w o r d s that will trigger y o u r m e m o r y . M a r k j u s t e n o u g h so that you can review w i t h o u t rereading the p a r a g r a p h . 3. U s e different color m a r k i n g s if it helps you r e m e m b e r .
Summarizing Write a brief s u m m a r y statement of k e y g e o l o g y processes in y o u r o w n w o r d s in the text margin. U s e these a n n o t a t i o n s to facilitate e x a m review.
1. U s e y o u r o w n w o r d s . Putting ideas into y o u r o w n w o r d s is a powerful learning strategy. Be neat. T h i s takes t i m e . But it will pay off later w h e n you review b e c a u s e y o u r annotation will be legible and easily read. 3. Organize y o u r annotations into categories. G r o u p i n g ideas into categories or bulleted lists m a k e s t h e m easier to r e m e m b e r . 4. U s e a n n o t a t i o n s during e x a m review. Avoid r e r e a d i n g the text word for word. T h i s will save y o u a lot of t i m e .
Annotating
Circled n u m b e r s in the margin indicate s e q u e n c e s , such as t h e p r o c e s s e s of the rock cycle. Question m a r k ? Put a question mark in the margin to remind yourself to ask your instructor about a point you do not understand. Asterisks *** U s e asterisks to m a r k ideas of special importance. U s e asterisks sparingly. Save t h e m for the t w o or three m o s t vital ideas in the entire chapter. T Q (test q u e s t i o n ) U s e T Q t o m a r k material i n the c h a p t e r that you k n o w will be covered on the e x a m . T h i s will r e m i n d y o u to pay particular attention to these i t e m s .
M a r k i n g the text will aid kinesthetic learning. G o o d text m a r k i n g will aid visual m e m o r y — so use caution w h e n m a r k i n g up y o u r text. Mark carefully and selectively. If you m e s s up the text with excessive and illegible m a r k i n g s , you m a y interfere with s o m e of the g o o d m e m o r y aids that are built into the careful design of the text. R e m e m b e r , a w e l l - m a r k e d text will m a k e e x a m r e v i e w far m o r e efficient b e c a u s e the m a r k s will focus y o u r a t t e n t i o n o n the key m a t e rial you need to find and review. If you are worried about the i m p a c t of m a r k i n g on y o u r textb o o k ' s resale value, be sure to ask y o u r b o o k s t o r e . M a n y b o o k s t o r e s h a v e policies that e n c o u r a g e effective text m a r k i n g .
TRY T H I S NOW! 1. Try underlining. C h o o s e a section you c o n s i d e r to be important, key material. R e a d the p a s s a g e several t i m e s until you are sure you u n d e r s t a n d it. T h e n m a r k the p a s s a g e following the directions in the table. 2. S u m m a r i z e that you think p r o c e s s in the in the table as
a geological p r o c e s s . C h o o s e a section that c o n t a i n s a g e o l o g i c a l p r o c e s s is i m p o r t a n t e n o u g h to be covered on the next e x a m . A n n o t a t e the m a r g i n in a m a n n e r that will help you r e m e m b e r it. U s e the directions a guideline.
3. Try s o m e a n n o t a t i n g . W h e r e w o u l d circled n u m b e r s be helpful in this c h a p t e r ? Put t h e m in. W h a t material do y o u n e e d to discuss with y o u r instructor? M a r k w i t h a q u e s t i o n mark. 4. F i g u r e out w h a t is the most important idea in the entire chapter. Put three asterisks (***) and a TQ beside that section to insure you m a s t e r it before the e x a m .
CHAPTER
3
How to Be Successful in Geology (and Just About Any Other Challenging Course)
A
c a d e m i c success is largely a matter of strategy. If you m a n a g e y o u r t i m e well a n d study strategically (rather than h a p h a z a r d l y ) , y o u will be successful. In this c h a p t e r we d i s c u s s successful studying strategies for y o u r g e o l o g y c o u r s e . T h e best w a y to u s e this c h a p t e r is to read each strategy and then try it out. We r e c o m m e n d r e a d i n g o n e section of this chapter each day at the beginning of a new semester. Try the strategy in that section before r e a d i n g further. F o l l o w i n g are the success strategies that will be c o v e r e d in this introduction: • L e a r n i n g style • Chapter preview • N o t e taking • N o t e review • E x a m preparation M a n y other learning strategies and hints about learning specific material are p r o v i d e d t h r o u g h o u t this Study G u i d e . L o o k for t h e m and try out as m a n y of the strategies as p o s s i b l e . L e a r n i n g strategically initially takes an investment of t i m e , but y o u will find that it is an investment that p a y s off. S t u d y i n g strategies m a k e y o u m o r e efficient so that y o u can learn m o r e in less t i m e . M o r e important, y o u get m o r e e n j o y m e n t out of y o u r learning b e c a u s e y o u k n o w exactly w h a t you are doing.
Customize Each Strategy to Your Learning Style E a c h of us learns differently. Successful p e o p l e in any activity tend to be t h o s e w h o find a w a y to e x p r e s s their o w n u n i q u e talents in the activity. M o s t of us w o r r y t o o m u c h a b o u t 9
10
PARTI
CHAPTER 3
c o m p e t i n g . We w o u l d do better to seek a w a y of u s i n g o u r best skills and talents. It is o u r individuality that m a k e s us stand out from the p a c k a n d b e c o m e a leader. It is o u r i n d i v i d u ality that leads u s t o b e successful academically. W e r e c o m m e n d that y o u s p e n d s o m e t i m e t h i n k i n g a b o u t y o u r individual style, y o u r strengths as a learner, a n d h o w y o u prefer to learn. H e r e ' s a s i m p l e b e g i n n i n g . W h i c h m o d e o f l e a r n i n g d o y o u prefer t o u s e a n d u s e b e s t ? •
Visual Learning
Visual learners learn by seeing. T h e y often h a v e a g o o d m e m o r y for
pictures and even w o r d s of text. •
Auditory Learning
A u d i t o r y learners learn by listening. T h e y are g o o d listeners a n d
r e m e m b e r best b y j u s t listening. •
Kinesthetic L e a r n i n g
Kinesthetic learners learn by m o v i n g . T h e y learn and r e m e m b e r
best w h e n they get to practice an activity.
T R Y T H I S NOW! Put a 1 in the b o x next to the m o d e of learning y o u prefer m o s t strongly, a 2 for y o u r next strongest m o d e , and a 3 beside the m o d e you think you use least well and least prefer to use. H i n t : Usually, the m o d e we prefer is the o n e we use best. If y o u are not sure w h i c h m o d e you prefer, try taking the Learning Style Inventory online at the University of A r i z o n a L e a r n i n g C e n t e r W e b site: www.ulc.arizona.edu (click on Self-Assessment).
T h r o u g h o u t the Study G u i d e w e will m a k e s u g g e s t i o n s a b o u t h o w particular strategies m i g h t w o r k best for those w h o prefer o n e of the a f o r e m e n t i o n e d learning styles.
Make Geology Lecture a High-Priority Activity T h i s is step 1 in any strategic a p p r o a c h to c o u r s e s u c c e s s . Your g e o l o g y c o u r s e c e n t e r s on lecture. G e o l o g y is a very content-intensive subject. T h e r e ' s so m u c h to learn that instructors have to m a k e s o m e difficult decisions a b o u t w h a t to cover. S o m e t i m e s the faculty in a g e o l o g y department try to address this p r o b l e m by m a p p i n g out required content, perhaps even creating a core syllabus for instructors to follow. E v e n then instructors c a n and do m a n a g e to c r e ate very different courses. In our conversations with other instructors we have b e e n quite i m p r e s s e d with the d e g r e e to w h i c h t e a c h i n g a p p r o a c h e s differ. T w o g e o l o g y instructors m a y use the s a m e text and p e r h a p s even the s a m e basic lecture outline yet still teach t w o very different c o u r s e s with very different exam questions. T h e r e f o r e , a t t e n d i n g lecture is f u n d a m e n t a l to s u c c e s s . M o s t students w h o fail g e o l o g y do so b e c a u s e they fail to attend lecture. Just attending will put y o u far ahead of t h o s e w h o fail. H o w e v e r , j u s t attending is not e n o u g h to e n s u r e that you get an A in geology. R e s e a r c h s h o w s that few of us have the attention span to listen actively and take g o o d notes for an entire hour. T h e average note taker gets m o s t of the m a i n points during the first 10 m i n u t e s of the lecture. T h e n attention w a n d e r s , confusion sets in, and less and less of w h a t is said gets into the n o t e s . I n d e e d , m o s t students a b s o r b less than 15 p e r c e n t of the p o i n t s that are c o v e r e d in the final 10 m i n u t e s of the lecture. T h i s is d o u b l y unfortunate since, inevitably, the lecture gets to the heart of the topic and the material you will certainly be tested on t o w a r d the end. W h a t y o u a c t u a l l y do in l e c t u r e is f u n d a m e n t a l to s u c c e s s in g e o l o g y . You n e e d to be a strategist. You n e e d t o target y o u r a p p r o a c h t o y o u r g e o l o g y c o u r s e o n g e t t i n g t h e m o s t p o s sible out of lecture. That is our best advice and we have adhered to it rigorously in
How to Be Successful in Geology (and Just About Any Other Challenging Course)
c o n s t r u c t i n g this g u i d e . T h e entire S t u d y G u i d e i s o r g a n i z e d a r o u n d lecture. E v e r y c h a p t e r b e g i n s w i t h w h a t y o u do before lecture for that p a r t i c u l a r c h a p t e r t o p i c . N e x t c o m e s a s e c tion w i t h tips on w h a t to do during lecture; after that, a c h e c k l i s t to h e l p y o u i m p r o v e y o u r notes after lecture; a n d finally, lots of e x e r c i s e s and r e v i e w q u e s t i o n s to h e l p y o u w i t h exam prep b e f o r e y o u r m i d t e r m . In this c h a p t e r we present general strategies you can use every day of y o u r c o u r s e to ensure s u c c e s s — a l l o r g a n i z e d a r o u n d lecture.
Before Lecture Strategy: Preview the chapter before going to the lecture To preview a chapter, use the C h a p t e r P r e v i e w questions. T h e s e questions are provided at the beginning of each chapter of the Study G u i d e . R e a d each question, then skim the chapter to find the relevant material. Feel free to annotate the brief a n s w e r s provided for each question. T h e idea is to go to lecture with a general idea of the a n s w e r s to the questions in mind. Why preview? Introductory courses can be difficult. T h e r e are lots of new terms, new ideas, and new skills during a first-semester geology course. But which ideas are " i m p o r t a n t " ? H o w do you focus y o u r effort in this new and seemingly strange terrain?
Picture a house under construction. Imagine that the contractor was very careless and neglected to construct whole sections of the frame. Finishing the areas lacking a frame would be impossible. You can't tack siding onto thin air! This metaphor describes your geology lecture. You are the contractor. You need to arrive at class with an overview of the lecture already in mind. You have already identified what geological processes will be explained and what key questions the lecture will answer. Going to class without a frame of key questions is like building a house with no supporting structure. You will have nothing on which to hang the lecturer's main points (information). Without the main points, the details are meaningless. As the lecture progresses you are likely to feel increasingly confused and bored. By the time the lecturer gets to the most important material, you may be completely lost.
W h e r e do y o u get the o v e r v i e w ? T h e a n s w e r is surprisingly simple. Just spend a few m i n u t e s before lecture p r e v i e w i n g the c h a p t e r that will be covered. P r e v i e w i n g is the m e t h o d by w h i c h you g e n e r a t e and m a s t e r a f r a m e w o r k for listening. H e r e ' s h o w to do it. S t e p 1. R e a d t h e p r e v i e w q u e s t i o n s . You will find C h a p t e r P r e v i e w q u e s t i o n s at the b e g i n n i n g of e a c h c h a p t e r of the Study G u i d e . Step 2. S k i m the text c h a p t e r for a n s w e r s . Quickly find and read only the material in Understanding Earth you need to understand and answer the questions. D o n ' t b o g down in details. D o n ' t try for complete understanding. D o n ' t read the w h o l e chapter. Your goal is to a gain just a general understanding of each q u e s tion. Do use the brief a n s w e r s provided in the Study G u i d e to cue y o u r reading. S t e p 3 . M e m o r i z e t h e q u e s t i o n s a n d brief a n s w e r s . You m a y f i n d that you need to add a few notes to the brief answer. A n n o t a t e the a n s w e r s j u s t e n o u g h to e n s u r e that t h e a n s w e r s m a k e sense to you. A n n o t a t e in a w a y that will e n s u r e that y o u m e m o r i z e the questions.
11
12
PART I C H A P T E R 3
Before you go to lecture, be sure to spend some time previewing. You will find that as little as 10 to 15 minutes of time spent previewing can m a k e a big difference in h o w m u c h you understand of the lecture. With the key points already in mind, y o u can focus in lecture on understanding the details. This, in turn, will help ensure you get an excellent set of notes.
TRY T H I S NOW! M o v e i m m e d i a t e l y to the c h a p t e r that will be covered in y o u r next lecture. P r e v i e w the chapter. T h e n return here to read the next strategy.
During Lecture Strategy: Note-Taking Checklist Your basic g o a l d u r i n g lecture is to take g o o d notes. T h e notes should a n s w e r the C h a p t e r P r e v i e w q u e s t i o n s in depth. To avoid getting lost in details, k e e p the big picture in m i n d . H a v e a c o p y of the C h a p t e r P r e v i e w q u e s t i o n s in front of y o u . Better yet, d e v e l o p p r e l i m i nary a n s w e r s t o the q u e s t i o n s and c o m m i t t h e m t o m e m o r y before t h e lecture. A n o t h e r w a y to k e e p t h e b i g p i c t u r e in m i n d is to b r i n g a c o p y of t h e k e y figure or f l o w c h a r t t o l e c t u r e t o refer t o . I n m a n y c h a p t e r s w e s u g g e s t w h i c h f i g u r e y o u s h o u l d h a v e handy. D u r i n g s o m e classes the lecturer m a y show r o c k formations a n d m a y pass a r o u n d s p e c i m e n r o c k s . You will get m o r e out of t h e s e d e m o n s t r a t i o n s if y o u sit c l o s e to the front of the r o o m w h e r e y o u c a n see the s a m p l e r o c k s a s the lecturer d i s c u s s e s t h e m . R e m e m b e r t o focus on clues the lecturer p r o v i d e s for r e c o g n i z i n g a particular s a m p l e in the field. F o c u s in particular on the texture of s a m p l e s and learn to r e c o g n i z e d i f f e r e n c e s , for e x a m p l e , t h e fine t e x t u r e of a volcanic r o c k and the c o a r s e texture of a p l u t o n i c rock. As y o u listen to lecture, identify q u e s t i o n s y o u need to ask to u n d e r s t a n d the m a t e r i a l . Try to formulate at least o n e g o o d question y o u can ask d u r i n g every lecture. T h e m o s t i m p o r t a n t task d u r i n g lecture is, of c o u r s e , taking a g o o d set of n o t e s . N o t e taking is not easy. It is a skill that i m p r o v e s with practice. H e r e are a few tips that will h e l p y o u .
Note-Taking Checklist • •
O r g a n i z e y o u r notes in a three-ring b i n d e r so that y o u c a n easily r e o r g a n i z e t h e m . L e a v e space in y o u r n o t e s for i m p o r t a n t visual material (flowcharts, s i m p l e s k e t c h e s , c o m p a r i s o n charts). To m a k e this easy, e m p l o y a d o u b l e - c o l u m n or d o u b l e - p a g e note-taking format. Take notes on the r i g h t - h a n d p a g e or c o l u m n . S a v e the facing left p a g e as a " s k e t c h p a g e . "
•
Sit n e a r the front of the r o o m — n o t to be a t e a c h e r ' s pet but s i m p l y so that y o u c a n h e a r and see better!
•
D a t e e a c h d a y ' s notes so that y o u c a n find material later.
How to Be Successful in Geology (and Just About Any Other Challenging Course)
•
Take notes in a format that m a k e s the m a i n topics and c o n c e p t s easy to identify. S o m e students a c c o m p l i s h this b y t a k i n g notes i n outline format. B u t m a n y other a p p r o a c h e s are p o s s i b l e . Visual learners m a y find it helpful to highlight m a i n points (after class) in color. A n o t h e r g o o d a p p r o a c h w o u l d be to u s e the questions we p r o v i d e in the C h a p t e r P r e v i e w as h e a d s . You can add t h e m d u r i n g class or d u r i n g y o u r after-class review session.
•
K e e p the p r e v i e w q u e s t i o n s in front of y o u during lecture. Be sure to leave class w i t h a g o o d a n s w e r to e a c h of the p r e v i e w questions.
•
M a r k areas w h e r e y o u n e e d to do follow-up w o r k w i t h y o u r text, instructor, or tutor w i t h a q u e s t i o n m a r k in the m a r g i n .
•
I n d i c a t e p o s s i b l e test q u e s t i o n s by writing TQ (test question) in the left margin w h e r e y o u can easily see it.
•
W r i t e a s s i g n m e n t s in the left c o l u m n w h e r e you can easily find t h e m . After class e n t e r the d u e date in y o u r p e r s o n a l p l a n n e r or calendar.
T R Y T H I S NOW! Take the n o t e - t a k i n g checklist with y o u to y o u r next lecture. Try to follow all the s u g g e s t i o n s . After lecture review y o u r notes and c h e c k off e a c h point that y o u actually followed. T h e n return h e r e to read t h e next strategy. H i n t : P h o t o c o p y the checklist a n d u s e it for every lecture until all the strategies b e c o m e habits. U s e d this way, the checklist b e c o m e s a visual record of y o u r p r o g r e s s as a skillful n o t e taker.
After Lecture Review Your Notes Immediately G o o d n o t e t a k i n g c o n t i n u e s after the lecture is over. Right after lecture, w h i l e the material is fresh in y o u r m i n d , is the perfect t i m e to review y o u r n o t e s . R e v i e w to be sure y o u u n d e r s t o o d the key points and w r o t e t h e m d o w n in a form that will be easy to review later. D o n ' t p o s t p o n e this activity. T h e best t i m e is right after lecture before you go to y o u r next class or activity. L e a r n i n g experts tell us that m o s t of us will forget 80 p e r c e n t of w h a t we h e a r d in a lecture by the following day. On the other hand, if y o u review right after the lecture, t h e r e will h a v e b e e n no interruptions. M u c h of w h a t w a s said will still be in y o u r shortt e r m m e m o r y . If y o u m i s s e d s o m e t h i n g , you can p r o b a b l y r e m e m b e r it and p u t it into y o u r notes. T h e basic i d e a of r e v i e w i n g y o u r notes is to fill in w h a t you m i s s e d and to add helpful visual m a t e r i a l from the text. U s e the following checklist as a g u i d e .
Check Your Notes: Have y o u . . . •
written legible notes? (Rewrite t h e m if you n e e d to!)
•
identified the important points clearly? (You should have h e a d i n g s in y o u r notes for e a c h of the questions in the C h a p t e r Preview.)
•
filled in the h o l e s ( m i s s i n g material) w h i l e the lecture w a s still fresh in y o u r shortterm memory?
•
m a r k e d areas w h e r e you d o n ' t r e m e m b e r w h a t w a s said for a f o l l o w - u p session with y o u r instructor, tutor or study partner?
•
indicated possible test questions ( T Q ) in the m a r g i n ?
•
a d d e d additional visual material?
•
r e w o r k e d notes into a form that is efficient for y o u r learning style?
•
created a brief big picture overview of this lecture (using a sketch or written outline)?
T R Y T H I S NOW! After the next lecture, review y o u r notes and improve t h e m using the points in the checklist as a guide. C h e c k off each point as you complete it. T h e n return here to read the next strategy. Hint: Consider photocopying the checklist so that you can u s e it repeatedly.
Intensive Study Session Ask yourself questions as you study. Then answer them. You should schedule at least o n e h o u r after e a c h lecture for intensive study. T h i s can occur a n y t i m e before the next lecture. (Short-term m e m o r y is no longer a p r o b l e m since you have c o m p l e t e d a note review and have g o o d notes.) W h y do you need an intensive study session? T h i n k about the house/construction e x a m ple that we mentioned earlier. You need frames before you can add the siding, so you construct a frame of questions before each lecture. During lecture, you add the s i d i n g — t h e a n s w e r s to the questions. After lecture, master the ideas and details during an intensive study session. N o w y o u have constructed the first story. But this g e o l o g y c o u r s e is a s k y s c r a p e r with 23 floors (one for each chapter). E a c h chapter supports those a b o v e it. If y o u d o n ' t c o m p l e t e l y m a s t e r a chapter, the next will be m o r e difficult. M a s t e r y is not gained by j u s t reading the text. M a s t e r y occurs as the result of a s k i n g yourself q u e s t i o n s and a n s w e r i n g t h e m . T o help you w e p r o v i d e P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s for every c h a p t e r of the text. T h i s interactive learning material is specifically d e s i g n e d to h e l p you m a s t e r the key c o n c e p t s of each chapter. T h e greater the n u m b e r
How to Be Successful in Geology (and Just About Any Other Challenging Course)
of these exercises a n d q u e s t i o n s you can w o r k into y o u r intensive study sessions early in t h e course, the easier s u b s e q u e n t chapters will b e . Plan to spend the majority of y o u r study t i m e (70 percent) on t h e s e exercises and questions. U s e y o u r text as a reference. R e a d it as n e e d e d to a n s w e r questions and m a s t e r material. W h e n y o u read, r e a d efficiently. R e a d with p u r p o s e and r e a d to find a n s w e r s to questions. Hint: To u s e y o u r text effectively you m u s t k n o w it like the b a c k of y o u r hand. Carefully read Part I, C h a p t e r 2, H o w to U s e Understanding Earth in this g u i d e .
T R Y T H I S NOW! E a c h c h a p t e r of the S t u d y G u i d e c o n t a i n s a section titled I n t e n s i v e S t u d y S e s s i o n . Turn n o w to t h e c h a p t e r y o u are currently w o r k i n g on and try out its Intensive S t u d y Session material. Do e n o u g h to get a feeling for h o w Intensive S t u d y S e s s i o n works. T h e n return h e r e and r e a d t h e next section.
Exam Prep Materials in this section are m o s t useful during y o u r preparation for m i d t e r m and final e x a m inations. F o r o p t i m a l p e r f o r m a n c e , m i d t e r m preparation should begin about eight days before the e x a m (see the E i g h t - D a y S t u d y P l a n in A p p e n d i x A ) . T h e basic i d e a is a systematic review of material divided into short study sessions.
Tips for Preparing for Geology Exams • U s e the clues y o u r instructor has provided in lecture about what is important. Even w h e n a d e p a r t m e n t agrees on a c o m m o n core of material (a very rare occurrence), each instructor carves out a course that is unique, has a particular character or flavor, and has distinct areas of e m p h a s i s . Your instructor is the ultimate guide on what is important. • Be sure y o u k n o w the format of the e x a m . M u l t i p l e c h o i c e ? T r u e - f a l s e ? E s s a y ? Thought problems? • R e v i e w y o u r notes m a r k e d TQ (test q u e s t i o n ) . • A s k y o u r instructor if e x a m s are available from the p r e v i o u s semester. R e v i e w t h e m to c h e c k t h e format of q u e s t i o n s , see w h a t areas of c o n t e n t are stressed, and find out w h a t types of p r o b l e m solving are included. D o n ' t m a k e the m i s t a k e of a s s u m i n g that the s a m e q u e s t i o n s will be a s k e d this semester. • Be sure to attend review sessions if they are offered. • If y o u h a v e tutors, p r e c e p t o r s , s u p p l e m e n t a l instruction leaders, or other peer h e l p e r s w h o h a v e taken the class, a s k for their suggestions about p r e p a r i n g for the e x a m . • O n c e y o u are clear a b o u t the nature of the e x a m , begin y o u r review. C o n d u c t review in an orderly, systematic m a n n e r that focuses on all t h e important material. T h e E i g h t D a y S t u d y P l a n ( A p p e n d i x A) is a g o o d m o d e l for an orderly review.
Test Taking In every college e x a m , a n u m b e r of students k n o w the material yet fail the test b e c a u s e they get a n x i o u s , p a n i c , and freeze u p . We have found over the years that the best w a y to o v e r c o m e or avoid test anxiety is by c o m i n g to the test well p r e p a r e d a n d confident a n d by w o r k i n g strategically on the test. E x a m p r e p will not be a p r o b l e m if y o u u s e the materials p r o v i d e d for this p u r p o s e in this Study G u i d e . In this section, we will s u g g e s t strategies to try out during y o u r e x a m . You m a y w a n t to return to this section a day or so before y o u r e x a m .
Test-Taking Tips for Different Learning Styles Visual L e a r n e r s • U s e written directions. • W h e n you get stuck on an item, close y o u r eyes and picture flowcharts, pictures, field e x p e r i e n c e s , or text. Auditory Learners • Pay attention to verbal directions. • R e p e a t written directions quietly to yourself ( m o v i n g y o u r lips should be e n o u g h ) . • If you get stuck, r e m e m b e r the lecturer's voice c o v e r i n g this section. Kinesthetic Learners • W h e n you get stuck, m o v e in y o u r chair or tap y o u r foot to trigger y o u r m e m o r y . • Feel yourself d o i n g a lab p r o c e d u r e . • Sketch a flowchart to unlock the m e m o r y of a p r o c e s s .
M a n y e x a m s are in m u l t i p l e - c h o i c e format. H e r e are s o m e tips to m a x i m i z e y o u r perf o r m a n c e on m u l t i p l e - c h o i c e questions.
Test-Taking Tips for Multiple-Choice Exams 1 0 . A n s w e r the q u e s t i o n s y o u k n o w first. Flag items w h e r e y o u get stuck and c o m e b a c k to t h e m later. Often y o u will find t h e a n s w e r you are l o o k i n g for e m b e d d e d in another, easier question. 9 . Try t o a n s w e r t h e i t e m w i t h o u t l o o k i n g a t t h e o p t i o n s . T h e n c h e c k t o see i f y o u r a n s w e r m a t c h e s any of the options. 8. E l i m i n a t e t h e o p t i o n s . Treat each alternative a n s w e r as a t r u e - f a l s e item. If "false," e l i m i n a t e it. 7. U s e c o m m o n sense. R e a s o n i n g is m o r e reliable than m e m o r y . 6. U n d e r l i n e key w o r d s in the s t e m . T h i s is a g o o d strategy w h e n y o u are stuck. It m a y h e l p you focus on w h a t question is really b e i n g asked. 5. If t w o alternatives l o o k similar, it is likely that o n e of t h e m is correct. 4 . A n s w e r all q u e s t i o n s . U n l e s s points are b e i n g subtracted for w r o n g a n s w e r s (rare), it pays to g u e s s w h e n y o u ' r e not sure. R e s e a r c h indicates that the i t e m with the m o s t w o r d s in the m i d d l e of t h e list is often the correct item. B u t be c a u t i o u s . Your professor m a y have read the research, t o o !
How to Be Successful in Geology (and Just About Any Other Challenging Course)
3 . D o n o t c h a n g e a n s w e r s . Particularly w h e n you are g u e s s i n g , y o u r f i r s t g u e s s i s often correct. C h a n g e a n s w e r s only w h e n you have a clear reason for d o i n g so. 2. If the first i t e m is correct, c h e c k t h e last. If it says "all (or n o n e ) of the a b o v e , " you obviously n e e d to read the other alternatives carefully. M i s s i n g an "all of the a b o v e " item is one of the most c o m m o n errors on a multiple-choice e x a m . It is easy to read carelessly w h e n you are a n x i o u s . 1. READ THE DIRECTIONS BEFORE YOU BEGIN!
Final Exam Week At the end of e a c h semester, in o n e or t w o w e e k s , you take an e x a m in each and every c o u r s e . Most of the e x a m s are c o m p r e h e n s i v e finals that cover the entire semester. D e a l i n g with finals week successfully is a major challenge. H e r e are s o m e tips that will ensure that y o u do y o u r best work d u r i n g final w e e k .
Tips for Surviving Finals Week B e o r g a n i z e d a n d s y s t e m a t i c . U s e the Final E x a m P r e p W o r k s h e e t ( A p p e n d i x B ) to help you get o r g a n i z e d for finals. U s e the E i g h t - D a y Study Plan ( A p p e n d i x A) for every c o u r s e w h e r e the final e x a m will be an important factor in d e t e r m i n i n g your g r a d e . Stick to priorities. Say no to distractions. Build in m o m e n t s of r e l a x a t i o n . Take regular short b r e a k s , exercise, and get e n o u g h sleep. Be confident. By n o w you have built up a g o o d set of study habits. You are a c o m p e t e n t learner.
Organizing for final e x a m s can be quite a c h a l l e n g e . As with any big project, s p e n d i n g organizing t i m e up front will pay big d i v i d e n d s . Modify the suggestions in the F i n a l E x a m Prep W o r k s h e e t to fit y o u r personal situation and n e e d s . We wish you s u c c e s s !
CHAPTER
1
The Earth System
• H o w w a s E a r t h ' s size first m e a s u r e d a n d w h a t is E a r t h ' s t o p o g r a p h i c relief? Brief a n s w e r : E r a t o s t h e n e s m e a s u r e d the c i r c u m f e r e n c e of the E a r t h s o m e t i m e a r o u n d 2 5 0 B.C. F i g u r e 1.2 s h o w s how. Key F i g u r e 1.3 shows E a r t h ' s t o p o g r a p h y with respect to sea level. T h e sense of scale provided by Key F i g u r e 1.3 is very important. • W h a t is E a r t h ' s internal s t r u c t u r e a n d c o m p o s i t i o n ? Brief answer: T h e crust, mantle, and core are the major concentric layers within the Earth. T h e s e layers and their subdivisions are thought to represent changes in composition and in physical state (solid versus liquid). Preview Figures 1.5, 1.7, and Key Figures 1.8 and 1.11. • W h a t are t h e m a j o r c o m p o n e n t s of the E a r t h S y s t e m ? Brief a n s w e r : Earth S y s t e m F i g u r e 1.10 illustrates Earth as a s y s t e m of interacting c o m p o n e n t s , including the a t m o s p h e r e , h y d r o s p h e r e , b i o s p h e r e , and interior. • W h a t w e r e s o m e k e y e v e n t s in the history of the E a r t h ? Brief a n s w e r : Figure 1.13 presents the highlights. F o c u s on the c o n n e c t i o n s b e t w e e n g e o l o g i c and life history.
During Lecture This will be your very first lecture for this course. Get off to a good start. Arrive 10 minutes early. Find the best seat in the house: close to the front of the room, where you can hear the lecture and see the slides. This section is referred to by some as the "A section." Quite apt, since there really is a correlation between where you sit in a lecture hall and the grade you are likely to receive. Test this out. Try sitting in different locations the first week. Notice where it is easiest to concentrate and where it is most difficult. In many lecture halls the back row is the worst place to be. It is the location chosen by students w h o arrive late and leave early. It may even be noisy. A l w a y s feel free to change locations if your view is impeded by projection equipment or if you c a n ' t hear because of the whispered conversations of other students. Talk to your instructor if you need to. Okay, you have g o o d seat. W h a t can you do w h i l e you wait for lecture to b e g i n ? • M o t i v a t e yourself to w a n t to listen to this l e c t u r e . O p e n y o u r text to C h a p t e r 1. T h u m b through the chapter. Take a g o o d l o o k at the p h o t o s and figures. L o o k for topics that interest you. C h a p t e r 1 is loaded with visual material that should m a k e it easy to find subjects of interest. A s k yourself w h a t y o u w o u l d like to k n o w about this chapter. W h a t w o u l d you ask y o u r teacher if this w e r e a o n e - o n - o n e tutorial? Finally, try to think of s o m e e x p e r i e n c e you h a v e h a d that relates to this chapter. M a y b e you visited a p l a n e t a r i u m or read a catchy version of the formation of the universe like Cosmic Comics by Italo C a l v i n o . Or m a y b e you saw a v o l c a n o on y o u r last vacation. Actively l o o k for e x p e r i e n c e s to h o o k y o u r interest and to c o n n e c t you personally with c o u r s e inform a t i o n . Notice that the m o r e you look at the c h a p t e r and the m o r e you let yourself think about the pictures, the m o r e y o u r interest builds. Five m i n u t e s should b e e n o u g h t o get y o u r "motivational e n g i n e " turned o n and h u m m i n g . • P r e p a r e y o u r m i n d for learning. M a s t e r football players w a r m up by r u n n i n g , stretching, and passing the football. M a s t e r learners w a r m up by focusing their attention on what will be covered during lecture. S p e n d a m i n u t e or t w o looking over the C h a p t e r P r e v i e w questions. Try to anticipate h o w these questions will be a n s w e r e d in lecture. If you have time, read the C h a p t e r S u m m a r y for C h a p t e r 1. W h e n you preview, the goal is not to learn the material but merely to formulate questions that you expect to be a n s w e r e d during the lecture. W i t h questions in m i n d you are ready to take notes.
Learning Tip Do a learning w a r m - u p before every lecture. Arrive 10 m i n u t e s early. A n t i c i p a t e questions that will be a n s w e r e d in lecture.
The Earth System
The
man
who
is afraid of asking
is ashamed of learning. — D A N I S H PROVERB
After Lecture Review Notes The perfect t i m e to r e v i e w y o u r notes is right after the lecture w h i l e the material is still fresh in your m i n d . R e v i e w to be sure y o u o b s e r v e d all the key points a n d w r o t e t h e m d o w n in a form that will be r e a d a b l e later. As y o u review, y o u c a n polish y o u r notes by a d d i n g useful visual material and a s u m m a r y .
Check Your Notes: Have y o u . . . . • added a s i m p l e sketch or t w o to clarify the key p o i n t s ? H i n t : Try s k e t c h i n g y o u r personal v e r s i o n of h o w c o n v e c t i o n drives plate tectonics b a s e d on F i g u r e 1.11. M a k e it s i m p l e — s o m e t h i n g y o u will easily picture a n d r e m e m b e r . • written a brief ( o n e - p a r a g r a p h ) s u m m a r y of the m o s t i m p o r t a n t c o n c e p t y o u learned from this lecture? Feel free to u s e y o u r notes a n d figures in y o u r text as n e e d e d . Reviewing preview questions may help.
You will spend s o m e of y o u r study t i m e r e a d i n g Understanding Earth, particularly sections of the text that are e m p h a s i z e d in lecture. T h e text is l o a d e d with tools a n d c l u e s to h e l p y o u learn. You will find o u r s u g g e s t i o n s a b o u t h o w to take a d v a n t a g e of t h e s e learning aids in Part I, Chapter 2, M e e t Your A u t h o r s , in this S t u d y G u i d e .
Web Site Study Resources http://www.whfreeman.com/understandingearth5e Check out the W e b site to get an idea of the study aids that are available there. You will find Concept Review, W e b R e v i e w Q u e s t i o n s , G r a d e d Q u i z z i n g , O n l i n e R e v i e w Exercises, a n d Flashcards (to help you learn n e w terms). T h e Chapter 2 O n l i n e R e v i e w Exercise: Identify the Plate Boundaries is a g o o d review of plate tectonics; use it for both C h a p t e r 1 and C h a p t e r 2.
Exam Prep Materials in this section are m o s t useful d u r i n g p r e p a r a t i o n for e x a m s . T h e C h a p t e r S u m mary and the P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s should simplify y o u r c h a p t e r review. R e a d the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful o v e r v i e w that should refresh y o u r m e m o r y . Next, w o r k o n the P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d an e x a m , to see h o w well y o u h a v e m a s t e r e d this chapter. After you a n s w e r the q u e s t i o n s , score t h e m . Finally, a n d m o s t i m p o r t a n t , r e v i e w e a c h q u e s tion you missed. Identify a n d correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the question incorrectly.
Chapter Summary How do geologists study Earth? • Field and lab observations, experiments, and the h u m a n creative process help geoscientists formulate hypotheses (models) for h o w the Earth works and its history. A " h y p o t h e s i s " is a tentative explanation that can help focus attention on
21
22
PART II C H A P T E R 1
plausible features and relationships of a working m o d e l . If a hypothesis is eventually confirmed by a large body of data, it m a y be elevated to a "theory." Theories are a b a n d o n e d when subsequent investigations show t h e m to be false. Confidence g r o w s in theories that withstand repeated tests and are able to predict the results of new experiments and c o m p u t e r m o d e l s .
How was Earth's size first measured and what is Earth's topographic relief? • F i g u r e 1.2 illustrates h o w E r a t o s t h e n e s m e a s u r e d E a r t h ' s size. • E a r t h ' s t o p o g r a p h y is m e a s u r e d with respect to sea level. T h e lowest point in the Pacific O c e a n is actually d e e p e r than M o u n t E v e r e s t rises a b o v e sea level; see Key F i g u r e 1.3.
What is Earth's internal structure and composition? • E a r t h ' s interior c o n s i s t s of c o n c e n t r i c layers d i s t i n g u i s h e d by differences in c o m p o s i t i o n and physical state (solid versus liquid) and a s s o c i a t e d c h a n g e s in density. • E a r t h ' s crust (continental and o c e a n ) is c o m p o s e d of low-density silicate r o c k s , w h i c h are rich in a l u m i n u m and p o t a s s i u m . Refer to F i g u r e 1.7. T h e crust essentially floats on the m a n t l e , like an iceberg floats in water. • T h e m a n t l e is c o m p o s e d of silicate r o c k s of h i g h e r d e n s i t y than the crust and contains m o r e m a g n e s i u m and iron. T h e M o h o i s the b o u n d a r y b e t w e e n the crust and m a n t l e . • T h e c o r e is t h o u g h t to be metallic, c o m p o s e d mostly of iron and nickel. • S e i s m i c w a v e s allow geoscientists to detect the p r e s e n c e s of a liquid outer c o r e and an inner solid c o r e . C h a n g e s (jumps) in density b e t w e e n E a r t h ' s major layers are c a u s e d by c h a n g e s in their c h e m i c a l c o m p o s i t i o n and can be studied using s e i s m i c w a v e s .
What are the major components of the Earth system? • T h e major c o m p o n e n t s of the Earth s y s t e m are c l i m a t e , plate t e c t o n i c s , and the geodynamo. • T h e c l i m a t e system is driven by heat from our S u n and is influenced by the h y d r o s p h e r e , the c r y o s p h e r e , the b i o s p h e r e , and the lithosphere. • T h e plate tectonic s y s t e m is primarily driven by E a r t h ' s internal heat, w h i c h e s c a p e s t h r o u g h the c o n v e c t i o n of material in E a r t h ' s solid m a n t l e . Tectonic plates consist of the lithosphere, w h i c h includes the continental and o c e a n i c crust, and the rigid, u p p e r m o s t m a n t l e . • T h e g e o d y n a m o s y s t e m involves interactions in the o u t e r c o r e that p r o d u c e E a r t h ' s m a g n e t i c field. • E a r t h ' s m a g n e t i c field reverses in polarity (the north and south p o l e s flip) at irregular intervals.
What were some key events in the history of the Earth? • Earth formed 4.5 billion years a g o . • Earliest life forms are found in rocks about 3.5 million y e a r s old.
The Earth System • By 2.7 billion years ago, photosynthesis by early plant life p r o d u c e d increasing levels of o x y g e n in E a r t h ' s a t m o s p h e r e . • Only one-half billion years a g o animals appeared, diversifying rapidly in an explosion of evolution. • Life history is m a r k e d by periodic m a s s extinctions. A b o u t 65 million years ago, d i n o s a u r s w e r e killed off in an extinction event that w a s caused by a large bolide impact. • Our species (Homo sapiens) arrived 2 0 0 , 0 0 0 years a g o — a tiny fraction of the 4.5-billion-year history of the Earth.
Practice Exercise Answers and e x p l a n a t i o n s are p r o v i d e d at the end of the Study G u i d e .
Earth's topography A. W h e r e is the highest point on Earth relative to sea level? B. W h e r e is the lowest t o p o g r a p h i c point on E a r t h ? C. Relative to sea level, h o w m u c h l o w e r is the lowest point c o m p a r e d to the highest point? D. W h a t is the total t o p o g r a p h i c relief on the E a r t h ? Hint: Refer to p a g e 4 and Key F i g u r e 1.3.
Review Questions Answers and e x p l a n a t i o n s are provided at the end of the Study G u i d e . 1. W h a t p u r p o s e d o e s a scientific h y p o t h e s i s serve? A. B. C. D.
It It It It
represents a w i d e l y a c c e p t e d explanation. provides an opportunity for f e e d b a c k and testing. serves little p u r p o s e as part of the scientific m e t h o d . typically results in u n a n i m o u s a g r e e m e n t b e t w e e n scientists.
2. Of the following statements about the scientific m e t h o d , w h i c h o n e is true? A. A hypothesis that has w i t h s t o o d m a n y scientific tests is called a theory. B. T h e o u t c o m e of scientific e x p e r i m e n t s c a n n o t be predicted by a hypothesis. C. For a h y p o t h e s i s to be a c c e p t e d , it m u s t be agreed on by m o r e than o n e scientist. D. After a theory is proven to be true, it m a y not be discarded. Hint: Figure 1.1 is an excellent o v e r v i e w of the scientific m e t h o d . 3. How did E r a t o s t h e n e s m e a s u r e the c i r c u m f e r e n c e of E a r t h ? A. He m e a s u r e d the t i m e it takes for a sailing ship to appear fully on the horizon. B. He m e a s u r e d the difference in the angle of sunlight hitting the Earth at the s u m m e r solstice at t w o different places in E g y p t . C. He used data logged by C o l u m b u s . D. He m e a s u r e d the c h a n g e in the position of stars at night at t w o different p l a c e s in Egypt.
23
4. T h e principle of uniformitarianism can be s u m m a r i z e d by w h i c h s t a t e m e n t ? A. T h e E a r t h ' s surface and life c h a n g e very slowly o v e r t i m e . B. H u m a n activity has a small but significant effect on the Earth. C. T h e present is the key to the past. D. T h e Earth s y s t e m is all parts of our planet and their interactions. 5. W h i c h of the following internal layers of the Earth is the m o s t m a s s i v e ? A. crust
C.
mantle
B . inner core
D.
outer c o r e
H i n t : Refer to Figure 1.5. 6. T h e crust is A. thickest in the continents. B. thickest in the o c e a n s . C. about the s a m e thickness in both the continents and o c e a n s . D. of a c o m p l e t e l y u n k n o w n thickness. H i n t : Refer to Figure 1.7. 7. W h i c h of the following e l e m e n t s is m o r e a b u n d a n t in the E a r t h ' s crust c o m p a r e d to the Earth as a w h o l e ? A. iron
C.
nickel
B. magnesium
D.
silicon
H i n t : Refer to Key Figure 1.8. 8. W h y do geoscientists think tectonic plates m o v e across the E a r t h ' s surface? A. Centrifugal force of E a r t h ' s rotation spins plates across the E a r t h ' s surface. B. Volcanic eruptions on the seafloor push tectonic plates apart. C. Tidal forces drive plate m o t i o n . D. M o v e m e n t of the plates is a result of convection in the m a n t l e . 9. T h e E a r t h ' s m a g n e t i c field is caused by A. the p e r m a n e n t m a g n e t i s m of the E a r t h ' s solid inner core. B. the flow of molten iron in the inner core. C. the g e o g d y n a m o , created by convection in the outer liquid core, and electrical currents. D. solar radiation b o m b a r d i n g the Earth. 10. E a r t h ' s m a g n e t i c poles A. reverse at irregular intervals. B. consist of four poles. C. are g e n e r a t e d by a p e r m a n e n t l y m a g n e t i z e d inner c o r e . D. are very stable and do not c h a n g e .
The Earth System
11. C o n v e c t i o n transfers heat (see Key F i g u r e 1.11) by the physical circulation of hot and cold matter. H o w d o e s it w o r k ? A. H e a t e d m a t t e r rises u n d e r the force of b u o y a n c y b e c a u s e it is less d e n s e . B. H o t m a t t e r within the m a n t l e sinks b e c a u s e it is denser. C. C o l d m a t t e r rises u n d e r the force of b u o y a n c y b e c a u s e it is less d e n s e . D. M e t e o r b o m b a r d m e n t acts to stir up the m a n t l e and drive convection. 12. W h e n did b i o l o g y ' s B i g B a n g (evolutionary e x p l o s i o n ) o c c u r ? A . about 6 5 million y e a r s a g o B . a b o u t 5 4 0 million years a g o C . a b o u t 2 2 0 0 million years a g o D . a b o u t 3 5 0 0 million years a g o
25
Plate Tectonics: The Unifying Theory
Before Lecture This is a particularly i m p o r t a n t chapter. Plate tectonics is the major c o n c e p t that guides m o d ern geology. Before y o u attend lecture be sure to spend s o m e t i m e previewing the chapter. Previewing will greatly increase y o u r u n d e r s t a n d i n g of the lecture (see the C h a p t e r P r e v i e w in Part I, C h a p t e r 3, H o w to Be Successful in G e o l o g y ) . F o r an efficient preview use the following questions.
Chapter Preview This chapter c o n t a i n s extraordinarily helpful figures and p h o t o s . T h e goal is to learn how the continents and o c e a n plates can float about the Earth. F o c u s y o u r C h a p t e r 2 study time on e x a m i n i n g the fascinating figures and p h o t o s of s o m e of E a r t h ' s m o s t amazing features. • W h a t is t h e t h e o r y of p l a t e t e c t o n i c s ? Brief answer: T h e theory of plate tectonics describes the m o v e m e n t of lithospheric plates and t h e forces acting b e t w e e n t h e m . It also explains the distribution of m a n y large-scale features that result from m o v e m e n t s at plate b o u n d a r i e s : m o u n t a i n c h a i n s , earthquakes, v o l c a n o e s , t o p o g r a p h y of the seafloor, and distribution of r o c k a s s e m b l a g e s a n d fossils. Refer to F i g u r e 2 . 5 , E a r t h ' s lithosphere is m a d e of m o v i n g plates, on p a g e 2 4 . • W h a t are s o m e of t h e g e o l o g i c c h a r a c t e r i s t i c s of plate b o u n d a r i e s ? Brief a n s w e r : Volcanoes a n d e a r t h q u a k e activity are c o n c e n t r a t e d along plate b o u n d a r i e s . W h e r e divergent plate b o u n d a r i e s are e x p o s e d on land, subsiding basins and v o l c a n i s m are typical. M o u n t a i n chains form along convergent and transform plate b o u n d a r i e s . It is i m p o r t a n t to visualize each major type of plate b o u n d a r y and their locations on Earth. U s e the following figures in the text to do so: F i g u r e s 2 . 5 , 2.6, 2.7, and 2.8 for divergent plate b o u n d a r i e s ; F i g u r e s 2.5 and 2.6 for convergent plate b o u n d a r i e s ; a n d F i g u r e s 2 . 5 , 2.6, a n d 2.9 for a transform b o u n d a r y . If y o u h a v e seen or lived near the San A n d r e a s , a v o l c a n o , or a rift valley, like the R i o G r a n d e in N e w M e x i c o , start with the actual feature. • H o w c a n t h e a g e of t h e seafloor be d e t e r m i n e d ? Brief answer: A pivotal discovery in the history of plate tectonics w a s the determination of the age of the seafloor u s i n g m a g n e t i c a n o m a l i e s . F i g u r e 2.10 s h o w s h o w the age was d e t e r m i n e d . • H o w is t h e h i s t o r y of plate m o v e m e n t r e c o n s t r u c t e d ? Brief answer: T r a n s f o r m b o u n d a r i e s indicate the directions of relative plate m o v e m e n t , and seafloor i s o c h r o n s reveal the positions of divergent b o u n d a r i e s in earlier t i m e s . Refer to F i g u r e s 2.11 and 2 . 1 2 . • W h a t drives p l a t e t e c t o n i c s ? Brief a n s w e r : E a r t h ' s internal heat creates convective currents (flow of r o c k material from hotter to c o o l e r areas) in the m a n t l e . C o n v e c t i o n , the force of gravity, a n d the existence of an a s t h e n o s p h e r e are all i m p o r t a n t factors in any explanation of plate m o v e m e n t . R e v i e w Key F i g u r e 1.11 to see h o w convection w o r k s .
Vital Information from Other Chapters Review Key Figure 1.11 in Chapter 1 to remind yourself h o w convection drives plate tectonics.
27
CHAPTER
2
Plate Tectonics: The Unifying Theory From time to time in the history of science, a fundamental concept appears that unifies a field of study by pulling together diverse theories and explaining a large body of observations. Such a concept in physics is the theory of relativity; in chemistry, the nature of the chemical bond; in biology, DNA; in astronomy, the Big Bang; and in geology, plate tectonics. —UNDERSTANDING EARTH,
FRANK P R E S S AND RAYMOND SIEVER
Plate Tectonics: The Unifying Theory
T h e r e are t w o i m p o r t a n t caveats h e r e . First, you n e e d to budget y o u r t i m e . F o c u s on very i m p o r t a n t material, such as the figures we suggested in the preview questions. You d o n ' t h a v e t i m e to r e d o all the a r t w o r k in Understanding Earth. S e c o n d , the art in the text is especially well d o n e . It includes m a n y details you cannot easily execute in your notes. A l s o t h e c a p t i o n s in the text are very helpful, so w h e n y o u are w o r k i n g on sketches be sure to refer to the text figure and caption. Get off to a g o o d start. Try this idea for C h a p t e r 2 and o b s e r v e h o w it w o r k s for you. Modify it to fit y o u r learning style.
Intensive Study Session Set priorities for studying this chapter. T h e r e is a lot to d o , probably m o r e than you will have time for in one intensive study session. Set priorities and always do the important things first. • Instructor. Pay particular attention to any exercises r e c o m m e n d e d by y o u r instructor d u r i n g lecture and a l w a y s a n s w e r t h e m first. Your instructor is also your best r e s o u r c e if you are w o n d e r i n g w h i c h material is m o s t important. • Practice E x e r c i s e s a n d R e v i e w Q u e s t i o n s . U s e the Study G u i d e Practice Exercises and R e v i e w Q u e s t i o n s . Be sure to do E x e r c i s e 1 b e c a u s e it involves the key information you need to learn in this chapter. • Text. W o r k on y o u r r e s p o n s e s to E x e r c i s e s 2, 4, and 5 and T h o u g h t Q u e s t i o n s 1, 3, and 6 at the end of C h a p t e r 2 in the textbook. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e the C o n c e p t S e l f - C h e c k e r and W e b R e v i e w Q u e s t i o n s . Pay particular attention to the e x p l a n a t i o n s of the a n s w e r s .
Exam Prep Materials in this section are m o s t useful d u r i n g y o u r preparation for e x a m s . T h e C h a p t e r S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r c h a p t e r review. R e a d the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y . Next, w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as you w o u l d an e x a m , to see h o w well you h a v e m a s t e r e d this chapter. After you a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t important, review e a c h q u e s tion you m i s s e d . Identify a n d correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the question incorrectly.
Chapter Summary What is the theory of plate tectonics? • For over the last century s o m e geologists have argued for the concept of continental drift because of the j i g s a w - p u z z l e fit of the coasts on both sides of the Atlantic, the geological similarities in rock ages and trends in geologic structures on opposite sides of the Atlantic, fossil evidence suggesting that continents w e r e j o i n e d at one time, and the distribution of glacial deposits as well as other paleoclimatic evidence.
29
Web Site Study Resources http://www.whfreeman.com/understandingearth5e Identify the Earth's Plates and Identify the Plate Boundaries are Interactive Exercises w o r t h c o m p l e t i n g before t h e first lecture on this topic.
During Lecture O n e goal for the lecture should be to leave the class with good a n s w e r s to the preview questions. • To avoid getting lost in details, k e e p the big picture in m i n d . C h a p t e r 2 tells the story of plate tectonics: Earth's moving plates, how the plates m o v e , and the g e o l o g i c a l features associated with converging and diverging plate b o u n d a r i e s . Plate tectonics underlies and explains m u c h about m o d e r n geology. In that sense, this chapter provides a preview of y o u r entire g e o l o g y c o u r s e . • Focus on understanding Figure 2.6. It will be helpful to have this figure handy during lecture. Annotate the text figures with c o m m e n t s m a d e by your instructor. • If you h a v e n ' t already d o n e so, read the discussion of n o t e t a k i n g in Part I, C h a p t e r 3, H o w to Be Successful in G e o l o g y . H i n t : You can u s e the N o t e T a k i n g C h e c k l i s t before you go to lecture as a o n e - m i n u t e r e m i n d e r of w h a t to do to i m p r o v e y o u r note-taking skills. After lecture, use it as a quality c h e c k .
After Lecture Review Notes T h e perfect t i m e to review y o u r notes is right after lecture. T h e following checklist c o n t a i n s both general review tips and specific suggestions for this chapter.
Check Your Notes: Have y o u . . . • m a r k e d areas w h e r e you d o n ' t r e m e m b e r w h a t w a s said? Do y o u n e e d a f o l l o w - u p session with y o u r instructor, tutor, or study partner to get w h a t y o u m i s s e d ? • a d d e d visual m a t e r i a l ? E x a m p l e : For the lecture on C h a p t e r 2, it is very i m p o r t a n t to distinguish the difference b e t w e e n diverging a n d converging plates ( F i g u r e 2.6). You could insert a visual c u e about this distinction into y o u r n o t e s . F o r e x a m p l e , the simplest possible representation of Figure 2.6 w o u l d be t w o a r r o w s p o i n t e d a w a y from e a c h other. You also w a n t to r e m e m b e r that d i v e r g e n c e can c a u s e b o t h r i d g e s and valleys. You could d r a w a r i d g e (zigzag lines) and a valley ( d r o p p i n g plates) b e t w e e n y o u r arrows
Study Tip: Learn by drawing S k e t c h i n g simplified versions of figures in y o u r notes is a helpful w a y to learn and r e m e m b e r . Visual learners will r e m e m b e r material best after they l o o k at and study a figure. Visual learners learn m o r e if they enrich their notes with visual c u e s . F o r kinesthetic learners m e m o r y is activated by the act of d r a w i n g , so you learn as y o u l o o k and draw. As you take notes, be sure to leave r o o m so that y o u can insert material later. T h e ultimate goal is notes you can study from.
Plate Tectonics: The Unifying Theory
31
typically f o r m a l o n g c o n v e r g e n t a n d t r a n s f o r m p l a t e b o u n d a r i e s . W h e r e divergent p l a t e b o u n d a r i e s are e x p o s e d o n land, s u b s i d i n g b a s i n s a n d volcanism are typical.
How can the age of the seafloor be determined? • T h e a g e and relative plate velocity are inferred from c h a n g e s in the p a l e o m a g n e t i c p r o p e r t i e s of the o c e a n floor. • Various o t h e r m e t h o d s a r e n o w u s e d to m e a s u r e the rate and d i r e c t i o n of plate m o v e m e n t s . R e l a t i v e p l a t e s p e e d s in m i l l i m e t e r s p e r y e a r are s h o w n in Figure 2.5.
How is the history of plate movement reconstructed? • Seafloor i s o c h r o n s p r o v i d e the basis for reconstructing plate m o t i o n s for about the last 2 0 0 million years. • Distinct a s s e m b l a g e s of r o c k s characterize each type of plate boundary. U s i n g diagnostic r o c k a s s e m b l a g e s e m b e d d e d in continents and p a l e o e n v i r o n m e n t a l data r e c o r d e d by fossils and s e d i m e n t a r y r o c k s , geologists have b e e n able to reconstruct ancient plate tectonic events and plate configurations.
What is the engine that drives plate tectonics? • Driven by E a r t h ' s internal heat, convection within the mantle, c o u p l e d with the force of gravity and the existence of a soft z o n e , called the a s t h e n o s p h e r e , are important factors in m o d e l s for h o w plate tectonics w o r k s .
Practice Exercises Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e .
Exercise 1: Characteristics of active tectonic plate boundaries Complete the table by filling in the blank spaces and b o x e s .
Convergent See Figures 2.5 and 2.9.
Divergent Characteristics
See Figures 2.5, 2.6, 2.7, and 2.8.
Examples
Topography
Volcanism
Ocean/ Ocean J a p a n e s e islands Marinas Trench Aleutian T r e n c h
oceanic ridge, rift valley, ocean basins, ocean floor features offset by transforms, seamounts
trench, island arc
Ocean/ Continental
Collision
Transform See Figures2.5, 2.6, and 2.9.
H i m a l a y a s and Tibetan Plateau
offset of creek beds and other topographic features that cross the fault
present
not characteristic
30
PART II C H A P T E R 2
• In the last half of the t w e n t i e t h c e n t u r y the m a j o r e l e m e n t s of the plate t e c t o n i c theory w e r e f o r m u l a t e d . Starting in the 1940s, o c e a n floor m a p p i n g b e g a n to reveal major g e o l o g i c features on the o c e a n floor. T h e n the m a t c h b e t w e e n m a g n e t i c a n o m a l y p a t t e r n s o n t h e seafloor with the p a l e o m a g n e t i c t i m e scale revealed that the o c e a n floor h a d a y o u n g g e o l o g i c age a n d w a s s y s t e m a t i c a l l y o l d e r a w a y from the o c e a n i c r i d g e s y s t e m s . T h e c o n c e p t s o f seafloor s p r e a d i n g , s u b d u c t i o n , and t r a n s f o r m faulting e v o l v e d f r o m t h e s e and o t h e r o b s e r v a t i o n s . • According to the theory of plate tectonics, the Earth's lithosphere is broken into over a dozen moving plates. T h e plates slide over a hot and w e a k asthenosphere, and the continents, e m b e d d e d in s o m e of the m o v i n g plates, are carried along. T h e assembly breakup of Pangaea (see Figure 2.12) provides a dramatic e x a m p l e of h o w the m o v e m e n t of plates creates geography and geological features such as the H i m a l a y a M o u n t a i n s and even changes climate as land masses travel about. (Note Antarctica's location during the late Proterozoic. Would Antarctica have been covered with ice at that time? W h a t would its climate have been like?)
What are some of the geologic characteristics of plate boundaries? • T h e r e are t h r e e major t y p e s of b o u n d a r i e s b e t w e e n l i t h o s p h e r i c p l a t e s : diverg e n t b o u n d a r i e s , w h e r e p l a t e s m o v e apart; c o n v e r g e n t b o u n d a r i e s , w h e r e p l a t e s m o v e t o g e t h e r and o n e plate often s u b d u c t s b e n e a t h t h e o t h e r ; a n d transform boundaries, w h e r e plates slide past each other. Volcanoes, e a r t h q u a k e s and mountains are concentrated along the active plate b o u n d a r i e s . M o u n t a i n s
Exercise 2: Construct a conceptual flowchart or diagram illustrating how plate tectonics works Construct a c o n c e p t u a l flowchart or d i a g r a m that illustrates m o d e r n ideas on h o w plate tectonics w o r k s . Include t h o r o u g h captions or a written description of h o w it w o r k s . Be sure the roles p l a y e d by the following important factors are a d d r e s s e d . • E a r t h ' s internal heat
• density differences
• convection
• lithosphere
• asthenosphere
• the push and pull of gravity
• divergent and convergent b o u n d a r i e s H i n t s : Refer to p a g e s 1 2 - 1 3 and 3 7 ^ 1 7 in the text. T h i s is a c o m p l e x task. Break it d o w n into pieces. First, m a k e a sketch for each factor on separate pieces of paper. T h e n ask yourself w h e r e (in w h i c h E a r t h layer) e a c h p r o c e s s o p e r a t e s and arrange y o u r sketches accordingly. Finally, try to c o m b i n e all this material into a single chart.
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. T h e m o s t important p r o c e s s in building the o c e a n floor is A. v o l c a n i s m . B . subduction.
C. e a r t h q u a k e activity. D . m a g n e t i c reversal.
2. T h e y o u n g e s t ocean crust is located A. along the o c e a n i c ridges.
C.
B. in the o c e a n i c t r e n c h e s .
D. on the abyssal seafloor.
a r o u n d hot spots.
3. Rates of seafloor spreading today are in A. m i l l i m e t e r s p e r year.
C.
m e t e r s per year.
B. c e n t i m e t e r s per year.
D. kilometers p e r year.
H i n t : Refer to F i g u r e 2 . 5 . 4. A l o n g a transform plate boundary, the t w o plates A. m o v e apart to create a w i d e n i n g rift valley. B. are being c o n s u m e d by subduction. C. are being forced together to p r o d u c e a m o u n t a i n s y s t e m . D. m o v e horizontally past e a c h other. 5. T h e
is an e x a m p l e of a divergent plate m a r g i n .
A. E a s t African Rift B. J a p a n Trench
C. H i m a l a y a s D. San A n d r e a s fault
6. All the following features m a r k plate b o u n d a r i e s except A. the R e d Sea.
C. the H a w a i i a n Islands.
B. the San A n d r e a s fault.
D. Iceland.
H i n t : Refer to Figure 2.5. 7. W h i c h of the following features is not associated with a t y p e of active plate b o u n d a r y ? A. Atlantic coast of North A m e r i c a B . n o r t h w e s t e r n North A m e r i c a
C. Gulf of California D. Himalayas
8. T h e n a m e given to the supercontinent that b r o k e up in the Early Jurassic (195 M a ) w a s A. India.
C. L a u r a s i a .
B . Atlantis.
D. Pangaea.
H i n t : Refer to Figure 2 . 1 3 .
Plate Tectonics: The Unifying Theory 9. M i d - o c e a n ridges, a c c o r d i n g to the plate tectonic theory, are A. B. C. D.
p l a c e s w h e r e o c e a n i c crust is c o n s u m e d . pull-apart z o n e s w h e r e new oceanic crust is p r o d u c e d . locations of plate c o n v e r g e n c e . t r a n s f o r m faults.
10. T h e
are an e x a m p l e of a collision z o n e b e t w e e n t w o
pieces of continental crust riding on converging lithospheric plates. A. H i m a l a y a M o u n t a i n s
C.
B. islands of J a p a n
D. A n d e s M o u n t a i n s in South A m e r i c a
A l e u t i a n Islands in A l a s k a
11. At w h i c h t y p e of plate b o u n d a r i e s are v o l c a n o e s least likely to form? A. divergent b o u n d a r i e s
C.
B. c o n v e r g e n t b o u n d a r i e s
D. hot spots
transform b o u n d a r i e s
12. D u r i n g the early Triassic (237 M a ) , A. India collided with A s i a to form the H i m a l a y a n M o u n t a i n s . B. the s u p e r c o n t i n e n t R o d i n i a formed. C. the s u p e r c o n t i n e n t P a n g a e a w a s m o s t l y a s s e m b l e d . D. the Atlantic O c e a n had already b e g u n to o p e n . H i n t : Refer to F i g u r e 2 . 1 3 . 13. South A m e r i c a lay closest to t h e S o u t h P o l e during the A. late P r o t e r o z o i c ( 6 5 0 M a ) .
C. late Jurassic (152 M a ) .
B. last 65 million y e a r s .
D. early D e v o n i a n (390 M a ) .
H i n t : Refer to F i g u r e 2 . 1 3 . 14. T h e significance of the m a g n e t i c a n o m a l y patterns discovered in association with the seafloor w a s that the a n o m a l y patterns A. c o u l d be m a t c h e d with the m a g n e t i c reversal c h r o n o l o g y to establish an e s t i m a t e d age for the seafloor. B. p r o v i d e d e v i d e n c e for m a n t l e convection, a driving m e c h a n i s m for plate tectonics. C. a l l o w e d g e o m a g n e t i s t s to reconstruct the supercontinent Rodinia. D. r e p r e s e n t e d absolute proof that the seafloor w a s spreading apart. H i n t : Refer to F i g u r e 2.10. 15. T h e oldest r o c k s on the seafloor are about A. 20 million years old.
C.
B. 2 0 0 million years old.
D. 1 billion years old.
5 0 0 million years old.
H i n t : Refer to F i g u r e 2.12. 16. T h e o c e a n i c crust A . b e c o m e s progressively y o u n g e r a w a y from the oceanic ridges. B . b e c o m e s progressively older a w a y from the oceanic ridges. C. is the s a m e age virtually e v e r y w h e r e . D. r a n g e s in age from Jurassic to P r e c a m b r i a n . 17. Volcanic island arcs like the J a p a n e s e islands are associated with A. c o n v e r g e n t b o u n d a r i e s .
C.
transform b o u n d a r i e s .
B. divergent b o u n d a r i e s .
D.
a chain of hot spots.
A. c o n v e r g e n t b o u n d a r i e s .
C.
transform b o u n d a r i e s .
B . divergent b o u n d a r i e s .
D . active continental m a r g i n s .
H i n t : Refer to F i g u r e 2.6. 18. Rift valleys are associated with
H i n t : Refer to F i g u r e 2.6.
33
34
PART II C H A P T E R 2
19. W i t h w h a t tectonic activity is a rift valley usually a s s o c i a t e d ? A. s u b d u c t i o n
C.
B. m o v e m e n t on a transform fault
D. c o n t i n e n t a l collision
continental r u p t u r e
2 0 . M a g n e t i c a n o m a l i e s in the seafloor are c a u s e d by A. m a g n e t i c reversals r e c o r d e d by lavas e r u p t e d at o c e a n i c s p r e a d i n g c e n t e r s . B. c h a n g e s in the a t o m i c structure of m i n e r a l s in r e s p o n s e to c h a n g i n g o c e a n d e p t h . C. the m e t a m o r p h i s m of d e e p - s e a s e d i m e n t s . D. the heating up of subducting oceanic lithosphere as it plunges d e e p e r into the mantle. H i n t : R e v i e w p a g e s 1 3 - 1 4 and F i g u r e 2.10. 2 1 . W h e r e d o the plate-driving forces originate? A. Tectonic plates are passively d r a g g e d by c o n v e c t i o n c u r r e n t s . B. Gravity pulls and/or p u s h e s old, cold, h e a v y l i t h o s p h e r e . C. Injection of m a g m a from the m a n t l e p u s h e s the l i t h o s p h e r e apart. D . E a r t h q u a k e s c a u s e the plates t o m o v e . 2 2 . E a r t h ' s lithosphere can be characterized as A. h a v i n g an average t h i c k n e s s of about 100 k m . B . i n c l u d i n g the crust and u p p e r m a n t l e . C. b e i n g solid and a b o v e the a s t h e n o s p h e r e . D. all of the a b o v e .
CHAPTER
3
Earth Materials: Minerals and Rocks
Before Lecture Chapter Preview • W h a t is a m i n e r a l ? Brief a n s w e r : A naturally occurring inorganic, crystalline solid. • H o w are r o c k s related to m i n e r a l s ? Brief a n s w e r : M i n e r a l s are the building b l o c k s of r o c k s . Study Figure 3.21 (p. 3 5 ) . • H o w d o a t o m s c o m b i n e t o f o r m the crystal s t r u c t u r e o f m i n e r a l s ? Brief answer: H o w atoms c o m b i n e (chemically bond) d e p e n d s on the electronic charge on their ion, the size of the ion, and the degree to which the b o n d is ionic or covalent. • W h a t is t h e a t o m i c s t r u c t u r e of m i n e r a l s ? H i n t : See Figures 3.3 (halite), 3.8 (graphite and d i a m o n d ) , 3.9 (silicates), 3.10 (calcite), and 3.15 (mica). • W h a t a r e t h e m a j o r r o c k - f o r m i n g m i n e r a l s a n d t h e i r physical p r o p e r t i e s ? Brief answer: T h e major minerals are silicates, carbonates, oxides, sulfides, and sulfates. Diagnostic properties include hardness, cleavage, luster, color, density, and the shape of the crystal. • W h a t d e t e r m i n e s t h e p r o p e r t i e s of r o c k s ? Brief a n s w e r : M i n e r a l content and texture. • W h a t are the three t y p e s of r o c k s a n d h o w a r e they f o r m e d ? Brief a n s w e r : I g n e o u s , sedimentary, and m e t a m o r p h i c r o c k s . • H o w do t h e three t y p e s of r o c k s interact with e a c h o t h e r via plate t e c t o n i c s , t h e c l i m a t e s y s t e m , a n d the rock c y c l e ? Brief a n s w e r : Refer to Figures 3.22, 3 . 2 3 , 3.25, and 3.26. • W h a t is an ore m i n e r a l ? Brief a n s w e r : Mineral deposits b e c o m e ore deposits w h e n they are rich and v a l u a b l e e n o u g h t o m i n e economically. • H o w d o ore d e p o s i t s o f m e t a l - b e a r i n g m i n e r a l s f o r m ? Brief answer: M a n y metal ore deposits are f o r m e d by m a g m a t i c a n d h y d r o t h e r m a l p r o c e s s e s that are closely linked to m o d e r n and ancient plate tectonic b o u n d a r i e s . Refer to F i g u r e 3.28.
Previewing Tips Tip 1 To preview a chapter quickly, focus on the figures. It is often p o s s i b l e to get a g o o d o v e r v i e w j u s t by e x a m i n i n g the figures and c a p t i o n s . N o t e that preview q u e s t i o n s often refer you to specific figures. As little as 5 or 10 m i n u t e s p r e v i e w i n g t h e figures before lecture will i m p r o v e the quality of y o u r n o t e taking. Tip 2 T h e m o r e material a c h a p t e r contains the m o r e i m p o r t a n t it b e c o m e s to chunk material into a s c h e m a that is meaningful to you and is s i m p l e e n o u g h to r e m e m b e r . E x a m p l e : T h i n k of C h a p t e r 3 as a n s w e r i n g three simple q u e s t i o n s : (1) W h a t is a m i n e r a l ? (2) H o w is a m i n e r a l related to a r o c k ? (3) H o w do the three k i n d s of r o c k s interact with each other in the system called t h e rock c y c l e ? If you can r e m e m b e r those three questions and use t h e m to tie the details of the c h a p t e r together, y o u are likely to find that the c h a p t e r suddenly s e e m s m u c h simpler and that the material s e e m s m u c h easier to r e m e m b e r . H i n t : R e a d the section on P r e v i e w i n g in C h a p t e r 3 of Part I of this Study G u i d e .
Earth Materials: Minerals and Rocks
Minerals are
the alphabet and rocks are
the
words. — T O M DILLEY
Vital Information from Other Chapters • R e v i e w F i g u r e 1.8 (the relative a b u n d a n c e of e l e m e n t s in the w h o l e Earth c o m p a r e d to that of e l e m e n t s in E a r t h ' s crust) on p a g e 9 of the textbook. • Plate tectonic processes drive the formation of igneous and m e t a m o r p h i c rocks plus the burial sediments. It will be particularly useful to review C h a p t e r 2, especially the text a n d figures regarding converging and diverging plate boundaries.
During Lecture Do a learning w a r m - u p . A r r i v e early. L o o k for pictures in the text that catch y o u r interest. L o o k over the p r e v i e w q u e s t i o n s and k e e p t h e m h a n d y for e a s y reference. Your b a s i c goal d u r i n g lecture is to take g o o d notes. T h e notes should contain i n - d e p t h a n s w e r s to the p r e v i e w q u e s t i o n s . To avoid getting lost in details, k e e p the " b i g p i c t u r e " in mind. C h a p t e r 3 e x p l a i n s h o w r o c k s are built out of m i n e r a l s a n d h o w m i n e r a l s are built out of a t o m s . K e e p in m i n d the typical g e o l o g i c c o n d i t i o n s n e e d e d for the formation of the t h r e e m a j o r rock t y p e s . T h i s is nicely s u m m a r i z e d in F i g u r e 3.22. O p e n y o u r b o o k to this figure a n d put a b o o k m a r k there. You w o n ' t h a v e t i m e d u r i n g lecture. T h e lecturer m a y s h o w slides of m i n e r a l crystals and m a y pass a r o u n d s p e c i m e n s of particular m i n e r a l s a n d r o c k s . You will get m o r e out of the s a m p l e s if y o u sit c l o s e to the front of the r o o m w h e r e y o u c a n see the s a m p l e s as they are d i s c u s s e d . R e m e m b e r to focus on clues the lecturer p r o v i d e s for r e c o g n i z i n g a particular s a m p l e in the field. As y o u listen, try to formulate at least o n e g o o d q u e s t i o n y o u can ask d u r i n g the lecture.
The
important thing
is not to stop
questioning. — A L B E R T EINSTEIN
Lecture Tip: Rock texture The lecturer m a y s h o w or pass a r o u n d s a m p l e s of the three types of rocks to h e l p you learn h o w they differ. P a y p a r t i c u l a r attention to the crystal or grain size (texture) and to the p r e s e n c e or a b s e n c e of layering, fossils, and patterns p r o d u c e d by the alignment of m i n e r a l s . You will h a v e a better c h a n c e of s e e i n g the s a m p l e s if y o u sit at the front of the r o o m .
After Lecture Review Notes Right after lecture w h i l e the material is fresh in y o u r m i n d is the perfect t i m e to review y o u r notes. R e v i e w to be sure you u n d e r s t a n d all the key points and w r o t e t h e m d o w n in a form that will be r e a d a b l e later. O n e g o o d c h e c k is that y o u r notes should p r o v i d e detailed a n s w e r s
37
Earth Materials: Minerals and Rocks
Everything you do in life is worth infinite care and infinite effort. — J.D.
MCDONALD
Here are a few suggestions for h o w you might do an intensive study session for this chapter. • R e v i e w Q u e s t i o n s . Start y o u r study session by d e t e r m i n i n g h o w m u c h y o u already k n o w . Try a n s w e r i n g the S t u d y G u i d e R e v i e w Q u e s t i o n s . N o t i c e that you can c h e c k the a n s w e r s to t h e s e q u e s t i o n s at the e n d of the Study G u i d e . Often w e p r o v i d e s o m e additional information with a n answer. T h e R e v i e w Q u e s t i o n s are a great w a y to start studying b e c a u s e trying to a n s w e r t h e m will help you focus on w h a t y o u n e e d to w o r k on. Afterward y o u can go b a c k a n d read the text c o n c e r n i n g any p o i n t s y o u m i s s e d . • Practice E x e r c i s e s . I m m e d i a t e l y before the R e v i e w Q u e s t i o n s you will find Practice E x e r c i s e s . T h e s e exercises a l w a y s focus o n s o m e key material that you will learn best by an interactive a p p r o a c h that requires y o u to think. F o r C h a p t e r 3 the exercises deal with u n d e r s t a n d i n g h o w the structure of a m i n e r a l d e t e r m i n e s its p h y s i c a l p r o p e r t i e s . • Text. W o r k on E x e r c i s e s 5 a n d 12 at the e n d of the chapter. A l s o w o r k on T h o u g h t Q u e s t i o n s 4, 8, 11 (see F i g u r e 3.26), a n d 13. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e A lot of the fun of g e o l o g y h a s to do with figuring out the h o w s and w h y s of interesting and beautiful natural p h e n o m e n a . Go online and explore h o w the wondrous blue color of the H o p e D i a m o n d is related to its atomic structure and composition by d o i n g the G e o l o g y in P r a c t i c e exercises. T h e Rock Cycle Review Online Review Exercise provides an excellent tour through the rock cycle and h o w it w o r k s in the context of plate tectonics. O t h e r useful study aids on the W e b site: • F l a s h c a r d s to h e l p y o u learn n e w t e r m s • An online C h e m i s t r y R e v i e w containing an interactive periodic table of the elements • Additional C h a p t e r R e v i e w q u e s t i o n s Obviously you d o n ' t have time to do all these things for every chapter. T h e idea is to try out some of the tools and then decide w h i c h will be m o s t helpful given your personal learning style. " C r y s t a l " c o m e s f r o m t h e G r e e k w o r d krustallos,
w h i c h m e a n s ice. I s i c e — w a t e r
ice—a m i n e r a l ? H i n t : A s k y o u r s e l f if ice h a s t h e t h r e e p r o p e r t i e s of a m i n e r a l . (1) Is ice naturally o c c u r r i n g ? (2) D o e s it h a v e a solid c r y s t a l l i n e s t r u c t u r e ? (3) D o e s it h a v e a definite c h e m i c a l c o m p o s i t i o n ? ( S e e p a g e 4 5 i n t h e t e x t b o o k . )
Exam Prep Materials i n this s e c t i o n a r e m o s t useful d u r i n g y o u r p r e p a r a t i o n for m i d t e r m a n d final examinations. T h e C h a p t e r S u m m a r y a n d the P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s should simplify y o u r c h a p t e r review. R e a d the C h a p t e r S u m m a r y to begin y o u r session. It provides a helpful o v e r v i e w that s h o u l d refresh y o u r m e m o r y .
39
to all the C h a p t e r P r e v i e w questions. T h e following checklist c o n t a i n s both general review tips and specific suggestions for this chapter.
Check Your Notes: Have y o u . . . • a d d e d a c o m p a r i s o n chart to h e l p you sort out the key differences b e t w e e n the three rock t y p e s ? (See the e x a m p l e . ) • a d d e d simplified sketches of key text figures? Trying to simplify a figure is often the best w a y of studying it, since you get involved in the i m p o r t a n t details. E x a m p l e : D r a w a simplified version of F i g u r e 3.22 that will h e l p you r e m e m b e r the difference in texture (crystal size) b e t w e e n intrusive and extrusive i g n e o u s r o c k s .
Differences Between the Three Rock Types Igneous S o u r c e of materials
M e l t i n g of r o c k s in crust and m a n t l e
Sedimentary
Metamorphic
W e a t h e r i n g and e r o sion of rocks e x p o s e d at the surface
High temperatures and p r e s s u r e s in d e e p crust and u p p e r m a n t l e , b r e a k s from faulting or i m p a c t
R o c k - f o r m i n g process
Crystallization (solidification of melt)
Sedimentation, burial, and lithification ( c o m p a c t i o n and cementation)
S h e a r i n g and r e c r y s tallizing in the solid state
Refer to Figure 3.22 for the three different types of rock.
Intensive Study Session Study Tip: Intensive study session T h e p r o c e s s of learning g e o l o g y is similar to the p r o c e d u r e for building a h o u s e . Before each lecture, construct a frame of q u e s t i o n s . D u r i n g lecture, attach details and ideas to the frame. After lecture, m a s t e r the ideas d u r i n g an intensive study session. By learning about plate tectonics you have laid a g o o d foundation, but g e o l o g y is a skyscraper with 23 floors (one for each c h a p t e r ) . E a c h floor (chapter) s u p p o r t s t h e o n e a b o v e it. If you d o n ' t c o m p l e t e l y m a s t e r this chapter, the next o n e will be m o r e difficult. S c h e d u l e about an h o u r after each lecture for intensive study. D e v o t e this t i m e to m a s t e r i n g key c o n c e p t s . M a s t e r y is not gained by j u s t r e a d i n g the text. M a s t e r y o c c u r s as the result of asking yourself q u e s t i o n s and a n s w e r i n g t h e m . T h e P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s are d e s i g n e d to h e l p you r e a c h m a s t e r y quickly. T h e m o r e of these exercises and q u e s t i o n s you can w o r k into y o u r study s c h e d u l e early in t h e c o u r s e , the easier s u b s e q u e n t c h a p t e r s will be.
40
PART II
CHAPTER 3
N e x t , w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . T o d e t e r m i n e h o w well y o u h a v e m a s t e r e d this chapter, c o m p l e t e t h e e x e r c i s e s a n d q u e s t i o n s j u s t as y o u w o u l d a m i d t e r m . After y o u a n s w e r the q u e s t i o n s , s c o r e t h e m . Finally, review e a c h q u e s t i o n y o u m i s s e d . Identify and correct t h e m i s c o n c e p t i o n ( s ) that r e s u l t e d i n y o u r a n s w e r i n g the q u e s tion incorrectly.
Chapter Summary What are minerals? • M i n e r a l s are naturally o c c u r r i n g inorganic solids with a specific crystal structure and c h e m i c a l c o m p o s i t i o n .
How do atoms combine to form the crystal structure of minerals? • M i n e r a l s form w h e n a t o m s or ions c h e m i c a l l y b o n d in an orderly, t h r e e d i m e n s i o n a l g e o m e t r i c a r r a y — a crystal structure. T h e c h a r a c t e r o f the c h e m i cal b o n d c a n be ionic, covalent, or metallic. Ionic b o n d s are the d o m i n a n t t y p e of c h e m i c a l b o n d s in mineral structures.
What is the atomic structure of minerals? • T h e a t o m i c structure is d e t e r m i n e d by h o w the a t o m s or ions p a c k t o g e t h e r to form a crystalline solid. T h e p a c k i n g of a t o m s d e p e n d s on their size a n d the characteristics of c h e m i c a l b o n d s b e t w e e n the a t o m s .
Conceptual flowchart illustrating the factors that influence the physical properties of minerals
Composition Temperature Pressure
What are the major rock-forming minerals and what are their physical properties? • T h e strength of the c h e m i c a l b o n d s and the crystalline structure d e t e r m i n e s m a n y of the physical p r o p e r t i e s of m i n e r a l s , s u c h as h a r d n e s s and cleavage. • Silicate m i n e r a l s are the m o s t a b u n d a n t class of m i n e r a l s in the
Crystal structure — the way atoms are packed together + Type of chemical b o n d s ionic vs. covalent bonds
E a r t h ' s crust and m a n t l e . Isolated and chain silicates—olivine, pyroxene, and hornblende Sheet s i l i c a t e s — m i c a s and clay m i n e r a l s F r a m e w o r k silicates—feldspar and quartz • Other c o m m o n mineral classes include carbonates, oxides, sulfates, sulfides, h a l i d e s , and native m e t a l s .
Physical properties of minerals: Crystal shape Hardness Cleavage Color Electrical Optical
Why study minerals? • T h e c o m p o s i t i o n of r o c k s can be d e d u c e d from the c o m p o s i t i o n of its m i n e r a l s . • T h e e n v i r o n m e n t in w h i c h the r o c k f o r m e d c a n be inferred b e c a u s e m o s t m i n e r a l s are stable u n d e r only certain c o n d i t i o n s of t e m p e r a t u r e and pressure.
Earth Materials: Minerals and Rocks
• For fun and profit. G e m s t o n e s are treasured for their beauty, color, and rarity. M a n y m i n e r a l s are used in industrial p r o c e s s e s and manufacturing.
What determines the properties of rocks? • T h e properties and n a m e s of r o c k s are d e t e r m i n e d by mineral content (the kinds and p r o p o r t i o n s of minerals that m a k e up rocks) and texture (the size, shapes, and spatial a r r a n g e m e n t of a r o c k ' s crystals or grains).
What are the three types of rocks, and how are they formed? • T h e r e are t h r e e m a j o r r o c k t y p e s . I g n e o u s r o c k s solidify from m o l t e n liquid ( m a g m a ) . C r y s t a l size w i t h i n i g n e o u s r o c k s i s largely d e t e r m i n e d b y t h e c o o l i n g rate o f t h e m a g m a body. S e d i m e n t a r y r o c k s are m a d e o f s e d i m e n t s formed from the w e a t h e r i n g and erosion of any preexisting rock. D e p o s i t i o n , b u r i a l , a n d lithification ( c o m p a c t i o n a n d c e m e n t a t i o n ) t r a n s form l o o s e s e d i m e n t s into s e d i m e n t a r y r o c k s . M e t a m o r p h i c r o c k s a r e p r o d u c e d by a l t e r a t i o n in t h e solid state of any p r e e x i s t i n g r o c k by h i g h p r e s sures a n d t e m p e r a t u r e s , w h i c h result in a c h a n g e in t e x t u r e , m i n e r a l composition, or chemical composition.
What is the association of different rock types with plate tectonics, the climate system, and the rock cycle? • T h e rock cycle is the result of interactions b e t w e e n plate tectonic and climate systems. T h e s c h e m a t i c o f t h e r o c k c y c l e p r e s e n t e d i n F i g u r e 3.26 s h o w s h o w the three major r o c k types are strongly associated with plate tectonic and climatic settings. You m a y want to c h e c k out the online review exercise on the rock cycle.
What is an ore mineral? • Mineral deposits are c o n s i d e r e d ore deposits w h e n they are rich and valuable enough to be m i n e d economically. • H y d r o t h e r m a l , m e t a m o r p h i c , c h e m i c a l , and m e c h a n i c a l w e a t h e r i n g and sedimentary p r o c e s s e s can enrich metal-bearing minerals to form e c o n o m i c a l deposits.
How do ore deposits of metal-bearing minerals form? • Many metal ore deposits are formed by m a g m a t i c and hydrothermal processes, which are closely linked to both m o d e r n and ancient plate boundaries. Knowledge of how mineral deposits form and the association of mineral deposits and plate boundaries has greatly facilitated the discovery of new deposits.
41
Earth Materials: Minerals and Rocks
Silicate structure B B a s i c crystal s t r u c t u r e
Example mineral
C l e a v a g e (Fill in the b o x with o n e of the cleavage s y m b o l s given in the key.)
Silicate structure C B a s i c crystal s t r u c t u r e
Example mineral
C l e a v a g e (Fill in the b o x with o n e of the cleavage s y m b o l s given in the key.)
Exercise 2: Major mineral classes Fill in the blank with the n a m e of the mineral class to w h i c h each m i n e r a l b e l o n g s . T h e c o m mon classes of minerals include silicates, carbonates, sulfates, sulfides, oxides, native elements, and halides. Hint: Refer to Figure 3.9 and p a g e 53 in the textbook. 1. hornblende
silicate
13. graphite
Hint: an a m p h i b o l e 2. spinel
14. p y r i t e .
3. clay minerals
15. m u s c o v i t e Hint: a mica
4. calcite
16. anhydrite
5. g y p s u m
17. enstatite
6. quartz_
18. pyrite
7. d i a m o n d
19. s a p p h i r e .
8. dolomite
2 0 . orthoclase . H i n t : typically a tan-to-pink feldspar
9. plagioclase
2 1 . silver
Hint: a pyroxene
.
Hint: a feldspar 10. ruby
2 2 . albite
11. halite
2 3 . olivine
12. hematite
24. biotite
H i n t : typically a w h i t e feldspar
H i n t : a black mica
. .
43
Practice Exercises A n s w e r s and explanations are p r o v i d e d at the e n d of the Study G u i d e .
Exercise 1: Crystal structures of some common silicate minerals T h r e e crystal structures of silicate m i n e r a l s are illustrated h e r e . T h e illustration p r o v i d e s a b i r d ' s - e y e view of a plane of a t o m s within the crystal structure of the m i n e r a l . T h e triangles, representing silica tetrahedra, are equivalent to the b l u e p y r a m i d s in F i g u r e 3.9. • W r i t e the n a m e for each major silicate structure (single chain, d o u b l e chain, sheet, framework, or isolated tetrahedra). • Give an e x a m p l e of a m i n e r a l with that crystal structure. • U s i n g the s y m b o l s given in the C l e a v a g e S y m b o l Key below, characterize the cleavage properties for minerals with this crystal structure. H i n t : Refer to F i g u r e 3.9 and p a g e s 6 0 - 6 6 .
Geology Puns W h y did the geologist skip the m i n e r a l o g y l u n c h e o n ? She d i d n ' t have any apatite. T h e g e o l o g y teacher asks a student if he can n a m e t h e felsic m i n e r a l w i t h crystal faces, c o n c h o i d a l fracture, and a h a r d n e s s of 7 on the M o h s ' scale. "Of quartz I can," says the student. T h e geologist is at the eye doctor, being fitted for a n e w pair of glasses. " D o y o u ever see d o u b l e ? " asks the doctor. " O n l y w h e n I look t h r o u g h calcite s a m p l e s in lab," says the geologist.
Exercise 3: Identifying minerals by their physical properties Give the n a m e for each of the mineral s a m p l e s described. Hint: Refer to figures and text in Chapter 3, Appendix 3 and to various links to minerals and g e m s provided on the W e b site http://www.whfreeman.com/understandingearth5e.
Mineral S a m p l e A T h i s mineral is colorless and has a silvery shine. It separates into thin sheets or flakes that can b e n d w i t h o u t b r e a k i n g . It is within a rock that c o n t a i n s large crystals of q u a r t z and feldspar. H i n t : T h i s mineral is n a m e d after M o s c o w w h e r e it w a s c o m m o n l y u s e d as a substitute for w i n d o w glass. Mineral n a m e : Mineral Sample B This mineral sample is pale yellow and occurs as a vein within a rock fracture. M a n y small cubic crystals of this mineral were exposed in the vein when the rock broke apart along the fracture. T h e powdered form (streak) of the mineral is black and has a sulfurlike smell. It is a c o m m o n metal sulfide ore mineral. H i n t : A c o m m o n n a m e for this mineral is "fool's gold." Mineral name: Mineral S a m p l e C T h i s mineral is on display at a mineral exhibition both as a mineral s p e c i m e n and cut and polished in pieces of jewelry. It consists of beautiful c o n c e n t r i c b a n d s of various shades of green. A knife blade can easily p o w d e r its surface but my fingernail c a n n o t scratch it. Acid reacts with the mineral, especially if p o w d e r e d . T h e s p e c i m e n w a s labeled as c o m i n g from a c o p p e r m i n e in Bisbee, A r i z o n a . H i n t : T h e mineral is a c o p p e r carbonate. Mineral n a m e : Mineral S a m p l e D T h i s tan-to-pink s a m p l e o c c u r s as m a n y 5- to 7-centimeter crystals s u r r o u n d e d by flakes of m i c a a n d g r a i n s of w h i t e - t o - c l e a r q u a r t z . T h e t a n - t o - p i n k c r y s t a l s are p r i s m a t i c — r e c t a n g u l a r or b o x shaped. It has a h a r d n e s s b e t w e e n that of a knife b l a d e and that of a steel file. H i n t : It is o n e of t h e m o s t c o m m o n m i n e r a l s in t h e E a r t h ' s c o n t i n e n t a l c r u s t . A s e m i p r e c i o u s variety, called m o o n s t o n e , is used in j e w e l r y . Refer to F i g u r e 3 . 2 1 . Mineral name:
Earth Materials: Minerals and Rocks
Mineral S a m p l e E Exhibiting excellent cleavage in three directions, this mineral breaks into beautiful r h o m b o h e d r a l - s h a p e d p i e c e s . S a m p l e s vary in color from clear to w h i t e to tan. A fingernail does not scratch the surface, but a knife blade easily p o w d e r s the surface. W e a k acid readily reacts with the surface of the m i n e r a l . Hint: M a r i n e o r g a n i s m s typically p r o d u c e this mineral from seawater to g e n e r a t e their s k e l e t o n s — s e a s h e l l s . Refer to F i g u r e 3.16. Mineral n a m e : Mineral S a m p l e F This s a m p l e w a s found as a series of thin layers with other layers of m u d and silt. My fingernail can easily scratch its surface and form a w h i t e powder. Water dissolves the powder, but the s a m p l e d o e s not react with acid. S a m p l e s c o m m o n l y exhibit a splintery aspect but do not separate into individual fibers, such as asbestos d o e s . T h e s a m p l e easily breaks (cleaves) along o n e plane but d o e s not form the thin sheets typical of mica. Hint: T h i s mineral is a major constituent of plaster of Paris and w a l l b o a r d u s e d in buildings and h o u s e s . Refer to Table 3.2. Mineral n a m e :
Exercise 4: Rock cycle review Information and figures in Chapters 2 and 4 will be helpful references for this exercise. Also refer to Figure 3.26. A. W h a t are the three plate tectonic settings for the generation of m a g m a ? hotspots,
like
Hawaii
B. Given the t w o types of i g n e o u s r o c k s , intrusive and extrusive, fill in the table with their characteristic c o o l i n g rates (fast vs. slow) and textures (fine grained vs. coarse grained).
Types of igneous rocks
Cooling rates
Textures
Extrusive
Intrusive
C. Rock and m i n e r a l particles are transported and laid d o w n as s e d i m e n t s by w h a t natural agents of transport? Hint: T h r e e different agents of transport are s h o w n in Figure 3.24.
D. W h a t are the t w o w a y s loose s e d i m e n t s are converted into solid s e d i m e n t a r y r o c k ?
45
E. W h a t are the t w o m a i n types of s e d i m e n t s , and w h a t are they m a d e of?
Type of sediment
What is it made of?
H i n t : Refer to p a g e 65 in t h e textbook.
F. W h a t are the four major conditions that result in m e t a m o r p h i c r o c k s ?
H i n t : Refer to Figure 3.25
G . A r e m e t a m o r p h i c rocks f o r m e d b y m e l t i n g ? A n s w e r yes o r n o .
Review Questions A n s w e r s and e x p l a n a t i o n s are provided at the e n d of the Study G u i d e . 1. T h e b o n d s b e t w e e n Na and CI in halite are strongly ionic. In ion form, CI h a s seven electrons in its outer shell and Na has one. After these t w o e l e m e n t s b o n d , CI has electrons in its outer shell and Na has electrons in its outer shell. A. six, t w o
C.
o n e , seven
B. four, four
D.
eight, eight
H i n t : Refer to F i g u r e 3.4. 2.
and are e x a m p l e s of minerals with identical c h e m i c a l c o m p o s i t i o n s but different crystal structures. A . Calcite, d o l o m i t e B. Hematite, magnetite C . Pyrite, g y p s u m D. Graphite, diamond
3. T h e term geologists use for a naturally o c c u r r i n g a g g r e g a t e of m i n e r a l s is A. element. B. rock.
C. compound. D. crystal.
4. W h e n a substance is m a d e of a t o m s that are a r r a n g e d in a fixed, orderly, a n d r e p e a t i n g pattern, it is said to be A. a m o r p h o u s . B . glassy.
C. crystalline. D . liquid.
5. W h i c h of the following statements is N O T true of m i n e r a l s ? A. B. C. D.
They They They They
are crystalline. p o s s e s s a definite c h e m i c a l c o m p o s i t i o n . are naturally occurring. are organic.
48
PART II C H A P T E R 3
16. C h e m i c a l b o n d i n g a l o n g c l e a v a g e p l a n e s w i t h i n t h e c r y s t a l s t r u c t u r e of a m i n e r a l typically A. is m o r e covalent. B. is m o r e ionic.
C. is m o r e m a g n e t i c . D . involves electron sharing b e t w e e n a t o m s .
H i n t : Refer to the section in the textbook titled Cleavage. 17. Iron and m a g n e s i u m ions are similar in size and both have a +2 c h a r g e . T h e r e f o r e we w o u l d e x p e c t iron and m a g n e s i u m to A . b o n d easily. B . share electrons. C. form m i n e r a l s with different crystal structures. D. substitute for e a c h other within the crystal structure of m i n e r a l s . H i n t : Refer to F i g u r e s 3.3 and 3.4 and the section in the t e x t b o o k titled How Do Minerals
Form?
18. T h e c h e m i c a l b o n d s b e t w e e n carbon a t o m s within d i a m o n d are p r e d o m i n a n t l y A. covalent.
C.
metallic.
B . ionic.
D.
nuclear.
19. W h i c h of the following rocks forms from molten material c o o l i n g and solidifying within the E a r t h ' s crust? A. volcanic
C.
sedimentary
B . plutonic
D.
metamorphic
2 0 . M o l t e n rock within the E a r t h ' s crust is called A . silica.
C.
magma.
B . lava.
D.
mica.
2 1 . C o a r s e - g r a i n e d i g n e o u s r o c k s , s u c h as granite, are e x p o s e d today at the E a r t h ' s surface d u e to A. uplift and erosion. B . quickly c o o l e d lavas e r u p t i n g from ancient v o l c a n o e s . C. silicate m i n e r a l s precipitated from rainwater. D. all of the a b o v e . 22. Lithification is the p r o c e s s that converts s e d i m e n t s into solid rock by A. c o o l i n g and crystallization. B. pressure cooking. C.
subduction.
D . c e m e n t a t i o n and c o m p a c t i o n . 2 3 . B e d d i n g (layering) is a major identifying characteristic of A. s e d i m e n t a r y r o c k s .
C.
intrusive r o c k s .
B. metamorphic rocks.
D.
n o n e o f the a b o v e .
24. Contact metamorphism A. o c c u r s in areas of very high t e m p e r a t u r e and p r e s s u r e , like s u b d u c t i o n z o n e s . B. e x t e n d s over very large areas. C. is characteristic of continental collision z o n e s . D . o c c u r s i n areas w h e r e m a g m a intrudes a n d m e t a m o r p h o s e s n e i g h b o r i n g rock. H i n t : Refer to F i g u r e 3.25. 2 5 . In the rock cycle, w e a t h e r i n g A. creates s e d i m e n t s .
C.
creates m o u n t a i n s .
B. results in burial and lithification.
D.
can c a u s e m e t a m o r p h i s m .
2 6 . In the r o c k cycle, m o u n t a i n s typically form as a result of A . w e a t h e r i n g and erosion.
C.
s e d i m e n t a t i o n piling u p rock.
B. metamorphism.
D.
s u b d u c t i o n and c o n t i n e n t s colliding.
H i n t : Refer to F i g u r e 3.26.
E a i Materials: Minerals and Rocks Earth
6. Which of the following objects is N O T a m i n e r a l ? A. native c o p p e r
C.
B. glass
D. water ice
diamond
7. T h e most c o m m o n m i n e r a l g r o u p on Earth is A. carbonates.
C. oxides.
B. silicates.
D. halides.
H i n t : Refer to F i g u r e 1.8 and p a g e 8. 8. T h e structure of feldspars, such as orthoclase and plagioclase, consists of A. B. C. D.
double c h a i n s of silica tetrahedra. a t h r e e - d i m e n s i o n a l f r a m e w o r k of silica tetrahedra. single chains of silica tetrahedra. isolated silica tetrahedra.
9. Micas are k n o w n for their t e n d e n c y to split into thin sheets; their silicate c r y s t a l : ture is A. B. C. D.
isolated silica tetrahedra. chains of silica tetrahedra. sheets of silica tetrahedra. a f r a m e w o r k a r r a n g e m e n t of silica tetrahedra.
10. W h i c h of the following d o e s N O T b e l o n g to the c a r b o n a t e class of m i n e r a l s ? A. calcite B. anhydrite
C. d o l o m i t e D. aragonite
11. W h i c h of the following minerals h a s a sheet silicate structure? A. clay minerals
C.
B. feldspar
D. graphite
amphibole
12. A m p h i b o l e s and p y r o x e n e s are mineral g r o u p s that b e l o n g to t h e class of mil called A. B. C. D.
chain silicates. framework silicates. sulfide m i n e r a l s . sheet silicates.
13. W h i c h of the following statements is N O T true of quartz and calcite? A. Calcite has excellent cleavage; quartz has p o o r to no cleavage. B. Q u a r t z is a c a r b o n a t e m i n e r a l . C. Both are often colorless. D. Quartz h a s a h a r d n e s s of about 7 and calcite a h a r d n e s s of about 3. 14. T h e characteristic of certain m i n e r a l s to b r e a k along p l a n e s of w e a k n e s s is called A. crystal s y m m e t r y . B. cleavage.
C. h a r d n e s s . D. luster.
15. Graphite and d i a m o n d are t w o very different minerals. W h y are the physical p r o p erties of graphite and d i a m o n d so different even though they are both m a d e from pure c a r b o n ? A. T h e c h e m i c a l b o n d s b e t w e e n the carbon a t o m s and their crystal structures are significantly different. B. G r a p h i t e h a s strikingly different physical properties from d i a m o n d b e c a u s e of impurities within its crystal structure, w h e r e a s d i a m o n d is p u r e carbon. C. Actually, g r a p h i t e and d i a m o n d are not m i n e r a l s b e c a u s e they are m a d e from carbon and all m a t t e r m a d e from carbon is organic. Their physical properties are different b e c a u s e they w e r e p r o d u c e d by very different living o r g a n i s m s . D. N o n e of the a b o v e . H i n t : Refer to F i g u r e Story 3.11 (a a n d b).
47
Earth Materials: Minerals and Rocks
Study Tip: Construct a Simplified Rock Cycle Figure 3.26 p a c k s m u c h information into o n e p a g e and provides an o v e r v i e w of h o w rock types are linked to E a r t h ' s plate tectonic and c l i m a t e s y s t e m s . As a review for an exam, construct y o u r own rock cycle flowchart. K e e p it relatively simple and focus on presenting the following information: • Major r o c k types • O n e or t w o important diagnostic characteristics of each rock type • Process(es) that transform a r o c k into a n o t h e r r o c k type Use sketches, w o r d s , p h r a s e s , or bulleted lists to describe e a c h c o m p o n e n t (each major rock type) and c o n n e c t i n g p a t h w a y s (processes that transform r o c k s into a n o t h e r t y p e ) . If you are a visual learner, c o n c e n t r a t e on d r a w i n g simple illustrations that characterize h o w e a c h rock type forms and h o w it is linked to other rock t y p e s . If you learn better from r e a d i n g or listening, then use arrows and describe the major e l e m e n t s of the rock cycle in w o r d s and read it out loud.
49
CHAPTER
4
Igneous Rocks: Solids from Melts
50
Igneous Rocks: Solids from Melts
Before Lecture Before you go to lecture, s p e n d s o m e t i m e p r e v i e w i n g C h a p t e r 4. We have m a d e it easier by identifying the five key q u e s t i o n s on i g n e o u s r o c k s for y o u (see C h a p t e r P r e v i e w ) . T h e s e questions constitute the f r a m e w o r k for u n d e r s t a n d i n g this chapter. W o r k i n g with the C h a p t e r Preview questions before lecture a n d c o m m i t t i n g t h e m to m e m o r y should h e l p you understand the lecture better and take excellent notes. N e e d a refresher on p r e v i e w i n g ? See C h a p t e r 3 of Part I, H o w to Be Successful in Geology, in this Study G u i d e .
Chapter Preview • H o w are i g n e o u s r o c k s classified? Brief answer: C o m p o s i t i o n and texture (cooling history). Preview F i g u r e s 4 . 1 , 4 . 3 , and 4.4 and Tables 4.1 a n d 4.2. H i n t : Table 4.2 s u m m a r i z e s t h e crucial information, but you will n e e d to l o o k at the figures in order to u n d e r s t a n d it. • How and where do m a g m a s form? Hint: Table 4.3 a n d F i g u r e 4.11 in t h e t e x t b o o k p r o v i d e an overview. • How does m a g m a t i c differentiation a c c o u n t for the great variety of i g n e o u s rocks? Hint: Figure 4.5 tells the classic differentiation story. Figure 4.6 adds important details. • W h a t are t h e f o r m s of i n t r u s i v e a n d extrusive i g n e o u s r o c k s ? Hint: Figures 4.7 and 4 . 1 0 show t h e basic intrusive and extrusive igneous rock bodies. • H o w do i g n e o u s r o c k s relate to plate t e c t o n i c s ? Brief answer: M a g m a s t e n d to form at divergent and convergent plate b o u n d a r i e s a n d hot spots. H i n t : Refer to F i g u r e s 4 . 1 1 , 4 . 1 3 , and 4 . 1 4 and the Activity, R o c k Composition, a n d Types of M a g m a s section of C h a p t e r 4 for m o r e details.
Preview of magma types in relation to plate tectonic settings Plate tectonic setting (examples)
Magma type (example rock type)
Spreading c e n t e r s • oceanic ridge ( M i d - A t l a n t i c R i d g e )
• mafic (basalt)
• continental rift (East African Rift)
• mafic to felsic ( s i l i c i c ) — m o r e variable b e c a u s e s o m e continental crust m a y melt.
Subduction z o n e s • oceanic island arc (Japan)
• mafic to i n t e r m e d i a t e
• continental volcanic arc ( C a s c a d e R a n g e , Mt. St. H e l e n s , M o u n t Rainier, and A n d e s in South A m e r i c a )
• mafic to felsic ( s i l i c i c ) — m o r e variable b e c a u s e continental crust m a y melt.
Note: N o t m u c h v o l c a n i s m occurs in association with collision plate b o u n d a r i e s . Intraplate m a n t l e p l u m e s ("hot spots") • oceanic hot spots ( H a w a i i )
• mafic (basalt)
• continental hot spots (Yellowstone)
• mafic to felsic ( s i l i c i c ) — m o r e variable b e c a u s e continental crust is melted.
51
Vital Information from Other Chapters R e v i e w Figures 3.22 and 3 . 2 3 .
Web Site Study Resources http://www.whfreeman.com/understandingearth5e R e v i e w the Rock Cycle Review in C h a p t e r 3 on the W e b site. Pay particular attention to inform a t i o n on melting, magmas, cooling, and igneous rocks.
During Lecture A lot of i m p o r t a n t visual material will be covered in this lecture. Be sure you p r e v i e w the c h a p t e r before lecture. B e c a u s e the material is s o m e w h a t technical, y o u will want to take notes in as organized a m a n n e r as possible. F o l l o w i n g are s o m e ideas you m a y find helpful. • B i g picture for C h a p t e r 4. To avoid getting lost in details, keep the big picture in m i n d t h r o u g h o u t the lecture. Chapter 5 explains how plate tectonics drives the formation of magma and how igneous rocks of varying composition and texture are produced at particular plate locations. For example, fine, textured volcanic rock of basaltic composition is produced at a diverging ocean plate (see Figure 4.13). • K e y figures for lecture. T h i s c h a p t e r introduces m a n y n e w w o r d s and c o n cepts to learn. It m a y be helpful to b o o k m a r k o n e or t w o of the m o s t i m p o r tant t e x t b o o k figures you p r e v i e w e d before c o m i n g to lecture. F i g u r e 4 . 3 will h e l p you with igneous rock classification. F i g u r e s 3.26, 4 . 1 1 , 4 . 1 3 , a n d 4 . 1 4 s u m m a r i z e w h e r e and h o w m a g m a s are formed. • You will also be developing a new skill: identifying igneous r o c k s . Often lecturers use slides or rock s a m p l e s to h e l p you learn this skill. T h e following tip and chart tool will h e l p you master this skill efficiently.
Note-Taking Tip: How to take notes on rock samples D u r i n g lecture the instructor m a y s h o w slides of i g n e o u s r o c k formations a n d m a y p a s s a r o u n d s p e c i m e n s of i g n e o u s r o c k s . T h e s p e c i m e n s will be easier to see if y o u sit close to the front of the r o o m . F o r the instructor, each s a m p l e tells a story. You can b e c o m e proficient at r e a d i n g r o c k stories, too. It j u s t takes practice and a little organization. F o r igneous r o c k s , a chart like the following e x a m p l e will h e l p you o r g a n i z e w h a t you see and hear. T h e chart focuses on the t w o things you need to pay t h e m o s t attention to: texture ( w h e t h e r or not you can see crystals) and composition (the likely mineral content of this rock). If the instructor tells an interesting story about the rock, m a k e a note or t w o ; stories help us r e m e m b e r details we m i g h t o t h e r w i s e forget. W h a t to d o . Before the lecture on i g n e o u s r o c k s , m a k e a chart like t h e o n e that follows on a full sheet of n o t e b o o k paper. L e a v e plenty of r o o m for notes about e a c h s a m p l e . Try to put s o m e t h i n g in every c o l u m n for every r o c k s a m p l e . If you m i s s a c o l u m n or w e r e u n a b l e to see s o m e of the m i n e r a l s in the rock, be sure to talk to the instructor right after lecture (while y o u r m e m o r y is fresh and the r o c k s a m p l e is h a n d y ) and fill in w h a t you need to c o m p l e t e the chart. H i n t : T h i s idea is also g o i n g to be useful in s u b s e q u e n t chapters on s e d i m e n t a r y and m e t a m o r p h i c r o c k s .
Igneous Rocks: Solids from Melts
53
Example: Rock sample-note taking chart (Set up on one full p a g e of y o u r n o t e b o o k . )
Rock name (Example) Granite
Texture clues (coarse, fine, etc.) Coarse
Composition clues (mineral clues, color, dark/light, etc.) I could m a k e out crystals of black h o r n b l e n d e , w h i t e and pink feldspars, and thin, shiny sheets of m i c a in a light gray matrix (quartz).
Story about the rock to help you remember it " C a m e from [you n a m e it] M o u n t a i n near o u r c a m p u s . I slipped and fell j u s t before I found this s a m p l e . " Stories will h e l p you r e m e m b e r information.
Other notes Crystals w e r e clearly visible in the slide. Crystals w e r e m o s t l y feldspar and m u s c o v i t e mica! What I thought was h o r n b l e n d e turned out to be biotite. I tried to scratch t h e r o c k with a p e n n y ; no luck, m u s t be k i n d of h a r d ( a b o v e 3 on the M o h s ' scale). M i n e r a l s in granite average greater than 5 on the M o h s ' scale. T o o b a d I d i d n ' t b r i n g my b o w i e knife to class.
Sample 1
Sample 2 and so on
After Lecture Review Notes Chapter 4 is a bit technical, so it will be particularly important to r e w o r k and i m p r o v e y o u r notes after lecture.
Check Your Notes: Have y o u . . . • used the five p r e v i e w q u e s t i o n s as section h e a d i n g s for y o u r n o t e s ? You should h a v e sections dealing with (1) classification, (2) m a g m a formation, (3) m a g m a differentiation, (4) igneous structures, a n d (5) plate tectonics a n d i g n e o u s r o c k formation.
54
PART II
CHAPTER 4
•
a d d e d exercise material from this g u i d e to y o u r n o t e s ? A great place to b e g i n is to c o m p l e t e E x e r c i s e 1 (see P r a c t i c e E x e r c i s e s ) . E x e r c i s e 1 will h e l p y o u u n d e r s t a n d w h a t is p e r h a p s the single m o s t f u n d a m e n t a l idea a b o u t i g n e o u s rock, n a m e l y rock texture. If you do the exercise sketches on n o t e b o o k paper, y o u can clip t h e m s o m e w h e r e near the b e g i n n i n g of y o u r notes on i g n e o u s r o c k s for future review.
• a d d e d visual material to y o u r n o t e s ? Key figures to c o n s i d e r a d d i n g are F i g u r e 4.4, classification model of igneous r o c k s ; F i g u r e 4 . 5 , fractional crystallization w h i c h explains the c o m p o s i t i o n of a basaltic intrusion; F i g u r e 4 . 7 , b a s i c extrusive and intrusive i g n e o u s structures; and F i g u r e 4 . 1 1 , plate tectonics and m a g m a f o r m a t i o n . R e m e m b e r , the idea is to d r a w simplified versions that e m p h a s i z e t h e features d i s c u s s e d in lecture. •
included a brief s u m m a r y of this c h a p t e r ? W r i t i n g a brief s u m m a r y of the e s s e n c e of a c h a p t e r is a g o o d w a y to help focus on w h a t is i m p o r t a n t a n d avoid g e t t i n g b o g g e d d o w n in less essential details.
Intensive Study Session You learn g e o l o g y m u c h as y o u w o u l d build a h o u s e . Before e a c h lecture, c o n s t r u c t a frame of q u e s t i o n s . D u r i n g lecture, attach details and ideas to the frame. After lecture m a s t e r the ideas during an intensive study session. N o w y o u have c o n s t r u c t e d the first few stories. But g e o l o g y is a skyscraper with 23 floors (one for e a c h c h a p t e r ) . E a c h c h a p t e r s u p p o r t s t h e o n e a b o v e it. If y o u d o n ' t c o m p l e t e l y master this chapter, the next will be m o r e difficult. • Test y o u r u n d e r s t a n d i n g of rock t e x t u r e a n d o t h e r i m p o r t a n t c o n c e p t s identified in t h e C h a p t e r Preview. T h e r e are s o m e great features built into this c h a p t e r that you c a n use to learn. O n e g o o d e x a m p l e is F i g u r e 4 . 2 . R e a d the section Texture at the b e g i n n i n g of the chapter. You will learn h o w J a m e s Hutton d e d u c e d the nature of igneous rock by assessing three lines of e v i d e n c e (clues 1-3). T h e n l o o k at F i g u r e 4.2. You c a n h a v e an e x p e r i e n c e of d i s c o v ery not unlike H u t t o n ' s . Study F i g u r e s 4 . 1 , 4 . 2 , and 4.3 until y o u think you u n d e r s t a n d texture. T h e n do E x e r c i s e 1 and really m a s t e r interpreting r o c k textures. M o v e o n t h r o u g h the chapter, p a y i n g particular attention t o F i g u r e s 4 . 3 , 4 . 4 , 4 . 5 , 4 . 7 , and 4 . 1 1 . • Practice Exercises and Review Questions
N e x t use the S t u d y G u i d e
Practice E x e r c i s e s and R e v i e w Q u e s t i o n s in the S t u d y G u i d e . Be sure to do E x e r c i s e 1. It involves the key information you need to learn in this chapter. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e Complete
the
online
exercises Bowen's Reaction
Series Review a n d Igneous
Rock Review. F l a s h c a r d s will help you learn new t e r m s . Try y o u r h a n d at b e i n g a field geologist by d o i n g the exercises in G e o l o g y in P r a c t i c e , an excellent review of basic information on i g n e o u s r o c k s .
Exam Prep M a t e r i a l s in this section are m o s t useful during y o u r p r e p a r a t i o n for e x a m s . T h e following C h a p t e r S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s s h o u l d simplify y o u r c h a p t e r review. R e a d the C h a p t e r S u m m a r y first. It p r o v i d e s a helpful o v e r v i e w that s h o u l d refresh y o u r m e m o r y .
Igneous Rocks: Solids from Melts
Next, work on the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions just as you w o u l d an e x a m , to see h o w well you have mastered this chapter. After you answer the questions, score them. Finally, and most important, review each question you missed. Identify and correct the misconception(s) that resulted in your incorrect answer.
Chapter Summary How are igneous rocks classified? • Igneous r o c k s can be divided into t w o b r o a d textural classes: (1) coarsely c r y s talline r o c k s , w h i c h are intrusive (plutonic) and therefore cooled slowly, and (2) finely crystalline r o c k s , w h i c h are extrusive (volcanic) and cooled rapidly. Within each of these b r o a d textual classes, the rocks are subdivided according to their composition. General compositional classes of igneous rocks are felsic, intermediate, mafic, and ultramafic, with d e c r e a s i n g silica and increasing iron and m a g n e s i u m content.
How and where do magmas form? • T h e lower crust and the upper m a n t l e are typical places w h e r e physical c o n d i tions induce r o c k to melt. H i g h t e m p e r a t u r e s , a reduction in p r e s s u r e , and the presence of w a t e r all c a u s e m e l t i n g . C o m p o s i t i o n is also a factor in the melting t e m p e r a t u r e of r o c k s .
How to melt a r o c k — t h e generation of magma • Increase the t e m p e r a t u r e . N o t all m i n e r a l s melt at the s a m e t e m p e r a t u r e (refer to Figure 4.5). T h e mineral c o m p o s i t i o n of the rock affects the m e l t i n g t e m p e r a t u r e . Felsic r o c k s
55
56
PART II C H A P T E R 4
with h i g h e r silica content m e l t at lower t e m p e r a t u r e s than do mafic r o c k s , w h i c h contain less silica and m o r e iron and m a g n e s i u m . • L o w e r the confining p r e s s u r e . A r e d u c t i o n in p r e s s u r e c a n i n d u c e a hot r o c k to melt. A r e d u c t i o n in c o n fining p r e s s u r e on the hot u p p e r m a n t l e is t h o u g h t to g e n e r a t e the basaltic m a g m a s that intrude into the o c e a n i c r i d g e s y s t e m to form o c e a n crust (refer to Figure 4.13). • A d d water. T h e p r e s e n c e of water in a r o c k c a n lower its m e l t i n g t e m p e r a t u r e by up to a few hundred degrees. Water released from rocks subducting into the m a n t l e a l o n g c o n v e r g e n t plate b o u n d a r i e s m a y b e a n i m p o r t a n t factor i n m a g m a g e n eration, especially at the convergent plate b o u n d a r i e s (refer to F i g u r e 4 . 1 4 ) .
How does magmatic differentiation account for the great variety of igneous rocks? • T h e r e is an amazing variety of igneous rocks on Earth. Two processes help to explain how the composition of igneous rocks can be so variable. They are partial melting and fractional crystallization (refer to Figures 4.5 and 4.6). • T h e B o w e n ' s reaction series in F i g u r e 4.5 is a flowchart d e s c r i b i n g h o w the very general bulk c o m p o s i t i o n of a m a g m a can c h a n g e as the m a g m a solidifies or as a r o c k m e l t s .
What are the forms of intrusive and extrusive igneous rocks? • N a m e s are given to i g n e o u s r o c k b o d i e s b a s e d on their size and s h a p e . F i g u r e 4.7 s u m m a r i z e s the c o m m o n i g n e o u s r o c k b o d i e s , such a s batholith, p l u t o n , dike, and sill.
How do igneous rocks relate to plate tectonics? • M i d - o c e a n ridges and s u b d u c t i o n zones are the major sites of m a g m a t i c activity (refer to F i g u r e s 4 . 1 1 , 4 . 1 3 , and 4 . 1 4 ) .
Igneous Rocks: Solids from Melts
Practice Exercises Answers and e x p l a n a t i o n s are p r o v i d e d at the end of the Study G u i d e .
Exercise 1: Igneous rock textures In each of the four b o x e s , sketch t h e i g n e o u s r o c k texture that is consistent with the origin of the rock as described a b o v e e a c h b o x . Fill in the blank with the appropriate texture t e r m from the list. Table 4 . 2 , p l u s F i g u r e s 3.23 and 4 . 3 , will be helpful. Texture t e r m s Fine grained (aphanitic) Intermediate grain s i z e s — v i s u a l grains but n o t very coarse grained Coarse grained (phaneritic) Mixture of c o a r s e and fine grains (porphyritic)
A. Draw the texture of an igneous rock from a pluton solidified at depth within the crust.
B. Draw the texture of an igneous rock from a shallow m a g m a body, like a dike or sill.
phaneritic N a m e of texture
N a m e of texture
C. Draw the texture of a lava erupted from a m a g m a c h a m b e r after s o m e minerals had
D. Draw the texture of a lava flow that erupted before the m a g m a c h a m b e r u n d e r w e n t
begun to crystallize.
N a m e of texture
any crystallization.
N a m e of texture
57
58
P A R T II
CHAPTER 4
Exercise 2: Distribution of igneous rocks within the Earth C o m p l e t e the table with t e r m s from the lists that follow t h e table. Refer to Table 4.2 and F i g u r e 4 . 4 i n y o u r t e x t b o o k . S o m e a n s w e r s are p r o v i d e d . N o t e that the E a r t h ' s c o r e i s not i n c l u d e d in this table b e c a u s e it is t h o u g h t to be c o m p o s e d m o s t l y of iron a n d nickel, not silicate m i n e r a l s .
Major layer in the Earth Continental crust
Example igneous rock
General compositional group
General chemical composition
Granite
(For c o n t i n e n t a l crust, t h e r e are t w o a p p r o p r i a t e a n s w e r s . ) Intermediate Ocean crust
Mantle
Less Si, Na, K More Fe, Mg, Ca
E x a m p l e i g n e o u s rocks
General compositional group
granite/rhyolite
ultramafic
basalt/gabbro
felsic
andesite/diorite
intermediate
peridotite
mafic
granodiorite/dacit
Figure 4 . 4 . Classification model of igneous rocks. The vertical axis measures the mineral composition of a given rock as a percentage of its volume. The horizontal axis is a scale of silica content by weight. Thus, if you know by chemical analysis that a coarsely textured rock sample is about 70 percent silica, you could determine that its composition is about 6 percent amphibole, 3 percent biotite, 5 percent muscovite, 14 percent plagioclase feldspar, 22 percent quartz, and 50 percent orthoclase feldspar. Your rock would be a granite. Although rhyolite has the same mineral composition, its fine texture would eliminate it.
Igneous Rocks: Solids from Melts
Exercise 3: Predicting the change in composition in crystallizing magma
Sandstone Basalt Mostly sodium-rich plagioclase feldspar; no olivine Calcium-rich plagiocla feldspar and pyroxene no olivine Olivine Basalt Sandstone
A basaltic m a g m a intruded b e t w e e n s a n d s t o n e layers to form the sill. As the m a g m a c o o l e d in place, fractions of the crystals that formed settled into l a y e r s — i t u n d e r w e n t fractional crystallization. Using F i g u r e s 4.4 and 4 . 5 , and Table 4.2, predict h o w the silica and iron content of the Palisades sill c h a n g e s . Indicate w h e t h e r the silica and iron content increase, d e c r e a s e , or stay the same in e a c h layer of the sill. E x p l a i n the reason(s) for y o u r a n s w e r s . Predict h o w the silica and iron c o n t e n t of the mafic Palisades sill c h a n g e s as the m a g m a c o o l s . Circle the correct answer. A. Olivine -> P y r o x e n e 1. Silica content: 2. Iron content:
C a l c i u m - r i c h p l a g i o c l a s e feldspar and p y r o x e n e with no olivine i n c r e a s e d / d e c r e a s e d / stay the s a m e increased / d e c r e a s e d / stay the s a m e
Explanation: B. Calcium-rich p l a g i o c l a s e -> s o d i u m - r i c h p l a g i o c l a s e feldspar and no olivine 1. Silica content: i n c r e a s e d / d e c r e a s e d / stay the s a m e 2.
Iron content: i n c r e a s e d / d e c r e a s e d / stay the s a m e
Explanation:
Exercise 4: Sequence of mineral crystallization in a solidifying magma Circle the a n s w e r s that correctly c o m p l e t e the following statements. A. T h e a t o m i c (crystal) structure of the earliest formed silicate m i n e r a l s in a m a g m a tend to be M O R E / L E S S c o m p l e x than the crystalline structures of m i n e r a l s f o r m e d during later stages in the solidification of the m a g m a . B. D u r i n g the solidification of a m a g m a , the m i n e r a l s with the h i g h e s t silica c o n t e n t will crystallize F I R S T / L A S T . C. In the last stages of solidification of a m a g m a , the r e m a i n i n g silicate melt will c o n tain M O R E / L E S S silica than the original melt. Hint: You do n o t n e e d to k n o w the bulk c o m p o s i t i o n (mafic, i n t e r m e d i a t e , felsic) of the m a g m a t o a n s w e r t h e s e q u e s t i o n s . Refer t o F i g u r e s 4.4 and 4.5 and Table 4 . 2 .
59
Exercise 5: Partial melting and magma composition Circle the a n s w e r s that correctly c o m p l e t e the following s t a t e m e n t s . C o m p a r e d to the bulk c o m p o s i t i o n of the rock, the m i n e r a l s with the l o w e r m e l t i n g temperature are A. H I G H E R / L O W E R in the B o w e n ' s reaction series. B. D E P L E T E D / E N R I C H E D in M g , F e , and Ca. C. D E P L E T E D / E N R I C H E D in Si, N a , K. H i n t : Refer to F i g u r e 4 . 5 .
Exercise 6: Predicting the composition of magma generated in subduction zones Circle the answer for each statement that correctly completes the following sentence: C o m p a r e d to the basaltic o c e a n crust, the m a g m a generated by partial m e l t i n g of the s u b d u c t i n g slab of ocean crust will
A B. C. D
h a v e M O R E / L E S S silica, have M O R E / L E S S iron and m a g n e s i u m , have M O R E / L E S S s o d i u m and p o t a s s i u m , b e M O R E / L E S S mafic.
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. I g n e o u s r o c k n a m e s are b a s e d on A. B. C. D.
texture and c o m p o s i t i o n . f i n e grain and coarse grain. intrusive and extrusive. w h e r e the m a g m a c h a m b e r erupts.
2. W h a t r o c k has the s a m e m i n e r a l o g y as granite but a fine-grained t e x t u r e ? A. andesite
C.
obsidian
B . basalt
D.
rhyolite
H i n t : Refer to Table 4.2. 3. W h i c h of the following r o c k s contains the m o s t silica? A. basalt B. rhyolit
C. fissure e r u p t i o n s D. dacite
Igneous Rocks: Solids from Melts
4. W h i c h of the following pairs of intrusive and extrusive rocks are m a d e from the s a m e minerals—i.e., have the s a m e c h e m i c a l c o m p o s i t i o n ? A. g a b b r o and basalt B. diorite and basalt
C. fissure eruptions D. g a b b r o and rhyolite
5. In the field or in h a n d - s i z e d s p e c i m e n s , intrusive and extrusive i g n e o u s r o c k s are distinguished by w h i c h characteristic? A. c o m p o s i t i o n B. color C. porphyritic versus n o n p o r p h y r i t i c texture D. grain size 6. W h i c h of the following iron-rich m i n e r a l s is most c o m m o n in basalt? A. p y r o x e n e B . quartz
C. D.
muscovite Na-plagioclase
7. Granite is m a d e up mainly of A. quartz, o r t h o c l a s e (K-feldspar), and N a - p l a g i o c l a s e . B. quartz, C a - p l a g i o c l a s e , N a - p l a g i o c l a s e , and a m p h i b o l e . C. quartz, p y r o x e n e , and m u s c o v i t e . D. quartz, orthoclase (K-feldspar), C a - p l a g i o c l a s e , and olivine. Hint: Refer to Table 4.1 and Figure 4.4. 8. An igneous rock m a d e of a m i x t u r e of both coarse- and fine-grained minerals is called porphyritic and is f o r m e d by A. rapid c o o l i n g followed by a period of slow cooling. B. slow c o o l i n g followed by a period of rapid cooling. C. very slow c o o l i n g of a water-rich m a g m a . D. very rapid c o o l i n g in the p r e s e n c e of water. 9. Only iron and m a g n e s i u m - r i c h m i n e r a l s are found in w h i c h of the following lists of minerals? A. p y r o x e n e , h o r n b l e n d e , K-feldspar, biotite B. plagioclase, biotite, p y r o x e n e , clay C. quartz, m u s c o v i t e , biotite, plagioclase D . biotite, p y r o x e n e , olivine, h o r n b l e n d e 10. Following B o w e n ' s reaction series, the later, l o w e r - t e m p e r a t u r e fractions of liquid m a g m a b e c o m e progressively A. d e p l e t e d in silica. B. e n r i c h e d in silica. C. enriched in m a g n e s i u m . D. depleted in p o t a s s i u m . Hint: Refer to F i g u r e 4 . 5 . 11. W h i c h of the following m i n e r a l s are the earliest h i g h e s t - t e m p e r a t u r e m i n e r a l s to c r y s tallize in B o w e n ' s R e a c t i o n Series? A. quartz and feldspar B . plagioclase and a m p h i b o l e C. plagioclase and olivine D . chert and m i c a 12. T h e formation of granitic batholiths occurs A. within the o c e a n crust. B. within the continental crust. C. along s p r e a d i n g centers in the o c e a n . D . u n d e r h o t spots.
61
13. C o n s i d e r i n g the following m i n e r a l s , w h i c h pairs w o u l d you predict w o u l d N O T b e found together in the s a m e igneous r o c k ? A. K-feldspar and biotite B . N a - p l a g i o c l a s e and m u s c o v i t e C. quartz and N a - p l a g i o c l a s e D. quartz and olivine H i n t : Refer to F i g u r e s 4.4 and 4 . 5 . 14. H o w w o u l d you distinguish a lava flow from a sill e x p o s e d at the E a r t h ' s surface? A. Sills tend to be coarser grained than lava flows b e c a u s e they cool slower. B. T h e c h e m i c a l c o m p o s i t i o n s of the lava flow and the sill (that is, t h e m i n e r a l s present) w o u l d be very different. C. Sills tend to be finer grained b e c a u s e of slower rates of crystallization. D. L a v a flows are coarser grained b e c a u s e of very rapid rates of c o o l i n g . 15. T h e r o c k t y p e of m o s t batholiths found in the continental crust is A. g a b b r o . B . obsidian.
C. D.
granite. basalt.
16. H o w d o e s a rising m a g m a m a k e s p a c e for itself as it m o v e s t h r o u g h t h e solid crust? A. B. C. D.
by by by by
b r e a k i n g off large b l o c k s of r o c k that sink into t h e m a g m a c h a m b e r w e d g i n g open the overlying r o c k melting surrounding rocks all these p r o c e s s e s
17. T h e source for m o s t mafic m a g m a s is t h o u g h t to be A. partial m e l t i n g of felsic and intermediate r o c k s in the u p p e r c o n t i n e n t a l crust. B. partial m e l t i n g of ultramafic r o c k s within the u p p e r m a n t l e . C. m e l t i n g of preexisting granites and s e d i m e n t s . D. the m o l t e n core of the Earth. 18. If lava flows on the slopes of a v o l c a n o are derived from o n e large m a g m a chamber, w h i c h crystallizes slowly and feeds eruptions over a period of m a n y t h o u s a n d s of years, h o w w o u l d y o u predict the gross c o m p o s i t i o n of the lava w o u l d c h a n g e as the lava flows become younger? A. Younger lava flows w o u l d b e c o m e progressively e n r i c h e d in iron and m o r e mafic. B. Younger lava flows w o u l d b e c o m e progressively e n r i c h e d in silica and m o r e felsic. C. L a v a flows w o u l d be the s a m e c o m p o s i t i o n since they all c a m e from the same m a g m a chamber. D. L a v a flows w o u l d alternate in c o m p o s i t i o n . H i n t : U s e B o w e n ' s reaction series in F i g u r e 4.5 and review the t e x t b o o k section Magmatic
Differentiation.
19. T h e production of basalt can be achieved by the partial m e l t i n g of A. B. C. D.
gabbro. ultramafic r o c k s . a m i x t u r e of g a b b r o and oceanic s e d i m e n t s . rhyolite.
H i n t : Refer to Figures 4.11 and 4 . 1 3 . 2 0 . A n d e s i t e s — i n t e r m e d i a t e m a g m a s — a r e typically associated with A. B. C. D.
divergent plate m a r g i n s , like the m i d - o c e a n i c r i d g e s . fractures in the crust that allow m a g m a s from the upper m a n t l e to rise to the surface. h o t spots, like H a w a i i . convergent plate m a r g i n s , like the western e d g e o f South A m e r i c a — t h e A n d e s .
2 1 . A n ophiolite suite contains the following c o m b i n a t i o n o f r o c k s : A. B. C. D.
granite, gneiss, s a n d s t o n e , l i m e s t o n e , and shale. d e e p sea sediments, pillow basalts, g a b b r o , and peridotites. dikes, sills, and plutons of peridotites. plutonic r o c k s and surrounding s e d i m e n t s characteristic of a m a g m a c h a m b e r .
Igneous Rocks: Solids from Melts
22. Which flowchart characterizes the m a g m a t i c p r o c e s s e s that occurr at spreading c e n t e r s ? A. peridotite -> d e c o m p r e s s i o n melting -* basalt B. ophiolite -* fluid-induced m e l t i n g -* basalt C. deep ocean s e d i m e n t s -*• fluid-induced melting -> andesites D. basalt decompression melting peridotite 23. Which of the following h y p o t h e s e s for the origin of granite is N O T r e a s o n a b l e ? A. B. C. D.
mixing of m a g m a s with different c o m p o s i t i o n s d e c o m p r e s s i o n m e l t i n g within a m a g m a p l u m e partial melting melting of s e d i m e n t a r y and m e t a m o r p h i c r o c k s
24. Which set of c o n d i t i o n s will result in basalt melting at the lowest t e m p e r a t u r e ? A. dry basalt at low pressure B. dry basalt at high p r e s s u r e C. wet basalt at low pressure D. wet basalt at high pressure
63
CHAPTER
5
Sedimentation: Rocks Formed by Surface Processes
Sedimentation: Rocks Formed by Surface Processes
• How does plate tectonics relate to s e d i m e n t a r y r o c k ? Brief answer: E n v i r o n m e n t s of deposition, c o m p o s i t i o n , and texture of s e d i m e n t s , as well as the s h a p e and d e p t h of s e d i m e n t a r y b a s i n s , are influenced by plate tectonics. Refer to Figure 5.4. • What is the r e l a t i o n s h i p b e t w e e n s e d i m e n t s a n d e n v i r o n m e n t s on t h e Earth's s u r f a c e ? Hint: Refer to F i g u r e 5.5 and Table 5.2. • What are t h e m a j o r t y p e s of s e d i m e n t s a n d s e d i m e n t a r y r o c k s ? Brief answer: siliciclastic ( s a n d s t o n e ) versus c h e m i c a l (evaporites) and b i o c h e m i c a l (limestone). Study Tables 5.3 and 5.4, and F i g u r e s 5.5, 5.12, and 5.16.
Vital Information from Other Chapters Review Figure 3.24 and p r e v i e w p a g e s 3 7 2 - 3 8 1 in C h a p t e r 16.
During Lecture Note-Taking Tip: Some features to watch for in sedimentary rock samples and formations In this lecture you will be introduced to a variety of s e d i m e n t a r y r o c k s , formations, and sedimentary e n v i r o n m e n t s . S i n c e they will p r o b a b l y be n e w to you, it is s m a r t to prepare. Spend a few m i n u t e s of y o u r preview t i m e b e c o m i n g familiar with the m o s t important sedimentary r o c k features: • Bedding and other s e d i m e n t a r y structures: p a g e s 1 1 1 - 1 1 3 • Bedding s e q u e n c e s (stratigraphy): F i g u r e 5.11 on p a g e 114 • Grain size: F i g u r e s 5.3 and 5.15 and Table 5.3 • Mineral content and major g r o u p s of s a n d s t o n e s : F i g u r e 5.16 on p a g e 118 • Chemical and b i o c h e m i c a l s e d i m e n t a r y rocks: F i g u r e 5.17 • Fossils: Figure 5.18
One goal for lecture should be to leave the r o o m with g o o d a n s w e r s to the preview q u e s t i o n s . To avoid getting lost in the details, k e e p the big picture in mind: C h a p t e r 5 tells the story of how sedimentary r o c k s are created by the processes of the rock cycle. It m a y be helpful to have Figure 5.1 with you so that you can refer to it d u r i n g lecture.
Note-Taking Tip: Use abbreviations to speed up your note taking bed -+ b e d d i n g carb -» c a r b o n a t e clast -> clastic chem -+ c h e m i c a l biochem -+ b i o c h e m i c a l env -» e n v i r o n m e n t
lith -+ lithification sed -» s e d i m e n t or s e d i m e n t a r y sed r -> s e d i m e n t a r y r o c k c a r b sed -*• c a r b o n a t e s e d i m e n t sed b a s e -*• s e d i m e n t a r y basin sed env -* s e d i m e n t a r y e n v i r o n m e n t
After Lecture Review Notes R i g h t after lecture, while the material is fresh in y o u r m i n d , is the perfect t i m e to review your notes. R e v i e w to be sure you u n d e r s t o o d all the key points a n d w r o t e t h e m d o w n in a form that will be r e a d a b l e later.
Check Your Notes: Have y o u . . . • i n c l u d e d the chapter o v e r v i e w ? F i g u r e 5.1 is the "big picture." C o n s i d e r a d d i n g y o u r o w n version, including the six s e d i m e n t a r y r o c k - f o r m i n g steps. • d e v e l o p e d a chart like the o n e in E x e r c i s e 1 to h e l p y o u learn t h e s e d i m e n t a r y environments?
Intensive Study Session: Focus on chunking the information into a solid overview In earlier c h a p t e r s we h a v e used the m e t a p h o r of building a h o u s e for studying geology. Chunking is a m e n t a l p r o c e s s that will h e l p you construct a g o o d frame. T h e idea is to group the important information. Later you h a n g details on this f r a m e w o r k of basic information. If y o u h a v e s o m e t i m e s w o n d e r e d h o w s o m e of y o u r professors can rattle off tons of complicated material without even looking at their lecture notes, c h u n k i n g (see b o x ) p r o v i d e s an explanation. O v e r the years they have c h u n k e d ( c o n n e c t e d ) m o r e and m o r e bits of information about geology. C h a p t e r 5 provides a great opportunity for y o u to d e v e l o p y o u r m e m o r y efficiency. U s e the clues and aids in the b o x to g r o u p the details of s e d i m e n t a r y r o c k s into a format that is easy to r e m e m b e r .
Memory and Learning Tip: Chunking It's easier to r e m e m b e r a three-item list than a 30-item list. If y o u h a v e 30 things to r e m e m b e r , t h e first thing y o u n e e d to do is to divide the long list into a small n u m b e r of shorter lists. G r o u p similar i t e m s together, n a m e the g r o u p s , and then associate the i t e m s in e a c h list with the g r o u p to w h i c h it b e l o n g s . Instead of trying to r e m e m b e r 30 things, you have r e d u c e d y o u r task to r e m e m b e r i n g a m u c h smaller n u m b e r . E x a m p l e : F i g u r e 5.5 illustrates the different s e d i m e n t a r y e n v i r o n m e n t s on Earth. N o t e that while 11 sedimentary environments are discussed, they are o r g a n i z e d into three general k i n d s of e n v i r o n m e n t s : continental, shoreline, and m a r i n e . You are likely to r e m e m b e r the information better if you c h u n k it into t h e three categories rather t h a n trying to absorb the details of all eleven e x a m p l e s . T h e authors of Understanding Earth m u s t agree. N o t i c e that they h a v e o r g a n i z e d the information to h e l p y o u think in t e r m s of just three general categories (continental, shoreline, marine) for sedimentary environments. N o t e also the color coding. W h y did the authors c h o o s e particular c o l o r s for e a c h e n v i r o n m e n t . U s e these clues to study m o r e effectively and i m p r o v e y o u r retention of the information.
Sedimentation: Rocks Formed by Surface Processes
First, overview the p r o c e s s e s involved in forming s e d i m e n t a r y r o c k s . Study Figure 5.1 and commit the six steps in the figure to m e m o r y . Next, w o r k on F i g u r e 5.5 on text p a g e 109, w h i c h illustrates c o m m o n s e d i m e n t a r y environments on Earth. N o t e that although 11 s e d i m e n t a r y e n v i r o n m e n t s are discussed, they are organized into j u s t three k i n d s of e n v i r o n m e n t s . For n o w focus m o s t of y o u r attention on understanding the three and d o n ' t get lost in the details of all 11. Move on to diagenetic p r o c e s s e s on p a g e s 1 1 4 - 1 1 6 . F i g u r e 5.12 provides a great visual overview. It c h u n k s lots of information. H e r e ' s a little m e m o r y trick: You can recall all the information in this figure story by r e m e m b e r i n g j u s t t w o w o r d s : process and sediments. Here's how it w o r k s . First, notice that Steps 1-5 describe the three simple p r o c e s s e s (pressure, compaction, and c e m e n t a t i o n ) that are a part of d i a g e n e s i s . N e x t look at Step 6. O b s e r v e how pressure transforms different s e d i m e n t s into different k i n d s of r o c k s with very different textures. T h e r e are four s e d i m e n t s (mud, sand, gravel, and organic matter). So you have t w o short lists to r e m e m b e r : the list of three p r o c e s s e s and the list of four s e d i m e n t s . If you c a n remember the s e d i m e n t s ( m u d , sand, gravel, organic matter) you can easily g u e s s the r o c k that each turns into. So you can c o m m i t the entire table to m e m o r y by m e m o r i z i n g a very short two-word list: processes (3) and sediments (4). You should be able to r e m e m b e r the rest of the table by association. Try it! Move on to classification on p a g e 116. S e d i m e n t a r y rock classification involves many, many details. N o t i c e , however, that you can r e d u c e all these details to t w o basic categories: (1) clastic s e d i m e n t a r y r o c k s (Table 5.3 and Figure 5.15) and (2) c h e m i c a l / b i o c h e m i c a l sedimentary rocks (Table 5.4 and F i g u r e 5.17). H i n t : If you are a visual or kinesthetic learner, it may help to w o r k up y o u r o w n simplified version of these tables. If you w a n t to test y o u r k n o w l e d g e of the chapter, the following materials can be used for self-testing and applying y o u r new k n o w l e d g e to s o m e very interesting geologic p r o b l e m s . • Practice Exercises a n d R e v i e w Questions. At the very least do Exercise 1. It gets to the key information y o u need to learn in this chapter. • Text. Read Earth Issues on page 124. Study it to learn more about the formation of coral reefs. T h i s is a m u s t if you like to scuba dive! A n s w e r E x e r c i s e s 1, 2, and 5. A l s o w o r k on T h o u g h t Q u e s t i o n s 3, 5, 7, 10, and 12 (refer to p a g e s 3 7 3 - 3 8 0 in C h a p t e r 16). W h e n y o u visit the G r a n d C a n y o n in A r i z o n a , you will be able to u n d e r s t a n d the g e o l o g y better and i m p r e s s y o u r traveling partners after c o m p l e t i n g these exercises and questions. • W e b Site S t u d y R e s o u r c e s http ://ww w. w h f r e e m a n .com/ u n d e r s t a n d i n g e a r t h 5 e In the G e o l o g y in P r a c t i c e exercises on the W e b site, you can try y o u r h a n d at identifying and r e c o n s t r u c t i n g the interesting story told by s e d i m e n t a r y rocks. Identifying Sedimentary Environments, Common Sedimentary Environments, and Depositional Environments are W e b site exercises that will also provide you an opportunity to practice using information and skills highlighted in this chapter.
Exam Prep Materials in this section are most useful during y o u r preparation for q u i z z e s and e x a m s . T h e Chapter S u m m a r y and the Practice Exercises and Review Q u e s t i o n s should simplify y o u r chapter review. R e a d the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y .
68
PART II C H A P T E R 5
N e x t , w o r k on the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and q u e s t i o n s j u s t as you w o u l d an e x a m , to see h o w well y o u have m a s t e r e d this chapter. After you a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t important, review e a c h question y o u m i s s e d . Identify a n d c o r r e c t the m i s c o n c e p t i o n ( s ) that r e s u l t e d in y o u r answering the q u e s t i o n incorrectly.
Chapter Summary What are the major processes in the formation of sedimentary rocks? • W e a t h e r i n g and e r o s i o n p r o d u c e the clastic particles and d i s s o l v e d ions that c o m p o s e s e d i m e n t . Water, w i n d , and ice transport the s e d i m e n t to w h e r e it is d e p o s i t e d . Burial and d i a g e n e s i s h a r d e n s e d i m e n t s into s e d i m e n t a r y r o c k s . Refer to F i g u r e 5 . 1 .
How does plate tectonics relate to sedimentary rock? • S e d i m e n t a r y e n v i r o n m e n t s , the c o m p o s i t i o n and texture of s e d i m e n t s , and the g e o m e t r y of the b a s i n s in w h i c h s e d i m e n t s a c c u m u l a t e are all related to plate tectonic settings. For e x a m p l e , the formation of a n e w o c e a n basin by rifting a l o n g a divergent plate b o u n d a r y creates a rift basin and ultimately a stable continental margin and o c e a n basin. Refer to F i g u r e 5.4.
What is the relationship between sediments and environments on the Earth's surface? • S e d i m e n t a r y e n v i r o n m e n t s are often g r o u p e d into three general l o c a t i o n s : on the c o n t i n e n t s , near the shoreline, and in the o c e a n . • F a c t o r s that influence the s e d i m e n t s in these e n v i r o n m e n t s include (1) k i n d and a m o u n t of water, (2) topography, and (3) biological activity. • S e d i m e n t a r y structures and fossils p r o v i d e information about the agent of transport (water, wind, or ice) and the e n v i r o n m e n t of deposition for the s e d i m e n t .
What are the major types of sediments and sedimentary rocks? • T h e t w o major t y p e s of s e d i m e n t s are siliciclastic and c h e m i c a l / b i o c h e m i c a l . Siliciclastic (also k n o w n as clastic) s e d i m e n t s are f o r m e d from r o c k particles a n d mineral fragments. C h e m i c a l and b i o c h e m i c a l s e d i m e n t s o r i g i n a t e from the ions dissolved in water. Chemical and biochemical reactions precipitate t h e s e dissolved ions from solution. • Classification and the n a m e of clastic s e d i m e n t s and s e d i m e n t a r y r o c k s (cong l o m e r a t e , sandstone, shale) are based primarily on the size of the g r a i n s within the rock. T h e classification and n a m e o f c h e m i c a l and b i o c h e m i c a l s e d i m e n t s (evaporites, l i m e s t o n e s , and d o l o s t o n e ) in s e d i m e n t a r y r o c k s are b a s e d p r i m a rily on their c o m p o s i t i o n .
How is sediment transformed into hard rock? • Burial and d i a g e n e s i s transform loose s e d i m e n t s into hard rock. Burial p r o m o t e s this transformation b e c a u s e buried sediments are e x p o s e d to increasingly h i g h e r pressures and t e m p e r a t u r e s . D i a g e n e s i s involves m a n y p h y s i c a l and chemical processes. For example, cementation is an important chemical
70
PART 11 C H A P T E R 5
Exercise 3: Clastic and chemical sediments and sedimentary rocks G i v e n the descriptive s t a t e m e n t s in c o l u m n 1, fill in c o l u m n s 2 and 3 of the table with the a p p r o p r i a t e s e d i m e n t types and rock t y p e s from the list of c o m m o n s e d i m e n t t y p e s and sedimentary rocks. Sediment types biochemical
Sedimentary rocks . arkose
dolostone
limestone
sandstone
clastic
chert
evaporite
peat
shale
chemical
conglomerate
graywacke
phosphorite
siltstone
Descriptive statement
Sediment type
Sedimentary rock example
chemical
dolostone
C o m p o s e d largely of r o c k fragments P r e c i p i t a t e d in the e n v i r o n m e n t of deposition I m p o r t a n t s o u r c e of coal Often f o r m e d by d i a g e n e s i s F o r m e d from a b u n d a n t skeleton fragments of m a r i n e or lake o r g a n i s m s , such as coral, seashells, and foraminifers Produced by physical weathering P r o d u c e d from rapidly e r o d i n g granitic and gneissic terrains in an arid or semi-arid c l i m a t e
and
phosphorite
Sedimentation: Rocks Formed by Surface Processes
diagenetic c h a n g e in w h i c h minerals are precipitated in the open spaces within clastic s e d i m e n t s , forming c e m e n t s that b i n d the s e d i m e n t s together. • Figure 5.12 s u m m a r i z e s h o w diagenetic p r o c e s s e s p r o d u c e c h a n g e s in c o m p o sition and texture.
Practice Exercises Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e .
Exercise 1: Common sedimentary environments Using Figures 5.1 and 5.5 as a g u i d e , fill in c o l u m n 2 in the following table with the clastic or chemical s e d i m e n t (e.g., sand, silt, m u d , salts, c a r b o n a t e , peat) that best m a t c h the environments of deposition.
Environment of deposition
Sediment deposited
Alpine or glacial river c h a n n e l Dunes in a desert Flood plain along a b r o a d m e a n d e r b e n d River delta along a m a r i n e shoreline Continental shelf Deep sea adjacent to a continental shelf Shoreline sand d u n e s Tidal flats Organic reef
Exercise 2: Grain sizes for clastic sedimentary rocks Using the terms for sediments and rocks listed in Table 5.3, fill in the blanks in c o l u m n s 3 and 4 with the appropriate n a m e s of the sediments and rocks that match the typical particle size of the common object. A few a n s w e r s have been filled in as a reference.
Grain size Coarse
Common object
Sediment
football or bus
boulder
p l u m or lime
Rock type
gravel conglomerate
pea or bean c o a r s e - g r o u n d p e p p e r or salt fine-ground p e p p e r or salt Fine
talcum powder or baby powder
sandstone
69
Sedimentation: Rocks Formed by Surface Processes
Review Questions Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. W h i c h of t h e following r o c k g r o u p s includes only clastic s e d i m e n t a r y r o c k s ? A. B. C. D.
d o l o m i t e , g y p s u m , and l i m e s t o n e cherts, s a n d s t o n e , and shale d o l o m i t e , coal, and l i m e s t o n e shale, s a n d s t o n e , and c o n g l o m e r a t e
2. W h i c h s e q u e n c e of r o c k n a m e s h a s t h e c o r r e c t a r r a n g e m e n t in o r d e r of d e c r e a s i n g particle d i a m e t e r s ? A. c o n g l o m e r a t e , shale, s a n d s t o n e B. shale, siltstone, s a n d s t o n e C. s e d i m e n t a r y breccia, shale, s a n d s t o n e , claystone D. conglomerate, sandstone, claystone 3. T h e grains in a s a n d s t o n e m a y include A. B. C. D.
r o c k fragments. quartz. feldspar. all of the a b o v e .
4. Sedimentary r o c k s are p r o d u c e d t h r o u g h w h i c h of the following s e q u e n c e s of events? A. B. C. D.
erosion, w e a t h e r i n g , weathering, erosion, erosion, w e a t h e r i n g , w e a t h e r i n g , erosion,
transportation, deposition, burial, and diagenesis transportation, burial, diagenesis, and deposition deposition, transportation, and c e m e n t a t i o n transportation, deposition, burial, and diagenesis
5. D o l o m i t e is the p r i m a r y m i n e r a l found in d o l o s t o n e . It is f o r m e d by A. B. C. D.
foraminifera extracting m i n e r a l s from seawater. diagenetic alteration of calcite. direct precipitation in lake water. coral reef e x p o s u r e to direct sunlight.
Test-Taking Tip for Multiple-Choice Exams When taking a multiple-choice test, treat each alternative a n s w e r as a true-false question. Rule out any alternative that is false. For e x a m p l e in Q u e s t i o n 4, A and C are false because erosion m u s t follow w e a t h e r i n g . B is false b e c a u s e deposition m u s t p r e c e e d burial and diagenesis. T h e r e f o r e , D m u s t be the correct answer.
6. W h a t are the t w o m o s t i m p o r t a n t diagenetic p r o c e s s e s that transform loose s e d i m e n t s into hard s e d i m e n t a r y r o c k s ? A. c o m p a c t i o n , c e m e n t a t i o n B. transportation, burial C. erosion, transportation D. uplift, erosion
72
PART II C H A P T E R 5
7 . W h a t are t w o c h e m i c a l o r b i o c h e m i c a l s e d i m e n t a r y r o c k s f o r m e d b y the diagenetic process? A. dolostone, phosphorite B . l i m e s t o n e , chert C. sandstone, limestone D . siltstone, g r a y w a c k e 8. W h e r e are m o s t s e d i m e n t s d e p o s i t e d ? A. on the continental shelf and adjacent o c e a n floor B. in lakes C. a l o n g s t r e a m s D. in deserts 9. In w h i c h of the following materials w o u l d y o u least e x p e c t to find cross-bedding? A. sand d u n e s B . delta s e d i m e n t C. river b a r deposits D . evaporites 10. T h e m o s t w i d e s p r e a d e n v i r o n m e n t of d e p o s i t i o n for c a r b o n a t e s in the world today is A. the d e e p sea, e.g., the Arctic O c e a n . B. the tidal flat e n v i r o n m e n t , e.g., the Mississippi Delta. C. in river c h a n n e l s . D. a w a r m , s h a l l o w - w a t e r m a r i n e e n v i r o n m e n t , e.g., the Florida K e y s . 1 1 . A s a n d s t o n e m a d e of p u r e quartz grains is likely to be d e p o s i t e d A. a l o n g the shore of a continent. B. in the d e e p sea. C. in river c h a n n e l s . D. a l o n g the e d g e s of a coral reef and atolls. H i n t : Refer to F i g u r e 5.16. 12. C h e m i c a l w e a t h e r i n g is m o s t d o m i n a n t in A . w a r m , dry c l i m a t e s . B . w a r m , wet c l i m a t e s . C. cool, dry c l i m a t e s . D. cool, wet climates. E. at the shoreline. 13. T h e c o a s t of A l a s k a is k n o w n for its high m o u n t a i n o u s relief, active v o l c a n o e s , and glaciers that r e a c h to the sea. W h a t t y p e s of s e d i m e n t s w o u l d y o u e x p e c t to be commonly d e p o s i t e d offshore from this l a n d s c a p e ? A. arkose B . c a r b o n a t e sand C. quartz sand D. graywacke H i n t : Refer to F i g u r e 5.16 and the text that p r e c e d e s it. —~ JL
14. W h e n a granite is subject to intense c h e m i c a l w e a t h e r i n g , it is m o s t likely to result in a B
s e d i m e n t c o m p o s e d of
R e v i e w C h a p t e r 16,
A. feldspar and clay.
W e a t h e r i n g , E r o s i o n , and
B.
q u a r t z and c a l c i u m c a r b o n a t e .
Mass Wasting.
C.
quartz and clay.
D.
quartz and feldspar.
Sedimentation: Rocks Formed by Surface Processes
15. Feldspar is the m o s t a b u n d a n t silicate m i n e r a l in the crust of the Earth; the m o s t c o m mon s e d i m e n t a r y r o c k is A. s a n d s t o n e . B. c o n g l o m e r a t e . C. l i m e s t o n e . D. shale. 16. A coarse s a n d s t o n e with a s y m m e t r i c a l ripples and s m a l l - s c a l e c r o s s - b e d d i n g is e x p o s e d in a cliff b e t w e e n layers of siltstone a b o v e and gravels below. W h a t is the e n v i r o n m e n t of deposition for the c o a r s e s a n d s t o n e layer? A. b e a c h B. river c h a n n e l C. lake D. offshore. H i n t : Refer t o F i g u r e 5 . 1 1 . 17. Reefs and atolls are built by coral and algae A. on s u b s i d i n g o c e a n i c v o l c a n o e s and continental m a r g i n s . B. on islands in the m i d d l e of lakes. C. in the d e e p o c e a n floor and later uplifted to sealevel. D. w h e r e d o l o s t o n e is t r a n s f o r m e d to limestone. 18. As seawater e v a p o r a t e s , precipitation of soluble salts occurs in w h i c h o r d e r ? A. halite, c a r b o n a t e s , c a l c i u m sulfate B. calcium sulfate, halite, c a r b o n a t e s C. iron o x i d e , quartz, peat D. c a r b o n a t e s , c a l c i u m sulfate, halite 19. M e c h a n i s m s by w h i c h plate tectonic p r o c e s s e s p r o d u c e s e d i m e n t a r y b a s i n s are A. rifting, thermal sag, and flexure of the lithosphere. B. heating and c o m p r e s s i o n of the crust. C. w e a t h e r i n g and erosion. D. diagenesis and turbation.
Figure 5.14. The relative abundance of the major sedimentary rock types. In comparison with these types, all other sedimentary rock types—including evaporates, cherts, and other chemical rocks—exist in only minor amounts.
73
CHAPTER
6
Metamorphism: Modification of Rocks by Temperature and Pressure
Metamorphism: Modification of Rocks by Temperature and Pressure
Before Lecture Chapter Preview • What c a u s e s m e t a m o r p h i s m ? Brief answer: M e t a m o r p h i s m — a l t e r a t i o n of preexisting r o c k s in the solid s t a t e — is caused by increases in pressure and t e m p e r a t u r e and by reaction with c h e m i c a l components introduced by migrating fluids. See F i g u r e 6 . 1 . • What are t h e v a r i o u s k i n d s of m e t a m o r p h i s m ? Brief answer: R e g i o n a l and c o n t a c t m e t a m o r p h i s m are the m o s t c o m m o n . See F i g u r e 6.3 and Figure Story 6.4. • What are the chief t y p e s of m e t a m o r p h i c r o c k s ? Brief answer: M e t a m o r p h i c rocks fall into t w o major textural classes called foliated with minerals oriented in s o m e preferred direction such as the grain in w o o d and nonfoliated with no preferred m i n e r a l orientation. See Table 6 . 1 . • How is m e t a m o r p h i s m l i n k e d to plate t e c t o n i c s ? Hint: Refer to F i g u r e 6 . 3 .
Vital Information from Other Chapters Review the sections M e t a m o r p h i c R o c k s and T h e R o c k C y c l e . T h e s e sections are short and well worth your review.
During Lecture Keeping up with a fast-speaking lecturer can be a c h a l l e n g e . • Take as m a n y notes as you can. • D o n ' t stop w r i t i n g w h e n y o u b e c o m e c o n f u s e d a n d y o u w a n t to p o n d e r a concept. You can do that later. • If you miss s o m e t h i n g , leave a space so that you can fill it in later.
Note-Taking Tip: Use abbreviations to speed up your note taking met rock -> m e t a m o r p h i c r o c k sed rock -» s e d i m e n t a r y r o c k si -+ slate or slaty si cleave -> slaty c l e a v a g e shl -> shale shst -> schist fol ->• foliated unfol -> unfoliated gran -> granular reg m -> regional m e t a m o r p h i s m con m -* contact m e t a m o r p h i s m Feel free to m a k e up your o w n abbreviations. T h e important thing is to develop shorthand that is meaningful to you, while being quick, easy to use, and easy to remember.
75
76
PART II C H A P T E R 6
After Lecture Review Notes R e v i e w y o u r n o t e s right after lecture w h i l e material is fresh in y o u r m i n d . H e r e is some m e t a m o r p h i c r o c k m a t e r i a l t o add t o y o u r n o t e s .
Check Your Notes: Have y o u . . . •
added key figures to focus y o u r attention? T h e m o s t i m p o r t a n t figures are F i g u r e 6.1 (pressure and t e m p e r a t u r e ) , F i g u r e 6.3 (types of m e t a m o r p h i s m ) , F i g u r e 6.4 (foliation). Table 6.1 (classification) is the m o s t i m p o r t a n t table.
•
a d d e d helpful s k e t c h e s ? S u g g e s t i o n for C h a p t e r 6: M e t a m o r p h i c r o c k s are classified in part by texture. D r a w simple s k e t c h e s of textures (slaty c l e a v a g e , phyllite, schist, g n e i s s ) to help y o u r e m e m b e r the g r a d e s of m e t a m o r p h i s m . C l o s e study of the photo a t t h e b e g i n n i n g o f t h e c h a p t e r a n d F i g u r e 6.4 will h e l p y o u s e e h o w t o d o this.
•
a d d e d a s u m m a r y of the g r a p h s ? C h a p t e r 6 c o n t a i n s s o m e i m p o r t a n t information in g r a p h i c form. R e v i e w P - T F i g u r e s 6.2, 6.7, 6.8, 6.9, and 6.10. N o t e that all show different g r a d e s of m e t a m o r p h i s m resulting from increasing p r e s s u r e and temperature. C a n y o u s u m m a r i z e all t h e s e figures on a single p a g e of y o u r n o t e s ? W h a t d o e s your s u m m a r y sketch tell y o u about the formation of m e t a m o r p h i c r o c k s ?
• created a brief big picture o v e r v i e w of this lecture (a sketch or written o u t l i n e ) ? S u g g e s t i o n for C h a p t e r 6: F i g u r e 6.3 is k e y to u n d e r s t a n d i n g t h e t e c t o n i c settings that drive m e t a m o r p h i s m . Sketch a simplified version that clearly s h o w s six geologic settings for m e t a m o r p h i s m . W r i t e a c a p t i o n for this figure in y o u r o w n w o r d s .
Intensive Study Session We r e c o m m e n d giving the highest priority to activities that involve a n s w e r i n g questions. A n s w e r i n g q u e s t i o n s w h i l e using y o u r text and lecture notes as reference material is far more efficient than r e r e a d i n g c h a p t e r s or g l a n c i n g over notes. As a l w a y s , y o u h a v e three sources from w h i c h to c h o o s e q u e s t i o n s . • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . U s e the P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s at the end of this Study G u i d e chapter. E x e r c i s e 1 will help you sort out the k i n d s of rock d i s c u s s e d in this chapter. N o w that y o u h a v e studied the three r o c k t y p e s in detail, it's t i m e to integrate the information. E x e r c i s e 2 will h e l p y o u rise a b o v e the details and gain an o v e r v i e w by c o m p a r i n g i g n e o u s , sedimentary, and m e t a m o r p h i c r o c k s . • Text. For this chapter, the most important figures are Figure 6.1 (pressure and temperature), Figure 6.3 (types of m e t a m o r p h i s m ) , and Figure 6.10 (plate tectonics). T h e most important table is Table 6.1 (classification). C o m p l e t e Exercises 1, 6, 8, and 9 and T h o u g h t Questions 5, 6, 8, and 10 at the end of the chapter. • W e b Site O n l i n e R e v i e w E x e r c i s e s a n d S t u d y Tools http://www.whfreeman.com/understandingearth5e C o m p l e t e the G r a d e d O n l i n e Q u i z and W e b R e v i e w Q u e s t i o n s . Pay particular attention to the e x p l a n a t i o n s for the hints and a n s w e r s . F l a s h c a r d s will h e l p y o u learn n e w t e r m s . T h e G e o l o g y i n P r a c t i c e e x e r c i s e s p r o v i d e a n opportunity t o practice identifying m e t a m o r p h i c r o c k s and r e v i e w i n g g e o l o g i c settings in w h i c h they occur. C o m p l e t e the O n l i n e R e v i e w E x e r c i s e s What happens rock,
where?
Metamorphism
and plate
tectonics,
Create
a
metamorphic
and Before and after: Metamorphic rocks to review and reinforce w h a t
y o u have learned in the text.
78
PART II C H A P T E R 6
the three most c o m m o n types of m e t a m o r p h i s m within the Earth's crust. Other types of m e t a m o r p h i s m are low-grade, high-grade, and shock m e t a m o r p h i s m .
Vhat are the chief types of metamorphic rocks? • M e t a m o r p h i c r o c k s fall into t w o major textural c l a s s e s : foliated (displaying a preferred orientation of m i n e r a l s , a n a l o g o u s to the grain in w o o d ) a n d t h e n o n foliated. T h e c o m p o s i t i o n o f t h e p a r e n t r o c k and the g r a d e o f m e t a m o r p h i s m are the m o s t important factors controlling the mineralogy of m e t a m o r p h i c rock. M e t a m o r p h i s m usually causes little to no c h a n g e in the b u l k c o m p o s i t i o n of rock. T h e k i n d s o f minerals and their orientation d o c h a n g e . M i n e r a l a s s e m b l a g e s w i t h i n m e t a m o r p h i c r o c k s are u s e d by geoscientists as a guide to the original composition of the parent rock and the conditions d u r i n g m e t a m o r p h i s m . Refer to 6.7. • M i n e r a l a s s e m b l a g e s in m e t a m o r p h i c r o c k s p r o v i d e a b a s i s for r e c o n s t r u c t i n g the c o n d i t i o n s that c a u s e d m e t a m o r p h i s m and u n d e r s t a n d i n g m o r e a b o u t t h e associated geologic setting. Figure 6.8 and Table 6.2 s u m m a r i z e major m i n e r a l s of m e t a m o r p h i c facies. F i g u r e s 6.7, 6.9, and 6.10 illustrate h o w g e o l o g i s t s study and interpret m e t a m o r p h i c r o c k s and reconstruct the c o n d i t i o n s that formed them.
tHow
is metamorphism linked to plate tectonics?
• R e g i o n a l and h i g h - p r e s s u r e m e t a m o r p h i s m o c c u r at c o n v e r g e n t plate b o u n d aries. Refer to F i g u r e s 6.4 and 6.10. • Seafloor m e t a m o r p h i s m o c c u r s at spreading c e n t e r s . • C o n t a c t m e t a m o r p h i s m o c c u r s in a variety of plate tectonic settings w h e r e m a g m a b o d i e s are g e n e r a t e d .
The
important thing
is not to stop questioning. — A L B E R T EINSTEIN
Figure 6 . 8 . Changes in the mineral composition of mafic rocks, metamorphosed under conditions ranging from low grade to high grade.
Metamorphism: Modification of R o c k s by Temperature and Pressure
Figure 6.2. Temperatures, pressures, and depths at which low- and high-grade metamorphic rocks form. The dark band shows common rates at which temperature and pressure increase with depth over much of the continents.
Exam Prep Materials in this section are m o s t useful d u r i n g y o u r preparation for e x a m s . T h e C h a p t e r Summary and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r chapter review. Read the C h a p t e r S u m m a r y to begin your session. It provides a helpful overview that should refresh your m e m o r y . Next, w o r k on the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d an e x a m , to see h o w well y o u have m a s t e r e d this chapter. After you answer the q u e s t i o n s score t h e m . Finally, and m o s t important, review e a c h q u e s t i o n that you missed. Identify and correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the question incorrectly.
Chapter Summary What causes metamorphism? • M e t a m o r p h i s m is the alteration in the solid state of preexisting r o c k s , including older m e t a m o r p h i c r o c k s . Increases in t e m p e r a t u r e and p r e s s u r e and reactions with c h e m i c a l - b e a r i n g fluids c a u s e m e t a m o r p h i s m . M e t a m o r p h i s m typically involves a r e a r r a n g e m e n t (recrystallization) of the c h e m i c a l c o m p o n e n t s within the parent rock. R e a r r a n g e m e n t of c o m p o n e n t s within m i n e r a l s is facilitated by (1) h i g h e r t e m p e r a t u r e s that increase ion mobility in the solid state; (2) higher confining p r e s s u r e that c o m p a c t s the rock; (3) directed p r e s s u r e associated with tectonic activity that c a n c a u s e the r o c k to s h e a r ( s m e a r ) , which orients m i n e r a l g r a i n s and generates a foliation; and (4) c h e m i c a l reactions with m i g r a t i n g fluids m a y r e m o v e or add materials and i n d u c e the growth of new m i n e r a l s .
What are the various kinds of metamorphism? • An overview of six types of m e t a m o r p h i s m is provided in Figure 6.3. Regional metamorphism (associated with convergent plate boundaries), contact m e t a m o r phism (caused by the heat from an intruding body of m a g m a ) , and seafloor metamorphism (caused by seawater percolating at mid-ocean spreading centers) are
77
Metamorphism: Modification of Rocks by Temperature and Pressure
79
Practice Exercises Answers and e x p l a n a t i o n s are p r o v i d e d at the end of the Study G u i d e .
Exercise 1: Classification of metamorphic rocks by texture Complete the table by filling in the blank spaces.
Parent rock
Texture (foliated/granoblastic)
Metamorphic rock
shale quartz-rich
foliated sandstone granulite
granite limestone homfels amphibolites
and
greenstones
migmatite
Study Tip: Putting it all together Now you have b e e n i n t r o d u c e d to all three major r o c k types, i g n e o u s , sedimentary, and metamorphic. T h i s is a g o o d t i m e to a s s e m b l e w h a t you have learned into a c o m p a r i s o n chart. A chart is an excellent w a y to e n s u r e that y o u r e m e m b e r details about these r o c k types. Exercise 2 will h e l p you do this.
Exercise 2: Comparing igneous, sedimentary, and metamorphic rocks Complete the t a b l e b y filling i n the b l a n k s p a c e s . N o t e that t h e r e m a y b e m o r e t h a n o n e reasonable a n s w e r for s o m e b l a n k s .
Major mineral composition
Texture
calcium carbonate
nonfoliated
quartz, K and Na feldspar,
phaneritic
Rock type (igneous, sedimentary, metamorphic)
Rock name (granite, sandstone, marble)
mica, and a m p h i b o l e clay
fine-grained
clastic (continued)
80
PART II C H A P T E R 6
Major mineral composition
Texture
Rock type (igneous, sedimentary, metamorphic)
p y r o x e n e , c a l c i u m feldspar,
Rock name (granite, sandstone, marble) basalt
and olivine quartz
monfoliated
p e b b l e s and c o b b l e s of a variety of r o c k t y p e s f r a g m e n t s of seashells and
sedimentary
fine m u d quartz, m u s c o v i t e , chlorite,
metamorphic
schist
and g a r n e t
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of t h e S t u d y G u i d e . 1 . W h e n existing r o c k s u n d e r g o m e t a m o r p h i s m , they b e c o m e c h a n g e d b y A. the w e a t h e r i n g p r o c e s s at or n e a r the surface. B . color and h a r d n e s s . C. m e l t i n g and crystallization from the melt. D. the application of heat and pressure. 2. M e t a m o r p h i c rocks exposed at the surface are mainly products of processes acting on rocks A. n e a r the E a r t h ' s surface. B. at d e p t h s r a n g i n g from the u p p e r to l o w e r crust. C . within the m a n t l e . D. within the center of continents. 3. G e n e r a l l y there are t w o types of m e t a m o r p h i c r o c k s : A. regional and contact. B. foliated and g r a n o b l a s t i c . C . c o m p a c t e d and c e m e n t e d . D . clastic and porphyritic. 4. Foliated m e t a m o r p h i c r o c k s typically o c c u r in association with regional m e t a m o r p h i s m b e c a u s e the A. orientation of r o c k s involved in regional m e t a m o r p h i s m favors t h e d e v e l o p m e n t of foliation. B. r o c k is softened by heat and s q u e e z e d by c o m p r e s s i v e forces. C. p a r e n t rock is the correct t y p e to p r o d u c e foliation. D. p r e s s u r e is very low, w h i c h allows foliation to d e v e l o p . H i n t : Refer to F i g u r e 6.4. 5. Foliation d e v e l o p s A. p e r p e n d i c u l a r to c o m p r e s s i v e forces. B . parallel t o c o m p r e s s i v e forces. C. d u e to high w a t e r content. D. d u e to low t e m p e r a t u r e s and p r e s s u r e s .
Metamorphism: Modification of Rocks by Temperature and Pressure
6. Some m e t a m o r p h i c rocks are distinguishable from igneous and sedimentary rocks b e c a u s e their constituent grains A. B. C. D.
interlock, f o r m i n g a c o n t i n u o u s m o s a i c . have quite different c h e m i c a l c o m p o s i t i o n s . tend to be lined up in a preferred direction. are r o u n d e d and c e m e n t e d together.
7. Chemical c o m p o s i t i o n s of m e t a m o r p h i c r o c k s are d e t e r m i n e d by the A. B. C. D.
pressures to w h i c h they h a v e b e e n subjected. effects of b o t h t e m p e r a t u r e and pressure. t e m p e r a t u r e to w h i c h they h a v e been raised. c o m p o s i t i o n of the original r o c k s and fluids that percolate t h r o u g h the r o c k during metamorphism.
8. M e t a m o r p h i s m affects A. only older i g n e o u s rock. B. any y o u n g e r i g n e o u s a n d m e t a m o r p h i c rock. C. only o l d e r s e d i m e n t a r y rock. D. any older i g n e o u s , sedimentary, or m e t a m o r p h i c r o c k s . 9. Which m e t a m o r p h i c s e q u e n c e correctly s h o w s increasing grain size? A. B. C. D.
schist -» gneiss ->• phyllite -> slate slate -> phyllite ->• schist -> gneiss gneiss -> phyllite-"- slate -> schist phyllite -> slate ->• gneiss -> schist
10. Starting with the lowest t e m p e r a t u r e z o n e , w h i c h series of index m i n e r a l s is characteristic of increasing m e t a m o r p h i c g r a d e ? A. chlorite, biotite, garnet, sillimanite B. garnet, chlorite, biotite, k y a n i t e C. biotite, garnet, chlorite, sillimanite D. chlorite, biotite, sillimanite, kyanite, garnet Hint: Refer to F i g u r e 6.7. 11. On the M o o n , t h e major c a u s e of m e t a m o r p h i s m is A. B. C. D.
burial. subduction. meteor impacts. very cold t e m p e r a t u r e s .
12. Of the m e t a m o r p h i c r o c k s listed, w h i c h o n e is formed at the h i g h e s t t e m p e r a t u r e ? A. slate B. schist C. phyllite D. gneiss Hint: Refer to F i g u r e 6.7. 13. The difference b e t w e e n a gneiss and a granite is that the gneiss A. B. C. D.
has a different b u l k c h e m i c a l c o m p o s i t i o n . shows a distinct foliation. has a different m i n e r a l c o m p o s i t i o n . is generally less c o a r s e grained.
14. Schist and slate are distinguishable in that A. B. C. D.
schist is fine grained, w h e r e a s slate is schist is foliated, w h e r e a s slate is not slate is fine grained, w h e r e a s schist is slate is foliated, w h e r e a s schist is not
c o a r s e grained. foliated. c o a r s e grained. foliated.
81
82
PART II C H A P T E R 6
15. R e g i o n a l m e t a m o r p h i s m is found in association with A. lava flows. B . hot springs. C. very low pressures. D . s u b d u c t i o n z o n e s and c o r e s o f m o u n t a i n r a n g e s . 16. If gneiss or a n o t h e r m e t a m o r p h i c r o c k is h e a t e d to a d e g r e e that it b e g i n s to melt, A. the quartz, K-feldspar, and N a - r i c h p l a g i o c l a s e w o u l d start to m e l t first. B. N a - C a p l a g i o c l a s e , biotite, and m i n e r a l s (such as garnet) w o u l d m e l t first, leaving a r e s i d u e rich in felsic m i n e r a l s . C. all the m i n e r a l s in t h e r o c k w o u l d start to m e l t at essentially the s a m e temperature to f o r m a m a g m a of the s a m e c o m p o s i t i o n of the g n e i s s . D. the f e r r o m a g n e s i a n m i n e r a l s w o u l d start to m e l t first. H i n t : Refer t o F i g u r e 4 . 5 . 17. As m a g m a intrudes into a h o s t or c o u n t r y r o c k (the p r e e x i s t i n g r o c k that is in contact with the intrusion), it is transformed into a new r o c k . W h a t is this p r o c e s s c a l l e d ? A . regional m e t a m o r p h i s m
C.
recrystallization
B . contact m e t a m o r p h i s m
D.
schistosity f o r m a t i o n
18. In the W e b G e o l o g y in P r a c t i c e exercise, Gravels to Metaconglomerate, oval quartz pebbles apparently b e c o m e converted into cigar-shaped features within a metaconglomerate. H o w would y o u explain the transformation? A. T h e c i g a r - s h a p e d features f o r m e d as larger quartz p e b b l e s w e r e t u m b l e d in a stream c h a n n e l before m e t a m o r p h i s m . B . A s the oval quartz p e b b l e s b e c a m e e x p o s e d t o elevated t e m p e r a t u r e s , they softened and stretched out in r e s p o n s e to directed p r e s s u r e . S o m e of the q u a r t z m a y have also recrystallized along directions p e r p e n d i c u l a r to the directed p r e s s u r e . C. Q u a r t z is an index mineral for h i g h - g r a d e m e t a m o r p h i s m a s s o c i a t e d w i t h burial of s e d i m e n t a r y r o c k s . T h e r e f o r e , this c o n g l o m e r a t e m u s t have b e e n uplifted from near the b o t t o m of the crust. D. T h e c o n g l o m e r a t e melted and the large cigar-shaped features are large quartz crystals. H i n t : T h e b o t t o m of F i g u r e 4.5 p r o v i d e s an i m p o r t a n t c l u e to the t e m p e r a t u r e at which m i n e r a l s begin to melt and solidify (crystallize). 19. You are on a s u m m e r b a c k p a c k i n g trip in A l a s k a with friends and find an o u t c r o p of mica schist with large garnet p o r p h y r o b l a s t s , like the s a m p l e shown in F i g u r e 6.6. Your friends quickly s c r a m b l e to collect s o m e garnet crystals. T h e y then ask y o u a b o u t the conditions u n d e r w h i c h the beautiful garnet crystals formed. W h a t is y o u r r e s p o n s e to their query? A. G a r n e t - b e a r i n g schists are formed from iron-rich m a g m a s that solidify underground. B. Garnet is an index mineral for low-temperature and low-pressure m e t a m o r p h i s m associated with meteor impact craters. So we m u s t be within an ancient i m p a c t crater. C. G a r n e t c o m m o n l y o c c u r s in m i c a schists and is an index mineral for intermediate- to h i g h - g r a d e m e t a m o r p h i s m , associated with regional m e t a m o r p h i s m . T h i s r o c k may h a v e at o n e t i m e b e e n in the roots of a h u g e m o u n t a i n . D. Garnets occur only in eclogites. Therefore, this rock m u s t have b e g u n at the base of the crust and b e e n exposed by extensive uplift and erosion occurring in this region. 2 0 . W h i l e studying s o m e m e t a m o r p h i c r o c k s a m p l e s for a n u n d e r g r a d u a t e r e s e a r c h project, y o u discover e v i d e n c e that the rocks w e r e e x p o s e d t o h i g h - g r a d e m e t a m o r p h i c c o n d i tions. F u r t h e r study reveals that the garnets within the r o c k s h o w a history of p r o g r a d e followed b y r e t r o g r a d e P - T p a t h s . You c h e c k the f i e l d n o t e s from the g e o l o g i s t w h o collected the s a m p l e s and are not surprised to find m e n t i o n of ophiolites in the region (refer to F i g u r e s 4 . 1 2 and 6.9). You c o n c l u d e from this i n f o r m a t i o n that m e t a m o r p h i c rocks formed when A . c o n t i n e n t s collided. B. m a g m a intruded into a v o l c a n o associated with a s u b d u c t i o n z o n e . C. a m e t e o r i t e hit the Earth. D. h y d r o t h e r m a l fluids altered r o c k in an area with n u m e r o u s hot springs and g e y s e r s .
Metamorphism: Modification of R o c k s by Temperature and Pressure
Figure 6.10. P-T p a t h s a n d rock a s s e m b l a g e s a s s o c i a t e d with (a) o c e a n - c o n t i n e n t plate convergence a n d ( b ) c o n t i n e n t - c o n t i n e n t plate c o n v e r g e n c e . lower g e o t h e r m a l g r a d i e n t s p r e s e n t i n s u b d u c t i o n z o n e s .
T h e P-T p a t h s differ i n illustrating t h e
R o c k s t r a n s p o r t e d t o similar d e p t h s —
and p r e s s u r e s — b e n e a t h m o u n t a i n b e l t s b e c o m e m u c h hotter a t a n e q u i v a l e n t d e p t h .
83
CHAPTER
7
Deformation: Modification of Rocks by Folding and Fracturing
Figure 7.19. Last stage in the development of a geologic province. Refer to Figure 7.15 (1) for a more regional perspective on this type of deformation.
Before Lecture B e f o r e y o u attend lecture, b e sure t o s p e n d s o m e t i m e p r e v i e w i n g t h e chapter. F o r a n efficient preview, u s e the S t u d y G u i d e C h a p t e r P r e v i e w q u e s t i o n s a s a f r a m e w o r k for u n d e r s t a n d i n g the chapter. P r e v i e w i n g w o r k s b e s t i f y o u d o i t j u s t b e f o r e l e c t u r e . W i t h t h e m a i n p o i n t s in m i n d , y o u will u n d e r s t a n d t h e lecture better. T h i s in t u r n will r e s u l t in a b e t t e r and m o r e c o m p l e t e set o f n o t e s .
84
Deformation: Modification of Rocks by Folding and Fracturing
Study Tip Rock deformation lectures are particularly visual. Slide material on folds and faults can be confusing if you have never seen these geologic features before. So be sure to preview the figures before attending lecture. For an overview, start with Figure 7.7. Notice h o w three kinds of force p r o d u c e three kinds of faults if the material is brittle. W h a t h a p p e n s if the material is ductile or plastic?
How m u c h t i m e should y o u devote to p r e v i e w i n g ? Obviously, m o r e t i m e is better than less. But even a brief (five- or t e n - m i n u t e ) p r e v i e w session j u s t before lecture will p r o d u c e a result that you will n o t i c e . F o r a refresher on w h y p r e v i e w i n g is so important, see Study G u i d e , Part 1, Chapter 3, H o w to Be Successful in G e o l o g y .
Chapter Preview • W h a t are g e o l o g i c m a p s a n d c r o s s s e c t i o n s ? Brief a n s w e r : G e o l o g i c m a p s represent the r o c k formations e x p o s e d at the E a r t h ' s surface. G e o l o g i c cross sections are d i a g r a m s s h o w i n g h o w the geologist interprets or projects t h e g e o l o g y at t h e surface into the subsurface. Refer to F i g u r e 7.4. • How do rocks deform (break or bend)? Brief a n s w e r : R o c k s typically break (fault) w h e n the t e m p e r a t u r e is low, burial is shallow, and the force is applied quickly. R o c k s typically b e n d (fold) w h e n the temperature is higher, burial is deeper, and the force is applied over a long period. See Figures 7.7 and 7.10. • W h a t g e o l o g i c features are p r o d u c e d w h e n rocks d e f o r m ? Brief a n s w e r : F o l d s , faults, and j o i n t s are c o m m o n geologic structures p r o d u c e d w h e n rocks are d e f o r m e d . • W h a t do g e o l o g i c s t r u c t u r e s p r o d u c e d by rock d e f o r m a t i o n tell geologists a b o u t the g e o l o g i c h i s t o r y of a r e g i o n ? Brief a n s w e r : T h e type(s) of folds, faults, and j o i n t s a n d their spatial orientation provide g e o l o g i s t s w i t h clues for d e c i p h e r i n g the k i n d s of forces affecting a region over t i m e . S e e F i g u r e 7.19.
Vital Information from Other Chapters After each lecture y o u n e e d to t h o r o u g h l y master the c o n c e p t s covered before you attend the subsequent lecture. T h e ideas of g e o l o g y are like a stack of b o x e s . E a c h new idea rests on all ideas (boxes) stacked b e n e a t h it. Another l o o k at C h a p t e r 3 w o u l d serve y o u well. S i n c e there is a strong c o n n e c t i o n between deformation a n d m e t a m o r p h i s m , review Figure 6.4 and t h e section on Plate Tectonics and M e t a m o r p h i s m in C h a p t e r 6.
Web Site Study Resource http://www.whfreeman.com/understandingearth5e The Online R e v i e w E x e r c i s e Tectonic Forces in Rock Deformation introduces you to the basic t e r m i n o l o g y and c o n c e p t s covered in this chapter.
85
86
PART II
CHAPTER 7
During Lecture O n e goal for lecture is to leave class with g o o d a n s w e r s to the p r e v i e w q u e s t i o n s . • To a v o i d g e t t i n g lost in d e t a i l s , k e e p t h e b i g p i c t u r e in m i n d . C h a p t e r 7 tells t h e s t o r y o f t h r e e k i n d s o f forces ( c o m p r e s s i v e , t e n s i o n a l , a n d s h e a r i n g ) a n d h o w g e o l o g i s t s find e v i d e n c e o f t h e s e forces i n r o c k s t r u c t u r e s (folds a n d faults). • D e f o r m a t i o n lectures are particularly visual. Slide material on folds and faults c a n be confusing if y o u h a v e never seen t h e s e g e o l o g i c features before. K e e p F i g u r e s 7.7 and 7.10 on h a n d during lecture.
Note-Taking Tip F i g u r e s of faults and folding can be d r a w n very simply o n c e you u n d e r s t a n d t h e m . B u t until y o u d o , m a k e it easy on yourself. P h o t o c o p y F i g u r e s 7.7 a n d 7.10. T h r e e - h o l e p u n c h t h e m for easy insertion into y o u r three-ring n o t e b o o k . If they are already in y o u r n o t e b o o k before lecture, you c a n n o t be distracted d r a w i n g t h e m d u r i n g lecture.
Education
is
a
voyage
in
self-discovery. —LAURENCE M.
GOULD
After Lecture Review Notes T h e perfect t i m e to review y o u r notes is right after lecture. T h e following c h e c k l i s t contains b o t h g e n e r a l review tips and specific s u g g e s t i o n s for this chapter.
Check Your Notes: Have y o u . . . ( I a d d e d visual material? S u g g e s t i o n : Test y o u r u n d e r s t a n d i n g of C h a p t e r 7 by a d d i n g to y o u r notes s i m p l e sketches of important geological features such as n o r m a l , reverse, and thrust faults, and an anticline, syncline, a s y m m e t r i c fold, and o v e r t u r n e d fold. A d d c a p t i o n s to help you k e e p things straight. (For e x a m p l e , " A s s o c i a t e the w o r d ' s i n k ' to the V in syncline. ' S i n k ' is w h a t a syncline r e s e m b l e s . " ) • created a brief big picture o v e r v i e w of this lecture (a sketch or written o u t l i n e ) ? S u g g e s t i o n : Figure 7.7 is a g o o d o n e - p a g e s u m m a r y of brittle rock d e f o r m a t i o n .
Intensive Study Session B e c a u s e t h e r e is a lot to learn in this chapter, be sure to set priorities for s t u d y i n g . T h e r e is p r o b a b l y m o r e material than y o u will h a v e t i m e to study in o n e intensive study session. We r e c o m m e n d that y o u give highest priority to activities that involve a n s w e r i n g q u e s t i o n s . A n s w e r i n g questions w h i l e u s i n g y o u r text and lecture notes as reference m a t e r i a l is far m o r e
Deformation: Modification of Rocks by Folding and Fracturing
efficient than r e a d i n g c h a p t e r s or glancing over n o t e s . As a l w a y s , you have three sources from which to c h o o s e q u e s t i o n s : • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s Be sure to do E x e r c i s e 1. It involves the key information y o u n e e d to learn in this chapter. • Text. This is a very visual chapter, so focus y o u r attention on understanding the figures. M o s t of the really essential material for this chapter is in Figures 7.7 and 7.10, so focus on understanding these illustrations first. A n s w e r Exercises 1, 4, and 5 at the end of the chapter. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e the O n l i n e R e v i e w Q u e s t i o n s . Pay particular attention to the hints if you get b o g g e d d o w n . T h e G e o l o g y in Practice exercise Folds, Faults, and Other Records of Rock Deformation provides an opportunity for you to practice your skills identifying and interpreting classic e x a m p l e s of r o c k deformation. T h e F l a s h C a r d s will h e l p y o u learn the new terminology. T h e O n l i n e R e v i e w Exercises Tectonic Forces in Rock Deformation and Types of Folds will help you organize y o u r k n o w l e d g e about the different features of folds and faults.
Exam Prep Materials in this section are m o s t useful during y o u r preparation for quizzes and e x a m s . T h e Chapter S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r chapter review. R e a d t h e C h a p t e r S u m m a r y to b e g i n y o u r session. It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y . Next, w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d an e x a m , to see h o w well you have m a s t e r e d this chapter. After you a n s w e r t h e q u e s t i o n s , score t h e m . Finally, and m o s t important, review each q u e s tion you missed. Identify and correct t h e m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the question incorrectly.
Chapter Summary What are geologic maps and cross sections? • Geologic m a p s represent the r o c k formations e x p o s e d at the E a r t h ' s surface. The orientation of r o c k layers is m e a s u r e d as strike and dip. Refer to Figure 7.3. • Geologic cross sections are d i a g r a m s s h o w i n g h o w the geologist interprets or projects the g e o l o g y at the surface into the subsurface. Refer to F i g u r e 7.4.
How do rocks deform (bend and break)? • All rocks m a y b e n d (ductile behavior) and b r e a k (brittle behavior) in r e s p o n s e to the application of forces. L a b o r a t o r y e x p e r i m e n t s have revealed that w h e t h e r a rock exhibits ductile or brittle b e h a v i o r d e p e n d s on its c o m p o s i t i o n , t e m p e r ature, d e p t h of burial (confining p r e s s u r e ) , and the rate with w h i c h tectonic processes apply force. • Ductile b e h a v i o r is m o r e likely w h e n a r o c k is s e d i m e n t a r y and e x p o s e d to higher t e m p e r a t u r e s , d e e p e r burial, and slower application of tectonic forces. Brittle b e h a v i o r is favored w h e n r o c k s are i g n e o u s or h i g h - g r a d e m e t a m o r p h i c and cooler, closer to the E a r t h ' s surface, and e x p o s e d to m o r e rapid application of tectonic forces.
88
PART II C H A P T E R 7
What geologic features are produced when rocks deform? • F o l d i n g is a r e s u l t of d u c t i l e d e f o r m a t i o n . F r o m t h e t y p e of fold and its orie n t a t i o n , g e o l o g i s t s c a n interpret t h e o r i e n t a t i o n o f t h e t e c t o n i c forces a n d c h a r a c t e r i s t i c s o f t h e r o c k layers d u r i n g d e f o r m a t i o n . • F a u l t i n g and j o i n t i n g are a result of brittle d e f o r m a t i o n . J o i n t i n g o c c u r s w h e n a r o c k fractures but there is little m o v e m e n t a l o n g the fracture p l a n e s . F a u l t s are fractures a l o n g w h i c h there i s a p p r e c i a b l e m o v e m e n t (offset). T h e t y p e a n d orientation of faults and j o i n t s p r o v i d e valuable i n f o r m a t i o n a b o u t t h e tectonic forces and the characteristics of the r o c k layers at the t i m e of d e f o r m a t i o n .
What do geologic structures produced by rock deformation tell geologists about the history of a region? • T h e t y p e of fold or fault p r o v i d e s a m e a n s for g e o l o g i s t s to i n t e r p r e t t h e t y p e o f t e c t o n i c force acting o n t h e r o c k d u r i n g d e f o r m a t i o n . T e c t o n i c forces c a n b e o f three t y p e s : c o m p r e s s i v e , t e n s i o n a l , and s h e a r i n g f o r c e s . T h e s e forces are active at all t h r e e t y p e s of plate t e c t o n i c b o u n d a r i e s : c o m p r e s s i v e forces d o m i n a t e at c o n v e r g e n t b o u n d a r i e s ( w h e r e plates collide or s u b d u c t ) ; t e n s i o n a l forces d o m i n a t e a t divergent b o u n d a r i e s ( w h e r e plates are p u l l e d a p a r t ) ; a n d s h e a r i n g forces d o m i n a t e at t r a n s f o r m faults ( w h e r e p l a t e s slide h o r i z o n t a l l y past each other). • G e o l o g i c structures such as folds, faults, and j o i n t s o c c u r on all scales from m i c r o s c o p i c to the size of a m o u n t a i n s i d e . G e o l o g i s t s d e d u c e the g e o l o g i c history of a r e g i o n in part by u n r a v e l i n g the history of d e f o r m a t i o n , t h e r e b y r e c o n s t r u c t i n g w h a t the r o c k units l o o k e d like before d e f o r m a t i o n . • R e g i o n a l d e f o r m a t i o n a l fabrics (the t y p e s of faults a n d folds in e v i d e n c e ) c a n h e l p geologists d e c i p h e r the plate tectonic history of the r e g i o n . F o r e x a m p l e , n o r m a l faulting suggests that a r e g i o n is b e i n g stretched like a r u b b e r b a n d by tensional forces. Refer to F i g u r e s 7 . 1 5 , 7.16, 7.17, 7.18, a n d 7.19. C o n s i d e r h o w t h e g e o l o g i c c i r c u m s t a n c e s i n F i g u r e 7.16 are similar but also s o m e w h a t different from t h o s e d e p i c t e d in F i g u r e 7.15 (1 Tensional t e c t o n i c s ) .
Figure 7.16. (b) A rift valley results from tensional forces and normal faulting. The African Plate, on which Egypt rides, and the Arabian Plate, bearing Saudi Arabia, are drifting apart. The tensional forces have created a rift valley, filled by the Red Sea. The diagram shows parallel normal faults bounding the rift valley in the crust beneath the sea.
D)eformation: eformation
Modification of Rocks by Folding and Fracturing
89
Practice Exercises ly G u i d e . Answers a n d e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e .
Exercise 1: Silly Putty at r o o m t e m p e r a t u r e , it exhibits Silly Putty is a p o p u l a r teaching aid with geologists b e c a u s e , at r o o m tei e putty quickly, it will snap into all three d e f o r m a t i o n characteristics of solids. If y o u pull on the putty qui molded into m a n y shapes. Plus, two pieces. H a n d l e d slowly, the putty can be easily b e n t and m o l d e d int operties of Silly Putty with the a ball of it t h r o w n on the floor will b o u n c e . C o m p a r e the properties ol behavior of r o c k s by c o m p l e t i n g the table.
Behavior of Silly Putty
Behavior of rock
Snaps into pieces
Type of of force force
Geologic structure produced by this style of deformation
tensional nal
Bends
ductile
Bounces
elastic •essional compressional ill ofofputty The ball putty is •essed by compressed by hthe with thej the floor. impact t with
R o c k s d o e x h i b i t e l a s t i c b e h a v i o r . M o r e o n this w h e n w e s t u d y e a r t hhquakes. q u a k e s . ((Earthquakes E
are attributed to the elastic
properties o f r o c k s . )
Exercise 2: Geologic structures For each of the following five illustrations of deformed rocks, nnaam mee the the I(A) geologic structure (normal fault, syncline); (B) type of force (compressional, tensional, sional, she shearing) responsible for producing each geologic structure; and (C) the plate tectonic boundary )oundary (i(convergent, divergent, or shear) with w h i c h the geologic structure is c o m m o n l y associated. iated.
A. Geologic structure B. Type of force C. C o m m o n l y associated plate tectonic b o u n d a r y
D. Geologic structure E. Type of force F. C o m m o n l y a s s o c i a t e d plate tectonic b o u n d a r y
90
PART II C H A P T E R 7
G . G e o l o g i c structure H. T y p e of force
.
I . C o m m o n l y a s s o c i a t e d plate tectonic b o u n d a r y
J.
G e o l o g i c structure
K. T y p e of force
.
L . C o m m o n l y a s s o c i a t e d plate tectonic b o u n d a r y
M . G e o l o g i c structure N . T y p e o f force O . C o m m o n l y a s s o c i a t e d plate tectonic b o u n d a r y
Exercise 3: Anticline versus syncline A. Briefly d e s c r i b e the d i a g n o s t i c differences b e t w e e n an anticline a n d a s y n c l i n e .
B. D r a w a picture of a typical outcrop pattern for a plunging syncline e x p o s e d at the surface. Refer to Figure 7.11.
Deformation: Modification of R o c k s by F o l d i n g and Fracturing
91
92
PART II C H A P T E R 7
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of the S t u d y G u i d e . 1. A r o c k that b r e a k s suddenly in r e s p o n s e to the application of forces is A. elastic. B . plastic. C. brittle. D . ductile. 2. T h e t w o m e a s u r e m e n t s that define the orientation of an e x p o s e d r o c k layer are A. strike and d i p . B. strike and slip. C. slip and d i p . D. fold axis a n d tilt. 3. T h e s e n s e of m o t i o n a l o n g the San A n d r e a s fault in California is A. left lateral strike slip. B. right lateral strike slip. C. dip slip. D. thrust. 4. As tensional forces are applied to a c o n t i n e n t a l region, t h e resulting g e o l o g i c feature will be a(n) A . anticline. B. rift valley. C. thrust fault. D. dome. H i n t : Refer to F i g u r e s 7.7 and 7.15. 5. W h e n no offset can be detected along a fracture in a rock, t h e fracture is called a A. stress p l a n e . B. joint. C.
fault.
D. rupture. 6. W h i c h of the following c o n d i t i o n s p r o m o t e s ductile d e f o r m a t i o n of r o c k s ? A. old i g n e o u s r o c k s within the interior of a c o n t i n e n t B. r o c k s d e f o r m i n g at a relatively low t e m p e r a t u r e C. stress building up rapidly to a very high level D . r o c k s subjected t o high confining p r e s s u r e s and t e m p e r a t u r e s 7. W h e n d e f o r m e d , w h i c h of the following r o c k s is m o r e likely to fracture brittlely instead of flow ductilely? A . basalt B.
shale
C. p u r e l i m e s t o n e D. muddy sandstone 8. As m o l t e n r o c k cools n e a r or at the surface, it c a n d e v e l o p s h r i n k a g e fractures that e x t e n d vertically d o w n into the r o c k body. T h e s e c r i s s c r o s s i n g , regularly p a t t e r n e d fractures create long thin r o d s of r o c k we call A . s h r i n k a g e palisades. B. tension faults. C. columnar joints. D. elongate joints. H i n t : Refer to t h e p h o t o at the b e g i n n i n g of C h a p t e r 4.
Deformation: Modification of Rocks by Folding and Fracturing
9. W h i c h of t h e following g e o l o g i c structures is c a u s e d by tensional forces? A. thrust fault B. reverse fault C. anticline D. n o r m a l fault 10. Thrust faults c o m m o n l y form A. B. C. D.
a r o u n d h o t spots. w h e r e c o n t i n e n t s are colliding. w h e r e continents are pulling apart. a l o n g a t r a n s f o r m fault.
11. T h e direction of dip is defined by a line A. at right angles to the strike line. B. north of t h e strike line. C. parallel to the strike line. D. parallel to p l u n g e . H i n t : Refer t o F i g u r e 7 . 3 . 12. F r o m an airplane y o u n o t i c e that t h e o u t c r o p s of tilted layers of r o c k m a k e a distinct zigzag pattern across a plain. You r e a s o n a b l y c o n c l u d e that the A . a r e a has b e e n cut b y n u m e r o u s n o r m a l faults. B. layers are folded into a series of p l u n g i n g folds. C. layers are folded into a series of n o n p l u n g i n g folds. D. layers h a v e b e e n tilted so that all the layers dip in t h e s a m e direction. H i n t : Refer t o F i g u r e s 7 . 1 1 .
CHAPTER
8
Clocks in Rocks: Timing the Geologic Record
Clocks in Rocks: Timing the Geologic Record
Before Lecture Time-Management Tip: Something is always better than nothing How m u c h time should you devote to previewing? Obviously, the more time, the better. However, even a brief (five or ten m i n u t e s ) preview session j u s t before lecture will produce n o t i c e a b l e results. Arrive ten m i n u t e s early for lecture. U s e the time to preview the c h a p t e r for the d a y ' s lecture. E v e n if you have only a m i n u t e or t w o you can read the preview q u e s t i o n s a n d brief a n s w e r s and gain a r o u g h idea of w h a t t h e c h a p t e r is about. The t i m e j u s t before lecture is p r e c i o u s , b e c a u s e w h a t e v e r you preview will r e m a i n in short-term m e m o r y and h e l p y o u u n d e r s t a n d lecture.
Chapter Preview • H o w c a n t h e relative a g e s of rocks be d e t e r m i n e d from rock o u t c r o p s ? Brief a n s w e r : T h e principles of superposition, fossils, and cross-cutting relationships provide a basis for establishing the relative age of a s e q u e n c e of rocks at an o u t c r o p . For e x a m p l e , in Figure 8.4 and Eaith Issues 8 . 1 . the y o u n g e s t s e d i m e n t a r y r o c k layers are on top and the oldest are at the b o t t o m . A n d in Figure 8.10, m a g m a intrusions and a fault are y o u n g e r than the rock they cut through. • H o w c a n t h e relative a g e s of rock o u t c r o p s at t w o or m o r e l o c a t i o n s be determined? Brief a n s w e r : Stratigraphic and fossil succession, plus radiometric dates of rock units, provide a basis for establishing how rock outcrops at different localities m a y be related to e a c h other (correlate), even if they are h u n d r e d s or t h o u s a n d s of miles apart. See Figure 8.5. • W h a t is t h e G e o l o g i c T i m e S c a l e a n d h o w is it c a l i b r a t e d ? Brief a n s w e r : T h e G e o l o g i c T i m e Scale is the internationally accepted reference for E a r t h ' s g e o l o g i c history. U s i n g relative and absolute dating m e t h o d s , geologists have calibrated (divided) E a r t h ' s history into four e o n s : H a d e a n , A r c h e a n , P r o t e r o z o i c , and P h a n e r o z o i c . B e c a u s e m o r e e v i d e n c e is available for the m o s t recent eon, the P h r a n e r o z o i c , it h a s been possible to divide it m o r e finely into eras, p e r i o d s , and epochs. See F i g u r e s 8.10 and 8.14. • H o w are t h e G e o l o g i c T i m e S c a l e a n d g e o c h r o n o l o g i c a l m e t h o d s , like r a d i o m e t r i c dating, a p p l i e d t o g e o l o g i c p r o b l e m s ? Brief a n s w e r : We u n d e r s t a n d E a r t h ' s history to the degree to w h i c h we can place the record of g e o l o g i c events in time. T h e G e o l o g i c T i m e Scale is the accepted standard for how g e o l o g i c t i m e is subdivided.
Vital Information from Other Chapters Review F i g u r e 1.13 (the r i b b o n of g e o l o g i c t i m e from the formation of the solar s y s t e m to present). R e v i e w the text section T h e R o c k C y c l e : Interactions b e t w e e n Plate Tectonic and Climate S y s t e m s in C h a p t e r 3. R e r e a d the A t o m i c Structure of A t o m s section in C h a p t e r 3 before reading about r a d i o m e t r i c dating m e t h o d s . A l s o preview F i g u r e 9.10 on p a g e 198.
95
Time is simply nature's way from happening at once.
of keeping
everything
— G R A F F I T I ON A WALL
During Lecture H e r e ' s an overview that should h e l p y o u take g o o d notes for this lecture. • B i g P i c t u r e . T h e big picture for this lecture is t h e entire G e o l o g i c T i m e S c a l e ! G e o l o g i c t i m e is w o n d r o u s l y h u g e . It is so e x p a n s i v e that at first it s e e m s i n c o m p r e h e n s i b l e . T h e lecturer will describe the 4.5-billion-year e x p a n s e of g e o l o g i c t i m e and m a y u s e e x a m p l e s and exercises d e s i g n e d t o h e l p y o u g r a s p geologic t i m e . • N e w T e r m s . It is difficult to talk about t h e G e o l o g i c T i m e Scale w i t h o u t referring to its t i m e intervals. So you m a y feel b a r r a g e d with n e w t e r m s : e p o c h s , periods, eras, e o n s , H o l o c e n e , P l e i s t o c e n e , P l i o c e n e . To avoid getting lost, k e e p a copy of Figures 8.11 and 8.14 at h a n d . Refer to F i g u r e 8.14 to c h e c k t e r m s as needed. N o t e h o w different-sized c h u n k s of t i m e are u s e d for different e o n s : h u g e c h u n k s (eons) are u s e d for events early in E a r t h ' s history w h e n there is less geological e v i d e n c e for w h a t h a p p e n e d , and smaller c h u n k s (epochs) are u s e d for intervals of t i m e closer to the present. B e c a u s e m o r e evid e n c e is available for the m o s t recent eon, the P h a n e r o z o i c , it h a s b e e n divided m o r e finely into eras, periods, and e p o c h s . See F i g u r e 8.14. • S u c c e s s i o n of G e o l o g i c E v e n t s . You will w o r k t h r o u g h h o w t h e relative ages of r o c k s are d e t e r m i n e d . By the end of lecture y o u will be able to u s e t w o basic principles (superposition and cross-cutting) to d e t e r m i n e t h e relative a g e of a s e q u e n c e of r o c k s such as that s h o w n in F i g u r e 8.10. • A b s o l u t e D a t i n g . C a r b o n - 1 4 has a short half-life. It w o r k s for dating y o u n g e r s a m p l e s of tissue attached to b o n e , charcoal, a n d w o o d b e c a u s e these m a t e r i als all c o n t a i n carbon. O t h e r isotopes ( u r a n i u m - 2 3 8 , p o t a s s i u m - 4 0 , r u b i d i u m 87) h a v e m u c h l o n g e r half-lives and are u s e d to date r o c k s that are g e o l o g i cally older. Table 8.1 shows h o w half-life is related to the effective dating r a n g e of each m e t h o d .
After Lecture Review Notes T h e perfect t i m e to review y o u r notes is right after lecture. T h e following checklist contains both general review tips and specific suggestions for this chapter.
Check Your Notes: Have y o u . . . • shown clearly h o w superposition and cross-cutting features can be used to s e q u e n c e rock units and geologic events? Hint: A simple sketch is the best w a y to s h o w this, s h o w n clearly the characteristics of (1) an unconformity and (2) an a n g u l a r u n c o n f o r m i t y ? C h e c k Figures 8.6, 8.7, and 8.8 against y o u r notes. A d d sketches to y o u r notes if you need to.
Clocks in Rocks: Timing the Geologic Record
Intensive Study Session The two big tasks for this study session are to m a s t e r the skill of dating outcrops using the principles of superposition and cross cutting and to learn the G e o l o g i c T i m e Scale. Begin y o u r w o r k on o u t c r o p s with Study G u i d e E x e r c i s e 1. T h e other exercises and t h e review q u e s t i o n s will allow y o u to test y o u r skills further. Refer to y o u r notes and relevant text figures to h e l p y o u .
We are like the butterflies who flutter for a day and think it is forever, — C A R L SAGAN
Unless you h a p p e n t o h a v e spent y o u r s u m m e r w o r k i n g o n a n a r c h e o l o g y d i g , tossing a r o u n d terms like P a l e o c e n e , E o c e n e , a n d O l i g o c e n e , the e p o c h s of the C e n o z o i c m a y l o o k like a steep m e m o r y c u r v e . H e r e a r e four different strategies for learning the G e o l o g i c T i m e Scale. 1. M a r k e r e v e n t s are simply interesting things that h a p p e n e d : a n i m a l s or plants that evolved, creatures that d o m i n a t e d the Earth, large extinction events. L o o k at F i g u r e 8.11. Select s o m e m a r k e r events you already k n o w about. E x a m p l e : C a n you g u e s s o n e of the p e r i o d s d u r i n g w h i c h d i n o s a u r s w e r e d o m i n a n t ? T h e m o v i e Jurassic Park has m a d e this an easy q u e s t i o n to answer. W h e n did c o m p l e x life b e g i n ? F i n d s o m e other m a r k e r events of particular interest to you. You m a y find yourself surprised at h o w early s o m e events occurred. M a r k e r events will h e l p y o u r e m e m b e r the G e o l o g i c T i m e Scale. Try E x e r c i s e 3. 2. Logical c h u n k s . G r o u p the information into short lists you can r e m e m b e r . S t u d y the groupings of the t i m e scale with F i g u r e s 8.11 and 8.14 in front of you. L e a r n it as a series of short lists. U n d e r s t a n d the following logic. • E o n s ( F i g u r e 8.14) are the b i g g e s t t i m e c h u n k s . T h e r e are only four ( H a d e a n , A r c h e a n , P r o t e r o z o i c , P h a n e r o z o i c ) t o r e m e m b e r . O n l y the m o s t recent ( P h a n e r o z o i c eon) is b r o k e n d o w n further. H a d e a n sound like H a d e s (hell), not a b a d description of the y o u n g planet w i t h its m o l t e n surface and asteroids crashing into it. • E r a s ( F i g u r e 8.11) are next biggest. You h a v e to learn eras only for the P h a n e r o z o i c eon. T h a t ' s b e c a u s e g e o l o g i c a l e v i d e n c e is too limited to justify the division of earlier e o n s . T h e r e are only three P h a n e r o z o i c eras to r e m e m b e r : O l d Life, M i d d l e Life and N e w Life. T h i n k of t h e m that w a y first; you can tack on the G r e e k stems later. • P e r i o d s ( F i g u r e 8.11) are next. All three eras of the P h a n e r o z o i c eon are further divided into p e r i o d s . No periods for earlier e o n s : not e n o u g h e v i d e n c e . • E p o c h s are t h e smallest divisions of g e o l o g i c t i m e . You h a v e to learn e p o c h s only for the m o s t r e c e n t era ( C e n o z o i c , or N e w Life). All e p o c h s of the C e n o z o i c e n d in -cene (for " C e n o z o i c " ) . Now that you u n d e r s t a n d t h e divisions, return to F i g u r e 8.14, w h i c h clarifies h o w they all fit together and adds t h e absolute dates that have b e e n d e t e r m i n e d by r e a d i n g the r a d i o m e t r i c rock clocks.
97
3. W o r d S t e m s . W o r d stems are clues to m e a n i n g . G r e e k and Latin s t e m s are u s e d a great deal by scientists. You can look t h e m up in any g o o d dictionary. A few helpful stems for t h e G e o l o g i c T i m e Scale follow. , Eras Paleo- = G r e e k : " o l d " Meso- = G r e e k : " m i d d l e " Ceno- = G r e e k : " n e w " -zoic = G r e e k : "life" E p o c h s : D o n ' t worry about the m i d d l e e p o c h s for now. Just r e m e m b e r the first and last ones. Paleo - G r e e k : " o l d " Pleisto = G r e e k : " m u c h . " R e m e m b e r that there w a s much ice in t h e P l e i s t o c e n e . Holo = Greek: "recent." R e m e m b e r that the H o l o c e n e is the present or most recent epoch. N o t e that all e p o c h s of the C e n o z o i c e p o c h end in -cene. 4. M n e m o n i c (catch p h r a s e ) . W h e n there are long lists of unfamiliar t e r m s to learn (such as the e p o c h s of t h e C e n o z o i c ) , m a n y learners find m a k i n g catch p h r a s e s helpful. Try this. D o E x e r c i s e 4 , G e o l o g i c T i m e Scale m n e m o n i c . W e b Site O n l i n e R e v i e w E x e r c i s e s a n d S t u d y Tools http://www.whfreeman.com/understandingearth5e Take the G r a d e d O n l i n e Q u i z and pay particular attention to the f e e d b a c k for answers. F l a s h c a r d s will h e l p you learn n e w t e r m s . W o r k with Field Relationships for Relative Time Dating and the Geologic Time Scale Review O n l i n e R e v i e w E x e r c i s e s .
No vestige of a beginning,
no prospect of an
end. —JAMES HUTTON
(1726-1797)
Exam Prep M a t e r i a l s in this section are m o s t useful during y o u r p r e p a r a t i o n for e x a m i n a t i o n s . The C h a p t e r S u m m a r y and P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify your c h a p t e r review. R e a d t h e C h a p t e r S u m m a r y to b e g i n y o u r session. It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y .
Chapter Summary How can the relative ages of rocks be determined from rock outcrops? • T h e principles of superposition and cross-cutting relationships p r o v i d e a basis for establishing t h e relative age of a s e q u e n c e of r o c k s at an o u t c r o p . U s i n g these t w o principles, geologists can order ( d e t e r m i n e w h a t h a p p e n e d first, second, third, and so on) the g e o l o g i c events represented by t h e r o c k s a n d g e o logic features in r o c k o u t c r o p s . T h e principle of original horizontality for sedi m e n t a r y layers p r o v i d e s a starting p o i n t for identifying s e q u e n c e s of s e d i m e n t a r y r o c k s affected by tectonic forces after they w e r e deposited.
Clocks in Rocks: Timing the Geologic Record
How can the relative ages of rock outcrops at two or more locations be determined? • To r e c o n s t r u c t the g e o l o g i c history of the Earth, geologists n e e d to correlate t h e g e o l o g i c events r e p r e s e n t e d by r o c k s at o n e locality with the g e o l o g i c events r e p r e s e n t e d by r o c k s at other localities. T h e stratigraphic and fossil succ e s s i o n and the r a d i o m e t r i c dates of rock units show h o w r o c k o u t c r o p s at different localities m a y be related to e a c h other, even if they are h u n d r e s or t h o u s a n d s of m i l e s apart.
What is the Geologic Time Scale and how is it calibrated? • T h e G e o l o g i c T i m e S c a l e is the internationally a c c e p t e d reference for the s e q u e n c e o f events r e p r e s e n t e d b y E a r t h ' s r o c k record. G e o l o g i s t s c o n s t r u c t e d it over a b o u t the last 2 0 0 y e a r s u s i n g m a i n l y fossils, superposition, and c r o s s cutting r e l a t i o n s h i p s to establish the relative a g e s of t h o u s a n d s of r o c k outcrops a r o u n d t h e world. I n a b o u t t h e last 6 0 y e a r s , the G e o l o g i c T i m e S c a l e has b e e n calibrated u s i n g r a d i o m e t r i c m e t h o d s derived from the discovery o f radioactive i s o t o p e s by physicists in the early part of t h e twentieth century. • T h e G e o l o g i c T i m e S c a l e , like t h e t h e o r y o f p l a t e t e c t o n i c s o r a n y o t h e r s c i entific t h e o r y o r t o o l , i s a l w a y s o p e n t o t h e c h a l l e n g e o f n e w e v i d e n c e . B u t i t i s i m p o r t a n t t o u n d e r s t a n d that t h e G e o l o g i c T i m e S c a l e i s r o o t e d d e e p l y not j u s t i n g e o l o g y b u t i n p h y s i c s , c h e m i s t r y , b i o l o g y , a n d p a l e o n t o l o g y . I t is o n e of s c i e n c e ' s g r e a t f o u n d a t i o n s , a c o n c e p t t h a t if s e r i o u s l y c h a l l e n g e d b y h a r d e v i d e n c e (it h a s n o t b e e n ) w o u l d force r e a p p r a i s a l o f m a n y d i s c i p l i n e s o f solid w o r k i n g s c i e n c e . F o r i t t o b e w r o n g , m u c h o f g e o l o g y w o u l d have to be wrong, and substantial areas of physics would have to be wrong, and m o s t of b i o l o g y and p a l e o n t o l o g y w o u l d have to be w r o n g . An error of this m a g n i t u d e is very unlikely. T h u s (for m o s t practical p u r p o s e s ) the G e o l o g i c T i m e S c a l e is a s s u m e d as fact by virtually all scientists and scientifically literate p e o p l e .
How are the Geologic Time Scale and geochronological methods like radiometric dating applied to geologic problems? • We u n d e r s t a n d E a r t h ' s history to the d e g r e e to w h i c h we c a n p l a c e the r e c o r d of g e o l o g i c events in t i m e . T h e G e o l o g i c T i m e Scale is t h e a c c e p t e d standard for h o w g e o l o g i c t i m e is subdivided. E a r t h Issues 8.1 p r o v i d e s an e x a m p l e .
Figure 8.12. The radioactive decay of rubidium to strontium.
99
Clocks in Rocks: Timing the Geologic Record U s i n g this information, unit 1 is a. older than unit 12 b u t y o u n g e r than unit 1 1 . b. y o u n g e r than unit 12 but older than unit 1 1 . c. older t h a n unit 11 but y o u n g e r than unit 12. 2 . E x p l a i n the logic y o u u s e d t o a n s w e r q u e s t i o n E - l .
Exercise 2: Ordering geologic events In the illustration, a g e o l o g i c o u t c r o p reveals three layers of s e d i m e n t a r y r o c k s , o n e fault, and a single i g n e o u s d i k e intrusion. A. O r d e r t h e s e q u e n c e of g e o l o g i c events from y o u n g e s t to oldest.
Youngest
Oldest
B. Briefly describe the geologic history represented by the rock sequence.
Exercise 3: Marker events for the Geologic Time Scale A. Enter each event in t h e list in the a p p r o p r i a t e eon b o x in the table. W h e n there is m o r e than one event in a b o x , order t h e m so that the oldest is at the b o t t o m and t h e y o u n g e s t is at the top. Significant ( M a r k e r ) E v e n t s i n E a r t h H i s t o r y Dinosaur extinction Earliest evidence of life End of heavy b o m b a r d m e n t of the Earth Evolutionary Big B a n g Humans evolve
Major phase of continent formation completed M o o n forms First nucleus-bearing cells develop O x y g e n buildup in atmosphere
B. Fill in the n a m e s of eras, periods, and epochs in the correct sequence from oldest at the bottom to youngest at the top. Refer to both Figures 1.13 and 8.11 to complete this exercise.
1 01
100
PART II C H A P T E R 8
Practice Exercises A n s w e r s a n d e x p l a n a t i o n s are p r o v i d e d at the end of the S t u d y G u i d e .
Exercise 1: Determining the succession of geologic events T h e f o l l o w i n g b l o c k d i a g r a m illustrates t h e g e o l o g y o f a n a r e a i n A r g e n t i n a . A n s w e r the q u e s t i o n s r e g a r d i n g t h e g e o l o g i c a l h i s t o r y of this area. C i r c l e t h e c o r r e c t a n s w e r when a choice is provided.
Unit 2 contains clasts of units 3 through 10. Unit 5 contains clasts of units 6 through 10. Units 5 through 10 are baked along their contacts with unit 1 1 . Units 1, 2 , 8 , % and 11 are baked along their contacts with dike 12. A . W h i c h unit i s the y o u n g e s t r o c k i n this area? B . W h i c h unit i s the oldest r o c k i n this area? C. 1. Is unit 3 older than unit 12?
Yes
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 3 4 5 6 7 8 9 1 0 1 1
No
12
12
N o t p o s s i b l e to k n o w
2 . E x p l a i n t h e logic y o u u s e d t o a n s w e r q u e s t i o n C - l .
D. 1. Is unit 1 y o u n g e r than unit 11 ?
Yes
No
N o t p o s s i b l e to k n o w
2 . E x p l a i n the logic y o u u s e d t o a n s w e r q u e s t i o n D - l .
E. 1. In an attempt to further w o r k out the g e o l o g i c age relationships in this area, samples of units 1, 1 1 , and 12 (igneous rocks) w e r e collected for r a d i o m e t r i c dating. The resulting counts of radioactive p a r e n t a t o m s and d a u g h t e r a t o m s are listed in the table. R o c k unit
N o . Parent atoms
No. Daughter atoms
1
500
500
11
250
750
12
750
250
j
Period
Epoch
Phanerozoic
Quaternary
Holocene
Humans
Tertiary
Pleistocene
Era
Eon
evolve
Mesozoic Jurassic
Pennsylvanian
Ordovician
Proterozoic
First
nucleus-bearing
cells
develop Archeon
Hadean
Earth
accretion
Nothing
begins
lives long but the
earth and the mountain. —CHARLES
BALLARD
Exercise 4: Geologic Time Scale mnemonic Construct a m n e m o n i c device for r e m e m b e r i n g the Geologic T i m e Scale n a m e s . T h e first letter of each word must match the first letter of the corresponding period or e p o c h in the proper order. You m a y use your native language, but be careful not to mix up the words w h e n you do so. E x a m p l e s (refer to Figures 8.11 and 8.14 for the G e o l o g i c T i m e S c a l e ) : Periods of the G e o l o g i c T i m e Scale Chronically Justify
Overworked
Contradictory
Student Decks Test
Monotonous
Physics
Professor
Questions.
E p o c h s of the C e n o z o i c : Please
Eat
Our Mushroom
Pot Pie
Hot.
N o w invent y o u r o w n m n e m o n i c t o h e l p you r e m e m b e r the G e o l o g i c T i m e Scale.
To
C l o c k s i n R o c k s : T i m i n g the G e o l o g i c R e c o r d
Review Questions Answers a n d e x p l a n a t i o n s are p r o v i d e d at t h e e n d of the Study G u i d e . 1. T h e p r i n c i p l e of superposition h o l d s that for any unfolded series of s e d i m e n t a r y layers, A. B. C. D.
overlying strata e x t e n d over a b r o a d e r area than t h e layers b e n e a t h t h e m . the layer at t h e t o p of the pile is a l w a y s y o u n g e r than the layers b e n e a t h it. s e d i m e n t s generally a c c u m u l a t e in the vertical s a n d s t o n e - s h a l e - l i m e s t o n e sequence. t h e layer at the t o p of the pile is a l w a y s older than those b e n e a t h it.
2. F r o m y o u n g e s t to oldest, t h e correct s e q u e n c e of eras dividing the P h a n e r o z o i c eon is A. Paleozoic, Cenozoic, Mesozoic. B. Mesozoic, Cenozoic, Paleozoic.
C. Mesozoic, Paleozoic, Cenozoic. D. Cenozoic, Mesozoic, Paleozoic.
3. T h e e p o c h s of t h e Tertiary p e r i o d p r o g r e s s from oldest to y o u n g e s t in w h i c h s e q u e n c e ? A. B. C. D.
Eocene, Oligocene, Paleocene, Eocene, Paleocene, Eocene, Paleocene, Eocene,
Paleocene, Miocene, Pliocene Oligocene, Miocene, Pliocene Miocene, Pliocene, Oligocene Oligocene, Miocene, Pliocene, Pleistocene
4. F r o m oldest to y o u n g e s t , t h e correct order of p e r i o d s for the P a l e o z o i c era is A . C a m b r i a n , O r d o v i c i a n , D e v o n i a n , Silurian, M i s s i s s i p p i a n , P e r m i a n , P e n n s y l v a n i a n . B . C a m b r i a n , O r d o v i c i a n , Silurian, D e v o n i a n , M i s s i s s i p p i a n , P e n n s y l v a n i a n , P e r m i a n . C . C a m b r i a n , Silurian, O r d o v i c i a n , P e n n s y l v a n i a n , M i s s i s s i p p i a n , D e v o n i a n , P e r m i a n . D. n o n e of the a b o v e . 5. S a n d s t o n e and shale r o c k layers i m m e d i a t e l y b e l o w and a b o v e an a n g u l a r u n c o n f o r m i t y imply a history of A. erosion, deposition, and deformation erosion. B. erosion, deformation, deposition, and erosion. C. d e f o r m a t i o n , erosion, deposition, and deformation. D. d e p o s i t i o n , deformation, erosion, and deposition. H i n t : Refer t o F i g u r e 8.8. 6. F r o m t h e d i a g r a m , o n e c a n infer a g e limits for r o c k layer 3 of A. B. C. D.
b e t w e e n 3 4 a n d 6 0 million y e a r s . b e t w e e n 3 0 a n d 6 0 million y e a r s . less t h a n 20 million y e a r s . more than 60 million years.
7. W h a t m a k e s t h e isotopes of a given e l e m e n t different from e a c h other? A. B. C. D.
their their their their
atomic numbers n u m b e r of electrons n u m b e r of n e u t r o n s n u m b e r of p r o t o n s
1 03
1 04
PART II CHAPTER 8
8. Naturally occurring that d e c a y ( s ) into other materials at known rates can be u s e d to estimate the actual age of a rock. A. organic m a t t e r B. minerals
C. radioactive e l e m e n t s D . silicon
9. T h e P h a n e r o z o i c t i m e is divided into three intervals: (1) t h e interval of old life, (2) the interval of m i d d l e life, and (3) the interval of m o d e r n life. T h e s e intervals correspond (from oldest to y o u n g e s t ) to the A . A r c h e a n , M e s o z o i c , and P a l e o z o i c . B . P a l e o z o i c , P r e c a m b r i a n , and P r o t e r o z o i c . C. P r e c a m b r i a n , C a m b r i a n , and N e o c a m b r i a n . D . P a l e o z o i c , M e s o z o i c , and C e n o z o i c . 10. W h i c h r a d i o m e t r i c dating m e t h o d w o u l d be m o s t effective in d e t e r m i n i n g the age of charcoal at an archeological site? A. rubidium-strontium C. uranium-lead B. radiocarbon D. potassium-argon H i n t : Refer to Table 8.1 in y o u r textbook. 1 1 . O n l y geologically y o u n g materials can b e dated using radioactive C - 1 4 isotopes because A. B. C. D.
t h e decay rate varies widely over t i m e . they h a v e a very short half-life. within d e c a d e s all the C - 1 4 is d e c a y e d away. they are a very rare isotope.
12. If the half-life of s o m e radioactive e l e m e n t is 1 billion years and a m a s s of r o c k originally c o n t a i n e d 1000 a t o m s of the radioactive e l e m e n t , h o w m a n y a t o m s of the radioactive e l e m e n t w o u l d be left after 3 billion years h a d p a s s e d ? A. 5 0 0 a t o m s B. 250 atoms
C. 125 a t o m s D . n o radioactive a t o m s
13. S m a l l p i e c e s of charcoal from an ancient ruin yield a c a r b o n - 1 4 date of 3 0 0 0 years. This age best represents the a p p r o x i m a t e interval of t i m e that has e l a p s e d since A. a fire b u r n e d the w o o d . B . h u m a n s inhabited the ruin.
C. h u m a n s cut t h e w o o d . D . t h e w o o d died.
14. Radiometric dates have been attached to the Geologic T i m e Scale by the determination of r a d i o m e t r i c ages of A. B. C. D.
i g n e o u s r o c k s y o u n g e r and older than s e d i m e n t a r y formations. shales. fossil skeletons. metamorphosed sediments.
15. F o r t h e m o s t part, r a d i o m e t r i c dates for r o c k s r e p r e s e n t only t h e last t i m e t h e r o c k A. crystallized from a m a g m a or was metamorphosed.
C. b e c a m e c e m e n t e d , D . w a s deposited.
B . w a s eroded. 16. O n e m e t h o d that geologists u s e to study buried s e d i m e n t s and u n c o n f o r m i t i e s is A. seismic stratigraphy. B . r a d i o m e t r i c stratigraphy. H i n t : Refer t o F i g u r e 8.15.
C. depositional stratigraphy. D . m e t a m o r p h i c stratigraphy.
17. If a r o c k is heated by m e t a m o r p h i s m and t h e d a u g h t e r a t o m s g e n e r a t e d by the d e c a y of the radioactive parent a t o m s m i g r a t e out of a m i n e r a l that is s u b s e q u e n t l y radiometrically dated, the date will be the actual age. A. y o u n g e r than B. older than
C. the s a m e as D. n o n e of the a b o v e
Clocks in Rocks: Timing the Geologic Record
18. W h i c h s a m p l e of basalt in the d i a g r a m is likely to yield the m o s t accurate K / A r r a d i o metric date?
A. A
B. B
C.
C
19. A layer of c o n g l o m e r a t e c o n t a i n s c o b b l e s of an i g n e o u s rock. O n e of the c o b b l e s w a s dated r a d i o m e t r i c a l l y at 35 million years old. F r o m this r a d i o m e t r i c age, t h e c o n g l o m erate layer can be inferred to be A. m o r e t h a n 35 million years old. B. less t h a n 35 million years old.
C. 35 million years old. D. n o n e of the above.
20. In t h e illustration, w h a t is t h e m o s t r e c e n t geological event d e p i c t e d ?
A. B. C. D.
eruption of the lava faulting intrusion of t h e pluton d e p o s i t i o n of shales, s a n d s t o n e s and l i m e s t o n e s
21. Pieces of c h a r c o a l w e r e found in a paleosoil layer c o v e r i n g an ancient fire pit with stone tools i n c l u d i n g a r r o w h e a d s and axes. T h e charcoal w a s radiometrically dated u s i n g carb o n - 1 4 and y i e l d e d an a g e of a p p r o x i m a t e l y 10,500 y e a r s . W h a t c a n y o u infer about the age of the a r c h e o l o g i c a l site? A. B. C. D.
The The The The
a r c h e o l o g i c a l site is about 10,500 years old. a r c h e o l o g i c a l site is y o u n g e r than 10,500 y e a r s . a r c h e o l o g i c a l site is older than 10,500 years. a g e of the a r c h e o l o g i c a l site is u n r e s o l v e d but m a y be older than 10,500 y e a r s .
1 05
CHAPTER
9
Early History of the Terrestrial Planets
Before Lecture Chapter Preview • H o w did o u r solar s y s t e m f o r m ? Brief answer: It accreted from gas and dust about 4.5 billion years ago. Figure 9.2 s h o w s this process beautifully. L o o k it over before lecture. • H o w did the E a r t h form a n d c h a n g e o v e r t i m e ? Brief a n s w e r : T h e E a r t h ' s core, mantle, crust, o c e a n s , and a t m o s p h e r e evolved as the interior of t h e planet heated u p , melted, a n d differentiated. S t u d y F i g u r e s 9.5 a n d 9.6. 106
Early History of the Terrestrial Planets
• W h a t a r e s o m e m a j o r e v e n t s i n the h i s t o r y o f o u r s o l a r s y s t e m ? Brief a n s w e r : T h e a g e of the solar s y s t e m as d e t e r m i n e d from meteorites is about 4.56 billion y e a r s . T h e major planets f o r m e d within about 10 million years, and they differentiated into a c o r e - m a n t l e - c r u s t layering in less than 100 million years. T h e M o o n formed from a giant impact at about 4.5 Ga. Minerals as old as 4.4 billion years have survived in t h e E a r t h ' s crust. • W h a t do p l a n e t a r y s u r f a c e s tell us a b o u t their a g e ? Brief a n s w e r : S a m p l e s from the surface of the M o o n have b e e n dated by isotopic m e t h o d s . T h e age of other planetary surfaces is estimated from the density of impact craters. • W h a t have we l e a r n e d f r o m recent p l a n e t a r y p r o b e s ? Brief a n s w e r : Venus is the only planet other than Earth that has active tectonics controlled by g l o b a l c o n v e c t i o n of its m a n t l e . Water is p r e s e n t on M a r s only as ice at its poles a n d in the shallow subsurface. In the past, water m a y h a v e b e e n present as a liquid on t h e M a r i a n surface.
After Lecture The perfect t i m e to review y o u r notes is right after lecture, w h i l e the material is still fresh in your mind. R e v i e w to be sure you noted all the key points and wrote t h e m d o w n in a form that will be r e a d a b l e later. As y o u review, you can also add useful visual material and a s u m m a r y .
Check Your Notes: Have y o u . . . • annotated y o u r notes for focused l e a r n i n g ? Usually the m o s t efficient w a y to do this is by (1) u n d e r l i n i n g or highlighting key points and (2) adding h e a d i n g s or key words in the m a r g i n (you do set up y o u r notes with a left-hand margin, d o n ' t y o u ? ) . Hint: P r e v i e w q u e s t i o n s m a k e great h e a d i n g s . For additional ideas a b o u t a n n o t a t i n g notes go to t h e following link: http://www.csbsju.edu/academicadvising/help/lec-note.htm • added a simple sketch or t w o to clarify the key points? • written a brief ( o n e - p a r a g r a p h ) s u m m a r y of the most important c o n c e p t you learned from this lecture? Feel free to use y o u r notes and the figures in y o u r text as needed. Reviewing p r e v i e w q u e s t i o n s m a y h e l p .
Intensive Study Session For this chapter the m o s t efficient study a p p r o a c h is to start with the t w o practice exercises. Practice Exercise 1 will h e l p y o u m a s t e r the basics of planet formation. T h e n turn to Practice Exercise 2. W h e n we are learning about a new subject, we all have a tendency to focus on the "what" (what we k n o w ) rather than the " h o w " (how do we k n o w it)—the supporting evidence. Yet to carry out an intelligent conversation and write a good response to a discussion question in homework or on an e x a m , we need to be able to explain the evidence for our statements and observations. T h e observation that there is water on M a r s begs for a detailed explanation. H o w do we know there is water on M a r s ? Before the next lecture, rough out an answer to Practice Exercise 2 (also refer to Exercise 6 at the end of Chapter 9 in the textbook). As questions arise about how a line of evidence serves to support the presence of water on M a r s , write the questions down and ask the instructor about t h e m via e-mail or during the next class meeting.
1 07
108
PART II C H A P T E R 9
Web Site Study Resources http://www.whfreeman.com/understandingearth5e C h e c k out the study aids available on the W e b site. You will find C o n c e p t Self-Checker, Web R e v i e w Q u e s t i o n s , G r a d e d O n l i n e Q u i z , O n l i n e R e v i e w E x e r c i s e s , and Flashcards (to help y o u learn new t e r m s ) . T h e O n l i n e R e v i e w E x e r c i s e Mars Field Trip provides an opportunity for y o u to e x p l o r e s o m e of t h e g e o l o g i c features on M a r s . You will learn how the surface features present today tell planetary geologists a b o u t c o n d i t i o n s and processes that have s h a p e d M a r s during its history.
Exam Prep M a t e r i a l s in this section are m o s t useful during y o u r p r e p a r a t i o n for e x a m s . The Chapter S u m m a r y and P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s s h o u l d simplify your chapter review. B e g i n y o u r review with the C h a p t e r S u m m a r y . It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y . N e x t , w o r k on the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises a n d q u e s t i o n s j u s t as y o u w o u l d an e x a m , to see h o w y o u have m a s t e r e d this chapter. After y o u a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t important, review e a c h question you m i s s e d . Identify and correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the question incorrectly.
Chapter Summary How did our solar system form? • O u r solar system p r o b a b l y formed w h e n a c l o u d of interstellar gas and dust c o n d e n s e d about 4.5 billion years a g o . T h e planets vary in c h e m i c a l c o m p o s i tion in a c c o r d a n c e with their distance from the Sun and with their size.
How did the Earth form and change over time? • E a r t h p r o b a b l y grew by accretion of colliding c h u n k s of matter. Very early after t h e Earth formed, it is t h o u g h t that our M o o n f o r m e d from material ejected from the Earth by the i m p a c t of a giant m e t e o r i t e . • H e a t g e n e r a t e d from the M o o n - f o r m i n g i m p a c t and the d e c a y of radioactive e l e m e n t s p r o b a b l y c a u s e d m u c h of the Earth to melt. M e l t i n g a l l o w e d iron and other d e n s e m a t t e r to sink t o w a r d the E a r t h ' s center and f o r m the c o r e . L o w e r density (lighter) m a t t e r floated u p w a r d to form t h e m a n t l e and crust. R e l e a s e of t r a p p e d gases (mostly water) from within t h e Earth g a v e rise to t h e oceans and an early a t m o s p h e r e . In this way, t h e Earth w a s t r a n s f o r m e d into a differentiated planet with c h e m i c a l l y distinct z o n e s : an iron c o r e ; a m a n t l e of mostly m a g n e s i u m , iron, silicon, and o x y g e n ; and a crust rich in o x y g e n , silicon, alum i n u m , c a l c i u m , p o t a s s i u m , s o d i u m , and radioactive e l e m e n t s . • As the E a r t h cooled, an outer relatively rigid shell called t h e lithosphere f o r m e d on top of a hotter and softer a s t h e n o s p h e r e . Volatiles t r a p p e d within the m a n t l e e s c a p e d t h r o u g h v o l c a n o e s t o form E a r t h ' s a t m o s p h e r e and o c e a n s . Life evolved and d e v e l o p e d the capability of extracting c a r b o n d i o x i d e out of surface e n v i r o n m e n t s and releasing o x y g e n as a b y - p r o d u c t of p h o t o s y n t h e s i s . Refer t o C h a p t e r 1 1 . T h e interaction of major c o m p o n e n t s of Earth's system continues to this day.
What are some major events in the history of our solar system? • T h e a g e of the solar system as d e t e r m i n e d from m e t e o r i t e s is a b o u t 4 . 5 6 billion y e a r s . T h e major planets formed within about 10 million years, and they
Early History of the Terrestrial Planets differentiated into a c o r e - m a n t l e - c r u s t layering in less than 100 million y e a r s . T h e M o o n f o r m e d f r o m a g i a n t i m p a c t at a b o u t 4.5 G a . M i n e r a l s as old as 4.4 billion years h a v e survived in the E a r t h ' s crust. Refer to F i g u r e 9.10.
What do planetary surfaces tell us about their age? • S a m p l e s from t h e surface of the M o o n h a v e b e e n dated by isotopic m e t h o d s . T h e a g e of other planetary surfaces is estimated from superposition and t h e density of i m p a c t craters.
What have we learned from recent planetary probes? • Venus is t h e only planet other t h a n E a r t h that h a s active tectonics controlled by global c o n v e c t i o n of its m a n t l e . Refer to F i g u r e 9.15. • Water is p r e s e n t on M a r s only as ice at its poles and in the shallow subsurface. In the past, w a t e r m a y h a v e b e e n p r e s e n t as a liquid on the M a r i a n surface. • The C a s s i n i - H u y g e n s p r o b e f o u n d that S a t u r n ' s rings e x t e n d e d h u n d r e d s of kilometers from t h e p l a n e t and w e r e m a d e up of billions of particles of ice a n d r o c k — p a r t i c l e s r a n g i n g in size from grains of sand to h o u s e s .
Practice Exercises Answers and e x p l a n a t i o n s are provided at the end of the Study G u i d e .
Exercise 1: The evolving early Earth Fill in the b l a n k s in t h e following flowchart, w h i c h characterizes a p o p u l a r h y p o t h e s i s for the early history of the E a r t h after the S u n and planets h a d f o r m e d (refer to parallel flowchart).
1 09
110
PART II
CHAPTER 9
Exercise 2: Evidence of water on Mars D i s c u s s four lines of e v i d e n c e revealed by recent M a r i a n p r o b e s of w a t e r (past a n d present) on Mars. H i n t : Start by m a k i n g a list of lines of e v i d e n c e m e n t i o n e d in this chapter. P i c k the four that y o u u n d e r s t a n d t h e b e s t and t h e n discuss h o w e a c h s u p p o r t s the p r e s e n c e of water on M a r s , b o t h in the past and at present.
Figure 9 . 2 6 . S t r a t i g r a p h y e x p o s e d a l o n g t h e flank o f E n d u r a n c e Crater, (a) T h e r e c o r d c a n b e read i n t h e o u t c r o p a n d d e p i c t e d a s a n interpretive drawing t h a t s h o w s e a c h s t a g e i n t h e h i s t o r y o f the e n v i r o n m e n t , ( b ) T h e vertical s u c c e s s i o n o f layers i n this o u t c r o p p r e s e r v e s a n excellent r e c o r d o f early M a r t i a n e n v i r o n m e n t s .
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of t h e S t u d y G u i d e . 1. D u r i n g the f o r m a t i o n of o u r solar s y s t e m , w h a t w a s the p r o c e s s that c a u s e d dust and c o n d e n s i n g m a t e r i a l t o accrete into p l a n e t e s i m a l s ? A. n u c l e a r fusion B. rapid spin of the p r o t o s u n C. h e a t i n g of g a s e s D . gravitational attraction and m a t e r i a l collisions 2. A major s o u r c e of internal h e a t in the E a r t h today is A . o c e a n tides. B . radioactivity. C. solar energy. D. volcanoes.
Test-Taking Tip W h e n taking a test, be alert to items that give away the a n s w e r s to other i t e m s . E x a m p l e : Q u e s t i o n 12 p r o v i d e s a hint for a n s w e r i n g q u e s t i o n 3.
Early History of the Terrestrial Planets
Figure 9.6. Early volcanic activity contributed enormous amounts of water vapor, carbon dioxide, and other gases to the atmosphere and oceans, and it contributed solid materials to the continents. Photosynthesis by microorganisms removed carbon dioxide and added oxygen to the primitive atmosphere. Hydrogen, because it is light, escaped into space.
3. W h i c h of t h e following p r o c e s s e s is t h o u g h t to be r e s p o n s i b l e for t h e formation of the present E a r t h ' s a t m o s p h e r e ? A. chemical breakdown of minerals B . c a p t u r e o f o x y g e n and nitrogen from space C. b o t h d e g a s s i n g of E a r t h ' s interior and liberation of o x y g e n by p h o t o s y n t h e s i s D. d e g a s s i n g of E a r t h ' s interior 4. F r o m r a d i o m e t r i c d a t e s of lunar r o c k s a m p l e s a n d m e t e o r i t e s , h o w old is the E a r t h estimated to be? A. 45 million y e a r s old B. 4.5 billion y e a r s old C. 10 billion y e a r s old D. 100 billion y e a r s old 5. G e o c h e m i s t s have d e t e r m i n e d that differences in the distribution of c h e m i c a l e l e m e n t s in the p r e s e n t E a r t h ' s crust, m a n t l e , and c o r e — i n contrast to the distribution of c h e m i cal e l e m e n t s in the initial solar n e b u l a — m a y be d u e in part to A. i n h o m o g e n e i t i e s within the initial interstellar dust cloud. B. early m e l t i n g and differentiation of materials of varying density. C. c o s m i c ray b o m b a r d m e n t over 4.5 billion years. D . nuclear synthesis w i t h i n o u r S u n . 6. W h y do g e o s c i e n t i s t s think t e c t o n i c plates m o v e across the E a r t h ' s surface? A. Centrifugal force of E a r t h ' s rotation spins plates a c r o s s the E a r t h ' s surface. B. Volcanic e r u p t i o n s on the seafloor p u s h tectonic plates apart. C. Tidal forces drive plate m o t i o n . D. M o v e m e n t of the plates is the surface manifestation of c o n v e c t i o n in the m a n t l e . H i n t : Refer t o F i g u r e 9 . 1 5 . 7. Within o u r solar s y s t e m , o n e big difference b e t w e e n the inner and outer p l a n e t s — besides size and position relative to the S u n — i s their A. density. B. color. C. rate of formation. D. t i m e of f o r m a t i o n .
111
112
PART II C H A P T E R 9
8. E a r t h ' s M o o n is t h o u g h t to h a v e formed by A. c a p t u r e of a large planetary object traveling p a s t E a r t h . B . accretion, j u s t like the p l a n e t s . C. material ejected from E a r t h by volcanic eruptions. D. the i m p a c t of a M a r s - s i z e d object on E a r t h very early in E a r t h ' s history. 9. T h e nebular origin of our solar system is characterized as A. a p r o v e n fact. B. a theory. C. a h y p o t h e s i s . D. pure guesswork. 10. B o m b a r d m e n t from space m a y be disastrous for life, but it is also an essential process in the history of a planet. W h y ? A. B o m b a r d m e n t is h o w a planet g r o w s , and residual h e a t from i m p a c t s m a y help to create a d y n a m i c planet. B. T h e o c e a n s and a t m o s p h e r e f o r m e d on E a r t h as a result of b o m b a r d m e n t . C. I m p a c t s drive plate tectonics. D . I m p a c t s k e e p planets from getting too big. 1 1 . T h e i m p a c t i n g object that c a u s e d the extinction of the d i n o s a u r s 65 million years ago is e s t i m a t e d to h a v e had a radius of about A. 100 m e t e r s . B.
1 kilometer.
C.
10 k i l o m e t e r s .
D . 100 k i l o m e t e r s . H i n t : Refer t o Table 9.2. 12. O x y g e n r e l e a s e d into E a r t h ' s a t m o s p h e r e by p h o t o s y n t h e s i s is vital for o u r existence not only b e c a u s e we need it to breathe but also b e c a u s e A. it is essential for all life on Earth. B. it c o m b i n e s with free h y d r o g e n gas in o u r a t m o s p h e r e to p r o d u c e water. C. life w o u l d not h a v e evolved w i t h o u t o x y g e n . D . i t forms a n o z o n e layer i n the u p p e r a t m o s p h e r e that p r o t e c t s u s from U V radiation.
It is in the stars. The stars above
us,
govern our condition. — W .
SHAKESPEARE
King Lear,
IV,
iii
CHAPTER 10
Evolution of the Continents
Figure 10.1. Major tectonic features of North America: Canadian Shield; interior platform; Cordilleran orogenic belt; Colorado Plateau; Appalachian fold belt; coastal plain.
Evolution of the Continents
Vital Information from Other Chapters To u n d e r s t a n d t h e evolution of the continents, y o u will n e e d to d r a w on diverse information from m a n y c h a p t e r s . W h i l e w o r k i n g on C h a p t e r 10, E v o l u t i o n of t h e C o n t i n e n t s , k e e p t h e following c o n c e p t s in m i n d a n d review t h e m as m u c h y o u need to. Plate Tectonics Divergent b o u n d a r i e s ( F i g u r e 2.6) C o n v e r g e n t b o u n d a r i e s (Figure 2.6) T h e forces that drive plate tectonics (Figure 1.11 and p a g e s 3 7 ^ - 0 ) Igneous Rocks M a g m a differentiation (Figures 4.5 and 4.11 a n d p a g e s 8 5 - 8 7 in text) F o r m a t i o n of i g n e o u s r o c k s at divergent and convergent plate b o u n d a r i e s (Figures 4 . 1 3 and 4 . 1 4 and p a g e s 9 2 - 9 7 ) Rock Deformation Folding and faulting (Figures 7.7) Thrust faulting (Figure 7.17) Folding ( F i g u r e 7.10) A p p a l a c h i a n Fold belt ( F i g u r e 7.18) D e v e l o p m e n t of a fictitious g e o l o g i c p r o v i n c e (Figure 7.19)
During Lecture One goal for lecture should be good answers to the preview questions. • To avoid getting lost in t h e details, k e e p the big picture in m i n d . C h a p t e r 10 e x a m i n e s t h e g e o l o g i c history of the continents. • M a k e a copy of Figure 10.18, the Wilson cycle, and have it handy during lecture.
After Lecture Review Notes The perfect t i m e to review y o u r notes is right after lecture. T h e following checklist c o n t a i n s both general review tips a n d specific s u g g e s t i o n s for this chapter.
Check Your Notes: Have y o u . . . • added notes or sketches of material from p r e v i o u s chapters that y o u n e e d to understand y o u r class n o t e s ? T h i s is a g o o d idea for this c h a p t e r b e c a u s e t h e c h a p t e r draws heavily on p r e v i o u s material. H i n t : Refer to the S t u d y G u i d e section Vital I n f o r m a t i o n f r o m O t h e r C h a p t e r s . T h e r e y o u will find a helpful list of material you may w a n t t o review o r i n c l u d e a s y o u r e w o r k y o u r n o t e s . • created a brief big picture o v e r v i e w of this lecture? H i n t : T h e W i l s o n cycle p r o v i d e s a useful o v e r v i e w of this chapter. Refer to F i g u r e 10.18 and c o n s i d e r a d d i n g sketches and text from that figure to y o u r n o t e s .
Intensive Study Session Set priorities for studying this chapter. Try to give h i g h e s t priority to activities that involve answering q u e s t i o n s . We r e c o m m e n d the following strategy for learning this chapter. • P r e v i e w t h e f o l l o w i n g F i g u r e s : 1 0 . 1 , 10.4, 10.8, 1 0 . 1 1 , 10.12, 10.13, 10.15, 10.16, 10.18, 10.20, a n d 10.24. You will n e e d to u n d e r s t a n d these figures to a n s w e r the R e v i e w Q u e s t i o n s .
115
1 14
PART II C H A P T E R 10
Before Lecture Before you attend lecture, be sure to spend s o m e t i m e p r e v i e w i n g the chapter. For an efficient preview, u s e the following q u e s t i o n s .
Chapter Preview • W h a t are t h e m a j o r g e o l o g i c features of N o r t h A m e r i c a ? Brief a n s w e r : R e v i e w F i g u r e 1 0 . 1 . Pay particular attention to the location and characteristics of the active tectonic belt in western N o r t h A m e r i c a and the relatively stable c o n t i n e n t of the ancient o r o g e n i c belts, platform, and shield. L o o k for patterns in age and location. • H o w do continents grow? Brief answer: T h e silica-rich, iron-poor rocks in continents are produced mostly in subduction zones by magmatic differentiation and metamorphism of silica-rich sediments. O n c e produced, the continental rocks are difficult to subduct and recycle into the mantle because they are more buoyant than mantle material. Rifting and transform faulting typically break continents into small pieces while terrane accretion and continent collisions assemble pieces into larger continents. Refer to Figures 10.11 and 10.12. • How does orogeny modify continents? Brief answer: M u c h as a styrofoam float resists being d r a g g e d under water, silica-rich, low-density continental crust is m o r e buoyant than the mantle, and therefore continents resist being subducted. Instead, continental crust is d a m a g e d (deformed) and piles up (thickens) at convergent plate b o u n d a r i e s . Intense folding, faulting, detachment, and thrust transport of sedimentary w e d g e s and the formation of h u g e granitic batholiths thicken the overriding continental lithosphere. T h e s e processes can deform continental crust h u n d r e d s of kilometers from the convergence z o n e . T h i c k e r continental crust tends to stand higher. • W h a t is e p e i r o g e n y ? Brief answer: Gradual d o w n w a r d and upward m o v e m e n t s of broad regions of the crust, without significant folding or faulting, involves a set of processes called epierogeny. Heat, loading and unloading, and flow within the m a n t l e m a y all cause epierogeny. • W h a t is the W i l s o n c y c l e ? Brief a n s w e r : T h e Wilson cycle characterizes the s e q u e n c e of events in the opening and closing of an ocean basin. A new o c e a n basin forms w h e n a c o n t i n e n t is rifted apart. As an ocean basin closes due to s u b d u c t i o n of its o c e a n lithosphere, continents grow. A few times in E a r t h ' s history, the c l o s u r e of a series of o c e a n b a s i n s resulted in the formation of a supercontinent, like P a n g a e a in the early P e r m i a n . Refer to F i g u r e s 10.17 and 10.18. • H o w h a v e t h e A r c h e a n c r a t o n s s u r v i v e d billions o f y e a r s o f p l a t e tectonics? Brief a n s w e r : L i k e giant sailboats, c o n t i n e n t s h a v e cratonic k e e l s that stabilize the raft of continental lithosphere against the effects of convective currents in the mantle and plate tectonic p r o c e s s e s . It is h y p o t h e s i z e d that the keels c o n s i s t e d of somewhat less d e n s e mantle rocks that are about the s a m e a g e as the A r c h e a n crust above t h e m . Refer to F i g u r e 10.24.
Study Tip W h e n confronted with n e w scientific t e r m i n o l o g y , s o m e t i m e s t h e d i c t i o n a r y c a n help. F o r e x a m p l e , if y o u l o o k up the t e r m s epeirogeny and orogeny y o u will find t h e following origins for the r o o t s of t h e s e t e r m s : e p e i r o g e n y — G k . epeiros, c o n t i n e n t o r o g e n y — G k . oros, m o u n t a i n
116
PART II C H A P T E R 10
• C o m p l e t e P r a c t i c e E x e r c i s e s 1 a n d 2. T h e s e exercises will h e l p y o u r e m e m b e r t h e m o s t i m p o r t a n t i d e a s i n this chapter. • W o r k in s o m e review time as you study the evolution of the continents. Reviewing is always a g o o d idea, and it is especially important for this chapter because it draws on so m a n y ideas in previous chapters. Refer to Vital Information f r o m O t h e r C h a p t e r s for a list of helpful figures to review. • A n s w e r the R e v i e w Q u e s t i o n s . Try answering e a c h question to c h e c k your understanding of the lecture. C h e c k y o u r answers as y o u go, but try to a n s w e r the questions before y o u look at the answers. Pay attention to the test-taking tips we provide. They will help you do better on your m i d t e r m . • S o m e t i m e before your e x a m , a n s w e r all the exercises at the e n d of the c h a p ter. T h e y require short answers and w o n ' t take long if y o u k n o w the material. A helpful animation, Accretion of a Buoyant Fragment of a Continent,
is provided
on the W e b site for Exercises 5 and 6 at the end of Chapter 10 in the textbook. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e the C o n c e p t S e l f - C h e c k e r and O n l i n e R e v i e w Q u e s t i o n s . Pay particular attention t o the e x p l a n a t i o n s for t h e a n s w e r s . T h e G e o l o g y i n P r a c t i c e E x e r c i s e s a d d r e s s the q u e s t i o n " D o c o n t i n e n t s float o n t h e m a n t l e ? " F l a s h c a r d s will h e l p y o u learn the n e w t e r m i n o l o g y in this chapter.
Exam Prep M a t e r i a l s in this section are m o s t useful d u r i n g y o u r p r e p a r a t i o n for q u i z z e s a n d e x a m s . The C h a p t e r S u m m a r y and the Practice Exercises and R e v i e w Q u e s t i o n s s h o u l d simplify chapter review. B e g i n y o u r session with the C h a p t e r S u m m a r y . It p r o v i d e s a helpful overview that s h o u l d refresh y o u r m e m o r y . N e x t , w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and q u e s t i o n s j u s t as y o u w o u l d for an e x a m , to see h o w w e l l y o u h a v e m a s t e r e d this chapter. After y o u a n s w e r the q u e s t i o n s , s c o r e t h e m . Finally, and m o s t i m p o r t a n t , review each q u e s t i o n y o u m i s s e d . Identify and correct the m i s c o n c e p t i o n ( s ) that r e s u l t e d in y o u r answering t h e q u e s t i o n incorrectly.
Chapter Summary What are the major geologic features of North America? • F i g u r e 10.1 s h o w s the m a j o r tectonic features, i n c l u d i n g t h e C a n a d i a n Shield, interior platform, C o r d i l l e r a a n d A p p a l a c h i a n m o u n t a i n belts, c o a s t a l plain, and continental shelves. • F i g u r e 10.3 s h o w s the s e d i m e n t a r y b a s i n s and d o m e s of N o r t h A m e r i c a . • T h e A p p a l a c h i a n fold belt and w e s t e r n C o r d i l l e r a m o u n t a i n belt are c h a r a c t e r ized in F i g u r e s 10.4, 10.5, and 1 0 . 1 1 .
How do continents grow? • M a g m a t i c differentiation. M a g m a s g e n e r a t e d in s u b d u c t i o n z o n e s t e n d to be m o r e silica-rich b e c a u s e (1) crastal material, like s e d i m e n t s , c a n b e i n c o r p o rated into the melt and (2) partial m e l t i n g and crystal settling differentiate the iron-rich and silica-rich materials.
Evolution of the Continents
• C o n t i n e n t a l accretion. Plate m o t i o n s accrete buoyant (silica rich) rocks to continental margins by (1) transfer of fragments from a subducting plate to a continental plate, (2) closure of marginal basins to add thickened island-arc crust to the continent, (3) lateral transport via strike-slip faulting along continental margins, and (4) suturing of t w o continental margins during their collision. Refer to Figures 1 0 . 1 1 , 10.12, 10.15, 10.17, and 10.18 for illustrations and e x a m p l e s .
What are epeirogeny and orogeny? • E p e i r o g e n e y refers to s i m p l e u p - a n d - d o w n m o v e m e n t s w i t h o u t significant folding and faulting. P r o p o s e d m e c h a n i s m s for vertical crustal m o v e m e n t s are (1) isostatic adjustments c a u s e d by loading or u n l o a d i n g of the crust via accum u l a t i o n and m e l t i n g of glacial ice and (2) heating or c o o l i n g the crust in association with continental rifting or a hot spot. Refer to F i g u r e 10.20. • O r o g e n y u s u a l l y r e s u l t s from p l a t e c o n v e r g e n c e a n d is c h a r a c t e r i z e d by sever d e f o r m a t i o n i n c l u d i n g e x t e n s i v e faulting a n d folding. E x a m p l e s o f orogeny include the Appalachian and Rocky Mountains of the United States. But the most spectacular example on the planet is the A l p i n e - H i m a l a y a n belt. R e f e r to F i g u r e s 10.15 a n d 10.16 and t h e related section of text d e s c r i b ing t h e f o r m a t i o n o f t h e H i m a l a y a s .
How does orogeny modify continents? • Like a styrofoam float, the buoyant continental crust resists being dragged under water; silica-rich, low-density continental crust is m o r e buoyant than the mantle, and therefore continents resist being subducted. Instead, continental crust is d a m a g e d (deformed) and piles up (thickens) at convergent plate boundaries. • Intense folding, faulting, d e t a c h m e n t and thrust transport of s e d i m e n t a r y w e d g e s and the f o r m a t i o n of h u g e granitic batholiths thicken the overriding continental lithosphere. • T h e s e p r o c e s s e s can d e f o r m continental crust h u n d r e d s of kilometers from t h e c o n v e r g e n c e z o n e . T h i c k e r continental crust tends to stand higher.
What is the Wilson cycle? • T h e W i l s o n cycle, n a m e d after J. T u z o Wilson, is a flowchart that s u m m a r i z e s the general s e q u e n c e of events in the evolution of continental crust. Refer to Figure 10.18. • T h e cycle begins w h e n the e d g e s of continental cratons are rifted during the breakup of a supercontinent such as Pangaea. Rifting creates a new ocean basin with passive m a r g i n s that collect ocean sediment. Eventually convergence begins, creating an active margin, with a c c o m p a n y i n g subduction, m o u n t a i n building, and terrain accretion. • F r o m t i m e to t i m e d u r i n g E a r t h ' s history, plate c o n v e r g e n c e p r o d u c e d supercontinents like R o d i n a and P a n g a e a .
How have the Archean cratons survived billions of years of plate tectonics? • The formation of silica-rich continental crust goes back at least 4.0 billion years. • Cratons (older and m o r e stable parts of the continents) have keels that, like the hull of a sailboat, extend into the m a n t l e .
117
Evolution of the Continents
119
Practice Exercises A n s w e r s and e x p l a n a t i o n s are provided at the end of the Study G u i d e .
Exercise 1: Evolution of the Continents C o m p l e t e the sentences in the flowchart with w o r d s from the list. Refer to F i g u r e s 10.12, 10.15, and 10.18. accretion and collision intrusion of m a g m a s subducts continental plate magmatism thick erosion o c e a n basin thrust faulting folding passive margin hot stable craton
118
PART II
C H A P T E R 10
• C r a t o n i c keels a p p e a r to consist of s o m e w h a t l o w e r density rock, called peridotite, that is depleted of heavier constituents, s u c h as the e l e m e n t iron and the m i n e r a l garnet. • Keels a p p e a r to f o r m at a b o u t the s a m e t i m e as t h e c o n t i n e n t a l crust a b o v e it. • It is h y p o t h e s i z e d that the cold, strong, m a n t l e k e e l m o r e t h a n 2 0 0 km thick h e l p s t o p r e s e r v e t h e c r a t o n s from d i s r u p t i o n b y m a n t l e c o n v e c t i o n and plate tectonic p r o c e s s e s . Refer to F i g u r e 10.24 .
Nothing lives long but the earth and the mountain. — C H A R L E S BALLARD
120
PART II
C H A P T E R 10
Exercise 2: Ocean crust versus continental crust Using information in Chapters 2, 4, and 10, complete the table.
Characteristics
Ocean crust
Continental crust
Composition Very
R o c k type(s)
heterogeneous—can
contain
any
rock,
but granitic and gneissic with a cover of sediments 3
Density
3.0
g/cm
Thickness
10 km 4
Topographic
Abyssal
features
Ridge
dominant.
g/cm
The
Age
are 3
2.7
ages
of continental
crust span
billion years.
floor with axial rift
Trenches Seamounts Hot-spot
island
chains
Plateaus Structure/Architecture
A
model for the structure
crust is
the
sediments,
ophiolite
basaltic pillow
and gabbro.
(Note:
of the
suite:
ocean
deep-sea
lavas
and dikes,
Peridotites are part of
the mantle lithosphere,
not the ocean crust.)
The
architecture
complex.
It
accreted
of the
consists
microplates,
arcs,
suture zones,
belts
representing
Sediments Orogenic existing plate
cover basement of the
crustal
boundaries
A n s w e r s a n d e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. T h e m a s s i v e , interior r e g i o n s of c o n t i n e n t s that h a v e b e e n stable for extensive p e r i o d s A. mountains.
C. cratons.
B. plateaus.
D. plains.
2. I n t e n s e d e f o r m a t i o n and accretion of continental crust o c c u r s at A . c o n v e r g e n t plate m a r g i n s .
C . h o t spots.
B. transform margins.
D . divergent plate m a r g i n s .
3. W h i c h of t h e following features is not a s s o c i a t e d with o r o g e n y ? A. thrust faulting
C.
B . intrusion o f p l u t o n s
D. metamorphism
passive c o n t i n e n t a l m a r g i n
4 . T h e oldest r o c k s found o n the continents are a b o u t A. 2 0 0 million y e a r s old.
C.
B.
D. 4.5 billion years old.
1 billion y e a r s old.
H i n t : Refer to F i g u r e s 10.8 a n d 10.22.
4 billion years old.
and
blocks
Review Questions
of t i m e are called
cratons, volcanic
arcs,
ophiolite suites,
processes
is
island ancient
interior platform Origin
continents of preexisting
orogenic rock
in
and zones. the
continent. accretion along
of pre-
convergent
Evolution of the Continents
5. W h i c h of the following p r o c e s s e s p r o d u c e s continental g r o w t h ? A.
suturing of m a r g i n s d u r i n g a continental collision
B. C.
transfer of crust d u r i n g subduction transfer of crust d u r i n g strike-slip faulting
D. all t h e a b o v e 6. T h e W i l s o n cycle is A. B. C. D.
a m o d e l for the evolution of l a n d s c a p e s . the s e q u e n c e of events for the differentiation of m a g m a s . t h e cycle of tectonic events related to continental evolution. a h i g h - p e r f o r m a n c e m o u n t a i n b i k e developed in the A p p a l a c h i a n M o u n t a i n s .
7. M u c h of the crust within the Cordillera of western N o r t h A m e r i c a over the last 2 0 0 million years. A. accreted C. subsided B. eroded D. subducted H i n t : Refer to F i g u r e s 10.11 and 10.12. 8. C r a t o n i c and other continental interior r o c k s are typically those r o c k s f o u n d at active continental m a r g i n s . A . m u c h y o u n g e r than B. older than H i n t : Refer to F i g u r e 10.8.
C . y o u n g e r than D. the s a m e age as
Test-Taking Tips for Multiple-Choice Exams* 10. A n s w e r t h e q u e s t i o n s y o u k n o w first. M a r k i t e m s w h e r e y o u get stuck. C o m e back to h a r d e r q u e s t i o n s later. Often you will find the a n s w e r you are looking for e m b e d d e d in another, easier question. 9. First try to a n s w e r the i t e m w i t h o u t l o o k i n g at the o p t i o n s . 8. E l i m i n a t e t h e d i s t r a c t e r s . Treat e a c h alternative as a t r u e - f a l s e item. If " F a l s e , " eliminate it. 7. Use c o m m o n s e n s e . R e a s o n i n g is m o r e reliable than m e m o r y . 6. U n d e r l i n e k e y w o r d s in the s t e m . T h i s practice can be helpful when you are stuck. It m a y h e l p y o u focus on w h a t q u e s t i o n is really b e i n g asked. 5. If t w o a l t e r n a t i v e s look similar, it is likely that o n e of t h e m is correct. 4. A n s w e r all q u e s t i o n s . U n l e s s points are being subtracted for w r o n g a n s w e r s (rare), it pays to g u e s s w h e n y o u ' r e not sure. R e s e a r c h indicates that items with the most w o r d s in t h e m i d d l e of t h e list are often the correct items. B u t be cautious. Your professor m a y h a v e r e a d the r e s e a r c h t o o ! 3. Do not c h a n g e a n s w e r s . Particularly w h e n you are g u e s s i n g , y o u r first g u e s s is often correct. C h a n g e a n s w e r s only w h e n you have a clear reason for d o i n g so. 2. If the first i t e m is c o r r e c t , c h e c k the last. If it says "all (or n o n e ) of the a b o v e , " you o b v i o u s l y need to read the other alternatives carefully. M i s s i n g an "all of t h e a b o v e " i t e m is o n e of the m o s t c o m m o n errors on a m u l t i p l e - c h o i c e e x a m . It is easy to r e a d carelessly w h e n y o u are a n x i o u s . 1. READ THE DIRECTIONS BEFORE YOU BEGIN! * For optimal exam performance, review and use the Final Exam Prep Worksheet (Appendix B). The idea is to organize a systematic review of material divided into short study sessions.
12
9. Interior continental shields, or cratons, are traversed by a n c i e n t o r o g e n i c belts, but y o u n g e r o r o g e n i c belts are different from ancient o r o g e n i c belts b e c a u s e A. B. C. D.
their s e d i m e n t s have b e e n subjected to extensive regional m e t a m o r p h i s m . the crust is m u c h thinner and cooler. they consist of hotter and thicker crust. they consist almost entirely of volcanic r o c k s .
10. A large portion of the Cordilleran o r o g e n i c belt m a y consist of A. accreted terranes. B . hot-spot v o l c a n i s m . H i n t : Refer t o F i g u r e 1 0 . 1 1 .
C. ophiolite suites. D . a n ancient craton.
11. O r o g e n y is taking p l a c e today A. along the east coast of N o r t h A m e r i c a . B. along the w e s t coast of South A m e r i c a . C. in the center of N o r t h A m e r i c a . D. in t h e C a n a d i a n Shield. 12. W h i c h of the following statements about o r o g e n i c s y s t e m s is N O T valid? A. B. C. D.
O r o g e n y is initiated by rifting as e x t e n s i o n begins to open up a n e w o c e a n basin. O r o g e n y is initiated by subduction and the evolution of an active convergent margin. L a r g e v o l u m e s of granite are intruded during o r o g e n i e s . F o l d i n g and thrusting of preexisting r o c k s contributes to crustal t h i c k e n i n g in the orogen.
13. T h e
w e r e p r o d u c e d b y c o n v e r g e n t plate boundary
p r o c e s s e s , including collision, during t h e P a l e o z o i c . A. Cascade Mountains of northwestern North America B. Rocky Mountains of Colorado C. A p p a l a c h i a n M o u n t a i n s along the eastern m a r g i n of N o r t h A m e r i c a D. C o r d i l l e r a of w e s t e r n N o r t h , Central, and South A m e r i c a H i n t : Refer to F i g u r e s 10.4 and 10.17. 14. T h e Cordilleran o r o g e n y w a s initiated by f o r m a t i o n of A. a continental rift in w e s t e r n N o r t h A m e r i c a . B. a s u b d u c t i o n z o n e a n d convergent plate b o u n d a r y along t h e w e s t e r n e d g e of North America. C. an i m p a c t of a c o m e t - s i z e object in the Pacific O c e a n adjacent to t h e w e s t e r n edge of N o r t h A m e r i c a . D. a line of m a n t l e or h o t spots, w h i c h causes d o m i n g a n d rifting t h r o u g h o u t western North America. 15. T h e w e s t e r n Cordillera of N o r t h A m e r i c a is t o p o g r a p h i c a l l y h i g h e r than t h e Appalachians m a i n l y b e c a u s e A. o r o g e n y o c c u r r e d m o r e recently in t h e Cordillera, so the crust is thicker. B. granite batholiths w e r e intruded in the Cordilleran belt. C. the Appalachians eroded faster because they consist mostly of soft, sedimentary rocks. D. the A p p a l a c h i a n s never r e a c h t h e elevation of the C o r d i l l e r a b e c a u s e collisions do not g e n e r a t e h i g h m o u n t a i n s . 16. Your a s s i g n m e n t as a geologist is to m a p o u t ancient o r o g e n i c z o n e s in w h a t is n o w the stable interior of a continent. R e c e n t epeirogenic uplift has resulted in g o o d exposures of the ancient b a s e m e n t r o c k s . W h i c h o f t h e following features w o u l d N O T b e evidence of an ancient o r o g e n i c z o n e ? A . intensely d e f o r m e d s e d i m e n t a r y r o c k s B. lava flows and thick layers of volcanic tuff C. m a n y granitic p l u t o n s that are all about t h e s a m e r a d i o m e t r i c age D. w i d e s p r e a d and relatively thick a c c u m u l a t i o n s of coral-rich l i m e s t o n e , sandstones, and shales
Evolution of the Continents
17. W h y are the Sierra N e v a d a M o u n t a i n s so m u c h higher t h a n the continental surfaces w e s t and east of t h e m ? A. T h e crust is proably thicker and/or hotter beneath the Sierra N e v a d a . B. T h e crust b e n e a t h the Sierra N e v a d a is very thin and hot. C. T h e crust beneath the Sierra Nevada is probably denser than that to the east and west. D. T h e Sierra N e v a d a s are m o s t p r o b a b l y part of an ancient s p r e a d i n g center that is no longer active. 18. M o u n t a i n s are b o t h the source and p r o d u c t of s e d i m e n t s b e c a u s e A. m o s t s e d i m e n t s are s u b d u c t e d with the o c e a n lithosphere and thereby contribute to subduction zone magmatism. B. m o s t s e d i m e n t s shed off m o u n t a i n s end up on the m a r g i n of a c o n t i n e n t and are eventually accreted to the edge of continental crust by orogenic processes associated with convergent plate m a r g i n s . C. the melting of sediments results in mafic igneous rocks characteristic of continental crust. D. m o s t s e d i m e n t s e n d up on the d e e p o c e a n floor w h e r e they sit for billions of y e a r s . 19. In the context of plate tectonics, a reasonable sequence of events for an orogenic "cycle" is A. hot spots -> s u b d u c t i o n -> rifting -»• orogeny. B. rifting -»• passive margin ->• s u b d u c t i o n -> o r o g e n y -» uplift. C. rifting -> collision ->• s u b s i d e n c e ->• erosion -+ uplift. D. transform faulting -> uplift -*• v o l c a n i s m -• orogeny. H i n t : Refer to F i g u r e 10.18. 2 0 . T h e E a r t h ' s oldest continental crust can be found A. in active o r o g e n i c z o n e s . B. on the o c e a n floor.
C. in continental shield r e g i o n s . D. along the m a r g i n s of the c o n t i n e n t s .
2 1 . T h e g r o w t h o f continents occurs a t A. hot spots. B. s u b d u c t i o n z o n e s .
C. rift z o n e s . D. transform faults.
22. W h e r e on Earth could you go today to find an orogenic system with strong similarities to the Cordillera of N o r t h A m e r i c a ? T h e study of this active orogenic s y s t e m w o u l d provide you with a better understanding of the geologic history of western N o r t h A m e r i c a . A. Andes B. H i m a l a y a s
C. Appalachians D. E a s t Africa Rift
2 3 . E p e i r o g e n y is associated with A. s u b d u c t i o n z o n e s w h e r e c o n v e r g e n c e causes rapid vertical uplift. B. stable interior platforms within continents w h e r e isostatic adjustments result in gradual uplift or s u b s i d e n c e . C. continental collisions w h e r e the highest m o u n t a i n s form. D. t h e W i l s o n cycle for the evolution of continents. H i n t : Refer to F i g u r e 10.20. 24. Continental cratons s e e m relatively i m m u n e from deformation by plate tectonic p r o c e s s e s because A. of d e e p , strong, m a n t l e keels b e n e a t h t h e m . B. they are always located at the center of the continent far from active plate b o u n d a r i e s . C. t h e continental crust is very strong. D. they are covered with s e d i m e n t s . H i n t : Refer to F i g u r e 10.24. 25. A modern example of how crust can be transported laterally along a continental margin is the A. a m a l g a m a t i o n of the s o u t h w e s t e r n Pacific islands. B. mid-Atlantic Ridge. H i n t : Refer to F i g u r e 10.6.
C. A p p a l a c h i a n orogeny. D. San A n d r e a s strike-slip fault.
1 23
126
PART II C H A P T E R 11
Vital Information from Other Chapters After e a c h lecture, y o u need to t h o r o u g h l y m a s t e r the c o n c e p t s c o v e r e d before y o u attend the s u b s e q u e n t lecture. T h e ideas of g e o l o g y are like a stack of b o x e s . E a c h n e w idea rests on all ideas (boxes) stacked b e n e a t h it. T h e r e is a g o o d d e a l of g e o l o g i c a l t i m e m a t e r i a l in this c h a p t e r in t h e s e c t i o n Geologic E v e n t s i n E a r t h ' s H i s t o r y . R e v i e w F i g u r e 1 1 . 1 3 i n t h e c o n t e x t o f F i g u r e s 1 1 . 1 3 , 8 . 1 1 , and 8.14; refer t o F i g u r e s 8.11 a n d 8.14 i n C h a p t e r 8 , C l o c k s i n R o c k s : T i m i n g t h e Geologic R e c o r d . T h e s e t h r e e figures s u p p l e m e n t e a c h other. I t i s vital t h a t y o u u n d e r s t a n d the m e t h o d o f r a d i o m e t r i c d a t i n g t h a t h a s s u p p l i e d t h e d a t e s for t h e r i b b o n o f t i m e (review T a b l e 8.1 a n d t h e s e c t i o n I s o t o p i c D a t i n g M e t h o d s ( p a g e s 1 8 2 - 1 8 3 ) . F o r a t m o s p h e r i c dati n g a n d c l i m a t e w o r k , different m e t h o d s a r e r e q u i r e d . R e v i e w C l o c k i n g t h e C l i m a t e Syst e m o n p a g e 186. R e v i e w p a g e s 1 4 - 1 6 i n C h a p t e r 1 . R e v i e w C h a p t e r 9 , E a r l y H i s t o r y o f t h e T e r r e s t r i a l P l a n e t s . P a g e s 1 9 4 - 1 9 5 a n d 2 0 3 - 2 0 4 are p a r t i c u l a r l y i m p o r t a n t .
During Lecture T h i s lecture is likely to c o v e r a series of p r o c e s s e s . Listen carefully and try to get the key inputs and outputs of e a c h p r o c e s s into y o u r n o t e s .
Note-Taking Tip C o p y F i g u r e 11.13 and paste it at the b e g i n n i n g of y o u r C h a p t e r 11 lecture notes. Refer to it to h e l p y o u stay oriented in geological time. D o n ' t get b o g g e d d o w n w i t h specific dates of events. W h e n we are talking about a billion years, a difference of a few million years isn't worth worrying about. Instead, think in t e r m of landmarks, and try to place them in the correct order. For e x a m p l e , single-celled life occurred a l o n g t i m e before multicelled life. O x y g e n h a d to be present before multicelled life occurred. All extinctions are in the last 5 0 0 million years.
After Lecture Review Notes T h e perfect t i m e to review y o u r n o t e s is right after lecture. T h e following c h e c k l i s t contains b o t h general review tips and specific s u g g e s t i o n s for this chapter.
Check Your Notes: Have y o u . . . •
a d d e d visual material? S u g g e s t i o n : Test y o u r u n d e r s t a n d i n g of C h a p t e r 11 by a d d i n g simplified sketches of the i m p o r t a n t m e t a b o l i c p r o c e s s e s and E a r t h / o r g a n i s m c y c l e s in this chapter. H i n t : S e e F i g u r e s 11.5 and 1 1 . 1 1 .
•
labeled t h e inputs and o u t p u t s of specific p r o c e s s e s that w e r e d i s c u s s e d ? If not, y o u n e e d to l o o k t h e m up in the text.
• indicated events in g e o l o g i c a l t i m e in an o r d e r y o u can m a k e s e n s e of? R e f e r to F i g u r e 11.13. • created a brief b i g picture o v e r v i e w of the lecture (using a s k e t c h or written o u t l i n e ) ?
Geobiology: Life Interacts with the Earth
Carefully r e w o r k y o u r notes for this lecture with great attention to p r o c e s s e s . For e a c h p r o c e s s discussed, it m a y be helpful to m a k e a simple flowchart specifying the input, the process, and the output. Process Input
-»
Output
For cycles such as the sulfur cycle (Figure 11.11), it m a y help you to reduce the figure to a very simple version that will help you r e m e m b e r the essential b i o s p h e r e part of the cycle elements. D e c o m p o s e r m i c r o o r g a n i s m s digest d e c a y i n g animal/plant matter - » h y d r o g e n sulfide + iron in t h e soil —» iron pyrite Be sure to add the figure n u m b e r (in this c a s e F i g u r e 11.11) so that you can review t h e p r o c e s s in its entirety.
Intensive Study Session Because there is a lot to learn in this chapter, be sure to set priorities for studying. Q u i t e likely, there is m o r e material than you will have t i m e to study in o n e intensive study session. We recommend that y o u give h i g h e s t priority to activities that involve a n s w e r i n g questions. A n s w e r ing questions w h i l e u s i n g y o u r text and lecture notes as reference material is far m o r e efficient than r e a d i n g c h a p t e r s or glancing over notes. As always, you h a v e three sources from which to c h o o s e q u e s t i o n s . • Text. W o r k out p r e l i m i n a r y a n s w e r s to E x e r c i s e s 1, 2, 3, 5, 6, 7, and 10 at the end of the t e x t b o o k chapter. R e a d each question. T h e n try to a n s w e r it. Finally, check y o u r a n s w e r against the text. C h e c k y o u r a n s w e r s with y o u r fellow students, t e a c h i n g assistant, and instructor. • Practice E x e r c i s e s a n d R e v i e w Q u e s t i o n s . L e a r n by testing yourself on the processes c o v e r e d in this chapter. C o m p l e t e the R e v i e w Q u e s t i o n s , referring freely to the text and r e c o m m e n d e d figures. Practice Exercise 1 will be a great help in sorting out s o m e of the most important w a y s the biosphere interacts with important geological processes. Start there. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e
Exam Prep Materials in this section are m o s t useful during preparation for q u i z z e s and e x a m s . T h e C h a p ter S u m m a r y and P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r c h a p t e r review. Read the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful o v e r v i e w that should refresh y o u r m e m o r y . Next w o r k o n t h e P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e t h e exercises and questions j u s t as y o u w o u l d an e x a m , to see h o w well y o u h a v e m a s t e r e d the chapter. After you a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t i m p o r t a n t of all, review e a c h question that y o u m i s s e d . Identify a n d correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r answering the q u e s t i o n incorrectly.
Chapter Summary What is the biosphere? • T h e E a r t h ' s b i o s p h e r e i s t h e p a r t o f o u r p l a n e t t h a t c o n t a i n s all its l i v i n g o r g a n i s m s . I t i n c l u d e s all t h e p l a n t s a n d a n i m a l s a s w e l l a s w h o l e k i n g d o m s o f n e a r l y i n v i s i b l e m i c r o o r g a n i s m s s u c h a s fungi a n d b a c t e r i a t h a t
1 27
128
PART II C H A P T E R 11
are astronomically m o r e n u m e r o u s than h u m a n s or the creatures that m a k e u p o u r v i s i b l e w o r l d . S o m e m i c r o o r g a n i s m live i n t h e m o s t e x t r e m e e n v i ronments on Earth. • B e c a u s e the b i o s p h e r e intersects with the lithosphere, h y d r o s p h e r e , and a t m o s p h e r e , it c a n influence or even control b a s i c g e o l o g i c and climatic p r o c e s s e s . G e o b i o l o g y is the study of the interaction of t h e s e b i o s p h e r e o r g a n i s m s with the Earth.
What is metabolism and how does metabolism affect the Earth? • O r g a n i s m s c a n be subdivided into p r o d u c e r s (autotrophs) or c o n s u m e r s (heterotrophs) a c c o r d i n g to the w a y they obtain their food. A u t o t r o p h s m a k e their o w n food u s i n g s o u r c e s of energy and nutrients. H e t e r o t r o p h s feed directly or indirectly on a u t o t r o p h s . • M e t a b o l i s m is t h e p r o c e s s that all o r g a n i s m s u s e to c o n v e r t i n p u t s ( e n e r g y a n d n u t r i e n t s ) t o o u t p u t s (stored e n e r g y a s c a r b o h y d r a t e s o r fat a n d w a s t e products). For example, a photosynthetic plant uses sunlight, carbon dioxide, a n d w a t e r to p r o d u c e c a r b o h y d r a t e s a n d o x y g e n gas as a b y p r o d u c t . R e f e r to T a b l e 11.2. • As their m e t a b o l i c p r o c e s s e s o p e r a t e ,
organisms continuously exchange
e n e r g y and m a t t e r w i t h their e n v i r o n m e n t . B i o c h e m i c a l cycles are p a t h w a y s that d e s c r i b e this e x c h a n g e or flow. I m p o r t a n t b i o c h e m i c a l c y c l e s i n c l u d e the c a r b o n c y c l e (Figure 11.4 a n d Table 11.2), the p h o s p h a t e cycle (Figure 11.5), and the sulfur c y c l e (Figure 11.11).
How important are microbes? • Single-celled o r g a n i s m s i n c l u d i n g bacteria, s o m e fungi a n d algae, a n d p r o t o z o a are k n o w n as m i c r o b e s . W h e r e there is water, there are m i c r o b e s . • M i c r o b e s are the m o s t a b u n d a n t and diverse g r o u p of o r g a n i s m s on E a r t h . • M i c r o b e s w e r e the first o r g a n i s m s to inhabit the Earth, and all other o r g a n i s m s are d e s c e n d e d from t h e m . Refer to F i g u r e 11.6. • E x t r e m o p h i l e s are m i c r o b e s that live in e n v i r o n m e n t s that w o u l d kill other o r g a n i s m s . Refer to Table 11.3. • M i c r o b e s play a critical r o l e in m a n y g e o l o g i c p r o c e s s e s , s u c h as m i n e r a l precipitation, m i n e r a l dissolution, and the flow of i m p o r t a n t e l e m e n t s t h r o u g h E a r t h ' s crust.
How did life originate, and what signs do we have of life on the young Earth? • G e o s c i e n t i s t s c o n t i n u e to e x p l o r e the m e c h a n i s m s for h o w life o r i g i n a t e d on the primitive E a r t h . L a b o r a t o r y e x p e r i m e n t s , like t h o s e b y Stanley M i l l e r (Figu r e 11.14), and studies of meteorites (Figure 11.24), the fossil r e c o r d (Figure 11.15), r o c k chemistry, and m o d e r n o r g a n i s m s (Figure 11.16) are all filling in p i e c e s of this p u z z l e . • T h e fossil r e c o r d tells us that m i c r o b e s originated first on E a r t h and that they evolved into all multicelled o r g a n i s m s . We c a n find fossil m i c r o b e s in r o c k s 3.5 billion y e a r s old.
Geobiology: Life Interacts with the Earth
Where did the oxygen in Earth's atmosphere originate? • E a r t h ' s a t m o s p h e r e is t h o u g h t to have b e e n o x y g e n a t e d by c y a n o b a c t e r i a that gave off o x y g e n gas as a b y p r o d u c t of p h o t o s y n t h e s i s . T h e fossils and the o c c u r r e n c e s of b a n d e d iron formations and red b e d s in the g e o l o g i c r e c o r d p r o v i d e e v i d e n c e for i m p o r t a n t m i l e p o s t s in the history of interactions b e t w e e n life and e n v i r o n m e n t . • An o x y g e n - r i c h surface e n v i r o n m e n t set the stage for the evolution of E u k a r y a , including all multicellular a n i m a l s .
What is the difference between radiation and extinction? • A radiation is the relatively rapid d e v e l o p m e n t of n e w types of o r g a n i s m s that derive f r o m a c o m m o n ancestor. In contrast, extinction o c c u r s w h e n g r o u p s of o r g a n i s m s are no l o n g e r able to a d a p t to c h a n g i n g e n v i r o n m e n t a l c o n d i t i o n s or c o m p e t e with a superior g r o u p of o r g a n i s m s . Refer to F i g u r e 1 1 . 2 1 . • T h e C a m b r i a n e x p l o s i o n , w h i c h m a r k s the origination of all major a n i m a l groups, is an e x a m p l e of a radiation. Refer to F i g u r e 11.19. • W h e n m a n y g r o u p s of o r g a n i s m s b e c o m e extinct at the s a m e time it is called a m a s s extinction. Refer to F i g u r e 1 1 . 2 1 , E a r t h Issues 1 1 . 1 , F i g u r e 9.18, and Table 9.2.
How do we search for life on other planets? • The p r e s e n c e of liquid w a t e r over l o n g p e r i o d s of t i m e ( h u n d r e d s of millions of years) is c o n s i d e r e d to be a p r e r e q u i s i t e for life. • On a p l a n e t t o o c l o s e to its star w a t e r will boil and b e c o m e a gas, w h i c h can be lost to s p a c e over t i m e . On a planet too far from its star, w a t e r will freeze into a solid. F o r every star, there is a h a b i t a b l e z o n e , m a r k e d by the distance away from the star to the p o i n t w h e r e w a t e r is stable as a liquid. If a planet is within the h a b i t a b l e z o n e , there is a c h a n c e that life m i g h t have originated there. Refer to F i g u r e 11.25.
Figure 11.25. S t a r s have h a b i t a b l e z o n e s where life on an o r b i t i n g p l a n e t c o u l d exist if t h e temperature were n o t too warm (too c l o s e t o t h e star) o r t o o c o l d ( t o o far away from t h e s t a r ) . T h e width o f t h e z o n e i s t h e d i s t a n c e over which water w o n ' t boil away or freeze.
1 29
130
PART II C H A P T E R 11
Extinction is a difficult concept to grasp. It is an eternal concept. It's not at all like the killing of individual life forms that can be renewed through normal processes of reproduction. Nor is it simply diminishing numbers. Nor is it damage that can somehow be remedied or for which some substitute can be found. Nor is it something that simply affects our own generation. Nor is it something that could be remedied by some supernatural power. It is rather an absolute and final act for which there is no remedy on earth or in heaven. A species once extinct is gone forever. — T H O M A S BERRY
Practice Exercise T h e a n s w e r and an e x p l a n a t i o n are p r o v i d e d at t h e e n d of the S t u d y G u i d e .
Exercise: How organisms and the Earth interact Life p r o c e s s e s influence w e a t h e r i n g , precipitate m i n e r a l s , a n d m o d i f y the composition of the a t m o s p h e r e a n d o c e a n s . R e v i e w s o m e w a y s life p r o c e s s e s affect t h e E a r t h by filling in the table.
Life's impact on Earth
Life process(es) generating the impact
Description of the interaction and impact
0 i n Earth's 2
atmosphere
Greenhouse
Extraction
effect: C o o l i n g
oceans
of carbon from
and
atmosphere
by
shell-producing
and
photo synthetic
organisms
Greenhouse
Respiration
and
effect:
of anaerobic
microbes
Warming
Table in
metabolism Refer
to
11.2 and page 246
textbook.
Mineral precipitation
Mineral dissolution
"Sulfate-eating
"
microbes
Hydrogen, methane
hydrogen gases
may
sulfide,
and
be produced.
Geobiology: Life Interacts with the Earth
Review Questions Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. A u t o t r o p h s are o r g a n i s m s that A. B. C. D.
m a k e their o w n food. get food by feeding directly or indirectly on p r o d u c e r s . are multicellular. live only in a n a e r o b i c (oxygen-free) e n v i r o n m e n t s .
2. Living cells are c o m p o s e d primarily of t h e e l e m e n t s A. c a r b o n , h y d r o g e n , o x y g e n , and nitrogen. B. silicon, o x y g e n , h y d r o g e n , and iron. C. carbon, p h o s p h o r o u s , iron, and c a l c i u m . D. silicon, nitrogen, iron, and m a g n e s i u m . 3. Photosynthesis is a m e t a b o l i c p r o c e s s that A. B. C. D.
releases energy, C 0 , and water. stores energy, releases water, and precipitates c a l c i u m c a r b o n a t e . stores e n e r g y as c a r b o h y d r a t e s (sugars) and uses C 0 , and water. b u r n s o x y g e n t o create e n e r g y and releases water and C 0 . 2
2
2
4. E u k a r y o t i c cells are A. very s i m p l e cells w i t h o u t nuclei. B. cells with a m o r e c o m p l i c a t e d internal structure, including a nucleus. C. the earliest life form to a p p e a r in the fossil record. D. p h o t o s y n t h e s i z i n g . 5. The earliest r e c o r d of e u k a r y o t e s o c c u r r e d during the A. P h a n e r o z o i c . B. P r o t e r o z o i c . C. Late A r c h e a n . D. Early A r c h e a n . H i n t : Refer to F i g u r e s 8.14 and 11.6, and the section C h e m o f o s s i l s and E u k a r y o t e s in Chapter 1 1 . 6. Deposition of s e d i m e n t a r y b a n d e d iron formations correlates with the a p p e a r a n c e of A. iron-rich basaltic lavas. B. animal life. C. coal s w a m p s . D. a b u n d a n t stromatolites. H i n t : Refer to t h e t e x t b o o k sections M i c r o b i a l M a t s , Stromatolites, and Origin of Earth's O x y g e n a t e d A t m o s p h e r e . 7. Banded iron f o r m a t i o n s ( B I F s ) are A. B. C. D.
a b u n d a n t in the P h a n e r o z o i c . precipitated by reacting with nitrogen. rare in the A r c h e a n . precipitated by reacting with o x y g e n .
8. Stromatolites r e p r e s e n t s e d i m e n t a r y structures constructed by A. single-celled algae (cyanobacteria). B . h o r n corals. C. snails. D. b u r r o w i n g trilobites. 9. E x t r e m o p h i l e s are m i c r o b e s that A. B. C. D.
have a strong preference for o x y g e n - r i c h e n v i r o n m e n t s . are exclusively E u k a r y o t e s . precipitate a b u n d a n t c a l c i u m carbonate. live in e n v i r o n m e n t s with h i g h salinity, acidity, t e m p e r a t u r e , or no o x y g e n .
131
132
PART II C H A P T E R 11
10. L i v i n g t h i n g s are able to g r o w and A. eat food. B. reproduce. C. calculate. D . precipitate. 1 1 . W h a t d o t h e M u r c h i s o n m e t e o r i t e and the M i l l e r e x p e r i m e n t h a v e i n c o m m o n ? A. B o t h w e r e e x p o s e d to nitrogen gas, a b y p r o d u c t of life p r o c e s s e s . B . B o t h c o n t a i n e d a b u n d a n t a m o u n t s o f water. C. B o t h p r o v i d e e v i d e n c e for an a t m o s p h e r e rich in o x y g e n on the primitive Earth. D. A m i n o acids, the f u n d a m e n t a l building b l o c k s of p r o t e i n s , are a b u n d a n t in the meteorite and the p r o d u c t s p r o d u c e d by the Miller e x p e r i m e n t .
Figure 11.14. This simple experiment design was used to convert ammonia ( N H ) , hydrogen ( H ) , water vapor ( H 0 ) , and small carbon-bearing molecules such as methane ( C H ) into amino acids—a key component of living organisms. 3
2
2
4
12. T h e C a m b r i a n E x p l o s i o n is A. the greatest m a s s extinction event in Earth history. B. a surprising r a p i d radiation of every m a j o r a n i m a l g r o u p . C. the first a p p e a r a n c e of life on Earth. D. t h e rapid radiation of stromatolites a c r o s s t h e E a r t h ' s surface. 13. As a planetary scientist studying potentially h a b i t a b l e planets outside o u r solar system, y o u w o u l d get very excited about finding e v i d e n c e for A. a p l a n e t a r y a t m o s p h e r e rich in nitrogen and a m m o n i a g a s e s . B. a b u n d a n t o x y g e n gas and traces of m e t h a n e in a p l a n e t ' s a t m o s p h e r e p l u s w a t e r ice on the surface. C. an a t m o s p h e r e rich in h y d r o g e n and h y d r o g e n sulfide g a s . D. a surface c o v e r e d with lava flows and few i m p a c t craters.
CHAPTER 12
Volcanoes
136
PART II C H A P T E R 12
Intensive Study Session R e a d y to w o r k ? Set priorities for studying this chapter. • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . B e sure t o d o E x e r c i s e 1 . I t involves t h e k e y information y o u n e e d to learn in this chapter. • Text. P a y particular attention to F i g u r e s 1 2 . 1 , 1 2 . 1 1 , 12.12, 12.20, 1 2 . 2 3 , a n d 12.25. T h e s e figures p r e s e n t t h e m o s t i m p o r t a n t ideas i n C h a p t e r 12. A n s w e r E x e r c i s e s 1 and 5 and T h o u g h t Q u e s t i o n 1 at the e n d of the chapter. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e D o the O n l i n e R e v i e w E x e r c i s e s W h e r e D o V o l c a n o e s O c c u r ? and F i n d i n g the Volcanic H o t Spots.
Motivation Tip C h a p t e r 12 c o n t a i n s material that should be of c o n s i d e r a b l e interest to a n y o n e living or p l a n n i n g to live in the n o r t h w e s t e r n U n i t e d States. T h e r e is i n f o r m a t i o n in this chapter that c o u l d literally save y o u r life. A l s o illustrated are l a n d f o r m s in H a w a i i a n d s o m e of o u r w e s t e r n national p a r k s , for e x a m p l e , M o u n t Rainier, C r a t e r L a k e , and Yellowstone. Q u i t e possibly y o u have visited o n e of t h e s e p l a c e s and w o n d e r e d at its b e a u t y . Give yourself permission to enjoy this particularly interesting chapter! B e g i n your study session by j u s t browsing the artwork in Chapter 12 for ideas that interest you. Start reading wherever your interest takes you. Give yourself fifteen or twenty m i n u t e s just to enjoy the interesting illustrations before y o u p l u n g e into studying text and d o i n g exercises
Exam Prep M a t e r i a l s in this section are m o s t useful d u r i n g p r e p a r a t i o n for q u i z z e s a n d e x a m s . The C h a p t e r S u m m a r y a n d the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s s h o u l d simplify your chapter review. R e a d the C h a p t e r S u m m a r y to b e g i n y o u r session. It p r o v i d e s a helpful o v e r v i e w that s h o u l d refresh y o u r m e m o r y . N e x t w o r k o n t h e P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and q u e s t i o n s j u s t as y o u w o u l d an e x a m , to see h o w w e l l y o u h a v e m a s t e r e d this chapter. After y o u a n s w e r the q u e s t i o n s , s c o r e t h e m . Finally, and m o s t i m p o r t a n t of all, review each q u e s t i o n that y o u m i s s e d . Identify and correct t h e m i s c o n c e p t i o n ( s ) that c a u s e d y o u to answer the q u e s t i o n incorrectly.
Chapter Summary Why does volcanism occur? • V o l c a n i s m o c c u r s w h e n m o l t e n r o c k inside the Earth rises b u o y a n t l y to the surface b e c a u s e it is less d e n s e than the s u r r o u n d i n g rock. V o l c a n i s m is a surface e x p r e s s i o n o f m a g m a g e n e r a t i o n w i t h i n the E a r t h .
Volcanoes
What are the three major lava types and how do they relate to eruptive style and volcanic landforms? • Silicate lavas can be classified in three major t y p e s — f e l s i c (rhyolite), interm e d i a t e (andesite), and mafic ( b a s a l t ) — b a s e d on d e c r e a s i n g a m o u n t of silica and increasing a m o u n t s of iron and m a g n e s i u m . • Eruption styles, volcanic deposits, landforms, and potential hazards are strongly linked to t h e c h e m i c a l c o m p o s i t i o n and gas content of the lava (refer to Exercise 1). B e c a u s e basaltic lavas are relatively fluid and dry, they typically exhibit less explosive eruptions and erupt as lava flows. R h y o l i t e lavas are very viscous and usually w e t . T h e r e f o r e , they typically erupt very explosively as pyroclastic flows or form d o m e s .
How is volcanism related to plate tectonics? • T h e r e is a strong c o n n e c t i o n b e t w e e n major types of v o l c a n i s m and crustal plate b o u n d a r i e s . Basaltic lavas o c c u r at divergent plate b o u n d a r i e s and hot spots. T h e o c e a n crust is created by basaltic v o l c a n i s m at the o c e a n ridge system. Basalt is t h o u g h t to be g e n e r a t e d by partial melting d u e to d e c o m p r e s s i o n of the ultramafic u p p e r m a n t l e . Basaltic, andesitic, and rhyolitic lavas erupt at convergent z o n e s d u e to fluid-induced m e l t i n g . T h e lavas g e n e r a t e d along any particular c o n v e r g e n t z o n e d e p e n d in large part on w h a t t y p e of r o c k s are b e i n g subducted and melted within the overriding crust.
How does volcanism interact with human affairs? • There are b o t h benefits and h a z a r d s associated with v o l c a n i s m . G e o t h e r m a l heat is g r o w i n g in i m p o r t a n c e for electric energy generation. E a r t h ' s o c e a n s and a t m o s p h e r e are t h o u g h t to have c o n d e n s e d from volcanic d e g a s s i n g of our planet's interior. Volcanic dust and g a s e s can i m p a c t global c l i m a t e . Volcanic eruptions and associated m u d f l o w s can have disastrous impacts on a region and its p e o p l e . I m p o r t a n t ore-forming p r o c e s s e s occur w h e n hot water circulates t h r o u g h the m a g m a c h a m b e r a n d s u r r o u n d i n g rock.
Practice Exercises Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e .
Exercise 1: Lava types—their properties, eruption styles, deposits, landforms, association with plate tectonics, and hazards Fill in the b l a n k s w i t h t h e typical characteristics of e a c h lava t y p e . K e e p in m i n d that different lavas exhibit a r a n g e of p r o p e r t i e s a n d b e h a v i o r s . G i v e t h e best a n s w e r that generally characterizes e a c h lava. S o m e a n s w e r s h a v e b e e n p r o v i d e d a s g u i d e l i n e s . Bullets m a r k spaces to fill in. This table will p r o v i d e y o u with a very useful study g u i d e for m u c h of C h a p t e r 12. It makes an ideal s u m m a r y of the c h a p t e r that should be very useful w h e n y o u return to this chapter in preparation for y o u r m i d t e r m e x a m .
137
1 38
PART II C H A P T E R 12
Lava types Characteristics
Basalt (mafic)
Andesite (intermediate)
Rhyolite (felsic)
intermediate
•
intermediate
high (~ 70%)
Properties
silica c o n t e n t
• •
gas c o n t e n t
low,
viscosity
low-fluid
t y p i c a l flow velocity
0.7 to
typical flow l e n g t h
10 to 160 km
8 km
less than 1.5 km
typical flow t h i c k n e s s
5 to 15 m
30 m
200 m
•
•
eruption temperature
up to a few percent
E r u p t i o n styles
typically
Deposits
flood
magma 30 m/minute
not basalt
very
explosive
variable
high (up to*> 15%)
intermediate
•
9
less than 9 m/day
m/day
lava
obsidian
flow
dome pyroclastic tuff and
summit
plate t e c t o n i c s Hazards
welded
composite
Landforms
A s s o c i a t i o n with
flow—
cone
caldera
small
caldera
cinder
hot
•
• •
spots
tuff
volcano crater
cinder
• explosive
flow
pyroclastic/ash explosive hot
• • •
cone
• lava
dome
• •
flow
blast
gases
hot
blast
gases
• •
mudflow
T h e longest w o r d in the English l a n g u a g e is s u p p o s e d l y p n e u m o n o u l t r a m i c r o s c o p i c s i l i c o v o l c a n o c o n i o s — a lung d i s e a s e c a u s e d by breathing in particles of volcanic m a t t e r or a similar fine dust.
Exercise 2: Volcanoes at plate tectonic boundaries C o m p l e t e this exercise by filling in the b l a n k s adjacent to t h e list of v o l c a n i c areas with the correct m a t c h of m a g m a t i c (plate tectonic) setting and characteristic m a g m a type. U s e Figures 12.11, 12.12, 12.19, 12.20, and 12.26 and an atlas as a reference. Chapter 2 and W e b site links provided at http://www.whfreeman.com/understandingearth5e will also help you. S a m p l e answers are provided. N o t e that a volcanic area m a y have hybrid characteristics.
Volcanoes
Volcano or volcanic area
Type of volcano (shield, composite, caldera)
Magma type (mafic, intermediate, felsic)
Magmatic (plate tectonic) setting—divergent, convergent, hot spot
composite
intermediate
convergent/'subduction
Hawaii Tonga Islands
and felsic
mafic
Columbia Plateau Santorini ( T h e r a ) , G r e e c e
hot
spot
caldera
Mayon, P h i l i p p i n e s divergent and
Iceland Yellowstone Krakatoa, I n d o n e s i a North Island, N e w Z e a l a n d Crater L a k e , O r e g o n Japan Aleutian Islands, A l a s k a Mariana Islands Kilimanjaro, Africa Pinatubo, P h i l i p p i n e s Katmai, A l a s k a
composite
and
caldera
Mount Rainier, W a s h i n g t o n Tambora, I n d o n e s i a
composite and caldera
Vesuvius, Italy
Review Questions Answers and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. Lavas are a l m o s t a l w a y s fine-grained r o c k s b e c a u s e A. t h e m i n e r a l s they are m a d e up of do not form large crystals. B. very little w a t e r is in the extruded m a g m a . C. the lava crystallizes u n d e r p r e s s u r e t o o l o w for large crystals to grow. D. they cool t o o rapidly for large crystals to grow. 2. W h i c h extrusive r o c k s contain t h e m o s t silica? A. B. C. D.
andesite rhyolite granite basalt
3. D u r i n g v o l c a n i c eruptions, t h e m o s t c o m m o n gas released is A. B. C. D.
hydrogen. water. nitrogen. carbon dioxide.
139
convergent
hot
spot
140
PART II C H A P T E R 12
4. A c l o u d of s u p e r h e a t e d s t e a m and hot ash that is p r o d u c e d d u r i n g a volcanic eruption and m o v e s rapidly parallel to t h e g r o u n d is called A. a w e l d e d tuff.
C . volcanic b o m b s .
B. a pyroclastic flow.
D. a cinder flow.
5. A g o o d e x a m p l e of shield v o l c a n o e s c a n be found in A. n o r t h e r n California, W a s h i n g t o n , and O r e g o n . B . the H a w a i i a n islands. C. the C a r i b b e a n islands, such as M t . P e l e e on M a r t i n i q u e . D. western South America. 6. C o m p o s i t e v o l c a n o e s (stratovolcanoes) are c o m p o s e d largely of A. basalt lava flows and basaltic c i n d e r s . B . pillow a n d p a h o e h o e lava flows. C. rhyolitic and i n t e r m e d i a t e lavas and pyroclastic flows. D. dikes and sills. 7. C o m p a r e d to basalt, rhyolite lava flows are very thick and tend to form d o m e s because rhyolite lava A. c o n t a i n s less gas than basalt lava and is therefore m o r e fluid. B. is richer in silica than basalt lava and is therefore less fluid. C. c o o l s m o r e q u i c k l y than basalt lava. D. is less d e n s e than basalt lava. 8. On a recent three-day hike up a gently sloping m o u n t a i n , y o u r friends describe to you features they e n c o u n t e r e d . F r e q u e n t l y they c r o s s e d lava flows and fissures, and occasionally they h a d to d e t o u r a r o u n d large cinder c o n e s . F r o m y o u r f r i e n d s ' description, y o u tell t h e m they w e r e h i k i n g on a A . caldera.
C.
g a b b r o pluton.
B. composite volcano.
D . shield v o l c a n o .
9 . C o m p o s i t e v o l c a n o e s are c o m m o n l y associated with w h i c h tectonic setting? A . passive continental m a r g i n s and d e e p o c e a n b a s i n s B. c o n v e r g e n t plate b o u n d a r i e s , s u c h as t h o s e of the circum-Pacific C. o c e a n spreading centers, such as Iceland D. transfer faults, such as the S a n A n d r e a s fault 10. Of the following states, w h i c h is essentially all volcanic r o c k ? A. Alaska
C.
Oregon
B. Hawaii
D.
California
1 1 . Typically, explosive volcanic e r u p t i o n s are a s s o c i a t e d w i t h A. basalt lavas. B . shield v o l c a n o e s . C. m a g m a s that are p o o r in b o t h silica and d i s s o l v e d g a s e s . D. m a g m a s that are h i g h in both silica and dissolved g a s e s . 12. Shield v o l c a n o e s and c o m p o s i t e v o l c a n o e s differ in s h a p e b e c a u s e of A. the different c o m p o s i t i o n s of their m a g m a . B. the particular part of the o c e a n that e a c h o n e forms in. C. the latitude at w h i c h e a c h o n e f o r m s . D. factors that are c o m p l e t e l y u n k n o w n to us at present. H i n t : Refer t o F i g u r e 1 2 . 1 1 .
Volcanoes
13. C a l d e r a s usually form A. from m o l t e n material from t h e c o r e that c o m e s very near the E a r t h ' s surface at a thin p o i n t in t h e crust. B. after a s t e a m e x p l o s i o n , w h e n m a g m a c o m e s into contact with a b u n d a n t underg r o u n d water. C. w h e n an eruption literally b l o w s the t o p of a v o l c a n o off. D. after large v o l u m e s of m a g m a erupt, leaving a void in the m a g m a c h a m b e r into w h i c h t h e superstructure of a v o l c a n o can collapse. H i n t : Refer t o F i g u r e 12.12. 14. Your friends h a v e d e s c r i b e d to y o u an e r u p t i o n that t o o k p l a c e at an u n d i s c l o s e d location. T h e l a v a t h e y d e s c r i b e d m e r e l y flowed o u t of a fissure a n d s p r e a d r a p i d l y o v e r a large area. You w o u l d i n f o r m y o u r friends that t h e r o c k t y p e b e i n g f o r m e d w o u l d be m o s t likely t o b e A. granite. B . andesite.
C. basalt. D . rhyolite.
15. You h a v e b e e n i n f o r m e d that an explosive volcanic eruption h a s t a k e n p l a c e at an u n d i s c l o s e d location and that a h u g e n u e e a r d e n t e (pyroclastic flow) flowed off t h e steepsided v o l c a n o . You c o u l d r e s p o n d that the m a g m a t y p e is likely to be A. basaltic. B . rhyolitic.
C. ultramafic. D. gabbroic.
16. W h e r e are andesitic v o l c a n o e s l o c a t e d ? A. B. C. D.
w h e r e diverging plate b o u n d a r i e s occur at t r a n s f o r m b o u n d a r i e s a l o n g t h e m i d - o c e a n i c ridge crest a l o n g converging plate b o u n d a r i e s
141
1 42
PART II C H A P T E R 12
17. A large resort located on a beautiful lake and k n o w n for its hot springs is experiencing a d r o p in b u s i n e s s d u e to publicity about the recent s w a r m of s m a l l e a r t h q u a k e s in the area. T h e lake is actually located within a caldera, and beautiful r o c k t o w e r s and spires of w e a t h e r e d volcanic tuff are found all a l o n g the e d g e of the lake. As the director of the resort, y o u ' r e c o n c e r n e d about the c h a n g e in b u s i n e s s and the potential risk to your g u e s t s . W h a t should y o u d o ? A. You c a n n o t be w o r r i e d b e c a u s e y o u k n o w a v o l c a n o c a n ' t b l o w up on you. B. You k n o w that e a r t h q u a k e s w a r m s can be a p r e c u r s o r to volcanic e r u p t i o n s and that very explosive eruptions h a v e h a p p e n e d at the p l a c e in the recent g e o l o g i c past, so y o u d e c i d e that the resort should c l o s e until the situation is safe. C. A d v e r t i s e the resort as the best p l a c e to see beautiful basalt lava fountains. D. E a r t h q u a k e s have o c c u r r e d occasionally a l o n g a nearby k n o w n fault, and there have b e e n n o historic volcanic e r u p t i o n s , s o y o u ' r e not c o n c e r n e d . 18. W h i c h t y p e of lava is m o s t likely to erupt at the m i d - o c e a n r i d g e ? Refer to F i g u r e 12.20. A . basalt
C.
B . andesite
D . diorite
rhyolite
19. W h i c h t y p e of lava is m o s t likely to erupt at a hot spot? Refer to F i g u r e 12.20. A. basalt
C. rhyolite
B . andesite
D . diorite.
2 0 . W h i c h t y p e of lava is m o s t likely to erupt at a c o n v e r g e n t b o u n d a r y a l o n g the edge of a c o n t i n e n t a l plate, as s h o w n in F i g u r e 12.20? A. basalt
C. rhyolite
B . andesite
D . diorite
Volcanoes
2 1 . W h i c h of t h e following volcanic deposits can be f o r m e d from felsic lava? A. pahoehoe B . flood basalt
C. D.
volcanic d o m e shield v o l c a n o
22. If lava flows of progressively y o u n g e r ages all erupted from a single large m a g m a c h a m ber, h o w w o u l d y o u e x p e c t their c o m p o s i t i o n t o h a v e progressively c h a n g e d ? A. B. C. D.
e n r i c h e d in iron as they get y o u n g e r e n r i c h e d in silica as they get y o u n g e r m o r e mafic as they get y o u n g e r m o r e fluid as they get y o u n g e r
23. W h i c h of the following statements about mafic r o c k s is true? A. B. C. D.
Mafic Mafic Mafic Mafic
rocks rocks rocks rocks
are richer in silica t h a n felsic r o c k s . crystallize at h i g h e r t e m p e r a t u r e s than felsic r o c k s . are m o r e viscous t h a n felsic r o c k s . t e n d to be lighter in color than felsic r o c k s .
24. L a r g e v o l c a n o e s c a n potentially affect global c l i m a t e w h e n they erupt b e c a u s e they release A. g e o t h e r m a l heat. B. v o l c a n i c dust, sulfur, and c a r b o n d i o x i d e gas. C. lahars and lava flows. D. n i t r o g e n and argon g a s e s .
143
CHAPTER 13
Earthquakes
Figure 13.12. Earthquakes indicate how tectonic plates interact at their boundaries.
144
Earthquakes
Before Lecture Before y o u attend lecture, be sure to spend s o m e t i m e p r e v i e w i n g the c h a p t e r with the following q u e s t i o n s .
Chapter Preview • W h a t is an e a r t h q u a k e ? Brief a n s w e r : an e a r t h q u a k e is a shaking of the g r o u n d c a u s e d by seismic w a v e s that radiate out from a fault that m o v e s suddenly. Elastic r e b o u n d explains w h y e a r t h q u a k e s occur. Refer to F i g u r e 13.1. • W h a t are t h e three t y p e s of s e i s m i c w a v e s ? Brief a n s w e r : P ( p r i m a r y / c o m p r e s s i o n a l ) w a v e s , S ( s e c o n d a r y / s h e a r ) w a v e s , and surface w a v e s . Refer to F i g u r e 13.5. • W h a t is earthquake magnitude and how is it measured? Brief a n s w e r : E a r t h q u a k e m a g n i t u d e is a m e a s u r e of the size of the e a r t h q u a k e . T h e R i c h t e r m a g n i t u d e is d e t e r m i n e d from the a m p l i t u d e of the g r o u n d m o t i o n . T h e m o m e n t m a g n i t u d e is closely related to the a m o u n t of e n e r g y radiated by the e a r t h q u a k e . T h e Mercalli Intensity Scale is a m o r e qualitative m e a s u r e of the d a m a g e d o n e by an e a r t h q u a k e . • W h e r e do most earthquakes occur? Brief answer: M o s t but not all earthquakes occur along active plate tectonic boundaries. Refer to F i g u r e 13.12.
Vital Information from Other Chapters A careful review of C h a p t e r 7, D e f o r m a t i o n : Modification of R o c k s by F o l d i n g and Fracturing, will p r o v i d e y o u with i m p o r t a n t prerequisite information. C h a p t e r 7 e m p h a s i z e s brittle and plastic styles of r o c k deformation. E a r t h q u a k e s are t h o u g h t to be the result of the elastic behavior of solid r o c k s , a n a l o g o u s to the s n a p - b a c k from a r u b b e r b a n d w h e n it b r e a k s . In particular, r e v i e w p a g e s 1 5 4 - 1 6 3 and F i g u r e 7.7.
145
1 46
PART II
C H A P T E R 13
Web Site Study Resources http://www.whfreeman.com/understandingearth5e Tectonic Forces in R o c k Deformation for Chapter 7 and Create an Earthquake for Chapter 13 are O n l i n e R e v i e w E x e r c i s e s worth completing before t h e first lecture on earthquakes.
During Lecture O n e goal for lecture should be to leave the r o o m with good answers to the preview questions. • To avoid getting lost in details, k e e p the big picture in m i n d . C h a p t e r 13 tells t h e story of e a r t h q u a k e s : h o w e a r t h q u a k e activity is m e a s u r e d , t h e seismic w a v e s that are g e n e r a t e d by e a r t h q u a k e s , and h o w e a r t h q u a k e activity is driven by plate t e c t o n i c s — t h e location and characteristics of e a r t h q u a k e s is greatly influenced by the t y p e of plate b o u n d a r y . • F o c u s on u n d e r s t a n d i n g the differences b e t w e e n the three k i n d s of seismic (P, S, surface) w a v e s .
Note-Taking Tip: Mark possible test items As the end of s e m e s t e r a p p r o a c h e s , y o u r instructor m a y m e n t i o n that certain material will be covered in the e n d - o f - s e m e s t e r e x a m . It will be helpful to have a systematic way of m a r k i n g such material so that y o u w o n ' t m i s s it w h e n you are s t u d y i n g y o u r notes for t h e final. You c o u l d tag the material with a standard abbreviation like TQ (test q u e s t i o n ) . It will o b v i o u s l y save t i m e if y o u r TQ m a r k is very visible. M a r k it in dark, large print a n d p u t it out in the m a r g i n w h e r e y o u r e y e will easily spot it d u r i n g review. F o r m a t t i n g tip: If y o u have a d o p t e d t h e strategy of leaving a c o l u m n or the entire left p a g e of y o u r n o t e b o o k b l a n k for inserting visual material and special notes to yourself, p u t TQ tags in the b l a n k c o l u m n . See the following e x a m p l e .
Example of How to Mark Your Notes for Possible Test Items
Blank column or page for visual material, T Q , and other additions to your notes Your notes W h a t is an e a r t h q u a k e ? ground shakes Seismic w a v e from a m o v i n g fault -*• G r o u n d shakes TQ
W h a t are the three types of seismic w a v e s ?
( F S 13.5)
P (primary) w a v e
F i n a l M a y 10!
S (secondary) w a v e surface w a v e s
Earthquakes
After Lecture Review Notes The perfect t i m e to review y o u r notes is right after lecture. T h e following checklist c o n t a i n s both general r e v i e w tips and specific suggestions for this chapter.
Check Your Notes: Have y o u . . . • identified t h e i m p o r t a n t p o i n t s clearly? H i n t : You should h a v e h e a d e r s in y o u r notes for e a c h of the q u e s t i o n s in t h e C h a p t e r Preview. • filled in points that you d i d n ' t have time to record during lecture? C m a r k e d possible test q u e s t i o n s ( T Q ) in the m a r g i n ? • indicated i m p o r t a n t figures to study later? • a d d e d visual m a t e r i a l ? Key visual material for C h a p t e r 13 includes F i g u r e Story 1 3 . 1 , Elastic R e b o u n d T h e o r y of an E a r t h q u a k e , Figure Story 13.5, S e i s m i c Waves, and the figure you will d e v e l o p in Practice Exercise 1 at the end of this Study G u i d e chapter. You c a n sketch s i m p l e v e r s i o n s of any aspect of these figures that will h e l p y o u r e m e m b e r the ideas. • m a d e a c o m p a r i s o n chart to h e l p you review key ideas? S u g g e s t i o n ; C o m p l e t e Practice E x e r c i s e 2, Characteristics of S e i s m i c Waves. A d d a copy of this exercise to your n o t e s . It will be e x t r e m e l y useful for r e v i e w i n g before an e x a m .
Intensive Study Session Set priorities for s t u d y i n g this chapter. G i v e h i g h e s t priority to activities that i n v o l v e answering q u e s t i o n s . W e r e c o m m e n d t h e f o l l o w i n g strategy for l e a r n i n g this chapter. • W h a t i s a n e a r t h q u a k e ? W h a t c a u s e s e a r t h q u a k e s t o o c c u r ? B e sure y o u have clear, accurate a n s w e r s to these q u e s t i o n s in m i n d before m o v i n g on to other material. It will be m u c h m o r e difficult to u n d e r s t a n d the discussions of seismic w a v e s and other e a r t h q u a k e p h e n o m e n a that are the major focus of C h a p t e r 13 if y o u are at all fuzzy about e a r t h q u a k e basics. • Test yourself. C o m p l e t e Practice E x e r c i s e 1, E a r t h q u a k e focus versus epicenter, and try to a n s w e r R e v i e w Q u e s t i o n s 1, 2, 9, and 13 w i t h o u t l o o k i n g at the a n s w e r s . T h e n c h e c k y o u r a n s w e r s . Finally, and m o s t important, explore the text m a t e r i a l s u g g e s t e d for any questions y o u m i s s e d or any areas w h e r e y o u feel u n c l e a r about the c o n c e p t s . • Text. P r e v i e w the key figures in the text: F i g u r e 13.1 (elastic r e b o u n d theory explains e a r t h q u a k e ) , F i g u r e 13.5 (seismic w a v e s ) , and F i g u r e 13.6 ( d e t e r m i n ing the e p i c e n t e r of an e a r t h q u a k e ) . You have to u n d e r s t a n d these figures to c o m p l e t e the P r a c t i c e E x e r c i s e s a n d a n s w e r t h e R e v i e w Q u e s t i o n s . S o m e time before the e x a m , a n s w e r all seven of the exercise questions at t h e e n d of C h a p t e r 13 in t h e text. T h e s e are s h o r t - a n s w e r q u e s t i o n s and w o n ' t take long if you k n o w the material. • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . You will get the greatest return on y o u r study t i m e by w o r k i n g on Practice E x e r c i s e s 1 and 2 b e c a u s e they will h e l p y o u r e m e m b e r t h e m o s t i m p o r t a n t i d e a s i n t h e chapter. T h e n try
147
1 48
PART II C H A P T E R 13
a n s w e r i n g e a c h of the review q u e s t i o n s to c h e c k y o u r u n d e r s t a n d i n g of the lecture. C h e c k y o u r a n s w e r s as y o u go, but try to a n s w e r t h e q u e s t i o n before y o u l o o k at the answer. • W e b Site S t u d y R e s o u r c e s http ://w w w. w h f r e e m a n . c o m / u n d e r s t a n d i n g e a r t h 5 e C o m p l e t e the Concept R e v i e w and W e b R e v i e w Questions. Pay particular a t t e n t i o n t o t h e e x p l a n a t i o n s for t h e a n s w e r s . F l a s h c a r d s will h e l p y o u learn n e w terms. T h e O n l i n e R e v i e w E x e r c i s e s Identify the Factors Contributing to
to
Earthquakes
Earthquakes
at
at
Plate
Convergent
Boundaries Plate
and
Boundaries
Identify are
Factors
Contributing
interactive
exercises
w o r t h d o i n g . A l s o , c o m p l e t e the G e o l o g y i n P r a c t i c e e x e r c i s e s t o l e a r n more about earthquakes.
Exam Prep M a t e r i a l s in this section are m o s t useful d u r i n g p r e p a r a t i o n for q u i z z e s and e x a m s . T h e Chapter S u m m a r y and the Practice E x e r c i s e s and R e v i e w Q u e s t i o n s s h o u l d simplify y o u r chapter review. R e a d the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful overview that should refresh y o u r m e m o r y . Next, w o r k on the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . To d e t e r m i n e h o w well you have m a s t e r e d this chapter, c o m p l e t e the exercises and questions j u s t as y o u w o u l d a midterm. After y o u a n s w e r the questions, score t h e m . Finally, review any question y o u missed. Identify and correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r i n g the q u e s t i o n incorrectly.
In times of change, learners inherit the earth, while the learned find themselves beautifully equipped to deal with a world that no longer exists. — E R I C HOFFER
Chapter Summary What is an earthquake? • An e a r t h q u a k e is a s h a k i n g of the g r o u n d c a u s e d by s e i s m i c w a v e s that e m a n a t e from a fault that m o v e s suddenly. W h e n the fault m o v e s , the strain built up over years of slow d e f o r m a t i o n by tectonic forces is r e l e a s e d in a few m i n u t e s as s e i s m i c w a v e s . • Elastic r e b o u n d theory explains w h y e a r t h q u a k e s occur. O v e r a p e r i o d of t i m e , t h e application of stress c a u s e s r o c k to slowly d e f o r m (bend) elastically until it b r e a k s , and the r o c k snaps b a c k as the fault m o v e s . T h i s stretching a n d b r e a k i n g is a n a l o g o u s to stretching a r u b b e r b a n d until it b r e a k s and s n a p s b a c k t o sting y o u r hand.
What determines the depth of an earthquake? • T h e focus is a point a l o n g the fault at w h i c h the e a r t h q u a k e initiates. • E a r t h q u a k e s o c c u r only in brittle rock, w h i c h can b r e a k and snap b a c k elastically. At high t e m p e r a t u r e s and confining pressures found at g r e a t e r d e p t h s , r o c k s are ductile and do not break to g e n e r a t e e a r t h q u a k e s .
Earthquakes
• E a r t h q u a k e s o c c u r within cold, brittle s u b d u c t i n g o c e a n i c lithosphere to a d e p t h of about 7 0 0 k m . B e l o w this depth, the r o c k is t o o hot and soft to break.
Where do most earthquakes occur? • M o s t but n o t all e a r t h q u a k e s o c c u r a l o n g crust plate b o u n d a r i e s . E a r t h q u a k e s at d i v e r g e n t p l a t e b o u n d a r i e s are usually shallow, h a v e l o w e r m a g n i t u d e , a n d are a c o n s e q u e n c e of tensional stress. C o n v e r g e n t p l a t e b o u n d a r i e s p r o d u c e shallow and d e e p e a r t h q u a k e s o f low t o h i g h m a g n i t u d e s and are c o m m o n l y c a u s e d b y c o m p r e s s i v e stress. T r a n s f o r m faults p r o d u c e shallow t o m o d e r ately d e e p e a r t h q u a k e s of l o w to h i g h m a g n i t u d e , usually in r e s p o n s e to shear stress. Refer t o F i g u r e 13.12.
What governs the type of faulting that occurs in an earthquake? • T h e stress applied to the lithosphere is largely d e t e r m i n e d by t h e t y p e of plate b o u n d a r y . Tensional, c o m p r e s s i o n a l , and shear stresses d e t e r m i n e the type of fault. Refer to F i g u r e 13.10.
What is earthquake magnitude and how is it measured? • E a r t h q u a k e m a g n i t u d e is a m e a s u r e of the size of the e a r t h q u a k e . T h e R i c h t e r m a g n i t u d e is d e t e r m i n e d from the a m p l i t u d e of the g r o u n d m o t i o n . T h e m o v e m e n t m a g n i t u d e is closely related to the a m o u n t of energy radiated by the e a r t h q u a k e . T h e modified Mercalli intensity scale is a qualitative m e a s u r e of the d a m a g e d o n e b y a n e a r t h q u a k e .
What are the three types of seismic waves? • T h e r e are three major types of seismic waves (Figure 13.5). Two types of waves travel through the E a r t h ' s interior: P (primary/compressional) waves, which m o v e through all forms of matter and m o v e the fastest, and S (secondary/shear waves) waves, w h i c h m o v e through solids only and at about half the speed of P waves. T h e third type, surface waves, need a free surface like the E a r t h ' s surface to ripple across, like waves on the ocean. T h e y m o v e m o r e slowly than the interior w a v e s but cause m o s t of the destruction associated with earthquakes.
149
150
PART II C H A P T E R 13
What causes the destructiveness of earthquakes? • T h e destructiveness of an e a r t h q u a k e does not d e p e n d on its m a g n i t u d e alone. In addition to g r o u n d m o t i o n , the duration of the e a r t h q u a k e , a v a l a n c h e s , fires, liquefaction, t s u n a m i s , p r o x i m i t y to p o p u l a t i o n centers, and the c o n s t r u c t i o n design of buildings can all amplify the destructiveness of an e a r t h q u a k e . Refer to Earth I s s u e s 1 3 . 1 .
What can be done to mitigate the damage of earthquakes? • T h e d a m a g e c a u s e d by e a r t h q u a k e s c a n be mitigated by regulating the c o n struction d e s i g n of buildings in e a r t h q u a k e z o n e s ; b o l t i n g h o u s e s to their foundation; s e c u r i n g a p p l i a n c e s and tall furniture to walls and k e e p i n g h e a v y items at low levels; and h a v i n g a c o m m u n i t y plan for d e a l i n g with e m e r g e n c i e s g e n erated by e a r t h q u a k e s . G e o l o g i s t s g e n e r a t e s e i s m i c risk m a p s to aid p u b l i c authorities with their evaluations. Refer to Earth Issues 13.2.
Can scientists predict earthquakes? • Scientists c a n characterize the d e g r e e of risk in a region, but they c a n n o t c o n sistently predict e a r t h q u a k e s .
Practice Exercises A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the end of this S t u d y G u i d e .
Exercise 1: Earthquake focus versus epicenter L a b e l the d i a g r a m by filling in the b l a n k s with the following t e r m s : fault scarp, earthquake focus, fault zone, earthquake epicenter. U s e arrows to s h o w the relative m o t i o n a l o n g the fault z o n e . T h e star m a r k s the location from w h i c h fault m o v e m e n t p r o p a g a t e d . W h a t t y p e of fault is this?
Earthquakes
151
Exercise 2: Characteristics of seismic waves C o m p l e t e t h e table b e l o w by filling in the b l a n k b o x e s . F i g u r e 13.5 and the S e i s m i c W a v e s section of the t e x t b o o k will be helpful.
Characteristic Relative speed
P (primary) waves
S (secondary) waves second
Surface waves
fastest
Motion of m a t e r i a l t h r o u g h
rolling/elliptical
which w a v e p r o p a g a t e s
sideways
Medium through which wave will p r o p a g a t e
confined to surface
Analogy with c o m m o n
S
wave propagation
is
wave forms
difficult to visualize. It is somewhat analogous to the way cards in a deck of playing cards slide over each other as you shuffle the deck.
and
motions the
Earth's
152
PART II C H A P T E R 13
It often happens that the wave flees the place of its creation, while the water does not; like the waves made in a field of grain by the wind, where we see the waves running across the field, while the grain remains in place. — L E O N A R D O D E VINCI
Exercise 3: Factors that amplify the damage caused by an earthquake 1.
2.
3.
4.
5.
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the end of this S t u d y G u i d e . 1. Elastic r e b o u n d t h e o r y says that e a r t h q u a k e s are p r o d u c e d w h e n A. r o c k s d e f o r m plastically a l o n g a fault to p r o d u c e anticlines a n d s y n c l i n e s . B. r o c k s abruptly slip past e a c h other after an e x t e n d e d p e r i o d d u r i n g w h i c h elastic d e f o r m a t i o n is built up in the r o c k s . C. m a g m a w i t h i n the Earth abruptly b e g i n s to flow and elastically d e f o r m s the surrounding rocks. D . abrupt m o v e m e n t along faults are c a u s e d b y tidal forces. 2. T h e actual r u p t u r e p o i n t within the crust that results in an e a r t h q u a k e is called t h e A. tsunami. B.
focus.
C.
epicenter.
D . static release. 3. T h e o r d e r of arrival of seismic w a v e s at a r e c o r d i n g station is A. P w a v e s , S w a v e s , surface w a v e s . B. S w a v e s , surface w a v e s , P w a v e s . C. P w a v e s , surface w a v e s , S w a v e s . D . simultaneous. 4. To locate an e a r t h q u a k e epicenter, a m i n i m u m of
s e i s m i c stations is r e q u i r e d .
A. one B. two C. three D . b e t w e e n f i v e and t w e l v e , d e p e n d i n g o n the location o f the e a r t h q u a k e H i n t : Refer t o F i g u r e 13.6.
Earthquakes
5. T h e R i c h t e r scale for e a r t h q u a k e m a g n i t u d e m e a s u r e s t h e A. damage caused by the earthquake. B. a m o u n t of e n e r g y released by the e a r t h q u a k e . C. amount of ground motion. D. d u r a t i o n of t h e e a r t h q u a k e . 6. T h e g r o u n d m o t i o n g e n e r a t e d by a R i c h t e r m a g n i t u d e 8 e a r t h q u a k e is a factor of t i m e s greater than a R i c h t e r 4 e a r t h q u a k e . A. 2 B. C. D.
100 1000 10,000
7. P r i m a r y seismic w a v e s (P w a v e s ) , like s o u n d w a v e s , A. travel only t h r o u g h solid material. B. travel only t h r o u g h liquids and gas. C. travel t h r o u g h solid, liquid, and gas. D. are t h e slowest seismic w a v e s . 8. S e c o n d a r y seismic w a v e s (S waves) A. B. C. D.
travel parallel to P w a v e s (parallel waves). travel only t h r o u g h solid material. travel t h r o u g h solid, liquid, and gas. are the fastest seismic w a v e s .
9. A significant finding that supports the theory of plate tectonics is that m o s t e a r t h q u a k e s occur A. r a n d o m l y in t h e m i d d l e of tectonic plates. B. at all active tectonic plate b o u n d a r i e s . C. only at tectonic plate b o u n d a r i e s that m o v e toward each other. D. only at tectonic plate b o u n d a r i e s that slide past each other. 10. You j u s t started a j o b as a c o u n t y p l a n n e r in C o l o r a d o w h e n t h e B o a r d of S u p e r v i s o r s m a n d a t e s e a r t h q u a k e risk a s s e s s m e n t . Your first task is to assess the potential for a major s e i s m i c event in an area that has e x p e r i e n c e d only a few m i n o r e a r t h q u a k e s . You d e c i d e to A. install a state-of-the-art seismic r e c o r d i n g station to m o n i t o r e a r t h q u a k e activity. B. d e v e l o p a s e i s m i c risk m a p s h o w i n g the likelihood of an e a r t h q u a k e b a s e d on t h e n u m b e r that have o c c u r r e d in the past. C. d e v e l o p an e a r t h q u a k e protection p l a n with local and state officials. D. investigate the r e c o r d s for t s u n a m i s . 11. Given four structures all built identically, the o n e built on w o u l d sustain t h e M O S T d a m a g e d u r i n g a n e a r t h q u a k e w h e n all four are located t h e s a m e distance from its epicenter. A. a hillside c o m p o s e d of unfractured granite B. a q u a r t z - c e m e n t e d s a n d s t o n e formation C. solid unfractured granite b e d r o c k D. a water-saturated s t r e a m delta H i n t : Refer t o the section H o w E a r t h q u a k e s C a u s e Their D a m a g e i n y o u r textbook. 12. Your seismograph has j u s t recorded an earthquake. You are curious whether the earthquake occurred in North A m e r i c a or s o m e w h e r e else in the world. Given the arrival t i m e of 5 m i n u t e s for the first P waves and 10 m i n u t e s for the first S waves, you determine that the a p p r o x i m a t e distance b e t w e e n you and the earthquake is kilometers. A. 100 C. 3 0 0 0 B. 1000 D. 7000 H i n t : U s e the g r a p h i n F i g u r e 13.6 t o m a k e the estimate.
153
1 54
PART II C H A P T E R 13
13. W h i c h of the following features d o e s N O T c h a r a c t e r i z e a divergent plate boundary? A . shallow-focus e a r t h q u a k e s B . basalt e r u p t i o n s C. d e e p - f o c u s e a r t h q u a k e s D. a rift valley H i n t : Refer to F i g u r e 13.12. 14. W h i c h of the following states has the lowest potential for s e i s m i c h a z a r d ? A. Washington B.
Utah
C. Texas D . N e w York H i n t : Refer to F i g u r e 13.18. 15. T h e first m o t i o n s of an e a r t h q u a k e as r e c o r d e d at four different stations are displayed on the strike-slip fault s h o w n below. U s i n g this first m o t i o n data and Figure 13.11, d e t e r m i n e the direction of relative m o t i o n along the fault at the t i m e of the earthquake.
Bird's-eye view of a fault zone with the first motion data for P waves arriving at four seismograph stations during an earthquake. The star marks the epicenter. The dashed line is a north-south reference line plotted perpendicular to the fault (solid line). A. T h e north side m o v e d east (right) and south side m o v e d w e s t (left). B . T h e n o r t h side m o v e d w e s t (left) and south side m o v e d east (right). C . T h e north side m o v e d d o w n and south side m o v e d u p . D . T h e north side m o v e d u p and south side m o v e d d o w n . 16. W h a t t y p e of fault p r o d u c e d the first m o t i o n s h o w n in q u e s t i o n 15? H i n t : M a t c h the illustrations in F i g u r e 13.10 and 13.11 to d e t e r m i n e the t y p e of fault.
CHAPTER 14
Exploring Earth's Interior (a) The pattern of P-wave paths through Earth's interior (solid blue lines). The dashed blue lines show the progress of wave fronts through the interior at 2-minute intervals. Distances are measured in angular distance from the earthquake focus. The P-wave shadow zone extends from 105° to 142°. P waves cannot reach the surface within this zone because of the way they are bent when they enter and leave the core.
(b) S-wave paths through Earth's interior (solid green lines). The larger S-wave shadow zone extends from 105° to 180°. Although S waves strike the core, they cannot travel through its fluid outer region and therefore never emerge beyond 105° from the focus.
Figure 14.2. Earth's core creates P-wave and S-wave shadows. -
Before Lecture P r e v i e w i n g will greatly increase y o u r u n d e r s t a n d i n g of the lecture. F o r an efficient p r e v i e w u s e the following q u e s t i o n s .
155
156
PART II C H A P T E R 14
Chapter Preview • W h a t d o s e i s m i c w a v e s reveal a b o u t t h e E a r t h ' s i n t e r i o r ? Brief a n s w e r : S e i s m i c w a v e s reveal that the E a r t h has a c o n c e n t r i c a l l y z o n e d internal structure. T h e felsic crust lies on a d e n s e r ultramafic m a n t l e c o m p o s e d mostly of peridotite. T h e crust and u p p e r m a n t l e m a k e u p t h e rigid lithosphere. B e n e a t h the lithosphere lies the asthenosphere, the w e a k layer of the m a n t l e across w h i c h the lithos p h e r e slides in plate tectonics. T h e liquid outer c o r e and solid i n n e r c o r e are mostly iron. Refer to F i g u r e s 1.7 and 14.6. • W h a t has seismic tomography revealed about structures in the mantle? T o m o g r a p h i c i m a g e s s h o w h o w tectonic plates vary from very thin u n d e r the m i d - o c e a n r i d g e s to very thick u n d e r continental c r a t o n s . M a n y features of mantle c o n v e c t i o n a r e also revealed. Refer to F i g u r e 14.9. • H o w h o t d o e s it get in E a r t h ' s I n t e r i o r ? Refer to F i g u r e 14.8. • W h a t d o e s E a r t h ' s gravity field a n d isostatic r e b o u n d tell us a b o u t t h e interior? Brief answer: T h e o b s e r v e d gravity field is in a g r e e m e n t with the pattern of mantle convection inferred from seismic t o m o g r a p h y . M e a s u r i n g the rate of post-glacial isostatic r e b o u n d provides information on the viscosity of the m a n t l e a n d h o w it affects rates of uplift and subsidence of the b u o y a n t lithosphere. Refer to E a r t h Issues 14.1. • W h a t d o e s E a r t h ' s m a g n e t i c field tell us a b o u t t h e fluid o u t e r c o r e ? Brief a n s w e r : T h e E a r t h ' s m a g n e t i c field is p r o d u c e d by convective m o t i o n s of electrically c o n d u c t i n g iron-rich fluid in the outer c o r e . • W h a t is p a l e o m a g n e t i s m a n d w h a t is its i m p o r t a n c e ? Brief a n s w e r : T h e E a r t h ' s m a g n e t i c f i e l d flips b a c k a n d forth over g e o l o g i c time. P r e s e r v e d in s o m e r o c k s is a r e c o r d of past c h a n g e s in the orientation of E a r t h ' s m a g n e t i c f i e l d . Refer t o F i g u r e 14.15.
Vital Information from Other Chapters It is very important to review the information on seismic waves presented in the text section Studying Earthquakes at the beginning of Chapter 13. Pay particular attention to the section Seism i c W a v e s a n d Figure 13.5, and be sure y o u u n d e r s t a n d the distinctions b e t w e e n P waves and S w a v e s . A quick review of m o d e l s for m a n t l e convection will also be helpful. T h e key inform a t i o n is covered in Figures 1.11, 2.14, and 12.23. Finally, take another look at Figure 2.10, w h i c h w a s y o u r first e x p o s u r e to p a l e o m a g n e t i s m in the text.
During Lecture O n e goal for lecture should be to leave the r o o m with good answers to the preview questions. • To avoid getting lost in details, k e e p the b i g p i c t u r e in m i n d : C h a p t e r 14 tells the story of the interior of the Earth, its structure and c o m p o s i t i o n , and h o w E a r t h ' s interior supplies h e a t energy to drive g e o l o g i c a l p r o c e s s e s . Key p o i n t s : • E a r t h ' s interior is a concentrically z o n e d structure. • C o n t i n e n t s float on the m a n t l e . • M a n t l e b e h a v e s like a v i s c o u s fluid. • P and S w a v e s reveal a liquid outer core and a solid inner core. • H e a t transfer o c c u r s via convection. • E a r t h ' s m a g n e t i c field is best u n d e r s t o o d as a g e o d y n a m o : C o n v e c t i v e m o v e m e n t (driven by E a r t h ' s internal heat) generates an e l e c t r o m a g n e t i c field.
Exploring Earth's Interior
• F o c u s on u n d e r s t a n d i n g F i g u r e s 14.2, 14.5, 14.6, and 14.8. If y o u have l o o k e d at t h e m before c o m i n g to lecture, it will be easy to follow the lecture: You can s i m p l y a n n o t a t e t h e figures with i m p o r t a n t n e w material p r o v i d e d b y y o u r instructor a n d u n d e r l i n e material in the captions.
Note-Taking Tip We all have m o m e n t s when we d o n ' t understand a point being m a d e in lecture. W h e n momentarily confused, continue taking notes. Hopefully, the necessary insight will c o m e to you. If it does not, the notes you take will provide a clue to what you need to investigate further in y o u r text or in a conversation with y o u r instructor.
After Lecture Review Notes Check Your Notes: Have y o u . . . • a n n o t a t e d figures in the text with important material discussed by y o u r instructor? Figures 14.2, 14.5, 14.6, and 14.8 are the important figures in C h a p t e r 14. • a d d e d visual m a t e r i a l ? Since this c h a p t e r d e p e n d s heavily on material from C h a p t e r s 1, 2, and 13, it m a y be useful to quickly sketch key ideas about P and S w a v e s and m a n t l e convection in y o u r n o t e s . See the Vital Information from O t h e r C h a p t e r s section on Study G u i d e p a g e 156 for suggested material. n a d d e d a brief big picture overview of this lecture in y o u r o w n w o r d s ?
Intensive Study Session Set priorities for studying this chapter. Give highest priority to activities that involve answering questions. We r e c o m m e n d the following strategy for learning this chapter. • Text. Study figures 1 4 . 2 , 1 4 . 5 , 1 4 . 6 , 1 4 . 8 , 1 4 . 9 , 1 4 . 1 2 , and 14.15, and Earth Issues 14.2. You h a v e to u n d e r s t a n d these figures to a n s w e r the R e v i e w Q u e s t i o n s . S o m e t i m e b e f o r e y o u r n e x t e x a m c o m p l e t e t h e text exercises a t t h e e n d o f t h e chapter. T h e s e are s h o r t - a n s w e r q u e s t i o n s and w o n ' t take long i f y o u k n o w t h e m a t e r i a l . N o t i c e that helpful a n i m a t i o n s are p r o v i d e d o n t h e W e b site for s o m e of t h e c h a p t e r exercises. • P r a c t i c e Exercises. C o m p l e t e Exercises 1 and 2. W o r k i n g on these exercises will h e l p y o u r e m e m b e r the m o s t i m p o r t a n t ideas in the chapter. • R e v i e w Q u e s t i o n s . A n s w e r e a c h of the review questions to c h e c k y o u r understanding of the lecture. C h e c k y o u r a n s w e r s as y o u g o , but do try to a n s w e r t h e q u e s t i o n before y o u l o o k at t h e answer. N o t i c e the test taking-tips that are i n t e r s p e r s e d w i t h t h e s e q u e s t i o n s . T h e y are d e s i g n e d t o h e l p y o u d o better o n your next exam. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e t h e C o n c e p t S e l f - C h e c k e r a n d O n l i n e R e v i e w Q u e s t i o n s . P a y particular attention to t h e e x p l a n a t i o n s for the a n s w e r s . D i d y o u k n o w that Cleopatra adored peridot, a gemstone from the upper mantle? Check out G e o l o g y i n P r a c t i c e t o f i n d out m o r e about " C l e o p a t r a ' s e m e r a l d s " (peridot) and t h e c o m p o s i t i o n of t h e u p p e r ultramafic m a n t l e . F l a s h c a r d s will help y o u review t h e n e w t e r m i n o l o g y in this chapter.
157
158
PART II C H A P T E R 14
Exam Prep Materials in this section are m o s t useful during preparation for e x a m s . T h e C h a p t e r S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r chapter review. Read the C h a p t e r S u m m a r y to begin y o u r session. It p r o v i d e s a helpful o v e r v i e w that should refresh y o u r m e m o r y . N e x t , w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d for an e x a m , to see h o w well y o u h a v e m a s t e r e d this chapter. After y o u a n s w e r the questions, score t h e m . Finally, and m o s t important, review e a c h question y o u m i s s e d . Identify and correct the m i s c o n c e p t i o n ( s ) that r e s u l t e d in y o u r a n s w e r i n g the q u e s t i o n incorrectly.
Chapter Summary What do seismic waves reveal about the layering of Earth's crust and mantle? S e i s m i c w a v e s reveal that t h e E a r t h h a s a c o n c e n t r i cally z o n e d internal structure. Felsic c o n t i n e n t a l and mafic o c e a n crusts lie on a d e n s e r ultramafic m a n t l e consisting of iron-rich silicates, like peridotite. Refer to F i g u r e 14.6. T h e M o h o o r M o h o r o v i c i c discontinuity i n s e i s m i c w a v e velocities m a r k s the b o u n d a r y b e t w e e n t h e crust and t h e m a n t l e . Refer to F i g u r e s 1.7 and 14.5. E a r t h ' s tectonic plates are large fragments of t h e lithosphere, w h i c h i n c l u d e s the crust and the u p p e r m o s t , rigid m a n t l e . B e l o w the l i t h o s p h e r e in the u p p e r m a n t l e is a w e a k (soft) z o n e called the a s t h e n o s p h e r e . Refer to F i g u r e 14.6. A b r u p t increases i n s e i s m i c w a v e velocities c o u p l e d with laboratory studies on h i g h - p r e s s u r e m i n e r a l s suggest that there are zones at progressively greater d e p t h s within the mantle where the crystal structures of m i n e r a l s c o l l a p s e ( c h a n g e p h a s e ) u n d e r t h e intense p r e s s u r e t o f o r m m o r e c o m p a c t a t o m i c structures and therefore different m i n e r a l s .
What do seismic waves reveal about the layering of Earth's core? • P-wave and S-wave shadow zones reveal a liquid o u t e r c o r e a n d a solid i n n e r c o r e . R e f e r t o F i g u r e 14.2. • P - w a v e v e l o c i t i e s in t h e c o r e , t h e n a t u r a l a b u n d a n c e Figure 14.6. T h e s t r u c t u r e o f t h e m a n t l e s h o w i n g t h e S - w a v e v e l o c i t y t o a d e p t h o f 9 0 0 km. C h a n g e s i n v e l o c i t y mark t h e s t r o n g l i t h o s p h e r e , t h e weak a s t h e n o s p h e r e , a n d two z o n e s i n w h i c h c h a n g e s o c c u r b e c a u s e i n c r e a s i n g
of i r o n in n a t u r e , t h e e x i s t e n c e of i r o n - n i c k e l m e t e o r i t e s , t h e E a r t h ' s s t r o n g m a g n e t i c field, a n d t h e n e e d for a v e r y d e n s e c o r e to a c c o u n t for t h e o v e r a l l m a s s
p r e s s u r e f o r c e s a r e a r r a n g e m e n t of t h e a t o m s into d e n s e r
o f the E a r t h all s u p p o r t a n i r o n - n i c k e l c o m p o s i t i o n
and more c o m p a c t crystalline structures.
for t h e E a r t h ' s c o r e .
CHAPTER 11
Geobiology: Life Interacts with the Earth
Geobiology: Life Interacts with the Earth
Before Lecture Before y o u attend lecture be sure to spend s o m e t i m e previewing the chapter. For an efficient preview use the C h a p t e r P r e v i e w questions, w h i c h constitute the framework for understanding the chapter. P r e v i e w i n g w o r k s best if you do it j u s t before lecture. With the main points in mind you will u n d e r s t a n d the lecture better. T h i s in turn will result in g o o d and c o m p l e t e notes.
Study Tip To u n d e r s t a n d this chapter, focus on s y s t e m s and cycles. Study inputs and outputs: w h a t g o e s into a n d w h a t c o m e s out o f e a c h p r o c e s s described.
How m u c h t i m e should y o u devote to p r e v i e w i n g ? Obviously, m o r e t i m e is better than less. But even a brief (five- or t e n - m i n u t e ) preview session j u s t before lecture will p r o d u c e a result you will notice. F o r a refresher on w h y p r e v i e w i n g is so important, see Part 1, C h a p t e r 3, H o w to Be Successful in G e o l o g y .
Chapter Preview • H o w do organisms and the Earth interact? Brief answer: Life processes influence weathering, precipitate minerals, and modify the composition of the atmosphere and oceans. Geobiology is the study of these interactions. • W h a t is t h e b i o s p h e r e ? Brief a n s w e r : T h e b i o s p h e r e is t h e part of o u r planet that c o n t a i n s all living o r g a n i s m s . It is c h a r a c t e r i z e d by m e t a b o l i c p r o c e s s e s that influence and even control geologic processes. • W h a t is m e t a b o l i s m ? Brief a n s w e r : M e t a b o l i s m is the process that all o r g a n i s m s use to convert inputs into o u t p u t s . F o r e x a m p l e , p h o t o s y n t h e s i s by plants u s e s three inputs (sunlight, c a r b o n d i o x i d e , a n d water) t o p r o d u c e t w o outputs, sugar and o x y g e n gas. • H o w i m p o r t a n t are m i c r o b e s ? Brief a n s w e r : M i c r o b e s are the m u s t a b u n d a n t and diverse o r g a n i s m s on Earth. T h e y regulate g e o l o g i c p r o c e s s e s such as w e a t h e r i n g of m i n e r a l s and r o c k s , precipitation of certain s e d i m e n t a r y m i n e r a l s , and p r o d u c t i o n of gases such as o x y g e n and c a r b o n dioxide. • H o w d i d life o r i g i n a t e , a n d w h a t signs d o w e h a v e o f life o n the y o u n g E a r t h ? Brief a n s w e r : S i m p l e m o l e c u l e s o f m e t h a n e , a m m o n i a , and water o n t h e primitive Earth are t h o u g h t to have c o m b i n e d to form a m i n o acids, w h i c h then c o m b i n e d to form p r o t e i n s . T h e oldest signs of life on E a r t h are s h o w n in F i g u r e 11.15. • W h a t is t h e difference b e t w e e n r a d i a t i o n a n d e x t i n c t i o n ? Brief answer: R a d i a t i o n is the relatively rapid d e v e l o p m e n t of n e w types of organisms. W h e n g r o u p s of o r g a n i s m s are no longer able to adapt to c h a n g i n g environmental conditions or to c o m p e t e with a superior g r o u p of o r g a n i s m s , they b e c o m e extinct. • C a n life exist on o t h e r p l a n e t s ? Brief a n s w e r : F o r every star, there is a habitable z o n e w h e r e w a t e r will be stable. If a p l a n e t is within a h a b i t a b l e z o n e , there is a c h a n c e that life m i g h t h a v e originated there. Refer to F i g u r e 11.25.
125
134
PART II C H A P T E R 12
Before Lecture Chapter Preview • W h y d o e s v o l c a n i s m o c c u r ? S e e F i g u r e 12.1 ( S t u d y G u i d e p a g e 1 3 3 ) . Volcanism o c c u r s w h e n m a g m a rises b u o y a n t l y t o the surface. • W h a t a r e t h e t h r e e m a j o r l a v a t y p e s a n d h o w d o t h e y relate t o e r u p t i v e style and volcanic landforms? Mafic (basalt), i n t e r m e d i a t e (andesite), and felsic (rhyolite) lava t y p e s p r o d u c e e r u p t i o n s that r a n g e from relatively gentle basaltic flows to h i g h l y e x p l o s i v e rhyolitic eruptions. See Figure 12.11. • H o w is v o l c a n i s m related to p l a t e t e c t o n i c s ? Volcanism is c o n c e n t r a t e d at convergent and divergent plate b o u n d a r i e s and hot spots. S e e F i g u r e 12.20. • W h a t are s o m e of t h e beneficial effects of v o l c a n i s m ? Volcanic p r o c e s s e s create new o c e a n floor, o u r o c e a n s a n d a t m o s p h e r e , o r e deposits, and g e o t h e r m a l energy.
Vital Information from Other Chapters R e v i e w the following sections in C h a p t e r 4: H o w Do I g n e o u s R o c k s Differ from O n e Another? and I g n e o u s Activity and Plate Tectonics.
Previewing Tip S e e the flowchart H o w to S t u d y G e o l o g y on S t u d y G u i d e p a g e 3 for a d d i t i o n a l ideas on p r e v i e w i n g . T h e m o s t useful a n s w e r to a p r e v i e w q u e s t i o n is o n e that y o u will actually r e m e m b e r d u r i n g lecture. Simplify t o s o m e t h i n g y o u c a n c o m m i t t o m e m o r y . P r e v i e w q u e s t i o n s are like a fresh, u n d e c o r a t e d C h r i s t m a s tree. D u r i n g lecture you d e c o r a t e the tree.
During Lecture O n e g o a l for lecture should be to leave the r o o m with g o o d a n s w e r s to the p r e v i e w questions. • F o c u s o n u n d e r s t a n d i n g h o w v o l c a n o e s w o r k . W h e r e the m a g m a c o m e s from (for e x a m p l e , iron-rich o c e a n crust or silica-rich c o n t i n e n t a l crust) h a s everything to do with the explosiveness of the eruption and the s h a p e of t h e resulting volcanic landform. • F o c u s particularly on u n d e r s t a n d i n g F i g u r e s 12.1 ( h o w a v o l c a n o s y s t e m w o r k s ) and 12.11 (volcanic eruptive styles and l a n d f o r m s ) . It m a y be helpful to b o o k m a r k t h e s e figures and have t h e m h a n d y d u r i n g lecture for q u i c k refe r e n c e . D o a n n o t a t e text figures w i t h c o m m e n t s m a d e b y y o u r instructor. • W a n t ideas on taking g o o d n o t e s ? If y o u h a v e n ' t already d o n e so, r e a d t h e d i s c u s s i o n of n o t e taking in Part I, C h a p t e r 3, of the S t u d y G u i d e . You c a n u s e the N o t e - T a k i n g C h e c k l i s t before y o u go to lecture as a o n e - m i n u t e r e m i n d e r of w h a t to do to i m p r o v e y o u r note-taking skill. After lecture, u s e the c h e c k l i s t as a quality c h e c k .
Volcanoes
Note-Taking Tip: Use abbreviations to speed up your note taking ig r o c k —* i g n e o u s r o c k strat —> s t r a t o v o l c a n o c o m p —» c o m p o s i t e v o l c a n o H P - * h o t spot V -* v o l c a n i s m or v o l c a n o sh v -* shield v o l c a n o B -» basalt rhy -* rhyolite or rhyolitic m a g —» m a g m a Put t h e s e abbreviations in the m a r g i n to d r a w y o u r attention to t h e m later. T Q (test q u e s t i o n ) ?
areas w h e r e y o u feel lost or u n c l e a r about the material. U s e these ? flags as a r e m i n d e r that y o u n e e d to follow up v i a further study or a conference w i t h y o u r instructor, tutor, or study partner.
After Lecture Review Notes Right after lecture, w h i l e t h e material is fresh in y o u r m i n d , is the perfect t i m e to review and improve y o u r n o t e s .
Check Your Notes: Have y o u . . . • clearly identified i m p o r t a n t p o i n t s ? E x a m p l e : You should have clear descriptions of t h e t h r e e lava t y p e s , information about the c h e m i c a l c o m p o s i t i o n and explosiveness of e a c h t y p e , a n d legible sketches of e a c h l a n d f o r m discussed d u r i n g t h e lecture. u a d d e d sketches of the l a n d f o r m s associated with each eruptive style? Refer to F i g u r e 1 2 . 1 1 . E x a m p l e : C o m p a r e the slope of a shield v o l c a n o with that of a c o m p o s i t e v o l c a n o by d r a w i n g a simplified sketch of each o n e s h o w i n g h o w steep e a c h v o l c a n o is and t h e relative area e a c h v o l c a n o is likely to cover. No fancy a r t w o r k is n e e d e d , j u s t a steep inverted V for o n e and a flattened inverted V for the other. T h e shield v o l c a n o w o u l d be flattened and spread out and the c o m p o s i t e (stratovolcano) steeper. S k e t c h i n g p r o v i d e s a g o o d c h e c k on h o w well y o u u n d e r s t a n d the differences. • i n c l u d e d a sketch of t h e basic features of a volcanic g e o s y s t e m ? U s e Figure 12.1 as a r e f e r e n c e . • i n c l u d e d a sketch similar to F i g u r e 12.20 that s h o w s h o w v o l c a n i s m relates to plate t e c t o n i c s ? • d e v e l o p e d a table that s u m m a r i z e s t h e three lava types in t e r m s of r o c k c o m p o s i t i o n a n d a s s o c i a t e d l a n d f o r m s ? C o m p l e t i n g the table in Practice E x e r c i s e 1 will give y o u a s u m m a r y of C h a p t e r 12.
135
Exploring Earth's Interior
What has seismic tomography revealed about structures in the mantle? • T h e s e C A T s c a n - l i k e i m a g e s of the E a r t h ' s interior reveal h o w tectonic plates vary from very thin u n d e r the m i d - o c e a n ridges to very thick u n d e r continental cratons. A l s o revealed are features associated with m a n t l e convection, such as sinking slabs of lithosphere a n d s u p e r p l u m e s . Refer to F i g u r e 14.9.
How hot does it get in Earth's Interior? • Within n o r m a l continental crust, temperature increases at a rate of 20° to 30° C per kilometer. T h e rate of t e m p e r a t u r e increase slows w a y d o w n in the m a n t l e and core. T h e m o s t rapid c h a n g e in t e m p e r a t u r e (steepest g e o t h e r m a l gradient) occurs in the o u t e r m o s t layer of o u r planet. T h i s is not surprising, if you c o n sider h o w rapidly t e m p e r a t u r e c h a n g e s from the outside to the inside of a kitchen oven door. Refer to F i g u r e 14.8. • Seismic a n d laboratory studies suggest that the t e m p e r a t u r e in t h e outer liquid core is h i g h e r t h a n 3 0 0 0 ° C , and at the E a r t h ' s center the t e m p e r a t u r e m a y reach 6000° to 8 0 0 0 ° C . Refer to F i g u r e 14.8.
What does Earth's gravity and isostatic rebound tell us about the interior? • M e a s u r i n g t h e rate of postglacial isostatic r e b o u n d p r o v i d e s information on the viscosity of t h e m a n t l e and h o w it affects rates of uplift a n d s u b s i d e n c e of the b u o y a n t lithosphere. Refer to Earth Issues 1 4 . 1 . • T h e o b s e r v e d gravity field is in a g r e e m e n t with the pattern of m a n t l e c o n v e c tion inferred from seismic t o m o g r a p h y . E a r t h Issues 1 4 . 1 .
What does Earth's magnetic field tell us about the fluid outer core? • E a r t h ' s m a g n e t i c field b a s i c a l l y l o o k s like a d i p o l a r b a r m a g n e t . Refer to F i g u r e 1.12. • A g e o d y n a m o e x p l a i n s h o w E a r t h ' s m a g n e t i c field is generated. R a p i d conv e c t i o n in t h e m o l t e n o u t e r c o r e is t h o u g h t to stir up electrical c u r r e n t s in t h e c o n d u c t i n g iron to create the major c o m p o n e n t of t h e m a g n e t i c field. • T h e m a g n e t i c field c h a n g e s strength, position, and polarity over t i m e . T h e r e is a n o n d i p o l e c o m p o n e n t . B o t h the dipole a n d n o n d i p o l e c o m p o n e n t s exhibit secular variations over t i m e spans of d e c a d e s . Reversals in polarity also occur over t i m e s p a n s of t h o u s a n d s of y e a r s . Refer to F i g u r e 14.15.
What is paleomagnetism and what is its importance? • P r e s e r v e d in s o m e r o c k s is a clear r e c o r d of past c h a n g e s in t h e orientation of E a r t h ' s m a g n e t i c f i e l d . T h e c h r o n o l o g y o f m a g n e t i c f i e l d reversals has b e e n w o r k e d out so that the direction of r e m n a n t m a g n e t i z a t i o n of a r o c k formation is often an indicator of its stratigraphic age. Refer to F i g u r e s 14.14 and 14.15. N o t e : T h e pattern o f m a g n e t i c a n o m a l i e s p r o d u c e d b y p a l e o m a g n e t i c reversals r e c o r d e d by o c e a n floor r o c k p r o v i d e d i m p o r t a n t e v i d e n c e for seafloor spreading. Refer to F i g u r e 2.10.
159
160
PART II C H A P T E R 14
Practice Exercises A n s w e r s a n d e x p l a n a t i o n s are p r o v i d e d at the e n d of the S t u d y G u i d e .
Exercise 1: The Earth's interior layers It is i m p o r t a n t to u n d e r s t a n d E a r t h ' s layered c o m p o s i t i o n . L a b e l t h e E a r t h layers in the foll o w i n g figure, taking special note of the information a b o u t t h e c o m p o s i t i o n of each. Refer to F i g u r e s 1.7, 14.5, and 14.6 a n d t h e a c c o m p a n y i n g text section T h e L a y e r i n g and Composition of the Interior.
Exercise 2: Characteristics of Earth's internal layers C o m p l e t e the table by filling in t h e b l a n k s and c o m p l e t i n g the s e n t e n c e s . S h a d e d boxes r e m a i n b l a n k . Refer to text and figures in C h a p t e r s 1, 4, and 14. K e y figures i n c l u d e 1.5, 1.7, 1.8, 4 . 2 , 4 . 1 1 , 14.6, and 14.8.
1 62
PART II C H A P T E R 14
Exercise 3: Evidence for the asthenosphere and its significance A. Briefly discuss o n e line of e v i d e n c e s u p p o r t i n g the p r e s e n c e of an asthenosphere in the upper mantle. B. W h a t is the significance of the a s t h e n o s p h e r e to plate tectonic t h e o r y ? H i n t : Information in both C h a p t e r s 2 and 14 will h e l p y o u c o m p l e t e this assignment.
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of the Study G u i d e . 1. E a r t h ' s c o r e has a r a d i u s that is about A. B.
1/8 1/4
of the E a r t h ' s r a d i u s . C. 1/2 D . 3/4
H i n t : Refer to F i g u r e 14.5. 2. T h e t h i c k n e s s of E a r t h ' s tectonic plates is A. the s a m e on the continents as u n d e r the o c e a n s . B. at its thinnest u n d e r the o c e a n s . C. at its thinnest in the c o n t i n e n t s . D. completely unknown. H i n t : Refer to F i g u r e 1.7 and 14.6. 3. T h e likely c o m p o s i t i o n of the u p p e r m a n t l e is A. felsic.
C.
B . mafic.
D. carbon (diamonds).
ultramafic.
4. W h i c h of the following constitutes the rigid, outer layer of E a r t h ' s tectonic plates? A. a s t h e n o s p h e r e
C. crust
B. lithosphere
D. mantle
5. C o n t i n e n t a l crust h a s an overall c o m p o s i t i o n c o r r e s p o n d i n g closely to that of A. ultramafic.
C.
B . mafic.
D . peridotite.
6. T h e lithosphere is a
felsic to i n t e r m e d i a t e .
layer, as o p p o s e d to the a s t h e n o s p h e r e .
A. plastic
C.
B . fluid
D . rigid
weak
7. T h e inference that E a r t h ' s outer core is liquid is s u p p o r t e d by t h e o b s e r v a t i o n that A. P w a v e s do not p a s s t h r o u g h it. B. S w a v e s do not p a s s t h r o u g h it. C. P w a v e s travel m o r e rapidly t h r o u g h it. D. S w a v e s travel m o r e slowly t h r o u g h it. 8. T h e h i g h e s t density c o m p o n e n t of the Earth is A . the m a n t l e .
C . the c o r e .
B . continental crust.
D . the w h o l e Earth.
9. E a r t h ' s north m a g n e t i c p o l e is located A. at the north g e o g r a p h i c p o l e . B. in A l a s k a . C. b e t w e e n G r e e n l a n d and Baffin Island. D. in C h i n a . H i n t : Refer to F i g u r e 14.10 and an atlas. 10. T h e E a r t h ' s m a g n e t i c field is t h o u g h t to be g e n e r a t e d by A . p e r m a n e n t m a g n e t i s m o f m i n e r a l s within the m a n t l e . B . p e r m a n e n t m a g n e t i s m o f the solid iron-rich inner c o r e . C. electrical currents generated by m o v e m e n t of the liquid outer c o r e . D. electrical currents g e n e r a t e d by c o n v e c t i o n in the a s t h e n o s p h e r e
Exploring Earth's Interior
1 1 . As o p p o s e d to t h e lithosphere, the a s t h e n o s p h e r e is a A. brittle B. weak
layer.
C. m o l t e n D . rigid
12. M i n e r a l grains in s e d i m e n t s b e c o m e m a g n e t i z e d by the E a r t h ' s m a g n e t i c field w h e n A. they are struck a sharp b l o w by a meteorite. B . iron-rich m i n e r a l s align parallel t o the E a r t h ' s m a g n e t i c f i e l d . C. t h e E a r t h ' s m a g n e t i c field reverses itself. D. electricity from lightning strikes passes through the lava b e d s . H i n t : Refer to F i g u r e 14.14. 13. W h i c h layer of the E a r t h experiences t h e m o s t rapid increase in t e m p e r a t u r e with increasing d e p t h ? A. lithosphere
C.
mantle
B. asthenosphere
D.
liquid outer c o r e
H i n t : Refer to F i g u r e 14.8. 14. On w h i c h b o u n d a r y in the E a r t h ' s interior does the greatest c h a n g e in composition occur? A. B. C. D.
lithosphere-asthenosphere boundary crust-mantle boundary mantle-core boundary b o u n d a r y b e t w e e n the outer and inner c o r e
Test-Taking Tips: Test taking and learning style K n o w i n g y o u r learning style can h e l p d u r i n g e x a m s . C o n s i d e r the following. Visual L e a r n e r s • If y o u are a visual learner you probably pay better attention to directions that are written o u t (visual) than to s p o k e n directions. Rely on written directions w h e n they are available. If y o u r e x a m proctor gives the directions verbally w i t h o u t a visual y o u m u s t c o m p e n s a t e . M a k e yourself listen and d o n ' t hesitate t o ask a s m a n y q u e s t i o n s a s y o u n e e d to to get the directions straight. • W h e n y o u get stuck on an item, activate y o u r visual m e m o r y . C l o s e y o u r eyes a n d picture flow charts, pictures, field e x p e r i e n c e s or even lines of text. Auditory Learners • You p r o b a b l y pay better attention to directions that are s p o k e n t h a n to directions that are written out. R e l y on s p o k e n directions w h e n they are available. If the directions are on a slide, c o m p e n s a t e ! M a k e yourself read t h e m . • R e p e a t written directions quietly to yourself ( m o v i n g y o u r lips is often e n o u g h ) . • W h e n you get stuck, r e m e m b e r y o u r lecturer's voice covering this section. Kinesthetic Learners • You p r o b a b l y do best w i t h directions that allow y o u to w o r k an e x a m p l e . Unfortunately, it is a rare c l a s s r o o m e x a m that p r o v i d e s e x a m p l e s or s a m p l e s as part of t h e d i r e c t i o n s . So y o u will n e e d to m a k e up you o w n . Take a m i n u t e to translate the directions into s o m e t h i n g y o u can d o , or a s k the instructor for a s a m p l e or e x a m p l e . B e sure t o interact. R e m e m b e r that you learn b y doing. T h e r e a r e a v a r i e t y of t h i n g s k i n e s t h e t i c l e a r n e r s find helpful w h e n t h e y g e t s t u c k on a test i t e m . • W h e n you get stuck, m o v e in y o u r chair or tap y o u r foot, to trigger m e m o r y . • Feel yourself d o i n g a l a b p r o c e d u r e . • Sketch a flowchart to u n l o c k m e m o r y of a p r o c e s s . • Stuck on a g e o l o g y p r o b l e m ? S k e t c h i n g w h a t is b e i n g described ( w h a t is " g i v e n " in the p r o b l e m ) m a y u n l o c k y o u r m e m o r y and get y o u started.
163
1 64
PART II C H A P T E R 14
15. T h e crust is typically thickest b e n e a t h A . h i g h m o u n t a i n r a n g e s and plateaus o n the c o n t i n e n t s . B. ocean-spreading centers. C. c o n t i n e n t a l interiors like the G r e a t P l a i n s in N o r t h A m e r i c a . D. passive m a r g i n s of c o n t i n e n t s w h e r e t o p o g r a p h y is very flat. H i n t : Refer to F i g u r e 1.7. 16. Significant increases in S-wave velocity at about 4 0 0 and 6 5 0 k i l o m e t e r s depth are e x p l a i n e d by A. c h a n g e s in the c h e m i c a l c o m p o s i t i o n of the m a n t l e . B. a c o l l a p s e of the crystal structures to m o r e c l o s e by p a c k e d f o r m s . C. c h a n g e s in the d e g r e e of partial melting within the m a n t l e . D. r a p i d increases in t e m p e r a t u r e . H i n t : Refer t o F i g u r e 14.6. 17. T h e E a r t h ' s c o r e is inferred to be c o m p o s e d of m o s t l y iron b e c a u s e A. iron is naturally very a b u n d a n t . B. m o s t m e t e o r i t e s r e p r e s e n t i n g the interstellar m a t t e r from w h i c h the planets formed are rich in iron. C. iron is very d e n s e , so its p r e s e n c e in the c o r e w o u l d a c c o u n t for the E a r t h ' s average density. D. all of the a b o v e . 18. W i t h o u t any k n o w l e d g e of w h a t seismic w a v e s tell us a b o u t the E a r t h ' s interior, why is it u n r e a s o n a b l e to a s s u m e that a large p o r t i o n of the l o w e r m a n t l e is m o l t e n ? A. Actually, it could be m o l t e n . We j u s t d o n ' t see e v i d e n c e for it b e c a u s e silicate magm a s are trapped within the Earth d u e to the confining p r e s s u r e s . B. D i r e c t m e a s u r e m e n t s s h o w that the t e m p e r a t u r e at the c o r e / m a n t l e b o u n d a r y is not high e n o u g h t o m e l t t h e l o w e r m a n t l e . C. T h e m a g n e t i c field strength w o u l d be greatly r e d u c e d if m o r e of t h e E a r t h ' s interior was molten. D . Silicate m a g m a s are less d e n s e and w o u l d rise t o the surface, s o w e s h o u l d observe w i d e s p r e a d volcanic activity across the E a r t h ' s surface. T h e m o l t e n iron-rich outer c o r e is too d e n s e to rise. 19. As r o c k s e x p e r i e n c e increased p r e s s u r e with depth, P w a v e s in g e n e r a l will
as
they m i g r a t e t h r o u g h t h e m . A. travel faster B . travel s l o w e r C. travel at the s a m e velocity D. rapidly die out 2 0 . W h e n a reversal of the E a r t h ' s m a g n e t i c field o c c u r s , A. the sense of rotation of the Earth is also reversed. B. the Earth flips over in its orbit. C. the m a g n e t i c polarity of the E a r t h reverses so that the north e n d of a m a g n e t i c c o m p a s s n e e d l e p o i n t s toward the south g e o g r a p h i c p o l e . D. a l m o s t all the i g n e o u s s e d i m e n t a r y r o c k s of the o c e a n floor reverses in m a g n e t i z a tion to m a t c h the n e w orientation of the m a g n e t i c field. 2 1 . T h e M o h o , o r M o h o r o v i c i c discontinuity b e t w e e n t h e crust and t h e m a n t l e , w a s f i r s t detected from A. the abrupt d e c r e a s e in seismic velocities as they cross the discontinuity. B. the abrupt increase in seismic velocities as they cross the discontinuity. C. the S-wave s h a d o w z o n e t h r o u g h w h i c h S w a v e s do not p a s s . D . the o b s e r v a t i o n that n o e a r t h q u a k e s o c c u r b e l o w the M o h o .
Exploring Earth's Interior
22. S u p p o r t i n g e v i d e n c e for heat transfer by convection within the m a n t l e c o m e s from A. t o m o g r a p h y a n d E a r t h ' s gravity field. B. the b u l k density of the Earth. C. postglacial isostatic r e b o u n d . D. Earth's magnet field. 23. T h e " C r e t a c e o u s quiet z o n e " is a n a m e for a(n) A. p e r i o d w h e n d i n o s a u r s w e r e very sedate. B . t i m e w h e n plate m o t i o n s slowed w a y d o w n . C. b r e a k in v o l c a n i c activity on E a r t h d u e to a lack of m a n t l e s u p e r p l u m e activity. D. especially l o n g p e r i o d of n o r m a l polarity of E a r t h ' s m a g n e t i c field. H i n t : Refer t o F i g u r e 14.15. 24. T h e P - w a v e s h a d o w z o n e is caused by the w a y the E a r t h ' s c o r e A. refracts t h e s e i s m i c w a v e s . B. reflects t h e s e i s m i c w a v e s . C. a b s o r b s t h e P w a v e s . D. b l o c k s the P w a v e s . 25. As the l i t h o s p h e r e cools slowly by the c o n d u c t i o n of heat, it b e c o m e s A. less d e n s e and rises. B. soft and w e a k . C. d e n s e r a n d rises. D. d e n s e r and s u b s i d e s . H i n t : Refer t o F i g u r e 14.7. 26. E a r t h ' s tectonic plates are located in the A. c o r e , w h e r e their m o v e m e n t is driven by intense heat and pressure. B. m a n t l e , w h e r e their m o v e m e n t is driven by convection. C. l i t h o s p h e r e , w h e r e their m o v e m e n t is driven by gravity. D. a t m o s p h e r e , w h e r e their m o v e m e n t is driven by the prevailing westerlies.
165
CHAPTER 15
The Climate System
The Climate System
Before Lecture Before you attend lecture, spend s o m e t i m e p r e v i e w i n g the chapter. For an efficient preview, use the C h a p t e r P r e v i e w questions.
Chapter Preview • W h a t is t h e c l i m a t e s y s t e m ? Brief a n s w e r : T h e major c o m p o n e n t s of the climate system are the a t m o s p h e r e , h y d r o s p h e r e , lithosphere, and b i o s p h e r e . It is important to understand how these s y s t e m s interact with each other via feedback m e c h a n i s m s . Refer to Figure 15.1. • W h a t is t h e g r e e n h o u s e effect? Brief a n s w e r : C a r b o n d i o x i d e and other trace a t m o s p h e r i c gases are transparent to sunlight but a b s o r b heat (infrared radiation), which w a r m s E a r t h ' s surface e n v i r o n m e n t s . Refer to Figure 15.7. • How has Earth's climate changed over time? Brief a n s w e r : T h e largest c h a n g e s are the 100,000-year glacial cycles. But there are also significant short-term climate cycles that average 1.000 and 10,000 years. Refer to F i g u r e 15.10. T h e r e are also r a n d o m c l i m a t e events, like El N i n o . N o t e that we are currently living in an exceptional t i m e , the m o s t p r o l o n g e d stable w a r m period in the last 4 0 0 , 0 0 0 y e a r s . • W h a t is t h e c a r b o n c y c l e ? Brief a n s w e r : G e o c h e m i c a l cycles trace the flux of E a r t h ' s e l e m e n t s like carbon from o n e reservoir to another. T h e carbon cycle describes the m o v e m e n t of carbon b e t w e e n four m a i n reservoirs: (1) the a t m o s p h e r e ; (2) the global ocean, including m a r i n e o r g a n i s m s ; (3) t h e land surface, including terrestrial a n i m a l s , plants, and soils; and (4) t h e d e e p e r lithosphere. T h e carbon cycle is particularly i m p o r t a n t b e c a u s e of its strong link to life p r o c e s s e s and climate c h a n g e . Refer to Figure 15.14. • H o w c a n internal g e o l o g i c p r o c e s s e s c a u s e c l i m a t e c h a n g e ? Brief a n s w e r : O v e r the short t e r m of a few years, sulfuric acid aerosols and volcanic ash ejected from large volcanic eruptions can absorb solar radiation before it r e a c h e s the l o w e r a t m o s p h e r e a n d thus d e c r e a s e global t e m p e r a t u r e s . O v e r the long term of millions of years, plate tectonic m o v e m e n t s can shift continents over the poles, stabilizing p o l a r ice c a p s ; b l o c k or o p e n g a t e w a y s to o c e a n currents; and c a u s e uplift, which alters w e a t h e r i n g s y s t e m s and rates of c h e m i c a l w e a t h e r i n g that d r a w d o w n atmospheric carbon dioxide. • Was the t w e n t i e t h - c e n t u r y w a r m i n g c a u s e d by h u m a n activities? Brief a n s w e r : Global w a r m i n g of about 0.6°C d u r i n g the twentieth century correlates with the significant rise i n a t m o s p h e r i c C 0 and other g r e e n h o u s e gases caused b y fossil-fuel b u r n i n g , deforestation, and other h u m a n activities. 2
Vital Information from Other Chapters Review the section Classification of C h e m i c a l and B i o l o g i c a l S e d i m e n t s and S e d i m e n t a r y Rocks i n C h a p t e r 5 . A l s o p r e v i e w p a g e s 5 2 3 - 5 2 8 i n C h a p t e r 2 1 , w h i c h discuss climate change d u r i n g the last series of ice a g e s . T h e g e o b i o l o g i c a l p r o c e s s e s y o u l e a r n e d in C h a p t e r 11 p l a y a significant r o l e in t h e climate s y s t e m . C a r b o n is r e m o v e d from t h e a t m o s p h e r e via t h e b i o c h e m i c a l p r o c e s s of photosynthesis ( F i g u r e 11.4) a n d r e t u r n e d t o t h e a t m o s p h e r e via a n o t h e r b i o c h e m i c a l
167
168
PART II C H A P T E R 15
p r o c e s s , a n i m a l respiration. O x y g e n m a k e s up 21 p e r c e n t of E a r t h ' s a t m o s p h e r e . Oxygen is supplied and m a i n t a i n e d h o m e o s t a t i c a l l y by living o r g a n i s m s via the m e t a b o l i c process of p h o t o s y n t h e s i s (see F i g u r e 11.4 and the section M e t a b o l i s m on p a g e s 2 4 5 - 2 4 6 ) . Microbes, s u c h as c y a n o b a c t e r i a , p r o d u c e d E a r t h ' s original o x y g e n a t e d a t m o s p h e r e (see the section O r i g i n o f E a r t h ' s O x y g e n a t e d A t m o s p h e r e o n p a g e s 2 5 7 - 2 5 8 ) . Finally, methane-producing m i c r o b e s c o n t r i b u t e d t h e m e t h a n e supplies that " b u r p e d " out of s e d i m e n t s , triggering global w a r m i n g a n d a n extinction event that m a r k s t h e P a l e o c e n e - E o c e n e b o u n d a r y . As y o u read C h a p t e r 15, k e e p in m i n d that the c l i m a t e s y s t e m is c o m p o s e d of many subs y s t e m s . C l i m a t e is a tapestry of c o m p l e x and fascinating interactions.
During Lecture • K e e p t h e b i g picture in m i n d as y o u take notes. C h a p t e r 15 tells the story of E a r t h ' s c l i m a t e s y s t e m and h o w global c l i m a t e results from interactions b e t w e e n four E a r t h s y s t e m c o m p o n e n t s : a t m o s p h e r e , h y d r o s p h e r e , b i o s p h e r e , a n d lithosphere. H u m a n activities are b e c o m i n g a n increasingly i m p o r t a n t factor influencing h o w E a r t h s y s t e m s function. F o c u s o n u n d e r s t a n d i n g the c o m p o n e n t s , fluxes, and f e e d b a c k s in e a c h s y s t e m . • B e c a u s e of the social i m p o r t a n c e of global c l i m a t e , there m a y be o p p o r t u n i t i e s for d i s c u s s i o n / d e b a t e activities. P r e v i e w i n g the c h a p t e r will p r e p a r e y o u to take part in t h e s e activities.
After Lecture Review Notes T h e perfect t i m e to review y o u r n o t e s is right after lecture. T h e following checklist contains b o t h g e n e r a l review tips a n d specific s u g g e s t i o n s for this chapter.
Check Your Notes: Have y o u . . . •
w r i t t e n a s u m m a r y of w h a t is c o v e r e d in this l e c t u r e ? Your s u m m a r y s h o u l d say s o m e t h i n g significant a b o u t h o w h u m a n activities c h a n g e the g l o b a l e n v i r o n m e n t and the p o t e n t i a l for g l o b a l w a r m i n g d u r i n g y o u r lifetime. S u g g e s t i o n : W r i t e a brief p o s i t i o n p a p e r on a C h a p t e r 15 issue that c o n c e r n s y o u . A s k y o u r s e l f w h a t E a r t h s y s t e m i n f o r m a t i o n in C h a p t e r 15 is relevant to t h e i s s u e . T r y to d e v e l o p a position that is r e a s o n a b l e and c o n s i s t a n t w i t h e x i s t i n g s c i e n c e .
•
a d d e d i m p o r t a n t visual material to y o u r n o t e s ? S u g g e s t i o n s : D r a w o v e r v i e w sketches of the climate s y s t e m (Figure 15.1) and the c a r b o n cycle (Figures 15.14 and 15.15). To understand h o w climate c h a n g e s over t i m e , pay particular attention to the different t i m e scales (years, d e c a d e s , centuries, t h o u s a n d s of years) used in F i g u r e s 15.10, 15.16, and 2 1 . 2 5 .
Intensive Study Session Set priorities for studying this chapter. Give highest priority to activities that involve answering questions. We r e c o m m e n d the following strategy for learning this chapter. • T e x t . Preview the key figures in C h a p t e r 15, Figure 15.7 (the g r e e n h o u s e effect) and Figure 15.14 (the carbon cycle). You have to understand these figures to a n s w e r the R e v i e w Q u e s t i o n s . Figures 15.2, 15.3, and 15.11 and Earth Issues 15.2, 15.13, and 15.16 also contain important information, relevant to Chapters 15, 2 1 , and 2 3 .
The Climate System
S o m e t i m e before y o u r e x a m , a n s w e r the exercises at the e n d of the chapter. T h e s e are s h o r t - a n s w e r questions and w o n ' t take long if you k n o w the m a t e rial. F o c u s on E x e r c i s e s 2, 4, and 7 and T h o u g h t Q u e s t i o n s 2, 4, and 6. • P r a c t i c e E x e r c i s e s . C o m p l e t e Practice E x e r c i s e s 1 and 2. T h e y will help y o u r e m e m b e r the m o s t i m p o r t a n t ideas in the chapter. • R e v i e w Q u e s t i o n s . Try a n s w e r i n g each of the review questions to c h e c k y o u r u n d e r s t a n d i n g of the lecture. C h e c k y o u r a n s w e r s as you g o , but do try to a n s w e r the q u e s t i o n before you look at the answer. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e the C o n c e p t R e v i e w and W e b R e v i e w Q u e s t i o n s . P a y particular attention to the e x p l a n a t i o n s for the a n s w e r s . T h e O n l i n e R e v i e w E x e r c i s e Identify the Parts of the Carbon Cycle will h e l p you inventory major c o m p o nents of this g e o c h e m i c a l cycle. Did you k n o w that it takes about 2 p o u n d s of coal to p r o d u c e the energy for y o u to c o p y a m e g a b y t e of m u s i c off t h e I n t e r n e t a n d that this r e l e a s e s 4 p o u n d s o f c a r b o n d i o x i d e into the a t m o s p h e r e ? T h e G e o l o g y i n P r a c t i c e e x e r c i s e s for C h a p t e r 15 e x p l o r e h o w this is possible.
Exam Prep Materials in this section are m o s t useful during y o u r preparation for e x a m s . T h e C h a p t e r S u m m a r y a n d the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r c h a p t e r review. R e a d the C h a p t e r S u m m a r y to b e g i n y o u r session. It p r o v i d e s a helpful o v e r v i e w that should refresh y o u r m e m o r y . N e x t , w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d for an e x a m , to see h o w well you have m a s t e r e d this c h a p ter. After y o u a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t important, review e a c h question y o u m i s s e d . Identify and correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r ing the question incorrectly.
Chapter Summary What is the climate system? • M a j o r c o m p o n e n t s of the E a r t h ' s c l i m a t e s y s t e m are the a t m o s p h e r e , h y d r o sphere, lithosphere, and b i o s p h e r e . Refer to F i g u r e 1 5 . 1 . • E a r t h ' s surface w o u l d be m u c h c o l d e r w i t h o u t the p r e s e n c e of g r e e n h o u s e g a s e s like water and carbon d i o x i d e in the a t m o s p h e r e . • O c e a n currents play a major role in distributing heat across the E a r t h b e c a u s e w a t e r h a s a very h i g h capacity for storing heat. • T o p o g r a p h y affects c l i m a t e by influencing the flow of our a t m o s p h e r e . • Volcanic eruptions affect climate by changing the composition of the atmospheric gases and by adding dust and haze that increase the albedo of the atmosphere. • Various factors m a y exert a positive or negative feedback on the climate syst e m . In s o m e c a s e s , f e e d b a c k m e c h a n i s m s can act to stabilize E a r t h ' s climate, and in o t h e r cases they m a y destabilize it by amplifying c l i m a t e c h a n g e .
What is the greenhouse effect? • C a r b o n d i o x i d e and other trace a t m o s p h e r i c g a s e s act like the glass w i n d o w s in a g r e e n h o u s e . T h e y are transparent to sunlight but absorb heat (infrared radiation), w h i c h w a r m s E a r t h ' s surface e n v i r o n m e n t s . Refer to F i g u r e 15.7.
1 69
1 70
PART II C H A P T E R 15
How has Earth's climate changed over time? • T h e largest c h a n g e s are the 100,000-year glacial cycles. • T h e alternation b e t w e e n glacial and interglacial ages o b s e r v e d d u r i n g the P l e i s t o c e n e e p o c h is explained best by the M i l a n k o v i t c h c y c l e s . Refer to Figures 15.11 and 2 1 . 2 5 . • T h e r e are also significant short-term c l i m a t e cycles that a v e r a g e 1,000 and 10,000 years. Refer to F i g u r e s 15.10 and 15.16, and note the regularity of t h e s e c y c l e s . N o t e a l s o that w e a r e c u r r e n t l y living i n t h e m o s t p r o l o n g e d stable w a r m period in the last 4 0 0 , 0 0 0 years. • S h o r t - t e r m variations of climate are associated with El N i n o events and large volcanic e r u p t i o n s .
What are geochemical cycles and how can they impact climate? • G e o c h e m i c a l cycles trace the flux of E a r t h ' s e l e m e n t s , like c a r b o n , from o n e reservoir to another. • U n d e r s t a n d i n g the c a r b o n and c a l c i u m cycles is i m p o r t a n t b e c a u s e of their strong link to life p r o c e s s e s , climate, and the g r e e n h o u s e effect. Refer to F i g u r e s 15.13 and 15.14. • T h e release of c a r b o n d i o x i d e by the b u r n i n g of fossil fuels is h a v i n g a significant i m p a c t on the flux of c a r b o n from the l i t h o s p h e r e into t h e a t m o s p h e r e . Refer to F i g u r e 15.15.
What caused the twentieth-century warming? • B e t w e e n the end of the n i n e t e e n t h century and the b e g i n n i n g of the t w e n t y first, the m e a n surface t e m p e r a t u r e r o s e by a b o u t 0 . 6 ° C . R e f e r to F i g u r e 15.16. • F r o m a variety of c l i m a t e indicators, scientists have c o n c l u d e d that c l i m a t e during the t h o u s a n d years that p r e c e d e d the twentieth c e n t u r y e x p e r i e n c e d an Figure 15.16. Earth heats up. A comparison of Earth's average annual surface temperature with C 0 concentrations in the le atmosphere, showing that the, recent warming trend correlates with the increase in C 0 caused by human activities since the industrial revolution. re
irregular but steady global c o o l i n g of a b o u t 0.2°C and the m a x i m u m fluctuation in m e a n t e m p e r a t u r e s d u r i n g any o n e of the n i n e p r e v i o u s c e n t u r i e s w a s p r o b a b l y less than 0 . 3 ° C .
2
2
• C l i m a t e m o d e l s that include c h a n g e s in a t m o s p h e r i c g r e e n h o u s e g a s e s as a factor m a t c h the global t e m p e r a t u r e rise and also r e p r o d u c e the pattern of t e m perature c h a n g e b o t h g e o g r a p h i c a l l y and w i t h altitude in the a t m o s p h e r e .
The Climate System
fit is virtually impossible
to change one thing
in a complex
system without affecting other parts of the system, unpredictable
often
in as yet
ways. —ANONYMOUS
How hard to realize that every camp of men or beast has this glorious starry firmament for a
roof! In such places standing
top it is easy to realize that... the world with spaces
without
the firmament for its leaving
any
alone
on
the mountain
we all dwell in a house of one room— roof—and are
sailing
the
celestial
track. —JOHNMUIR, 1890
Practice Exercises Answers a n d e x p l a n a t i o n s are p r o v i d e d at the end of the S t u d y G u i d e .
Exercise 1: Conceptual map/Flowchart of a climate factor Construct a c o n c e p t u a l map/flowchart characterizing o n e other factor besides clouds that can affect Earth's climate. A l s o provide a brief written explanation, as in the clouds e x a m p l e illustrated. Follow these guidelines. • B e s u r e y o u u n d e r s t a n d a n d c l e a r l y e x p l a i n h o w t h e factor c a u s e s c l i m a t e t o change. • I n c l u d e in y o u r f l o w c h a r t p o s s i b l e positive and negative f e e d b a c k s y s t e m s . For e x a m p l e , t h e c o o l i n g effect o f low c l o u d s , like s t o r m c l o u d s , will r e d u c e e v a p o r a t i o n rate a n d m a y h a v e a s h o r t - t e r m negative f e e d b a c k on a d d i t i o n a l cloud f o r m a t i o n .
E x a m p l e : T h e following c o n c e p t u a l m a p / f l o w c h a r t s h o w s h o w c l o u d s i m p a c t c l i m a t e .
Climate C h a n g e F a c t o r : C l o u d s
Explanation o f flow c h a r t In a simple model, as air temperature increases, more moisture is evaporated and held in the atmosphere. This usually leads to more cloud cover. Clouds increase the albedo of the atmosphere and may cool the Earth's surface by reducing the amount of sunlight reaching the surface. Increasing cloud cover potentially would have a negative effect on global warming. Clouds may also have a warming effect on the Earth's surface by reducing the loss of heat to space during the night. Note: How clouds affect climate is still debated. High clouds have been shown to cause warming and low clouds cooling at the Earth's surface. There is growing evidence that clouds have an overall but not very great net cooling effect.
1 71
1 72
PART II C H A P T E R 15
Conceptual Map/Flowchart showing how
impacts climate
Explanation
Exercise 2: Release of carbon dioxide from the burning of fossil fuels A. E x p l a i n w h y the release of c a r b o n d i o x i d e from the b u r n i n g of fossil fuels might lead to an i n c r e a s e in c l o u d cover
B. W o u l d an increase in c l o u d cover h a v e a positive or negative affect on c l i m a t e ? Explain. F o r e x a m p l e , w o u l d an increase in cloud c o v e r e n h a n c e or r e d u c e the effect of increasing c a r b o n d i o x i d e in t h e a t m o s p h e r e ? S e e p a g e s 3 5 3 - 3 5 5 in the t e x t b o o k for examples of f e e d b a c k m e c h a n i s m s that c a n help to b a l a n c e or stabilize the c l i m a t e s y s t e m .
The Climate System
The balance influence on
of evidence suggests global climate.
a
discernible
1 73
human
— U N CLIMATE COMMITTEE, I P C C ,
1995, p . 4
Exercise 3: Flow of carbon through Earth's systems and reservoirs Carbon dioxide is an important greenhouse gas that can greatly impact Earth's surface temperatures. B e c a u s e the Earth is a closed system, the carbon cycle and distribution of carbon in the various reservoirs is a very important c o m p o n e n t of any m o d e l for h o w climate c h a n g e s . C o m p l e t e t h e t a b l e to s u m m a r i z e the flow of c a r b o n through E a r t h ' s s y s t e m s and reservoirs. Flux is t h e a m o u n t of e n e r g y or m a t t e r flowing through a given area or reservoir in a given time. Refer to F i g u r e s 15.7, 15.13, 15.14, and 15.15, and the a c c o m p a n y i n g text in Chapter 15.
Carbon fluxes Photosynthesis and precipitation of carbonates
Brief description of flux Carbon
is fixed in
organisms, contribute sediments,
Sedimentation
which to
ultimately
and
combine
to produce
carbonate
into
which
ions
can
precipitate
limestone
help
rock particles.
other
Climate cools. Carbon dioxide is drawn out of the oceans and atmosphere. The loss of CO from the oceans results in a reduction of CO in the atmosphere.
calcium
and collect to form cement
Carbon flows from the atmosphere and oceans rock—the lithosphere.
and oil.
Calcium carbonate,
living
organic matter in
coal,
Climatic impact/ implications
Direction of flux
or
2
2
Volcanism
Chemical weathering
C0 in rainwater combines with minerals in rock to form calcium carbonate.
Carbon flows from the atmosphere and oceans the crust.
Metamorphism
Heating, recystallization, and decomposition of rocks during metamorphism can release large amounts of C0 •
Carbon flows from the rocks (the crust) into the atmosphere and oceans.
The burning of fossil fuels releases large amounts of C0 into the atmosphere.
Carbon flows from the lithosphere (coal, oil, and gas) into the oceans and atmosphere.
2
into
2
Human activities: Combustion of fossil fuels
2
Refer to this W e b site for the latest information on global w a r m i n g : http://www.ngdc.noaa.gov/paleo/globalwarming/home.html
Climate cools. Carbon is being drawn out of surface environments and stored in the crust. Uplift of high plateaus and mountains may enhance this flux. Climate warms. Increased levels of C0 in the atmosphere enhance the greenhouse effect, which acts to trap heat energy and slow down the loss of heat to space. 2
1 74
PART II C H A P T E R 15
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the e n d of this S t u d y G u i d e . 1. W h i c h of the following gases is m o s t a b u n d a n t in the E a r t h ' s p r e s e n t atmosphere? A . nitrogen B. carbon dioxide C.
water
D. oxygen 2. W h y is c a r b o n d i o x i d e c o n s i d e r e d a g r e e n h o u s e g a s ? A. It a b s o r b s heat. B. It reflects radioactivity. C. It a b s o r b s UV light. D. It reflects sunlight. 3. E a r t h ' s global t e m p e r a t u r e trend is clearly A. u p w a r d over the last century. B. d o w n w a r d over the last century. C. u n c h a n g e d over the last few d e c a d e s . D . variable, but there has b e e n n o overall c h a n g e . H i n t : Refer to F i g u r e 15.16. 4. It has b e e n s u g g e s t e d that the uplift of the H i m a l a y a n m o u n t a i n s a n d the Tibetan plateau c o u l d h a v e c o n t r i b u t e d to or even c a u s e d a global c o o l i n g . T h e link b e t w e e n the H i m a l a y a n m o u n t a i n s and climatic c o o l i n g is p r o b a b l y related to A . t h e collision o f I n d i a with Asia, triggering v o l c a n i s m and i n c r e a s i n g the C 0 con2
centration in t h e a t m o s p h e r e . B. the uplift intensifying the m o n s o o n and a s s o c i a t e d p h y s i c a l and c h e m i c a l weathering, w h i c h resulted in a d r a w - d o w n of c a r b o n d i o x i d e from the a t m o s p h e r e . C. t h e fact that h i g h m o u n t a i n s generate m o r e c l o u d s , a n d their a l b e d o (reflectivity) cools t h e E a r t h ' s surface. D . E l N i n o and the N o r t h Atlantic d e e p - w a t e r current. 5. W h i c h of the following is N O T a s s o c i a t e d with El N i n o e v e n t s ? A. trade w i n d s s l a c k e n i n g or reversing direction B . volcanic activity C. c h a n g e in o c e a n circulation p a t t e r n s D . w o r l d w i d e a n o m a l o u s w e a t h e r patterns H i n t : Refer to Earth Issues 15.2. 6 . A s the o c e a n s b e c o m e w a r m e r ,
C0
2
i s r e l e a s e d from t h e o c e a n s into the
a t m o s p h e r e , resulting in a
effect.
A. more/positive
C.
B . less/negative
D . less/positive
more/negative
H i n t : I s C 0 m o r e o r less soluble i n w a r m e r w a t e r ? Refer t o C h a p t e r 2 0 , D e e p - S e a 2
S e d i m e n t a t i o n , and F i g u r e 2 0 . 2 4 . 7. T h e increase of the a v e r a g e t e m p e r a t u r e on E a r t h is linked to b u r n i n g fossil fuels b e c a u s e the A. burning process consumes oxygen. B. burning process consumes C 0 . 2
C . b u r n i n g p r o c e s s generates C 0 . 2
D. s m o k e given off by b u r n i n g insulates the E a r t h .
The Climate System
8. T h e surface t e m p e r a t u r e s on Venus, Earth, a n d to a lesser extent M a r s are all well a b o v e w h a t c a n be e x p l a i n e d by their distance from the Sun. W h a t other factor significantly contributes to elevated surface t e m p e r a t u r e s for these inner planets? A. t h e p r e s e n c e of g r e e n h o u s e g a s e s , like c a r b o n d i o x i d e B . interior heat C. dust from w i n d s t o r m s and v o l c a n o e s , w h i c h acts to trap heat D. p r e s e n c e of a r g o n and nitrogen in the a t m o s p h e r e 9. At p r e s e n t t h e greatest flux of carbon d i o x i d e occurs b e t w e e n our a t m o s p h e r e and A. oceans. B. volcanoes. C. living o r g a n i s m s . D. humans. H i n t : Refer to F i g u r e s 15.14 and 15.15. 10. W h i c h of t h e following contributes the m o s t c a r b o n to the a t m o s p h e r e ? A . h u m a n deforestation and agriculture B. p l a n t u p t a k e of c a r b o n C. o c e a n air g a s e x c h a n g e D. b u r n i n g of fossil fuel by h u m a n s H i n t : Refer to F i g u r e 15.15. 11. M o s t of t h e m a s s of the a t m o s p h e r e , w h e r e w e a t h e r forms and w h a t c o m m e r c i a l j e t s fly in, is t h e A. troposphere. B . stratosphere. C. mesosphere. D. thermosphere. H i n t : Refer to F i g u r e 15.2. 12. 7 5 % of t h e w o r l d ' s fresh w a t e r is stored in A. B. C. D.
lakes. rivers and s t r e a m s . caves. continental glaciers a n d ice sheets.
13. T h e r m o h a l i n e circulation refers to A. c o n v e c t i o n in the l o w e r a t m o s p h e r e . B. g l o b a l o c e a n i c circulation driven by differences in t e m p e r a t u r e and salinity (salt content). C. c h a n g e s in the t r a d e w i n d s that are t h o u g h t to c a u s e El N i n o events. D. a g l o b a l o c e a n i c current driven by w i n d . H i n t : Refer t o F i g u r e 15.3. 14. T h e g e o c h e m i c a l cycle of c a l c i u m is linked to l o n g - t e r m c l i m a t e c h a n g e by t h e fact that A. w e a t h e r i n g of c a r b o n a t e r o c k s r e m o v e s carbon d i o x i d e from the a t m o s p h e r e and lithosphere. B. c a l c i u m is precipitated as g y p s u m in evaporite deposits. C. p h o t o s y n t h e s i s precipitates calcium. D. h e a t r e l e a s e d by the reaction of c a l c i u m with b i c a r b o n a t e ion w a r m s the surface w a t e r of t h e o c e a n s .
1 75
1 78
PART II C H A P T E R 16
• H o w does chemical weathering work? Brief a n s w e r : Water, o x y g e n , acids, and physical w e a t h e r i n g facilitate chemical w e a t h e r i n g reactions that alter and break d o w n m i n e r a l s to form n e w minerals, oxides, and salts and release silica. • H o w d o e s physical w e a t h e r i n g w o r k ? Brief answer: Physical w e a t h e r i n g b r e a k s rocks into fragments. P r o c e s s e s that aid physical w e a t h e r i n g include c h e m i c a l w e a t h e r i n g , frost w e d g i n g , and the growth of plant roots. • H o w do soils f o r m as p r o d u c t s of w e a t h e r i n g ? Brief a n s w e r : Soils form in rock materials by c h e m i c a l w e a t h e r i n g . Soil formation is influenced by the c o m p o s i t i o n of the rock, stability of the w e a t h e r i n g surface ( t o p o g r a p h y ) , t i m e , and m o s t important, c l i m a t e . Life p r o c e s s e s and their by-products are also i m p o r t a n t factors in soil formation and soil type. • W h a t is m a s s w a s t i n g ? Brief a n s w e r : M a s s w a s t i n g , also called m a s s m o v e m e n t , is the d o w n - s l o p e movement of rock material. • W h y do mass movements occur? Brief a n s w e r : T h e three m o s t important factors e n h a n c i n g the potential for mass m o v e m e n t s are the steepness of the slope, the nature of the rock m a k i n g up the slope, and the w a t e r c o n t e n t of the slope material. • H o w can d a m a g e from mass m o v e m e n t s be minimized? Brief a n s w e r : Careful e n g i n e e r i n g and the restriction of land use c a n m i n i m i z e the h a z a r d s associated with m a s s m o v e m e n t s .
When we try to pick up anything by itself, we find it entwined with
everything
else
in
the
universe. — J O H N MUIR
Learning Warm-Up S c i e n c e and art are synergistic; that is, e a c h e n h a n c e s the other. F o r e x a m p l e , m a n y a s p e c t s of n a t u r e that we c o n s i d e r beautiful are the p r o d u c t of a w e a t h e r i n g p r o c e s s . B e f o r e lecture s p e n d five m i n u t e s s c a n n i n g t h e text p h o t o s for beautiful features a n d effects created b y w e a t h e r i n g . A s y o u enjoy the p h o t o s , y o u m i g h t a s k yourself, " H o w did w e a t h e r i n g p r o c e s s e s create the c o l o r s and s h a p e s i n this p h o t o ? "
Vital Information from Other Chapters R e v i e w t h e R o c k - F o r m i n g M i n e r a l s section and Table 3.1 and the S e d i m e n t a r y R o c k s section and F i g u r e 3.24 in C h a p t e r 3. T h e c o m p o s i t i o n and internal fabric of r o c k s significantly influence the r o c k ' s strength and the potential for m a s s m o v e m e n t . Therefore, a review of the c o m p o s i t i o n and especially of the different k i n d s of fabrics (textures) of i g n e o u s , sedimentary, a n d m e t a m o r p h i c r o c k s will p r o v i d e y o u with vital information for u n d e r s t a n d i n g the different c i r c u m s t a n c e s that cause m a s s m o v e m e n t s . W h i l e reviewing the b a s i c r o c k textures d e s c r i b e d in C h a p t e r s 4, 5, 6, and 7, ask yourself w h a t textures and d e f o r m a t i o n fabrics m i g h t c o n t r i b u t e to a w e a k e r rock and an e n h a n c e d potential for m a s s m o v e m e n t .
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics
Previewing Tip It will be very helpful to w o r k on the P r a c t i c e E x e r c i s e 2 before g o i n g to lecture. C o m p l e t e t h e exercise and take it to class w i t h y o u . Your lecturer will p r o b a b l y s h o w slides to h e l p y o u u n d e r s t a n d the different k i n d s of m a s s w a s t i n g . You will u n d e r s t a n d these differences better if you h a v e w o r k e d on t h e m before lecture.
During Lecture As you take n o t e s d u r i n g this lecture, be sure to get details on h o w w e a t h e r i n g w o r k s . O n e goal for lecture should be to leave class w i t h g o o d a n s w e r s to the C h a p t e r Preview questions. • F o c u s on u n d e r s t a n d i n g specific c h e m i c a l w e a t h e r i n g p r o c e s s e s such as oxidation and dissolution. Be sure to distinguish clearly b e t w e e n t h e c h e m i c a l p r o c e s s e s (for e x a m p l e , dissolution, oxidation) and the physical p r o c e s s e s (for e x a m p l e , frost w e d g i n g , exfoliation, etc). • F o c u s on u n d e r s t a n d i n g F i g u r e 16.12, w h i c h s h o w s the t w o basic soil-forming p r o c e s s e s , translocation and transformation. • To avoid getting lost in details, k e e p t h e big picture in mind. C h a p t e r 16 tells the story of w h a t c a u s e s m a s s w a s t i n g . M a s s wasting is about classification, and y o u r j o b is to u n d e r s t a n d the differences b e t w e e n the types of w a s t i n g . Be sure to p a y close attention to c o m m e n t s t h e lecturer m a y m a k e about h o w each k i n d of m a s s w a s t i n g differs from the others. H i n t : In general these differences will be a b o u t the steepness of t h e slope, the kind of rock in the slope b e i n g m o v e d , and t h e water content of t h e slope material. • You m a y not be familiar with the k i n d s of m a s s wasting (rock avalanche, creep, earthflows, and so o n ) . To help y o u visualize this process, the lecturer m a y s h o w slides of various kinds of m a s s wasting. S o m e of the slides m a y be very dramatic (for e x a m p l e , pictures of landslides). Enjoy the d r a m a and excitement! • If y o u c o m p l e t e d P r a c t i c e E x e r c i s e 2 before class, y o u can refer to it as the lecturer talks a b o u t the different k i n d s of m a s s w a s t i n g .
After Lecture Review Notes The perfect t i m e to review y o u r notes is right after lecture. If you wait even o n e day m o s t (80 p e r c e n t ) of w h a t y o u h e a r d will h a v e d i s a p p e a r e d from m e m o r y .
Check Your Notes: Have y o u . . . • clearly identified the important points? E x a m p l e : You should have clear descriptions of each t y p e of c h e m i c a l and physical weathering. • a d d e d visual m a t e r i a l ? C h u n k i n g material is a g o o d learning strategy. T h e r e are a lot of c h e m i c a l and physical weathering processes in C h a p t e r 16. It m a y be useful to m a k e a list of p r o c e s s e s that y o u can l o o k at as a g r o u p all in o n e place in y o u r n o t e b o o k . U s e a t w o - c o l u m n format for this list, in w h i c h both chemical and physical w e a t h e r processes are listed in o n e c o l u m n and details you need to r e m e m b e r about each process are briefly described in a second c o l u m n to the right. • a d d e d s i m p l e s k e t c h e s ? C h a p t e r 16 i n c l u d e s lots of material that is best learned by visualization. S k e t c h e s will h e l p you r e m e m b e r the key aspect of each kind of m a s s wasting. H i n t : Your sketch need not be artistic to be useful. Sketch only the features you n e e d to r e m e m b e r . E x a m p l e : F o r a r o c k a v a l a n c h e y o u could d r a w a steep slope (one line at 4 5 ° angle) with a pile of large b l o c k s at the b o t t o m to d e s i g n a t e large m a s s e s of b r o k e n rock (see F i g u r e 16.19).
1 79
180
PART II
C H A P T E R 16
•
a d d e d a c o m p a r i s o n chart that will h e l p y o u m a s t e r the classification of different kinds of mass wasting? Hint: See Exercise 2.
• created a brief b i g picture o v e r v i e w of this lecture (using a sketch or written outline) s h o w i n g h o w w e a t h e r i n g w o r k s ? S u g g e s t i o n for C h a p t e r 16: W r i t e a brief summary of the m o s t i m p o r t a n t points you have learned from this c h a p t e r that might influence y o u r c h o i c e of future h o m e sites.
Intensive Study Session Set priorities for studying this chapter. We r e c o m m e n d giving t h e h i g h e s t priority to activities that involve a n s w e r i n g q u e s t i o n s . Pay particular attention to any e x e r c i s e s recommended by the instructor during lecture and a n s w e r t h o s e first. T h e instructor is a l s o y o u r best resource if y o u are w o n d e r i n g w h i c h material is m o s t i m p o r t a n t . A n s w e r i n g q u e s t i o n s w h i l e u s i n g y o u r text and lecture notes as reference material is far m o r e efficient than r e r e a d i n g chapters or g l a n c i n g over n o t e s . As a l w a y s , y o u have three s o u r c e s from w h i c h to c h o o s e q u e s t i o n s . • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . Be sure to do the exercises, as they will focus on the key information y o u need to learn in this c h a p t e r — n a m e l y , h o w w e a t h e r i n g w o r k s , h o w soils form, factors that e n h a n c e the potential for m a s s wasting, and the classification of m a s s m o v e m e n t s . • Text. A n s w e r E x e r c i s e s 2, 4, 5, and 8. A l s o c o m p l e t e T h o u g h t Q u e s t i o n s 1, 2, 4, 6, 9, and 10. • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e Do the O n l i n e R e v i e w E x e r c i s e s Soils, Rock Mass Movements and Unconsolidated Mass Movement. T h e G e o l o g y in P r a c t i c e exercises will allow y o u to apply w h a t y o u learned a b o u t w e a t h e r i n g to u n d e r s t a n d i n g the beautiful form a t i o n s of B r y c e C a n y o n N a t i o n a l Park. T h e r e is also a G e o l o g y in P r a c t i c e exercise o n assessing potential h a z a r d s from m a s s w a s t i n g . C o m p l e t e the G r a d e d O n l i n e Q u i z a n d O n l i n e R e v i e w Q u e s t i o n s . P a y particular attention to the e x p l a n a t i o n s of the a n s w e r s . T h e C h a p t e r 16 I n t e r a c t i v e E x e r c i s e M i n e r a l Stability on t h e W e b site will h e l p y o u review the susceptibility of c o m m o n m i n e r a l s to c h e m i c a l w e a t h e r i n g . N o t e that the susceptibility of c o m m o n silicate m i n e r a l s to c h e m i c a l w e a t h e r i n g is closely related to their silicate crystal structure. Q u a r t z , a f r a m e w o r k silicate, is very stable on the E a r t h ' s surface, w h e r e a s olivine, w i t h an isolated silica tetrahedral crystal structure, is very s u s c e p t i b l e to c h e m i c a l weathering. Refer to F i g u r e 3.9. As y o u study the m a n y different types of m a s s m o v e m e n t illustrated in the figures, assess w h a t factors are m o s t i m p o r t a n t in c a u s i n g slope instability. R e m e m b e r that t h e three most i m p o r t a n t factors e n h a n c i n g the potential for m a s s m o v e m e n t s are the s t e e p n e s s of t h e slope, the n a t u r e of t h e rock material in the slope, and the w a t e r c o n t e n t of t h e s l o p e material. Man can
live without gold,
but not without salt. —FALVIUS MAGNUS CASSIODORUS
A R O M A N POLITICIAN OF THE FIFTH CENTURY A . D .
Exam Prep O r g a n i z a t i o n is key to successful e x a m p r e p a r a t i o n . It b e g i n s with n o t e t a k i n g and t i m e m a n a g e m e n t . Start t h i n k i n g n o w a b o u t h o w to o r g a n i z e y o u r t i m e for y o u r next e x a m . H e r e are a few tips that s h o u l d m a k e y o u r e x a m p r e p m o r e efficient.
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics
181
Tips for Preparing for Exams • U s e t h e c l u e s t h e instructor p r o v i d e s in lecture about w h a t is i m p o r t a n t . E v e n w h e n a d e p a r t m e n t a g r e e s on a c o m m o n c o r e of material (rare), e a c h instructor carves out a c o u r s e that is u n i q u e and has a particular c h a r a c t e r or flavor and distinct areas of e m p h a s i s . T h e instructor is the ultimate g u i d e to the question " W h a t is i m p o r t a n t ? " • Be sure y o u k n o w the format of the e x a m . Multiple c h o i c e ? T r u e - f a l s e ? E s s a y ? T h o u g h t p r o b l e m s ? All of the a b o v e ? • R e v i e w t h e n o t e s m a r k e d TQ (test q u e s t i o n ) . • A s k the instructor if e x a m s are available from the previous semester. Review t h e m to c h e c k the format of q u e s t i o n s , to see what areas of content are stressed, and to see w h a t types of p r o b l e m solving are included. D o n ' t m a k e the m i s t a k e of a s s u m i n g that the s a m e q u e s t i o n s will be asked this semester. • Be sure to attend review sessions if they are offered. A t t e n d i n g a review session will raise y o u r e x a m s c o r e . • If your class has tutors, preceptors, supplemental instruction leaders, or other peer helpers w h o have taken the class, ask for their suggestions about preparing for the exam. • O n c e y o u are clear a b o u t the n a t u r e of the e x a m , begin y o u r review. C o n d u c t the review in an orderly, systematic m a n n e r that e n s u r e s focused review of all the important m a t e r i a l . Be sure to take a look at the E i g h t - D a y S t u d y Plan in A p p e n d i x A. This plan is a great m o d e l for productive review.
Materials in this section are particularly useful d u r i n g y o u r p r e p a r a t i o n for q u i z z e s and examinations. T h e C h a p t e r S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s will simplify y o u r c h a p t e r review. Read the C h a p t e r S u m m a r y to begin y o u r session. It provides a helpful o v e r v i e w that will refresh y o u r m e m o r y . Next, w o r k o n t h e P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and q u e s t i o n s j u s t as y o u w o u l d on an e x a m , to see h o w well y o u have m a s t e r e d this chapter. After y o u a n s w e r the q u e s t i o n s , score t h e m . Finally, and m o s t important, review e a c h q u e s tion that y o u m i s s e d . Identify and correct t h e m i s c o n c e p t i o n ( s ) that resulted in y o u r a n s w e r ing the q u e s t i o n incorrectly.
Figure 16.16. Landslide at Turnagain Heights, Anchorage, Alaska, triggered by the earthquake of 1964. This is a classic example of the role of liquefaction in enhancing the potential for mass wasting.
1 82
PART II C H A P T E R 16
Chapter Summary What is weathering? • Physical weathering breaks rock into smaller pieces, and chemical weathering alters and dissolves the minerals in the rock. T h e principal factors that influence weathering are the composition of the parent rock, topography (stability of the land surface), and climate. T h e r e is a positive feedback b e t w e e n physical and chemical weathering, w h e r e o n e enhances the other if conditions are favorable. A g o o d e x a m p l e of this positive feedback is soil formation. As physical and chemical weathering proceed to alter a stable surface of r o c k material, a soil forms. T h e formation of soil p r o m o t e s weathering by increasing the availability of moisture and producing acidic chemical conditions. Soils also p r o m o t e the growth of plants, w h i c h aid both physical and chemical weathering.
How does chemical weathering work? • H o w c h e m i c a l w e a t h e r i n g w o r k s is well illustrated by three e x a m p l e s . First, t h e c h e m i c a l w e a t h e r i n g of feldspars, w h i c h are the m o s t a b u n d a n t silicate mineral in the E a r t h ' s crust, illustrates h o w w a t e r with t h e h e l p of c a r b o n i c acid can transform feldspars into clay m i n e r a l s and d i s s o l v e silica a n d salts (cations). Refer to F i g u r e 16.4. S e c o n d , the reaction of o x y g e n w i t h t h e iron in f e r r o m a g n e s i u m m i n e r a l s like p y r o x e n e illustrates oxidation. Refer to F i g u r e 16.5. Third, the reaction of calcite and other c a r b o n a t e m i n e r a l s that m a k e up l i m e s t o n e exemplifies the role naturally acidic w a t e r p l a y s in d i s s o l v i n g rock.
How does physical weathering work? • Physical w e a t h e r i n g involves a variety of p r o c e s s e s that b r e a k r o c k into fragments. Physical weathering is promoted by chemical weathering, which weakens grain b o u n d a r i e s within the rock. Physical w e a t h e r i n g also p r o m o t e s c h e m i c a l w e a t h e r i n g by increasing the surface area of the b r o k e n r o c k fragm e n t s . F r o s t w e d g i n g , mineral crystallization, and life p r o c e s s e s play a m a j o r role in b r e a k i n g r o c k apart.
How do soils form as products of weathering? • Soils are a p r o d u c t of c h e m i c a l w e a t h e r i n g of rock that has r e m a i n e d in p l a c e for a period of t i m e . Soil formation is m o s t affected by c l i m a t e . T h e c o m p o s i tion of the p a r e n t rock, c l i m a t e , t o p o g r a p h y , life p r o c e s s e s , and t i m e are also i m p o r t a n t factors in soil formation. Soil formation involves the t r a n s f o r m a t i o n and translocation of materials. Refer to F i g u r e 16.12. Soils, water, a n d t h e air w e b r e a t h e are the three m o s t basic natural r e s o u r c e s .
What is mass wasting? • M a s s m o v e m e n t s (also called m a s s w a s t i n g ) are slides, flows, or falls of large m a s s e s of r o c k material d o w n slopes w h e n the pull of gravity e x c e e d s the strength of the slope m a t e r i a l s . M a s s m o v e m e n t s c a n be t r i g g e r e d by earthq u a k e s , a b s o r p t i o n of large quantities of w a t e r from torrential rainfall, u n d e r cutting by flooding rivers, h u m a n activities, or other g e o l o g i c p r o c e s s e s .
Why do mass movements occur? • T h e t h r e e m o s t i m p o r t a n t factors i n f l u e n c i n g t h e p o t e n t i a l for m a s s m o v e m e n t s are t h e s t e e p n e s s o f t h e s l o p e , t h e n a t u r e o f t h e r o c k m a k i n g u p the
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics
s l o p e , a n d t h e w a t e r c o n t e n t o f t h e s l o p e m a t e r i a l . S e e Table 16.4. A l t h o u g h steep s l o p e s are p r o n e to m a s s m o v e m e n t s , s l o p e s of o n l y a few d e g r e e s can also fail c a t a s t r o p h i c a l l y b e c a u s e of t h e s e o t h e r factors. • Slopes b e c o m e unstable w h e n they b e c o m e steeper than the angle of repose (the m a x i m u m slope angle that unconsolidated material will assume). Slopes in consolidated material m a y also b e c o m e unstable w h e n they are oversteepened or d e n u d e d of vegetation. Erosion by rivers and glaciers and h u m a n activities can oversteepen slopes and thereby increase the potential for m a s s movement. • T h e c o m p o s i t i o n , texture, and g e o l o g i c structure of the slope material is another i m p o r t a n t factor influencing the potential for slope failure. F o r e x a m p l e , rocks with h i g h clay c o n t e n t t e n d to be w e a k and m a y liquefy. Tilted layers of s e d i m e n t a r y or v o l c a n i c r o c k s are m o r e likely to fail along b e d d i n g planes w h e n the b e d d i n g parallels the slope. Failure of foliated m e t a m o r p h i c r o c k s is m o r e likely to o c c u r parallel to the direction of foliation. • Water a b s o r b e d by t h e slope material contributes to instability in t w o w a y s : (1) by l o w e r i n g internal friction (and thus resistance to flow) a n d (2) by lubricating p l a n e s of w e a k n e s s in the slope.
How are mass movements classified? • Material that m a k e s up t h e slope, the w a y it m o v e s , and the rate of m o v e m e n t are u s e d to classified m a s s m o v e m e n t s . Refer to Figure 16.17 and t h e m a n y other figures that illustrate types of m a s s m o v e m e n t .
How can damage from mass movements be minimized? • T h e h a z a r d s a n d d a m a g e associated with m a s s m o v e m e n t s can be m i n i m i z e d by careful g e o l o g i c a l a s s e s s m e n t , e n g i n e e r i n g , and land u s e policies that restrict d e v e l o p m e n t on u n s t a b l e slopes. Of particular i m p o r t a n c e is the avoidance of s t e e p e n i n g or u n d e r c u t t i n g slopes a n d m i n i m i z i n g the a m o u n t of water that c a n infiltrate t h e slope material. In s o m e areas particularly p r o n e to m a s s m o v e m e n t s , d e v e l o p m e n t m a y h a v e t o b e restricted.
Table 16.4 Factors That Influence Mass Movements Nature of slope material
Water content
UNCONSOLIDATED Loose sand or s a n d y silt Unconsolidated m i x t u r e of sand, silt, soil, and r o c k fragments
CONSOLIDATED Rock, j o i n t e d and d e f o r m e d Rock, m a s s i v e
Steepness of slope
Stability of slope
dry wet
angle of repose
high moderate
dry wet
moderate
high low
dry wet
steep
high low
dry or w e t
m o d e r a t e to steep
moderate
dry or w e t dry or wet
moderate steep
high moderate
1 83
184
PART II C H A P T E R 16
Practice Exercises A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at t h e e n d of the S t u d y G u i d e .
Exercise 1: Physical and chemical weathering A n s w e r the q u e s t i o n s in t h e flowchart.
Exercise 2: Inventory of the different kinds of mass wasting T h e authors discuss eight different kinds of m a s s wasting. As an aid to learning the circumstances that favor each one, use your textbook to complete the table. Textbook figures and figure captions will help you. H i n t : You p r o b a b l y h a v e n ' t seen m a n y o f t h e s e features before, s o b e sure t o examine t h e p h o t o s a n d figures of e a c h t y p e of m a s s w a s t i n g in y o u r t e x t b o o k . If y o u are a visual learner, this activity m a y be vital. A l s o , to get a kinesthetic feel for t h e s e m o v e m e n t s , imagine yourself trying to o u t r u n e a c h m o v e m e n t . Indicate in the s p a c e l a b e l e d " S p e e d " w h e t h e r you c o u l d e s c a p e t h e m a s s m o v e m e n t by w a l k i n g , r u n n i n g , or m o v i n g as fast as a s p e e d i n g auto.
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics
Kind of mass wasting Rock
Composition of slope (consolidated vs. unconsolidated and wet vs. dry)
185
Characteristics Speed: Running or a speeding auto Slope angle: Steep slopes Triggering event(s): Earthquakes N o t e s : Occur in mountainous regions where rock is weakened by weathering, structural deformation, weak bedding, or cleavage planes
avalanche
Speed:
Creep
S l o p e angle: Any angle Triggering event(s): None Notes: Earthflows
Speed: S l o p e angle: Any angle Triggering event(s): Intense rainfall N o t e s : Fluidlike movement
Debris
Speed: S l o p e angle: Any angle Triggering event(s): Notes:
flow
Debris
avalanche
Mostly finer rock materials with some coarser rock debris with large amounts of water
Speed: Slope angle: Triggering event(s): Intense rainfall or catastrophic melting of ice and snow by a volcanic eruption. N o t e s : Contains large amounts of water
Water-saturated
Speed: Slope angle: Triggering event(s): Notes:
soil
and
rock
Speed: Walking S l o p e angle: Any angle Triggering event(s): Rainfall Notes:
Slump
Surface layers of soil
Speed: Walking S l o p e angle: Any angle Triggering event(s): N o t e s : Occurs only in cold regions when water in the surface layers of the soil alternately freezes and thaws; water cannot seep into the ground because deeper layers are frozen.
Exercise 3: Water's role in mass wasting Water e n h a n c e s the potential for m a s s w a s t i n g in m a n y w a y s . U s i n g y o u r t e x t b o o k as a g u i d e , briefly d e s c r i b e five different w a y s water e n h a n c e s the potential for m a s s m o v e m e n t s . 1. 2. 3. 4. 5.
186
PART II C H A P T E R 16
A . D i s c u s s three factors that e n h a n c e the p o t e n t i a l for m a s s m o v e m e n t a t the h o m e site in the figure. 1.
2.
2.
B. Given that the h o u s e is already built on this site, briefly discuss t w o p o s s i b l e ways of r e d u c i n g the risk of d a m a g e to the h o u s e d u e to slope failure. 1.
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics 2.
Review Questions A n s w e r s and e x p l a n a t i o n s are p r o v i d e d at the end of t h e Study G u i d e . 1. Of t h e following a g e n t s , w h i c h is N O T involved in the p r o c e s s of c h e m i c a l w e a t h e r i n g ? A. water B. oxygen
C. carbon d i o x i d e D . nitrogen
2 . T h e following p r o d u c t s all result from c h e m i c a l w e a t h e r i n g E X C E P T A. feldspar. B . iron o x i d e s .
C. silica in solution. D . clay m i n e r a l s .
3. W h i c h of t h e following m i n e r a l s does N O T c h e m i c a l l y w e a t h e r into a clay m i n e r a l ? A. muscovite B . K-feldspar
C. pyroxene D . quartz
4. Of t h e following materials, w h i c h o n e w o u l d m a k e the longest lasting t o m b s t o n e ? A. B. C. D.
limestone shale granite s a n d s t o n e c e m e n t e d with c a l c i u m carbonate.
5. An e x a m p l e of c h e m i c a l w e a t h e r i n g is A. B. C. D.
rusty streaks on a r o c k wall. a n g u l a r b l o c k s of r o c k rubble in the m o u n t a i n s . p o t h o l e s in p a v e m e n t . r o c k s w e d g e d apart b y tree roots.
6. A l t h o u g h w a t e r is an i m p o r t a n t agent of c h e m i c a l w e a t h e r i n g in its o w n right, it b e c o m e s m o r e effective if small a m o u n t s of c a r b o n i c acid are present. C a r b o n i c acid is formed when A. c a r b o n from coal b e d s or from graphite deposits is pulverized along a fault or fracture and then a d d e d to water. B. c a r b o n d i o x i d e from the a t m o s p h e r e or from organic d e c o m p o s i t i o n is a d d e d to water. C. sulfur from coal-fired p o w e r plants is a d d e d to water. D. w a t e r c o m e s in contact with the calcite in a l i m e s t o n e layer. 7. T h e potential for c h e m i c a l w e a t h e r i n g can be greatly e n h a n c e d by physical w e a t h e r i n g b e c a u s e p h y s i c a l w e a t h e r i n g increases A. the surface area of t h e r o c k particles. B. t h e availability of c h e m i c a l agents. C. d r a i n a g e and thereby r e d u c e s c o n t a c t with water. D. t h e size of the r o c k particles. 8. L i m e s t o n e and other c a r b o n a t e r o c k s w e a t h e r relatively fast in a A. B. C. D.
dry dry wet wet
/ / / /
climate d u e to
oxidation hydrolysis p h y s i c a l w e a t h e r i n g reaction dissolution p r o m o t e d by c a r b o n i c acid
9. G e o r g i a soil, along w i t h that of other w a r m , h u m i d regions, is d e e p red in color. T h i s color is d u e to A. iron o x i d e s . B . quartz.
C. feldspar. D . clay m i n e r a l s .
187
1 88
PART II C H A P T E R 16
10. C l a y m i n e r a l s , like kaolinite, are a p r o d u c t of and are a r a w material for
w e a t h e r i n g of
minerals
.
A. c h e m i c a l / silicate / pottery B. p h y s i c a l / sulfate / c e m e n t C. c h e m i c a l / sulfide / asphalt D. physical / c a r b o n a t e / fertilizers H i n t : Refer to F i g u r e 16.4. 11. W h i c h of the following climatic regions experiences the m o s t rapid chemical weathering? A. hot, low precipitation B. e x t r e m e l y cold, low precipitation C. hot, high precipitation D. e x t r e m e l y cold, high precipitation 12. Soil p r o d u c t i o n is often d e s c r i b e d as a "positive f e e d b a c k " p r o c e s s . W h y ? A. C a r b o n d i o x i d e in r a i n w a t e r is u s e d up by o r g a n i s m s , so w e a t h e r i n g of underlying r o c k is i m p e d e d . B. R a i n w a t e r b e c o m e s m o r e acidic as it percolates t h r o u g h t h e soil, so weathering of u n d e r l y i n g rock is p r o m o t e d . C. O n c e a l a y e r of soil is f o r m e d , t h e u n d e r l y i n g r o c k is p r o t e c t e d f r o m further weathering. D. P l a n t g r o w t h r e d u c e s the potential for w e a t h e r i n g and therefore of soil development. 13. Of the following minerals, the o n e m o s t rapidly altered by c h e m i c a l weathering would be A. m i c a (sheet silicate), such as biotite. B . a m p h i b o l e ( d o u b l e chain silicate), s u c h a s h o r n b l e n d e . C. p y r o x e n e (single chain silicate), such as augite. D. isolated silica t e t r a h e d r a m i n e r a l , such as olivine. H i n t : Refer to Table 16.2. 14. A c i d rain
the potential for c h e m i c a l w e a t h e r i n g .
A. neutralizes
C.
B. d o e s not affect
D. d e c r e a s e s
increases
H i n t : Refer to p a g e s 5 6 9 - 5 7 0 in C h a p t e r 23 of the textbook. 15. W h a t h a p p e n s c h e m i c a l l y to the q u a r t z sand grains in a c a l c i t e - c e m e n t e d s a n d s t o n e that is undergoing moderate chemical weathering? A . T h e y c o m b i n e with water. B . T h e y dissolve. C. T h e y oxidize. D. Virtually n o t h i n g — t h e y b e c o m e grains of q u a r t z sand. 16. R o c k material that tends to result in the m o s t fertile soils is
weathered.
A. not at all B . very w e a k l y C.
moderately
D . intensely 17. P h y s i c a l and c h e m i c a l w e a t h e r i n g in the w a r m , w e t climates of the E a r t h ' s surface alter e x p o s e d granite to A. q u a r t z and feldspar sand. B. olivine sand. C. iron-rich soil. D. quartz sand and clay.
Figure 16.5. The general course of chemical reactions by which an iron-rich mineral, such as pyroxene, weathers in the presence of oxygen and water.
Weathering, Erosion, and Mass Wasting: Interface Between Climate and Tectonics
18. In t h e d a y s of t h e p h a r a o h s of E g y p t , a c h e r i s h e d status s y m b o l w a s t h e obelisk, a s t o n e c o l u m n d e c o r a t e d w i t h h i e r o g l y p h s ( d e s i g n s c a r v e d into t h e s t o n e — u s u a l l y s a n d s t o n e ) . I n 1 8 7 9 t h e o b e l i s k o f T h o t h m e s III from t h e t e m p l e o f H e l i o p o l i s w a s m o v e d t o C e n t r a l P a r k i n N e w York City. W i t h i n a b o u t sixty y e a r s t h e h i e r o g l y p h s w e r e b a r e l y v i s i b l e o n t h e obelisk, w h i l e its c o u n t e r p a r t still s t a n d i n g i n E g y p t r e m a i n s i n n e a r l y perfect c o n d i t i o n i n t h e d e s e r t sun after a l m o s t 4 , 0 0 0 y e a r s . W h y d i d t h e s t o n e o b e l i s k d e t e r i o r a t e s o q u i c k l y w h e n i t w a s m o v e d t o N e w York C i t y ? H i n t : Refer to Figures 16.1, 16.11, and 23.18.
19. W h i c h hillside h o m e s i t e is the best l o n g - t e r m i n v e s t m e n t ?
A. site A B. siteB C. s i t e C D. siteD 20. W h a t force drives m a s s w a s t i n g ? A. B. C. D.
heat gravity friction convection
2 1 . M a s s w a s t i n g tends t o o c c u r w h e n A. B. C. D.
a slope b e c o m e s steeper d u e to u n d e r c u t t i n g by a river or o c e a n w a v e s . t h e m a s s on a slope d e c r e a s e s by draining water from the ground. friction is i n c r e a s e d by draining water from t h e g r o u n d . friction is d e c r e a s e d by taking water out of the ground.
22. Talus consists largely of A. clay a n d other very fine r o c k particles. B. a m i x t u r e of p o w d e r e d r o c k a n d ice. C. c o a r s e , a n g u l a r r o c k fragments. D. alternate layers of sand, silt, and clay. 23. T h e a n g l e of r e p o s e is the A. B. C. D.
a n g l e at w h i c h r o c k material is m o s t stable. a n g l e at w h i c h lava flows solidify w i t h o u t spreading out. a n g l e of a slope that will no longer support large b o u l d e r s a n d r o c k pillars. m a x i m u m slope at which loose material lies without cascading down.
1 89
190
PART II C H A P T E R 16
2 4 . An i m p o r t a n t factor in m a s s w a s t i n g is the orientation of r o c k layers, foliation, or jointing. For layered s e d i m e n t a r y and volcanic r o c k s , w h i c h c o n d i t i o n is the least stable? A. R o c k layers are at right a n g l e s to t h e slope. B. R o c k layers are parallel to the slope. C. R o c k layers are horizontal to the slope. D . R o c k layers stand vertical.
Test-Taking Tip T i m e p e r m i t t i n g , it is s o m e t i m e s helpful to sketch the alternatives to a test question. For e x a m p l e , if you a r e n ' t sure about the a n s w e r to R e v i e w Q u e s t i o n 2 4 , then y o u could sketch r o c k layers at right a n g l e s to the slope, r o c k layers parallel to the slope, and so o n . S k e t c h i n g can be particularly useful if y o u are a kinesthetic learner, as t h e action o f d r a w i n g m a y j o g o r u n l o c k y o u r m e m o r y w h e n y o u are stuck.
2 5 . W h i c h o f t h e following w o u l d b e m o s t subject t o m a s s m o v e m e n t s ( a s s u m i n g slope and c l i m a t e are the s a m e in e a c h c a s e ) ? A . h i g h - g r a d e gneiss, with highly c o n t o r t e d foliation B . q u a r t z - c e m e n t e d s a n d s t o n e , with layering p e r p e n d i c u l a r t o the slope C. shale, with b e d d i n g parallel to the slope D . m a s s i v e granite b e d r o c k 2 6 . Solifluction usually o c c u r s in A . cold r e g i o n s . B. very cold r e g i o n s like A n t a r c t i c a . C. any area w h e r e there is a lot of s u n s h i n e . D . tropical r e g i o n s . 2 7 . W h i c h of the following options is the m o s t effective w a y to stabilize an active landslide? A. piling additional r o c k and soil material on the l a n d s l i d e n e a r its top B. saturating the l a n d s l i d e itself with w a t e r C. d r a i n i n g t h e w a t e r a w a y from and out of the l a n d s l i d e area D. cutting a w a y the toe (base) of the landslide 2 8 . R o a d s t h r o u g h m o u n t a i n o u s r e g i o n s tend t o b e u n s t a b l e and r e q u i r e m o r e maintenance if they are built on A. b e d r o c k such as granite. B . horizontal lava flows. C. r o c k layers that dip p e r p e n d i c u l a r to the hillslope. D. r o c k layers that dip parallel to the hillslope. 2 9 . Your beautifully l a n d s c a p e d h o u s e , built on an idyllic G e o r g i a hillside site of small, irregularly u n d u l a t i n g knolls and d e p r e s s i o n s , with trees tilted at interesting angles, has d e v e l o p e d a bad case of b r o k e n and shifting foundation. T h e p r o b a b l e c a u s e for the foundation p r o b l e m is A . m e l t i n g permafrost. B. that the h o u s e is built on an active earthflow. C. m u d f l o w from an active n e a r b y v o l c a n o . D. r o o t w e d g i n g from the trees. 30. H o m e o w n e r s in California w h o s e houses survived recent wildfires are not quite out of t h e w o o d s yet. W i t h the a p p r o a c h i n g rainy season their next p r o b l e m will b e A. increased potential for m u d f l o w s and debris flows. B . accelerated soil erosion. C. flash floods. D. all of the a b o v e .
CHAPTER 17
The Hydrologic Cycle and Groundwater
Figure 17.2. The hydrologic cycle.
Before Lecture Before you attend lecture, be sure to spend s o m e time previewing the chapter. For an efficient preview use the C h a p t e r P r e v i e w questions, a framework for understanding the chapter. Previewing works best if you do it just before lecture. With the main points in mind, you will understand the lecture better and take better notes. H o w m u c h t i m e s h o u l d y o u devote t o p r e v i e w ? O b v i o u s l y , m o r e t i m e i s better than less. B u t even a brief (five- or t e n - m i n u t e ) p r e v i e w session j u s t before lecture b e g i n s will p r o d u c e 191
192
PART li C H A P T E R 17
a result y o u will notice. For a refresher on w h y previewing is important, see Part I, Chapter 3, H o w t o B e Successful i n G e o l o g y .
Chapter Preview •
H o w d o e s w a t e r m o v e a r o u n d a n d i n the E a r t h ? Brief answer: T h e h y d r o l o g i c cycle is a m o d e l for the m o v e m e n t of water on Earth. Refer to Figure 17.2.
• H o w does water move below the ground surface? Brief a n s w e r : Porosity and permeability are the principle factors that control the infiltration and How of groundwater. (Skim the textbook section H o w Water Flows.) • W h a t factors g o v e r n o u r use of g r o u n d w a t e r r e s o u r c e s ? Brief a n s w e r : Water, like soil and air, is a basic natural r e s o u r c e . S o m e of the m o s t i m p o r t a n t factors g o v e r n i n g our ability to use g r o u n d w a t e r are the d e p t h to the g r o u n d w a t e r table, springs and artesian s y s t e m s , the b a l a n c e b e t w e e n r e c h a r g e and d i s c h a r g e , D a r c y ' s law, and w a t e r quality. • W h a t g e o l o g i c p r o c e s s e s are affected b y g r o u n d w a t e r ? Brief a n s w e r : G r o u n d w a t e r is the g e o l o g i c a g e n t r e s p o n s i b l e for c a v e s , karst t o p o g r a p h y , and the formations that d e c o r a t e caves. G r o u n d w a t e r is a vital component of g e o t h e r m a l s y s t e m s . S e e F i g u r e s 17.19, 17.20, and 17.23.
Figure 17.1. The distribution of water on Earth.
During Lecture Warm-Up Activity S p e n d 5 to 10 m i n u t e s j u s t before lecture b r o w s i n g Earth I s s u e s 1 7 . 1 , W a t e r Is a P r e c i o u s R e s o u r c e : W h o S h o u l d G e t It? and 17.2, W h e n D o e s G r o u n d w a t e r B e c o m e a N o n r e n e w a b l e R e s o u r c e ? W h i c h issue interests y o u the m o s t ? After b r o w s i n g , ask yourself w h a t y o u w o u l d m o s t like to learn from this c h a p t e r and lecture.
O n e goal for lecture should be to leave the r o o m with good answers to the preview questions. • To avoid getting lost in details, k e e p the big picture in m i n d . C h a p t e r 17 is a survey of w a t e r in and a r o u n d the Earth. It tells the story of the h y d r o l o g i c cycle: h o w w a t e r m o v e s a r o u n d and in the Earth and m a i n t a i n s a b a l a n c e d waterflow budget. • First focus on u n d e r s t a n d i n g F i g u r e 17.2 (the h y d r o l o g i c cycle). • T h e n w o r k on u n d e r s t a n d i n g the factors that govern the flow of g r o u n d w a t e r , illustrated in the figures on p a g e s 4 0 8 ^ 1 1 1 .
j
The Hydrologic Cycle and Groundwater
After Lecture Review Notes The perfect t i m e to r e v i e w y o u r notes is right after lecture. T h e following checklist contains both general r e v i e w tips and specific suggestions for this chapter.
Check Your Notes: Have y o u . . . • included a clear representation of the hydrologic cycle s o m e w h e r e in y o u r notes (see F i g u r e 17.9)? IZ clearly identified factors that govern our use of g r o u n d w a t e r , for e x a m p l e , depth of the g r o u n d w a t e r table, springs and artesian s y s t e m s , the b a l a n c e b e t w e e n recharge and d i s c h a r g e , D a r c e y ' s law? • a d d e d visual material to h e l p you understand the m a t e r i a l ? Suggestion: S k e t c h a simple version of Figure 17.7 (porosity of rock materials; m a k e a copy of Practice Exercise 1 after you h a v e c o m p l e t e d the chart and insert it into y o u r notes. (It will be great aid for e x a m review, it s u m m a r i z e s all you will need to k n o w about porosity.) • created a brief big picture o v e r v i e w of this lecture (using a sketch or written outline)? Figure 17.2 p r o v i d e s a good visual s u m m a r y of the chapter. Activate kinesthetic learning by a d d i n g a simplified version to y o u r notes.
Intensive Study Session Set priorities for studying this chapter. Give the highest priority to activities that involve answering questions. We r e c o m m e n d the following strategy for learning this chapter. • Text. R e v i e w the important figures in Chapter 17. T h e y are Figures 17.2, 17.7, 17.9, 17.10, 1 7 . 1 1 , 17.12, 17.14, 17.15, and 17.20. Insert simple sketches of these figures into y o u r lecture notes. S o m e t i m e before the next e x a m , c o m p l e t e the exercises at the end of the text chapter. T h e exercises require short a n s w e r s and w o n ' t take long if y o u k n o w the m a t e r i a l . N o t e that helpful a n i m a t i o n s are p r o v i d e d on the W e b site for E x e r c i s e s 3, 5, and 6. • P r a c t i c e E x e r c i s e s a n d R e v i e w Q u e s t i o n s . C o m p l e t e Practice E x e r c i s e s 1 and 2. T h e s e exercises will h e l p y o u r e m e m b e r i m p o r t a n t ideas in the chapter. T h e n w o r k on a n s w e r i n g each of t h e R e v i e w Q u e s t i o n s to c h e c k y o u r understanding of t h e lecture. C h e c k y o u r a n s w e r s as y o u go, but do try to a n s w e r the q u e s t i o n before you l o o k at t h e answer. P a y attention to the test-taking tips. T h e y will h e l p y o u d o better o n q u i z z e s and e x a m s . • W e b Site S t u d y R e s o u r c e s http://www.whfreeman.com/understandingearth5e C o m p l e t e the O n l i n e R e v i e w Q u e s t i o n s . P a y particular attention t o t h e explanations for t h e a n s w e r s . Be sure to c h e c k out t h e a n i m a t i o n s of an aquifer and a confined aquifer and the d y n a m i c b a l a n c e b e t w e e n r e c h a r g e and discharge. T h e G e o l o g y in P r a c t i c e exercises involve y o u in an o n g o i n g c a s e study involving g r o u n d c o n t a m i n a t i o n .
Exam Prep Materials in this section are m o s t useful d u r i n g y o u r preparation for q u i z z e s and e x a m s . T h e Chapter S u m m a r y and the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s should simplify y o u r chapter review. R e a d the C h a p t e r S u m m a r y to b e g i n y o u r session. It p r o v i d e s a helpful overview that s h o u l d refresh y o u r m e m o r y . Next, w o r k o n the P r a c t i c e E x e r c i s e s and R e v i e w Q u e s t i o n s . C o m p l e t e the exercises and questions j u s t as y o u w o u l d an e x a m , to see h o w well you have m a s t e r e d this chapter.
1 93
194
PART II C H A P T E R 17
After y o u a n s w e r the questions, score t h e m . Finally, and m o s t important, review e a c h question that y o u m i s s e d . Identify and correct the m i s c o n c e p t i o n ( s ) that resulted in y o u r answering the q u e s t i o n incorrectly.
Chapter Summary How does water move around and in the Earth? • T h e h y d r o l o g i c c y c l e is a flowchart or m o d e l for the distribution and m o v e m e n t s of w a t e r on and b e l o w the surface of the Earth. T h e major reservoirs of the h y d r o l o g i c cycle are o c e a n s , glaciers, g r o u n d w a t e r , lakes and rivers, the a t m o s p h e r e , and the b i o s p h e r e , in d e c r e a s i n g v o l u m e s . W a t e r m o v e s in a n d out of t h e s e reservoirs by various p a t h w a y s and at varying r a t e s . O v e r the short t e r m a b a l a n c e is m a i n t a i n e d a m o n g the major reservoirs at and n e a r the E a r t h ' s surface. However, c l i m a t e c h a n g e and l o n g e r - t e r m tectonic p r o c e s s e s such as m o u n t a i n building and h u m a n activity c a n
