Student Study Guide for ''Understanding Earth'' (Fifth Edition)

Student Study Guide for Grotzinger, Jordan, Press, and Siever' UNDERSTANDING EARTH Fifth Edition Peter L. Kresan Unive...

Author: Peter L. Kresan | Reed Mencke | Ph.D.

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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


/ 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


/ 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.


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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

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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


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.

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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.


•

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


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 !


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

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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.

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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

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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


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.


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


• 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


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 .


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


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.


• 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.

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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

. .

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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:


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.

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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


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


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 .


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 .


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


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 ) .


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

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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


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.


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 .

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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


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 .


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

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CHAPTER

5

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.


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


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


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.


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


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


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.


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.

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CHAPTER

6

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


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 .


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


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

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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



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 .


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.

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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 .

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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


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.

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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


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.

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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


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


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.


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


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.


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.


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.

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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

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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


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 .

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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


• 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.

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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).

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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.


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


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.


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 .

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1 14


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


• 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.


• 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


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


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


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.

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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.


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

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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.


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


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.


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


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


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.


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


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


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

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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.

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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

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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.

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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


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


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


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


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



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.

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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.

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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.

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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.

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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


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.

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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 .


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.

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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

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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.


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.

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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.

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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

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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

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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 .

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• 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).

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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.


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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.

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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


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

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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.


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.

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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 .

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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.


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.

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190


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


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

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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 alter the rate of w a t e r m o v e m e n t b e t w e e n reservoirs and c a n affect the size of the r e s e r v o i r s .

How does water move below the ground surface? • T h e infiltration of w a t e r into the g r o u n d and g r o u n d w a t e r flow are largely c o n trolled by the porosity and p e r m e a b i l i t y of t h e r o c k materials and t o p o g r a p h y . A g r o u n d w a t e r aquifer is in 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 (the a m o u n t of w a t e r that infiltrates into the aquifer) and d i s c h a r g e , w h i c h c a n o c c u r from springs or w e l l s .

What factors govern our use of groundwater resources • D a r c y ' s law describes the g r o u n d w a t e r flow rate in relation to the s l o p e of the water table and the p e r m e a b i l i t y of the aquifer. • H u m a n d e m a n d for g r o u n d w a t e r has i n c r e a s e d to a level w h e r e p u m p i n g disc h a r g e s from m a n y aquifers e x c e e d s the natural rates of r e c h a r g e . As a result, aquifers are b e i n g depleted and g r o u n d w a t e r tables are l o w e r i n g to a p o i n t w h e r e d e p e n d a b l y h i g h - q u a l i t y g r o u n d w a t e r is b e c o m i n g m o r e and m o r e of a c h a l l e n g e to supply. • W a t e r quality m a y be c o m p r o m i s e d by b o t h natural and h u m a n s o u r c e s of c o n t a m i n a t i o n . Various factors like r e c h a r g e rate and aquifer size influence the a m o u n t of effort and effectiveness of a t t e m p t s to clean up c o n t a m i n a t i o n .

What geologic processes and features are associated with groundwater? • C a v e s , s i n k h o l e s , and associated karst t o p o g r a p h y are a result of t h e dissolution of c a r b o n a t e r o c k s (limestone) by g r o u n d w a t e r . K a r s t t o p o g r a p h y is well d e v e l o p e d in r e g i o n s of high rainfall, a b u n d a n t vegetation, u n d e r l y i n g fractured l i m e s t o n e , and an a p p r e c i a b l e h y d r o l o g i c g r a d i e n t to e n h a n c e g r o u n d w a t e r flow rates. E n v i r o n m e n t a l p r o b l e m s a s s o c i a t e d w i t h karst r e g i o n s include surface s u b s i d e n c e from collapse of u n d e r g r o u n d s p a c e and catastrophic cave-ins and s i n k h o l e formation. • All r o c k s b e l o w the g r o u n d w a t e r table are saturated with water. W i t h i n c r e a s ing depth, porosity and p e r m e a b i l i t y typically d e c r e a s e as confining p r e s s u r e i n c r e a s e s . W a t e r t e m p e r a t u r e increases progressively with i n c r e a s i n g d e p t h and, as a result, the w a t e r dissolves m o r e solids. H o t springs and g e y s e r s are surface e x p r e s s i o n s of the circulation of h y d r o t h e r m a l w a t e r s over a m a g m a b o d y or a l o n g a d e e p - s e a t e d fault.


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 the S t u d y G u i d e .

Exercise 1: Evaluating rock materials as potential aquifers You have recently purchased a rustic country cabin and need to drill a new well for a dependable water s u p p l y for the c a b i n . T h e g e o l o g y a r o u n d y o u r cabin is c o m p l e x b e c a u s e of ancient mountain-building events. B e c a u s e the rocks are folded and faulted, it is difficult to predict w h a t rock might be encountered as the water well is drilled. W h i c h of the rock materials in the table has the potential of yielding groundwater to your well? Fill in the blank parts of the table. H i n t : Keep in m i n d that permeability generally, but not always, increases as porosity increases. P e r m e ability also d e p e n d s on the sizes of the pores, h o w well they are connected, and h o w tortuous a path the water m u s t travel to pass through the material. Refer to Figure 17.7 and Table 17.2.

Rock material

Porosity (high, medium, low)

Potential as an aquifer (good, moderate, poor)

Loose, w e l l - s o r t e d , c o a r s e sand Silt and clay

Low

Granite and g n e i s s

Poor

Highly fractured granite Sandstone

Medium

Shale Highly j o i n t e d l i m e s t o n e

Moderate

to

good

Figure 17.7. Pores in rocks are normally filled partly or entirely with water.

1 95

1 96


Exercise 2: Evaluating groundwater wells

E v a l u a t e t h e potential characteristics of e a c h w e l l as 'high, Well A — p o t e n t i a l for

low,

or

none.

1. pollution 2. artesian flow 3. discharge

Well B — p o t e n t i a l for

4. pollution 5. artesian flow 6. d i s c h a r g e 7. l o n g - t e r m supply

Well C — p o t e n t i a l for

8. pollution 9. artesian flow 10. discharge

Americans

now

47 gallons

of soda pop

drink

more to

only

soda

pop

than

water from

37 gallons of water per person

the

kitchen

tap—

each year.

—WORLD

WATCH,

1990

Review Questions 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 . 1. In the h y d r o l o g i c cycle, h o w d o e s the evaporation rate from the l a n d surface c o m p a r e with t h e evaporation rate from the o c e a n s ? A . T h e evaporation rate i s m u c h greater from the l a n d t h a n f r o m t h e o c e a n s . B . T h e evaporation rate i s m u c h greater from the o c e a n s t h a n f r o m t h e land. C. E v a p o r a t i o n rates from the land and o c e a n s are e q u a l . D. T h e r e is no r e a s o n a b l e c o m p a r i s o n b e c a u s e it is t o o w e t over the o c e a n s for evaporation to occur. H i n t : Refer t o F i g u r e 1 7 . 1 . 2. T h e o c e a n s c o n t a i n by far the m o s t a m o u n t of w a t e r on E a r t h . W h a t is the s e c o n d largest reservoir for w a t e r on E a r t h ? A. lakes

C. rivers

B. groundwater

D . p o l a r ice and glaciers


3. T h e o c e a n s c o n t a i n about h o w m u c h of the water in the h y d r o s p h e r e ? A. 9 6 % C. 5 0 % B. 80%

D. 3 5 %

4. W h a t h a p p e n s to t h e porosity as the grain size gets smaller? A. It increases. C. It d e c r e a s e s . B. It remains unchanged. D . n o n e o f the above 5. T h e water table is A. the top of t h e u n s a t u r a t e d z o n e . B. t h e top of t h e saturated z o n e . C. generally p r e s e n t only in moist climates. D. t h e c o n t a c t b e t w e e n an aquifer and an u n d e r l y i n g , i m p e r m e a b l e layer of rock. 6. T h e ability of a solid, such as rock, to allow fluids to p a s s t h r o u g h it is A. d i s c h a r g e . B . capillary fringe

C. p e r m e a b i l i t y D . porosity.

7. An iciclelike d e p o s i t h a n g i n g from the ceiling of a cave is a A. stalactite. C. stalagmite. B . karst formation. D . quartzite. 8. Of t h e following r o c k t y p e s , w h i c h is the m o s t susceptible to g r o u n d w a t e r solution, therefore m a k i n g it t h e formation m o s t likely to h a v e caves? A. granite B. sandstone

C. l i m e s t o n e D . shale

9. A r o c k or soil layer that is water-bearing is a(n) A. p e r c h e d w a t e r table. C. stratum. B. z o n e of aeration. D. aquifer. 10. T h e potential for g e o t h e r m a l energy is h i g h e s t in a region that has n u m e r o u s A. surface lakes. C. h o t springs. B. caves. D . sinkholes. 11. An A. B. C. D.

a q u i c l u d e is a confined aquifer. a l w a y s located at the t o p of t h e water table. a r o c k layer that p r o v i d e s a g o o d flow of water into a well. an i m p e r m e a b l e r o c k layer that d o e s not allow water to flow t h r o u g h it.

12. W h i c h of the following w o u l d m a k e the best aquifer?

A. B. C. D.

rock A rockB rockC rock D

Porosity

Permeability

5% 10% 30% 35%

high medium low medium

13. If all other c o n d i t i o n s are equal, g r o u n d w a t e r m o v e s faster A. w h e r e sand grains are very well c e m e n t e d . B. t h r o u g h l o o s e s a n d t h a n t h r o u g h clay. C. w h e r e p e r m e a b i l i t y of the aquifer is lower. D. t h r o u g h clay t h a n t h r o u g h sand. H i n t : Refer to F i g u r e 17.7. 14. A p e r c h e d w a t e r table will m o s t likely d e v e l o p on top of A. shale. C. gravel. B . highly fractured granite. D. sandstone. H i n t : Refer t o F i g u r e 1 7 . 1 1 . 15. Rivers a n d streams that flow all year long, even during long p e r i o d s w i t h o u t rain, are p r o b a b l y fed b y A. sinkholes. B . springs. H i n t : Refer t o F i g u r e 1 7 . 1 1 .

C. w e l l s . D . karst conditions.

197

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Test-Taking Tip W h e n you are u n s u r e about an answer, you c a n s o m e t i m e s m a k e a correct g u e s s just by looking at the r e s p o n s e s . L o n g r e s p o n s e s are m o r e likely to be correct than short o n e s . T h i s o c c u r s b e c a u s e p e o p l e w h o write test i t e m s have t o b e sure that t h e correct alternative is clear and accurate, and that m a y s o m e t i m e s take m o r e w o r d s . Obviously, this is a strategy to use only as a last resort, w h e n y o u h a v e no c l u e w h a t the correct a n s w e r is. T h e safer strategy is to learn the material thoroughly.

16. An artesian w e l l will flow if the A. t o p of t h e well is l o w e r t h a n t h e w a t e r table in the r e c h a r g e area. B. t o p of t h e w e l l is h i g h e r than t h e w a t e r table in t h e r e c h a r g e area. C. b o t t o m of t h e well is lower than the land surface in the r e c h a r g e area. D. b o t t o m of the w e l l is l o w e r than the w a t e r table in t h e r e c h a r g e area. H i n t : Refer t o F i g u r e 17.10. 17. W h i c h w e l l will exhibit artesian flow?

Test-Taking Tip: Leveraging correct answers Be alert for i t e m s that test the s a m e i d e a or fact. Often an i t e m y o u are s u r e a b o u t can c l u e a c o r r e c t r e s p o n s e for a n o t h e r item y o u are n o t so s u r e a b o u t . E x a m p l e : I t e m s 1 6 a n d 1 7 b o t h test the s a m e c o n c e p t , n a m e l y , w h a t m a k e s a n a r t e s i a n well flow. L e t ' s say the p i c t u r e h e l p e d y o u figure o u t i t e m 17 but y o u had left i t e m 16 b l a n k or w e r e n ' t s u r e of y o u r a n s w e r . N o w , after w o r k i n g i t e m 17, y o u s h o u l d b e able t o g o b a c k a n d a n s w e r i t e m 1 6 c o r r e c t l y . I n e s s e n c e , y o u arc l e a r n i n g as y o u t a k e the test. L e a r n i n g by a n s w e r i n g q u e s t i o n s is o n e of the best ways to master material. Sound familiar?

18. W h i c h materials w o u l d m a k e the best aquifer? A. clay and silt

C.

B . gravel and sand

D . highly c e m e n t e d s a n d s t o n e

unfractured granite

19. At a shallow depth, a w e l l will m o s t likely e n c o u n t e r a g o o d w a t e r s u p p l y in which of the following l o c a t i o n s ? A. in granite on a r i d g e top B. in s a n d s t o n e on a r i d g e top C. in shale in a valley b o t t o m D. in s a n d s t o n e in a valley b o t t o m H i n t : M a k e a sketch illustrating e a c h situation.


Test-Taking Tip: Eliminating incorrect answers Unsure which is the right a n s w e r ? Try going d o w n the alternatives one at a time. Cross out each a n s w e r that y o u think is incorrect. Hopefully, only one alternative will remain. It will probably be the correct answer. E x a m p l e : In 19, did you eliminate the two items that mentioned "ridge top"? If so, you narrowed your choice to shale and sandstone. If you c o m p l e t e d the Practice Exercises, the choice between shale and sandstone w a s easy. If not a n d this w e r e a real test, you w o u l d have to guess. But b e c a u s e you eliminated t w o of the incorrect items, y o u r o d d s of guessing correctly would have doubled!

1 99

200


A. the spring B.

wellB

C.

wellC

D . the s t r e a m 2 4 . A s s u m e that y o u are dealing with the s a m e aquifer at four different localities and that the cross-sectional area t h r o u g h w h i c h the w a t e r flows (A) and the h y d r a u l i c conductivity (K) are the s a m e for e a c h site. Given the following data on vertical d r o p (h) and flow dist a n c e (1), w h i c h well will p r o d u c e w a t e r at the highest rate ( Q ) ? Refer to F i g u r e 17.15. A. h = 20 m e t e r s a n d 1 = 5 0 0 m e t e r s B. h = 30 m e t e r s a n d 1 = 1 k i l o m e t e r C. h = 3 0 0 m e t e r s a n d 1 = 6 k i l o m e t e r s D. h = 6 0 0 m e t e r s a n d 1 = 100 k i l o m e t e r s

Figure 17.15. Darcy's law describes the rate of groundwater flow down a slope between two points, Elevation A and Elevation B. The volume of water flowing at a certain time (Q) is proportional to the difference in height (h) between the high and low points of the slope (here shown as the drop in elevation of the water table between the two points), divided by the flow distance between them (the hydraulic gradient, I) and by K a constant proportional to permeability of the aquifer. The symbol A represents the cross-sectional area through which the water flows.

CHAPTER 18

Stream Transport from Mountains to Oceans / do not know much about gods, but I think that the river is a strong brown god—sullen, untamed, and intractable. —T.

S. ELIOT,

THE DRY SALVAGES

Figure 18.1. A river flows in a channel that moves over a broad, flat floodplain in a wide valley eroded from uplands. Floodplains may be narrow or absent in steep valleys.

202


Before Lecture As y o u p r e v i e w this chapter, k e e p in m i n d the o v e r a r c h i n g q u e s t i o n this c h a p t e r addresses: H o w d o streams w o r k ?

Chapter Preview • H o w do stream valleys and their channels a n d floodplains evolve? Brief a n s w e r : As a s t r e a m flows, it c a r v e s a valley. If t h e valley is w i d e e n o u g h , the c h a n n e l will be s u r r o u n d e d by a floodplain into w h i c h the c h a n n e l will overflow d u r i n g t i m e s of high water, d r o p p i n g its s e d i m e n t load as the w a t e r s p r e a d s out and slows. C h a n n e l s m a y be straight, m e a n d e r i n g ( w i n d i n g ) , or b r a i d e d (divided, interlacing c h a n n e l s ) . • H o w d o d r a i n a g e n e t w o r k s w o r k a s collection s y s t e m s , a n d h o w d o d e l t a s work a s d i s t r i b u t i o n s y s t e m s for w a t e r a n d s e d i m e n t ? Brief answer: Rivers and tributaries constitute an u p s t r e a m b r a n c h i n g d r a i n a g e network that collects the runoff for a particular d r a i n a g e basin. T w o notable e x a m p l e s are the Mississippi River basin and the C o l o r a d o River basin. T h e n a t u r e of a d r a i n a g e pattern d e p e n d s on the basin topography, r o c k type, and g e o l o g i c structure. S e e F i g u r e 18.7. Deltas form at the m o u t h s of rivers as the river b r a n c h e s into n u m e r o u s distributaries and d r o p s (distributes) its load of sediment. S e e Figures 18.7, 18.17, and 18.18. • H o w d o e s f l o w i n g w a t e r in s t r e a m s e r o d e solid rock a n d t r a n s p o r t a n d deposit sediments? Brief answer: F l o w i n g w a t e r e r o d e s rock by physical and 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 . Turbulent flow is r e s p o n s i b l e for transporting s e d i m e n t s . W h e n a stream flow s l o w s , the stream loses its c o m p e t e n c e to carry s e d i m e n t and d e p o s i t s it. Refer to F i g u r e 18.15. • H o w d o e s a s t r e a m ' s l o n g i t u d i n a l profile r e p r e s e n t t h e e q u i l i b r i u m b e t w e e n erosion and sedimentation? Brief a n s w e r : A s t r e a m ' s gradient (slope or longitudinal profile), base level, velocity ( s p e e d ) , d i s c h a r g e ( a m o u n t of water), and the availability of s e d i m e n t (load) d e t e r m i n e w h e t h e r a stream e r o d e s , d e p o s i t s , or r e a c h e s a b a l a n c e b e t w e e n erosion and d e p o s i t i o n — a n e q u i l i b r i u m , called a graded stream. C h a n g e s in these s t r e a m characteristics can be used to predict the b e h a v i o r of a stream w h e n it is d a m m e d or there is s o m e other c h a n g e in the d r a i n a g e .

Vital Information from Other Chapters C h a p t e r s 17 and 18 are a p a c k a g e . Be sure to review material in the first half of C h a p t e r 17. Pay particular attention to the r o l e that s t r e a m s play in the h y d r o l o g i c c y c l e ( F i g u r e 17.2) and h o w s t r e a m s interact with t h e g r o u n d w a t e r table (Figures 17.9, 1 7 . 1 1 , and 17.20).

During Lecture • K e e p the big picture in m i n d . D u r i n g this lecture y o u w a n t to learn h o w s t r e a m s work. You will learn h o w w a t e r flows in c u r r e n t s , h o w s t r e a m s b r e a k u p and e r o d e solid rock, a n d h o w s t r e a m s , c h a n n e l s , and entire d r a i n a g e s y s t e m s evolve over t i m e . • In this lecture it will be particularly helpful to 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 d u r i n g the lecture. N o t i c e w h i c h p r e v i e w q u e s t i o n is b e i n g a d d r e s s e d in e a c h s e g m e n t of the lecture. • It m a y be helpful to b o o k m a r k s o m e of the key figures so that y o u c a n refer to t h e m and a n n o t a t e y o u r t e x t b o o k a s they are d i s c u s s e d d u r i n g lecture. T h e

Stream Transport from Mountains to Oceans following are a l m o s t certain to be referred to: F i g u r e 18.1 ( c o m p o n e n t s of a s t r e a m s y s t e m ) , F i g u r e 18.4 (formation of natural levees by river floods), F i g u r e 18.7 (typical d r a i n a g e n e t w o r k s ) , F i g u r e 18.15 (velocity versus particle size), and F i g u r e 18.19 (stream n e t w o r k s ) . A n y figure from the text that is disc u s s e d in detail d u r i n g lecture should be b o o k m a r k e d and p r o m p t l y sketched in y o u r lecture n o t e s . Be sure to leave s p a c e in y o u r notes to add sketches of t e x t b o o k figures. If y o u are n o t g o o d at sketching or r u n out of time, cut and paste a c o p y of t h e figure into y o u r n o t e s .

Note-Taking Tip: Leave plenty of room for visual material There is a lot of visual material in this lecture. It is important to take notes in a format that allows plenty of r o o m to go back after lecture and m a k e sketches that will help you understand what you w r o t e in y o u r notes. If you take notes in a looseleaf n o t e b o o k , leaving r o o m is easy. Take notes on the right page, then you can go b a c k after lecture and add simplified sketches on t h e b l a n k left p a g e . U s e the text figures as m o d e l s . If you take notes in a spiral n o t e b o o k , you can divide the p a g e into t w o c o l u m n s by d r a w i n g a vertical line. Take notes in the right c o l u m n and use the left c o l u m n for sketches.

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 contains both general review tips and specific suggestions for this chapter.

Check Your Notes: Have y o u . . . • explained h o w s t r e a m valleys and their flood plains evolve'? explained h o w d r a i n a g e s y s t e m s w o r k ? • explained how sediment loads are m o v e d in w a t e r ? • explained the relationship b e t w e e n stream slope, velocity, discharge, and s e d i m e n t transport? I 1 filled in from m e m o r y a n y t h i n g you d i d n ' t have time to write d o w n d u r i n g the lecture? • added visual material to y o u r notes? Suggestion: Be sure to include simple sketches of t h e types of d r a i n a g e n e t w o r k s with an annotation about the factor(s) that influence the formation of each type.

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. R e v i e w the key figures in Chapter 18. You have to understand these figures to a n s w e r the review questions. Focus on Figure 18.1 (components of a stream system), Figure 18.3 (channel patterns), Figure 18.4 (the formation of natural levees by river floods), Figure 18.7 (typical drainage networks), Figure 18.8 (antecedent stream), Figure 18.9 (superposed stream), Figure 18.22 (the longitudinal profile), and Figures 18.23 and 18.24 (base level changes). This is a long list of key figures, so feel free to go right to the questions and refer to the figures as you a n s w e r them. An excellent review strategy is to w o r k through all ten exercises at the end of the chapter s o m e t i m e before y o u r next exam. T h e s e are shorta n s w e r exercises and w o n ' t take long if you k n o w the material.

203

204


• P r a c t i c e Exercises and Review Questions. Complete Practice Exercise 1. T h i s is an easy w a y to m a s t e r o n e of the m o s t i m p o r t a n t ideas in t h e c h a p t e r (stream velocity). T h e n d o the other Practice E x e r c i s e s and t h e R e v i e w Q u e s t i o n s . C h e c k y o u r a n s w e r s a s y o u go, but d o try t o a n s w e r e a c h 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://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 . Pay 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 F l a s h c a r d s will h e l p y o u learn the t e r m s in this chapter. Do the O n l i n e R e v i e w E x e r c i s e s Identify the Parts of a Marine Delta,

Flooding,

and

Create a Flood.

A l b e r t Einstein o n c e a d d r e s s e d the question, W h y d o r i v e r s m e a n d e r ? F i g u r e 18.3 illustrates steps in the formation of river m e a n d e r s . W h y m e a n d e r s form is still d e b a t e d . T h e G e o l o g y in P r a c t i c e exercises e x p l o r e ideas for the formation of river m e a n d e r s and their characteristics.

Exam Prep Tip Study C h a p t e r s 17 and 18 as o n e integrated unit. T h i n k of w a y s g r o u n d w a t e r and stream flow are linked.

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 and e x a m s . The C h a p t e r S u m m a r y and 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 s h o u l d simplify your 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 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 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 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 , 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 e a c h 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 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.

Mark Twain,

noting how muddy the Missouri was,

proclaimed it

"too thick to navigate,

to

but too thin

cultivate."

Chapter Summary How do stream valleys and their channels and floodplains evolve? • T h e physical features (drainage pattern, stream channel, floodplain, meand e r b e n d s i n t h e c h a n n e l , a l l u v i a l fans a n d d e l t a s ) o f a s t r e a m s y s t e m e v o l v e over time. • W h e n s e d i m e n t - l a d e n w a t e r o v e r f l o w s t h e b a n k s of a c h a n n e l d u r i n g flood stage, the water drops sediment, which builds up natural levees and floodplain deposits.

Stream Transport from Mountains to Oceans

How do drainage networks work as collection systems and deltas as distribution systems for water and sediment? • D r a i n a g e n e t w o r k s exhibit different patterns d e p e n d i n g on t o p o g r a p h y , r o c k type, and g e o l o g i c structure in the d r a i n a g e area. N e a r its m o u t h , a river tends to b r a n c h d o w n s t r e a m into distributary c h a n n e l s forming a delta. • Deltas are major sites of sediment deposition. W h e r e waves, tides, and shoreline currents are strong, deltas m a y be modified or even absent. Tectonics controls delta formation by uplift in the drainage basin and subsidence in the delta region.

How does flowing water in streams erode solid rock and transport and deposit sediments? • Streams

erode,

transport,

and

deposit

sediments.

Turbulent stream flow allows water to e r o d e and transport s e d i m e n t by suspension, saltation, rolling, and sliding. T h e tendency for particles to be carried in suspension is c o u n t e r e d by gravity, pulling t h e m to the b o t t o m and m e a s u r e d by the settling velocity. Refer to F i g u r e 18.14. • D e p o s i t i o n of s e d i m e n t s occurs w h e n the velocity of the s t r e a m d e c r e a s e s . Refer to F i g u r e 18.15.

How are the stream's gradient (slope), velocity (speed), discharge (amount of water), and sediment transport linked? • W h e t h e r a s t r e a m is d o m i n a n t l y e r o d i n g or d e p o s i t i n g its l o a d ( s e d i m e n t s ) is d e t e r m i n e d by s t r e a m velocity. S t r e a m v e l o c i t y i n turn d e p e n d s o n t h e s t r e a m ' s g r a d i ent ( s l o p e ) , d i s c h a r g e ( a m o u n t o f w a t e r i n t h e s t r e a m ) , load (sediment in transport), and channel characteristics. A s t r e a m ' s d r a i n a g e p a t t e r n s , t h e s t r e a m c h a n n e l , and t h e f l o o d p l a i n e v o l v e in r e s p o n s e to c h a n g e s in s t r e a m velocity, g r a d i e n t , s e d i m e n t load, and d i s c h a r g e and t h e c h a r a c t e r i s t i c s o f t h e l a n d s c a p e o v e r w h i c h the s t r e a m f l o w s . A l l u v i a l fans f o r m at m o u n t a i n fronts in r e s p o n s e t o a n a b r u p t w i d e n i n g o f t h e s t r e a m valley a n d a c h a n g e in s l o p e . • T h e longitudinal profile represents the stream gradient. It is a plot of the elevation of the stream channel bottom at different distances along the stream's course. T h e longitudinal profile is controlled by local (the river into which the stream flows or a lake) and regional (the ocean) base levels. Refer to Figure 18.23. Streams cannot cut below base level, because b a s e level is the b o t t o m of the hill.

205

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. W h e r e do rivers obtain the p o w e r to e r o d e and transport s e d i m e n t s ? A. heat B. gravity

C. electricity D. friction

2. T h e v o l u m e of water that flows past a given point along a s t r e a m c h a n n e l in a given interval of t i m e is the A. velocity. C. capacity.

B . discharge. D . gradient.

3. If the gradient (slope) of a s t r e a m is increased, w h a t h a p p e n s to the velocity of the water? A. B. C. D.

increases decreases r e m a i n s unaffected m a y increase o r d e c r e a s e

4. S t r e a m erosion and deposition are primarily controlled by a river's A. width. B . velocity. H i n t : Refer t o F i g u r e 18.15.

C. depth. D. channel shape.

5. You are c a n o e i n g a river in r e m o t e A l a s k a and y o u r G P S s h o w s an elevation of 2500 feet. After p a d d l i n g for five d a y s , you calculate that you h a v e traveled 2 0 0 miles. Your G P S n o w s h o w s an elevation of 2 3 0 0 feet. W h a t is t h e s t r e a m ' s gradient in feet/mile for the stretch y o u j u s t c a n o e d ? A. 1 foot/mile B. 2 feet/mile

C. 5 feet/mile D. 10 feet/mile

6. A trellis d r a i n a g e pattern forms on A. horizontal lava flows. B. folded and tilted s e d i m e n t a r y r o c k layers of varying resistance. C. a d o m e . D . horizontal s e d i m e n t a r y r o c k s . H i n t : Refer to F i g u r e 18.7 (3). 7. S t r e a m c o m p e t e n c e is m e a s u r e d by the A. B. C. D.

largest particle size the s t r e a m can transport. a m o u n t of material in t h e dissolved load. m a x i m u m w i d t h of the c h a n n e l along the floodplain. total a m o u n t of s u s p e n d e d and b e d load.

8. Particles t e n d to settle out of the s u s p e n d e d load in w h i c h o r d e r ? A. clay, sand, p e b b l e s B. p e b b l e s , sand, silt

C. sand, p e b b l e s , c o b b l e s D. clay, p e b b l e s , sand

9. Active erosion in a m e a n d e r b e n d takes p l a c e A. in t h e center of the stream. B. along the outer b a n k of a b e n d . C. along the inside b a n k of a b e n d . D. near a s t r e a m ' s h e a d w a t e r s . 10. E n t r e n c h e d m e a n d e r s like those s h o w n for t h e S a n J u a n River in F i g u r e 18.2 are evid e n c e of a A. B. C. D.

d e c r e a s e in stream gradient. d e c r e a s e in discharge. c h a n g e such that the river has r e n e w e d ability to e r o d e . d e c r e a s e in stream velocity.


1 1 . If a d a m is p l a c e d across a stream that has b e e n carrying a large v o l u m e of sediment, the s t r e a m w o u l d probably A . d e p o s i t d o w n s t r e a m and e r o d e u p s t r e a m from the d a m . B . e r o d e d o w n s t r e a m and not c h a n g e u p s t r e a m from the d a m . C . e r o d e d o w n s t r e a m and gradually deposit u p s t r e a m from the d a m . D . n o t c h a n g e d o w n s t r e a m but deposit u p s t r e a m from the d a m . H i n t : Refer to F i g u r e 18.24. 12. T h e regular d u m p i n g of c r u s h e d r o c k and coal w a s t e into a s t r e a m leads to A. B. C. D.

erosion u p s t r e a m a n d deposition d o w n s t r e a m from the d u m p . erosion both u p s t r e a m and d o w n s t r e a m from the d u m p . deposition both u p s t r e a m and d o w n s t r e a m from the d u m p . d e p o s i t i o n u p s t r e a m and erosion d o w n s t r e a m from the d u m p .

13. If flood control engineers straighten out a meandering stream channel, the stream probably A. flows m o r e slowly in the straightened stretch. B. deposits in t h e straightened stretch. C. d o w n c u t s in the straightened stretch. D. d o w n c u t s and flows m o r e rapidly in t h e straightened stretch. 14. If a flood is classified as a fifty-year flood, A. B. C. D.

it h a s b e e n fifty years since the last flood that large occurred. a flood at least that large h a s o c c u r r e d every year within the last fifty years. the flooded area is safe from a serious flood for at least fifty years. a flood that large occurs on t h e average of o n c e every fifty years but also has a c h a n c e of o c c u r r i n g d u r i n g any year. H i n t : Refer t o F i g u r e 1 8 . 2 1 .

15. S t r e a m velocity generally increases d o w n s t r e a m even t h o u g h stream gradient decreases because A. B. C. D.

the river c h a n n e l typically b e c o m e s r o u g h e r d o w n s t r e a m . c h a n n e l s typically m e a n d e r less d o w n s t r e a m . t h e a m o u n t of s e d i m e n t d e c r e a s e s d o w n s t r e a m . stream discharge typically increases d o w n s t r e a m as tributaries contribute their water.

16. If t h e regional b a s e level of a s t r e a m is lowered, A. the s t r e a m will d e p o s i t to raise the b a s e level to its former position. B. the s t r e a m will begin to d o w n c u t at its h e a d w a t e r s . C. the s t r e a m will begin to d o w n c u t at its d o w n s t r e a m end, and d o w n c u t t i n g will p r o g r e s s u p s t r e a m until the s t r e a m c h a n n e l is graded with the n e w b a s e level. D. it will h a v e no effect on the stream. 17. W h e r e streams e m e r g e from a n a r r o w m o u n t a i n c a n y o n onto a flat plain, alluvial fans form b e c a u s e A. the increase in the a m o u n t of water from tributary c a n y o n s results in deposition. B. s t r e a m velocity d e c r e a s e s d u e to a w i d e n i n g of the stream c h a n n e l and a d e c r e a s e in gradient. C. s t r e a m velocity increases d u e to a d e c r e a s e in gradient. D. all of t h e a b o v e . H i n t : Refer to F i g u r e 18.25. 18. W h a t s t r e a m feature(s) can d e v e l o p as a result of regional uplift and e r o s i o n ? A. accelerated d o w n c u t t i n g in s t e a m s B . s t r e a m terraces C . incised m e a n d e r s D. all of t h e a b o v e H i n t : Refer to F i g u r e s 18.25 and 18.26.

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C H A P T E R 18

CHAPTER 19

Winds and Deserts The wind grew stronger, whisked under stones, carried up straws and old leaves, and even little clods, marking its course as it sailed across the fields. The air and sky darkened, and through them the sun shone redly, and there was a raw sting in the air. — J O H N STEINBECK, THE GRAPES OF WRATH,

1939

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Before Lecture Before y o u attend lecture be sure to spend s o m e time previewing the chapter. For an efficient preview, use the questions below. C h a p t e r Preview questions constitute the basic framework for understanding the chapter. Preview w o r k s best if you do it just before lecture. With the main points in m i n d , y o u will understand the lecture better. This will in turn result in a better and more c o m p l e t e set of notes.

Chapter Preview • W h e r e d o w i n d s f o r m a n d h o w d o t h e y flow? Brief a n s w e r : W a r m air rises at the equator, c a u s i n g c l o u d i n e s s and a b u n d a n t rain in the tropics. A i r flows t o w a r d t h e p o l e s a n d at a b o u t 30 d e g r e e s n o r t h and 30 degrees south the c o o l e d air sinks, w a r m s up, a b s o r b s m o i s t u r e , and often p r o d u c e s d o m i n a n t l y clear skies and deserts. Refer to Figures 19.1 and 19.15. • H o w do winds erode and transport sand and finer-grained sediments? Brief a n s w e r : Turbulent airflow and forward m o t i o n c o m b i n e to lift finer particles into t h e w i n d and carry t h e m b y s u s p e n s i o n , sliding, rolling, and saltation. • H o w do winds deposit sand dunes and dust? Brief a n s w e r : A d e c r e a s e in w i n d velocity and gravity c a u s e s d e p o s i t i o n of sediment b e i n g transported by w i n d . T h e formation and shape of sand d u n e s d e p e n d s on the supply of sand and the strength and variability of w i n d direction. • W h a t f a c t o r s c o n t r i b u t e t o the e x i s t e n c e o f d e s e r t r e g i o n s o n E a r t h ? Brief a n s w e r : G l o b a l a t m o s p h e r i c circulation patterns, distance from w a t e r (oceans), t o p o g r a p h i c barriers like m o u n t a i n s , cold o c e a n c u r r e n t s , and polar c l i m a t e s contribute to the formation of desert c o n d i t i o n s on Earth. •

W h a t features are c h a r a c t e r i s t i c of d e s e r t l a n d s c a p e s ? S u g g e s t i o n : M a n y d i a g n o s t i c features of deserts are d e s c r i b e d in this chapter. O n e q u e s t i o n to ask yourself as you read about t h e s e features is, " W h a t features might a geologist look for in the rock record that w o u l d serve as e v i d e n c e for past desert c o n d i t i o n s at localities that are no longer d e s e r t s ? "

How much time do you have before lecture begins? 30 minutes or more

How to use it W i t h this m u c h t i m e y o u c a n dig d e e p into the chapter. D o a s m a n y o f t h e following a s your time allows. / R e a d the C h a p t e r P r e v i e w q u e s t i o n s and brief a n s w e r s . / R e a d the D u r i n g L e c t u r e s u g g e s t i o n s . / S t u d y the key figure(s) for this chapter (usually s h o w n at the b e g i n n i n g of the S t u d y Guide chapter). / S t u d y a n d a n n o t a t e any additional figures, hints, or s u g g e s t i o n s alluded to in the C h a p t e r Preview. / If t i m e allows, do 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 .

15-20 minutes

Do a brief but intense preview. / R e a d the C h a p t e r P r e v i e w q u e s t i o n s a n d brief a n s w e r s . / R e a d the D u r i n g L e c t u r e s u g g e s t i o n s . y S t u d y t h e key figure for this c h a p t e r ( a l w a y s s h o w n at the b e g i n n i n g the S t u d y G u i d e chapter).

5-10 minutes

R e a d the C h a p t e r Preview questions and brief answers. Focus on getting the questions clearly in mind. Then listen for answers during lecture. Even five minutes of previewing h e l p s !

Winds and Deserts

Vital Information from Other Chapters Review the section Hydrology and Climate in Chapter 17. Pay particular attention to Figure 17.3.

During Lecture O n e goal for lecture should b e t o leave the r o o m with g o o d a n s w e r s t o the C h a p t e r P r e v i e w 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 19 tells t w o related stories. First is the story of w i n d : E a r t h ' s a t m o s p h e r i c circulation p a t t e r n a n d h o w w i n d transports s e d i m e n t and creates sand d u n e s . S e c o n d is t h e story of deserts: h o w E a r t h ' s circulation patterns p r o d u c e deserts 15 to 30 d e g r e e s a w a y from t h e e q u a t o r a n d h o w u n i q u e features of a desert l a n d s c a p e (for e x a m p l e , desert p a v e m e n t , sand d u n e s , and p e d i m e n t s ) evolve. • F o c u s o n u n d e r s t a n d i n g F i g u r e 19.1 ( a t m o s p h e r i c circulation) and F i g u r e 19.12 ( d u n e t y p e s ) .

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 contains both g e n e r a l r e v i e w tips a n d specific suggestions for this chapter.

Check Your Notes: Have y o u . . . • a d d e d y o u r o w n sketches of the four types of sand d u n e s ? • created a brief big picture overview of this lecture (using a sketch or written outline)? S u g g e s t i o n s : S k e t c h a simple figure that integrates the information in Figures 19.1 and 19.15. Your sketch should a n s w e r the question " W h y do deserts tend to o c c u r b e t w e e n 15 to 30 d e g r e e s on either side of the e q u a t o r ? "

Intensive Study Session Set p r i o r i t i e s 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 involve a n s w e r i n g 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. • Text. Review the key figures in the chapter. Focus your attention on understanding Earth's atmospheric circulation pattern (Figure 19.1) and h o w it is related to the formation of deserts, as shown in Figure 19.15. T h e n m o v e on to understanding other features of deserts. Pay particular attention to the types of sand dunes (Figure 19.12) and to the formation of mountain pediments (Figure 19.20). • 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 . E x e r c i s e 1 will h e l p y o u r e m e m b e r t h e different types of sand d u n e s . T h e n a n s w e r t h e 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 o f the lecture. C h e c k y o u r a n s w e r s a s 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 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 t h e W e b 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 explan a t i o n s for the a n s w e r s . Do 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 World's Major Deserts and Understanding Global Air Circulation Patterns. E x p l o r e t h e link b e t w e e n dust storms, rainforests, and coral reefs by d o i n g the G e o l o g y in P r a c t i c e exercises.

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Exam Prep M a t e r i a l s i n this s e c t i o n 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 e x a m s . T h e Chapter 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 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 . 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 have 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 each 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 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 Where do winds form and how do they flow? • Prevailing w i n d s on Earth are largely controlled by z o n e s of rising and sinking air and the Coriolis effect. H o t (less d e n s e ) air rises and carries m o i s t u r e up to w h e r e it c o n d e n s e s and falls as precipitation. As air radiates heat to s p a c e in the u p p e r a t m o s p h e r e , it cools, b e c o m e s denser, sinks to the surface, and flows b a c k t o w a r d the equator. S i n k i n g air is typically very dry b e c a u s e it c o m e s from the cold and dry u p p e r a t m o s p h e r e . B e c a u s e of t h e rotation of Earth, the Coriolis effect deflects air flow in b o t h h e m i s p h e r e s .

What factors contribute to the existence of desert regions on Earth? • D e s e r t s are r e g i o n s w h e r e evaporation e x c e e d s precipitation. • D e s e r t r e g i o n s on Earth are the result of (1) global air circulation patterns, w h i c h g e n e r a t e a relatively stationary z o n e of d e s c e n d i n g , w a r m , dry air at a b o u t 15° to 30° north and south of the e q u a t o r ; (2) d i s t a n c e from large b o d ies of w a t e r like the o c e a n s ; (3) the rain s h a d o w g e n e r a t e d by high m o u n t a i n s , w h i c h b l o c k s the flow of m o i s t u r e - r i c h air; (4) cold o c e a n c u r r e n t s , w h i c h r e d u c e air t e m p e r a t u r e s and r e d u c e the transport of m o i s t u r e by the air; a n d (5) polar c l i m a t e s , w h e r e the air is so cold that it h o l d s very little m o i s t u r e at all.

How do winds erode and transport sand and finer-grain sediments? • W i n d is a m a j o r erosional and d e p o s i t i o n a l agent, m o v i n g e n o r m o u s quantities of sand, silt, and dust. Turbulent air flow w i t h i n w i n d e r o d e s and t r a n s p o r t s particles by s u s p e n s i o n , sliding, rolling, and saltation. As velocity d e c r e a s e s , s e d i m e n t is pulled out of air by gravity and is d e p o s i t e d as a b l a n k e t or as d u n e s of sand and dust.

How do winds deposit sand dunes and dust? • O n l y a b o u t 2 0 % of the area of desert r e g i o n s is c o v e r e d by sand. T h e distinctive t y p e s of sand d u n e s are g o v e r n e d by t h e a m o u n t of sand available, t h e strength of the w i n d , and the variability of w i n d direction. L o e s s , w i n d - b l o w n dust, is another i m p o r t a n t w i n d deposit.

What features are characteristic of desert landscapes? • G e o l o g i c and t o p o g r a p h i c features associated w i t h desert r e g i o n s i n c l u d e sand d u n e s , loess deposits, evaporite (salt) d e p o s i t s , desert p a v e m e n t , ventifacts (sandblasted r o c k s ) , alluvial fans and alluvial sands and gravels, p e d i m e n t s , m e s a s , distinctive soils ( p e d o c a l s , rich in c a l c i u m c a r b o n a t e ) , a n d rusty o r a n g e - b r o w n c o l o r s of w e a t h e r e d r o c k surfaces. S o m e of t h e s e features are p r e s e r v e d in the r o c k r e c o r d and p r o v i d e geologists c l u e s to ancient desert r e g i o n s that no l o n g e r exist.

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Practice Exercise The a n s w e r a n d an e x p l a n a t i o n is provided at the end of this Study G u i d e .

Exercise: Sand dune types C o m p l e t e the table. F i g u r e 19.12 and the t e x t b o o k section D u n e Types will be helpful.

Dune type

Characteristics

Sand supply

Wind direction/strength

Barchan Transverse Blowout

Limited to moderate

Linear (See F i g u r e 19.8.)

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 end of this St Study G u i d e , 1. W i n d belts on E a r t h are largely controlled rolled by A. r e g i o n s of sinking air. B. r e g i o n s of rising air. C. the Coriolis effect. D. all of the a b o v e . 2 . D e s e r t s m a y b e c a u s e d b y all o f the following following E X C CEEP T A. rising air. B . p r o x i m i t y t o cold o c e a n currents. C. great d i s t a n c e from the o c e a n . D. d e s c e n d i n g air. 3. As A. B. C. D.

a dune advances, sand e r o d e s from the w i n d w a r d slope. s a n d is deposited on the leeward slope. particles m o v e over t h e crest by saltation. all of t h e a b o v e occur.

4. As the A. B. C. D.

a d u n e g r o w s in height, w i n d streamlines over dune become less c o m p r e s s e d and their velocity d e c r e a s e s . less c o m p r e s s e d and their velocity increases. m o r e c o m p r e s s e d a n d their velocity d e c r e a s e s . m o r e c o m p r e s s e d and their velocity increases.

5. A dried lake b e d that h a s a b u n d a n t salt deposits and that is flat e n o u g h for a s p a c e shuttle to land on is called a(n) A. pediment. B . desert p a v e m e n t .

C . playa. D . o x b o w lake.

6. L o e s s is A. fine dust transported by w i n d in s u s p e n s i o n and d e p o s i t e d on land. B . dust transported b y w i n d , deposited, and then later e r o d e d a n d r e d e p o s i t e d by water. C. fine sand transported and deposited by w i n d . D. fine-grained salt particles e r o d e d by w i n d off a p l a y a surface.

Unidirectional/gusty

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7. D e s e r t p a v e m e n t m a y be the p r o d u c t of A. deposition of gravel by flash floods. B. concentration of coarser particles as w i n d r e m o v e s finer m a t e r i a l . C. deposition of m u d by flash floods. D . desert varnish. H i n t : Refer to F i g u r e 19.7. 8. R o c k material carried in s u s p e n s i o n by w i n d is mostly the size of A. sand.

C. gravel.

B. silt and clay.

D. p e b b l e s .

9. T h e Coriolis effect deflects w i n d s in the northern h e m i s p h e r e to the A. left ( w e s t w a r d ) .

C.

north.

B . right ( e a s t w a r d ) . D .

south.

H i n t : Refer to F i g u r e 19.1 and the section W i n d Belts in the text. 10. You are lost in the G o b l i n Desert. T h e nearest t o w n is d u e south. W i n d s b l o w from the south to t h e north but there is no w i n d b l o w i n g today. F u r t h e r m o r e , t h e sun is totally o b s c u r e d by c l o u d s . W i t h o u t a c o m p a s s , in w h i c h direction are y o u g o i n g to hike, given the orientation of the b a r c h a n d u n e s h o w n in the illustration?

11. W h i l e hiking through a dune-filled coastal plain on a w i n d l e s s m o r n i n g , y o u b e c o m e surr o u n d e d by a d e n s e fog and realize that you are lost. You k n o w y o u are near a shoreline and that the b e a c h will lead y o u b a c k to c a m p , but y o u d o n ' t k n o w w h i c h direction it is in. You r e c o g n i z e the crescent-shaped d u n e s that w r a p m o d e r a t e d e p r e s s i o n s as blow-out d u n e s . A c c o r d i n g to your c o m p a s s , the tapered a r m s of the d u n e s p o i n t south. Then, r e m e m b e r i n g that in this region strong, gusty w i n d s c o m e onto the coastal plain off the ocean, y o u i m m e d i a t e l y r e m e m b e r w h i c h direction you should head to get to the beach: A. n o r t h

C.

B . south

D . west

east

H i n t : Refer to F i g u r e 19.12. 12. W h y are evaporites significant geological d e p o s i t s ? A. T h e y are a g o o d p a l e o e n v i r o n m e n t a l indicator of ancient desert c o n d i t i o n s . B. T h e y are a major source of c h e m i c a l s like b o r a x . C. T h e y are a source of salt for the d i n n e r table. D. all of the a b o v e 13. P e t r o g l y p h s , early Native A m e r i c a n artwork, is scratched t h r o u g h

.

CHAPTER 20

Coastlines and Ocean Basins ... over all the face of Earth Main ocean flowed, not idle; but, with warm Prolifick humour softening all her globe . . . — J O H N M I L T O N , PARADISE LOST,

B O O K V I I , LL. 2 7 8 - 2 8 0

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Before Lecture Chapter Preview • H o w d o e s the g e o l o g y of the o c e a n s differ f r o m that of t h e c o n t i n e n t s ? Brief answer: O c e a n basins are created at o c e a n ridges (diverging m a r g i n s ) by volcanism and are destroyed in the brief period of several h u n d r e d million years by s u b d u c t i o n at converging m a r g i n s . • W h a t processes shape the shoreline? Brief answer: Waves and tides interact with tectonics to shape the shoreline. W i n d s blowing over the sea create waves. Waves approaching shallow water along the shoreline are transformed into breakers and refracted into longshore currents and longshore drift, w h i c h transports sand along the beach. Tides deposit sediment on longshore flats. Tectonic uplift creates cliffs and headlands, which are smashed by w a v e action, leaving behind cliff remnants called seastacks. Tectonic subsidence creates areas of long, wide beaches and low-lying coastal plains and sandbars, w h i c h m a y evolve into barrier islands. • H o w do h u r r i c a n e s affect c o a s t a l a r e a s ? Brief a n s w e r : E x t r e m e l y high w i n d s and a s t o r m surge are characteristic of hurricanes. T h e storm surge can flood low-lying areas up to depths of 10 m and c a n be devastating over a very broad area of the coastline. • W h a t are the m a j o r c o m p o n e n t s o f the c o n t i n e n t a l m a r g i n s a n d a d j a c e n t o c e a n floor? Brief a n s w e r : Continental m a r g i n s are flooded p o r t i o n s of t h e c o n t i n e n t . T h e c o n tinental slope m a r k s t h e e d g e of the c o n t i n e n t and a transition to d e e p e r w a t e r and the o c e a n floor. Turbidity currents transport fine s e d i m e n t s off t h e c o n t i n e n t a l shelf and o n t o the adjacent a b y s s a l o c e a n floor. • H o w is d e e p seafloor f o r m e d ? Brief answer: T h e d e e p seafloor is constructed by basaltic volcanism at o c e a n ridges and o c e a n hot spots and by deposition of fine-grained clastic and biochemically precipitated sediments. P r o m i n e n t features of the d e e p seafloor landscape include s e a m o u n t s , guyots, and abyssal plains and hills. Refer to Figures 20.20 and 20.22. O c e a n i c trenches (Figure 20.22) are formed w h e r e ocean plates converge. • W h a t are the c h a r a c t e r i s t i c s of a m i d - o c e a n r i d g e ? Brief answer: A rift valley runs along the crest, basaltic volcanism and e a r t h q u a k e activity are c o m m o n along the ridge crest, and s m o k e r s (hydrothermal springs) percolate through cracks on the flanks of the ridges. Refer to Figures 20.19(b) and 2 0 . 2 1 . •

W h a t k i n d s of s e d i m e n t a t i o n o c c u r in a n d n e a r the o c e a n s ? Terrigenous (note the similarity to the w o r d terrain) sediments are m u d s and sands e r o d e d from the c o n t i n e n t and d e p o s i t e d b y turbidity c u r r e n t s a l o n g t h e c o n t i n e n t a l shelf. Biochemical sediments result from deposition of c a l c i u m c a r b o n a t e from shells and coral reefs. Open-ocean (pelagic) sediments result from clays and o o z e s of c a l c i u m c a r b o n a t e and silica shells of m i c r o o r g a n i s m s (see Figure 2 0 . 2 3 ) . Evaporite sediments result from intense evaporation in shallow tropical seas. S e d i m e n t s derived from volcanic ash and lava flows are deposited near s u b d u c t i o n z o n e s . T h e c a r b o n a t e c o m p e n s a t i o n depth ( C C D ) is the level in the o c e a n b e l o w w h i c h c a l c i u m c a r b o n a t e d i s s o l v e s . Refer t o F i g u r e 2 0 . 2 4 .

Vital Information from Other Chapters C h a p t e r s 2, 4, and 5 contain key information on o c e a n b a s i n s and r o c k m a t e r i a l s associated with m a r i n e e n v i r o n m e n t s . In C h a p t e r 2, review F i g u r e s 2 . 3 , 2.6, 2.12. In C h a p t e r 4, I g n e o u s R o c k s , review p a g e s 9 1 - 9 7 . In C h a p t e r 5, review F i g u r e s 5.5 and 5.18, Tables 5.2 and 5.4, and Earth Issues 5 . 1 . A q u i c k r e r e a d of C h a p t e r 5 w o u l d h e l p i m m e n s e l y .

Coastines and Ocean Basins

During Lecture One goal for lecture s h o u l d be to leave the r o o m w i t h g o o d a n s w e r s to the C h a p t e r P r e v i e w questions. To avoid getting lost in details, k e e p the big picture in mind: C h a p t e r 20 tells the story of t h e o c e a n d e p t h s . You will learn about various landform features of the d e e p o c e a n , continental m a r g i n s , and shoreline a n d learn about t h e geological p r o c e s s e s that create t h e m .

Note-Taking Tip There is a lot of n e w t e r m i n o l o g y in this chapter. B e c a u s e t h e c h a p t e r is so t e r m i n o l o g y rich, the lecturer m a y use t e r m s you are not familiar with. M a r k , circle, or u n d e r l i n e them in y o u r notes so that you can c h e c k t h e m out later. Put the abbreviation def. {define) in the margin to r e m i n d yourself to do this.

. . . though we know the sea to be an everlasting terra incognita, so that Columbus sailed over numberless unknown worlds. . . . —HERMANN

MELVILLE,

MOBY-DICK

(1851)

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 c o n t a i n s both 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 m a t e r i a l ? S u g g e s t i o n s : D r a w a profile sketch similar to Figures 2 0 . 2 0 and 2 0 . 2 2 that s h o w s the major features of the d e e p ocean floor. • r e w o r k e d y o u r notes into a form that is efficient for y o u r learning style? Visual learners n e e d to see illustrations and m a y w a n t to c o p y key figures and insert t h e m into their notes w h e r e the figures are explained. Kinesthetic learners m a y find it m o r e beneficial to sketch t h e figures t h e m s e l v e s , b e c a u s e t h e act of d r a w i n g helps t h e m r e m e m b e r better.

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 involve a n s w e r ing q u e s t i o n s . We r e c o m m e n d t h e following strategy for learning this chapter. • Text. R e v i e w t h e key figures in C h a p t e r 2 0 . T h e s e q u e n c e of figures tells the story of the o c e a n and its seafloor topography, continental m a r g i n s , w a v e s , tides, and b e a c h e s . You will n e e d a general u n d e r s t a n d i n g of these figures to c o m p l e t e 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 . F i g u r e 20.2 explains h o w w a v e action (breaking and refraction) w o r k s along the shoreline of an o c e a n . F i g u r e 20.3 explains h o w the m o o n and sun p r o d u c e the tides. Figures 2 0 . 1 1 and 2 0 . 1 2 explain the structure and sand budget of an ocean b e a c h . F i g ures 2 0 . 1 5 a n d 2 0 . 1 6 explain the g e o l o g y of continental m a r g i n s Figures

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2 0 . 2 0 , 2 0 . 2 1 , and 2 0 . 2 2 will h e l p y o u learn a b o u t seafloor t o p o g r a p h y a n d l a n d f o r m s . 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 t h e e n d of the t e x t b o o k chapter. T h e y 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 t a k e l o n g if y o u k n o w the material. N o t e that there is an interesting a n i m a t i o n for E x e r c i s e 2 in the M e d i a L i n k Q u e s t i o n s section o f the W e b site. • 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 Study G u i d e Practice Exercises 1 and 2. T h e s e exercises will h e l p you r e m e m b e r s o m e of the m o s t important ideas in the chapter. T h e n a n s w e r the R e v i e w Q u e s t i o n s to c h e c k your u n d e r s t a n d i n g of the lecture. C h e c k your a n s w e r s as y o u go, but try to a n s w e r the question 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 ://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 C o n c e p t Self-Checker and W e b R e v i e w Q u e s t i o n s . Pay particular attention to the explanations for the answers. Do the O n l i n e R e v i e w E x e r cises Identify the Parts of a Beach and

Understanding Waves and Currents. T h e

G e o l o g y in Practice exercise Why Are the Oceans Salty? provides a g o o d review of the links a m o n g weathering, erosion, and the composition of seawater.

Study Tip N o w is the t i m e to look up the terms you d i d n ' t u n d e r s t a n d . S k i m the margin of your notes. (You did m a r k unfamiliar terms def d i d n ' t y o u ? ) T h e text p r o v i d e s t w o helpful aids for dealing w i t h t e r m s y o u d o n ' t know. Key T e r m s and C o n c e p t s at t h e e n d of each text c h a p t e r lists all n e w t e r m s a n d h a n d i l y p r o v i d e s the p a g e n u m b e r of a t e r m so that y o u c a n look it u p . Alternatively, use the G l o s s a r y at the e n d of the text.

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 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 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 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 . 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 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 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 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 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 How does the geology of the oceans differ from that of the continents? • V o l c a n i s m and s e d i m e n t a t i o n s h a p e the o c e a n floor. In contrast, folding, faulting, w e a t h e r i n g , and e r o s i o n play i m p o r t a n t parts in s h a p i n g the c o n t i n e n t s .

What processes shape the shoreline? • W a v e s and tides s h a p e the shoreline. W a v e s are created by the w i n d b l o w i n g over the surface of the water. O c e a n tides on Earth are a result of centrifugal force and gravitational forces acting a m o n g the Earth, M o o n , and S u n .

• T h e b e a c h is a result of the d y n a m i c b a l a n c e a m o n g w a v e s , l o n g s h o r e currents that e r o d e a n d transport r o c k material along the coast within the surf z o n e , and the supply of s a n d from rivers to t h e surf z o n e . Refer to F i g u r e 2 0 . 1 2 . • L o n g s h o r e currents result from the zigzag m o v e m e n t of water on a n d off the b e a c h . W a v e s typically splash o n t o shore at an angle in part d u e to w a v e refraction. T h e b a c k w a s h — o f f the b e a c h — r u n s d o w n the b e a c h slope at a small but o p p o s i t e a n g l e to the swash. T h e net result of this s w a s h and b a c k w a s h of water on a n d off the b e a c h slope is a l o n g s h o r e current that transports sand parallel to the b e a c h within t h e surf z o n e .

How do hurricanes affect coastal areas? • E x t r e m e l y h i g h w i n d s and a s t o r m surge are characteristic of hurricanes. T h e s t o r m surge can flood low-lying areas up to d e p t h s of 10 m and can be devastating over a very b r o a d area of the coastline. • T h e h u r r i c a n e intensity scale (Saffir-Simpson scale) is u s e d to estimate the potential p r o p e r t y d a m a g e and flooding e x p e c t e d along the coast from hurric a n e landfall. Refer t o Table 2 0 . 1 .

What are the major components of the continental margins? • C o n t i n e n t a l m a r g i n s are flooded portions of the continent. T h e continental slope and rise m a r k the e d g e of the continent and m a r k a transition to d e e p e r water a n d the o c e a n floor. • Passive continental m a r g i n s form w h e r e rifting and seafloor spreading carry continental m a r g i n s a w a y from active plate b o u n d a r i e s . Active continental m a r g i n s form w h e r e o c e a n i c lithosphere is s u b d u c t e d b e n e a t h a continent, or w h e r e a transform fault coincides with the continental m a r g i n . • C o n t i n e n t a l shelves are b r o a d and relatively flat at passive continental m a r g i n s and are n a r r o w and u n e v e n at active m a r g i n s . • Turbidity currents transport fine sediments from the continental shelf and to the adjacent abyssal o c e a n floor. Turbidity currents can b o t h e r o d e and transport s e d i m e n t s . S u b m a r i n e c a n y o n s and fans are formed by turbidity currents. • Coral reefs and atolls are c o n s t r u c t e d by coral and other m a r i n e o r g a n i s m s . T h e reef c o n s t r u c t plays an i m p o r t a n t role in m o d u l a t i n g w a v e e n e r g y and creating a favorable e n v i r o n m e n t for shallow m a r i n e life. Refer to F i g u r e 5.18 and E a r t h Issues 5 . 1 .

How is the deep seafloor formed? • V o l c a n i s m along o c e a n spreading centers (divergent plate b o u n d a r i e s ) creates n e w seafloor, w h i c h eventually is recycled b a c k into the m a n t l e by s u b d u c t i o n at the t r e n c h e s (convergent plate b o u n d a r i e s ) . • Basaltic v o l c a n i s m , i n d u c e d by pressure-release m e l t i n g in the u p p e r m a n t l e b e n e a t h s p r e a d i n g centers, generates an o c e a n crust that is thin and iron-rich relative to continental crust. R e v i e w p a g e s 9 1 - 9 7 in C h a p t e r 4.

What are the characteristics of a mid-ocean ridge? • A rift valley m a r k s the crest of an o c e a n ridge. • H y d r o t h e r m a l springs form on the rift valley floor as seawater percolates t h r o u g h the new, h o t o c e a n i c crust. • T r a n s f o r m faults offset ocean ridges at m a n y places to a c c o m m o d a t e different s p r e a d i n g rates a l o n g offset s e g m e n t s of the ridge. Refer to F i g u r e 2.5.

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What kinds of sedimentation occur in and near the oceans? • In the d e e p sea, fine-grained pelagic t e r r i g e n o u s and b i o c h e m i c a l l y precipitated s e d i m e n t s settle to the seafloor. Foraminiferal o o z e s , c o m p o s e d of tiny foraminiferal shells, are the m o s t a b u n d a n t b i o c h e m i c a l c o m p o n e n t of pelagic s e d i m e n t s . Foraminiferal and other c a r b o n a t e o o z e s are a b u n d a n t at depths less than about 4 k m . B e l o w a certain depth, called the c a r b o n a t e c o m p e n s a tion depth, c a r b o n a t e s e d i m e n t s dissolve in d e e p seawater. D e e p o c e a n water is colder, contains m o r e c a r b o n d i o x i d e , and is u n d e r h i g h e r p r e s s u r e . All these factors increase the solubility of c a r b o n a t e s e d i m e n t s . Silica o o z e is p r o d u c e d by s e d i m e n t a t i o n of t h e silica shells of d i a t o m s and radiolaria.

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: Profile from the Atlantic shoreline to the ocean floor A. Fill in the b l a n k s correctly to label this profile. H i n t : Refer to F i g u r e 2 0 . 1 5 .

Exercise 2: Passive versus active continental margins C h a r a c t e r i z e e a c h coastal locality either as a passive or as an active (Andean type) or active (Marianas type) continental m a r g i n . F i g u r e s 2 0 . 1 5 , 2 0 . 1 7 , 2 0 . 2 0 , 2 0 . 2 2 are useful references. A. coastline far from an active plate b o u n d a r y B. east coast of N o r t h A m e r i c a C. w e s t coast of S o u t h A m e r i c a D. c o a s t l i n e with a very broad, featureless continental shelf E. California coast a l o n g the San A n d r e a s Fault F.

continental m a r g i n with no volcanic activity for millions of years


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 this Study G u i d e . 1. O c e a n w a v e s are g e n e r a t e d m o s t l y by A. tides. B . ships.

C. the wind. D. earthquakes.

2. Geologically, the e d g e of a c o n t i n e n t is considered to be A. B. C. D.

on the o c e a n side of the seafloor t r e n c h e s . the c o n t i n e n t a l shelf. the continental rise and slope. the shoreline.

3. W h i c h of the following p r o c e s s e s is m o s t i m p o r t a n t for building the ocean floor? A. volcanism B. metamorphism C. precipitation of c a r b o n a t e r o c k s D. deposition of s e d i m e n t derived from the land 4. T h e b e n d i n g of w a v e s as they a p p r o a c h shore is called A. B. C. D.

w a v e reflection. wave erosion. l o n g s h o r e drift. w a v e refraction.

5. An erosional coast is characterized by A. sea cliffs, sea stacks, and wave-cut terraces. B . barrier islands. C. coral reefs. D . estuaries. 6. O c e a n - f l o o r r o c k is m a d e up of A. B. C. D.

basalt and p e l a g i c s e d i m e n t s . granite and g n e i s s . o b s i d i a n a n d sand. rhyolite and c a r b o n a t e s e d i m e n t s .

7. T h e r o c k that m a k e s up a s e a m o u n t is A. basalt. B. limestone.

C. granite. D . m a r i n e s e d i m e n t a r y rock.

8. S e a stacks are A. piles of s e d i m e n t a r y r o c k s near the shore. B. the e r o s i o n a l r e m n a n t s of sea cliffs. C. f o r m e d w h e r e a river drains o n t o the coastline. D. f o r m e d by rapidly g r o w i n g corals on a reef. H i n t : Refer t o F i g u r e 2 0 . 1 . 9. T h e d e e p abyssal o c e a n plain lies at a water d e p t h of A. B. C. D.

b e t w e e n 100 and 5 0 0 m e t e r s . 6 0 0 t o 1000 m e t e r s . 2000 to 3000meters. 4000 to 6000 meters.

H i n t : Refer to the text section Profiles A c r o s s T w o O c e a n s . 10. R e c e n t precise satellite m e a s u r e m e n t s of the sea surface shows that t h e sea level A. B. C. D.

rises a few m i l l i m e t e r s per year. does not change. d r o p s a few m i l l i m e t e r s p e r year. rises and falls with the seasons.

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1 1 . W h a t is s e d i m e n t - c h a r g e d w a t e r that flows rapidly d o w n the continental slope called? A. turbidity current B . l o n g s h o r e current

C. tidal current D. tsunami

12. T h e highest tides o c c u r w h e n A. the S u n , M o o n , and E a r t h are all aligned. B. the S u n and M o o n are at right a n g l e s to the Earth. C. the Earth is closest to t h e S u n . D. the M o o n is in either its first or its last quarter. H i n t : Refer t o F i g u r e 2 0 . 3 . 13. You live on a beachfront. Your up-current neighbors are p l a n n i n g to build a groin to halt erosion and increase the width of their beach. W h a t effect will this have on your beach? A. T h e groin will p r o b a b l y c a u s e y o u r b e a c h to grow. B. It is likely that y o u r b e a c h will r e m a i n u n c h a n g e d . C . T h e groin will p r o b a b l y c a u s e y o u r b e a c h t o e r o d e . D. n o n e of the a b o v e H i n t : Refer to Earth Issues 2 0 . 2 . 14. H e a d l a n d s and points e x p e r i e n c e greater e r o s i o n than b a y s and inlets b e c a u s e of A. l o n g s h o r e drift. B . w a v e refraction. C. w a v e reflection. D. the m o r e resistant r o c k in the h e a d l a n d . H i n t : Refer t o F i g u r e 2 0 . 2 (6). 15. If a river delivers sand to a shoreline faster than currents c a n t r a n s p o r t the sediment, the result is A. e r o s i o n of the sand to form a rocky coastline. B. formation of a m a r i n e terrace. C. e x p a n s i o n of the s a n d y b e a c h . D. formation of sea stacks and pillars. 16. If y o u accidentally get c a u g h t in a rip current that is c a r r y i n g y o u o u t to sea, what should y o u d o ? A. S w i m parallel to the shore. B. R e s t and float with the rip far out to sea and then s w i m b a c k . C. S w i m to shore, b e c a u s e this is the shortest distance. D . S c r e a m for help. H i n t : Refer t o F i g u r e 2 0 . 2 . R i p currents are rapidly m o v i n g b a c k f l o w s . 17. C o r a l atolls f o r m in tropical o c e a n s by the u p w a r d g r o w t h of coral A. from deep, submarine mountains. B. on a c o n t i n e n t a l shelf that is gradually s u b s i d i n g . C. on subsiding volcanic islands. D . o n e x p o s e d sections o f the o c e a n i c r i d g e s y s t e m . H i n t : Refer to F i g u r e 5.18 and Earth Issues 5.1 in C h a p t e r 5, S e d i m e n t a t i o n : Rocks F o r m e d b y Surface P r o c e s s e s . 18. W h a t is t h e m o t i o n of individual w a t e r m o l e c u l e s as a w a v e travels? A . T h e m o l e c u l e s travel a l o n g with the w a v e . B. T h e m o l e c u l e s follow a straight u p - a n d - d o w n path. C. T h e m o l e c u l e s follow a r o u g h l y circular path. D . T h e m o l e c u l e s travel laterally along l o n g s h o r e currents. 19. A result of w a v e refraction is that A. w a v e energy is c o n c e n t r a t e d on h e a d l a n d s and p o i n t s . B. w a v e e n e r g y is c o n c e n t r a t e d in the b a y s . C. s e d i m e n t is d e p o s i t e d in the vicinity of h e a d l a n d s , m a k i n g t h e m larger. D. w a v e energy is largely dissipated u n i f o r m l y a l o n g the coastline.


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CHAPTER 21

Glaciers: The Work of Ice

Before Lecture Before you attend lecture, be sure to spend s o m e time previewing the chapter. For an efficient preview, use t h e C h a p t e r P r e v i e w q u e s t i o n s .

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Chapter Preview • H o w d o g l a c i e r s f o r m a n d h o w d o they m o v e ? Brief a n s w e r : G l a c i e r s form w h e r e snow a c c u m u l a t i o n e x c e e d s snow melting. Refer to F i g u r e 2 1 . 9 . G l a c i e r s m o v e by a c o m b i n a t i o n of plastic flow and slip at the b a s e of the ice. S e e Figure 2 1 . 1 1 . • W h y do ice s h e l v e s float? Brief a n s w e r : Ice floats on the ocean in exactly the s a m e way that ice c u b e s float in a glass of water. Ice is less d e n s e than water. See Earth Issues 2 1 . 1 . • H o w do glaciers erode bedrock, transport and deposit sediments, and shape the landscape? Brief a n s w e r : G l a c i e r s e r o d e by scraping, p l u c k i n g , and grinding rock. Refer to F i g u r e 2 1 . 1 6 . T h e rock debris deposited by a glacier is called till. Till may contain debris from the size of rock flour to giant b o u l d e r s . Glaciers sculpt a distinctive l a n d s c a p e with U - s h a p e d a n d h a n g i n g valleys, aretes, cirques, m o r a i n e s , d r u m l i n s , k a m e s , and other features. Refer to Figures 2 1 . 1 5 , 2 1 . 1 7 , 2 1 . 1 8 , and 21.19. • W h a t a r e t h e ice a g e s a n d w h a t c a u s e d t h e m ? Brief a n s w e r : D u r i n g the Pleistocene e p o c h , m a n y cycles of a d v a n c e and retreat of continental ice sheets occurred. T h e ice sheets of the last major a d v a n c e w e r e g o n e by about 10,000 years a g o , the b e g i n n i n g of the H o l o c e n e e p o c h . A p o p u l a r theory for the c a u s e of t h e P l e i s t o c e n e ice ages relies on h o w variations in the E a r t h ' s orbit effect the intensity of the solar e n e r g y reaching Earth. Refer to F i g u r e 1 5 . 1 1 .

Vital Information from Other Chapters A review of p a g e s 3 5 5 - 3 5 9 is essential to understanding w h a t caused the glacial and interglacial cycles during the Pleistocene Epoch. A l s o review Figure 17.1, Distribution of water on earth.

During Lecture • K e e p the big p i c t u r e in m i n d to h e l p with n o t e taking. T h i s c h a p t e r tells the story of ice: h o w glaciers form, m o v e , a n d create a l a n d s c a p e of u n i q u e features. T h i s story h a s a l o n g history: the m a n y ice ages of t h e P l e i s t o c e n e epoch. T h e c h a p t e r s u g g e s t s a future w h e n our d e s c e n d e n t s will a l m o s t certainly have to face the return of ice and m a s s i v e glaciation. • Understanding glacial landscapes is one of the main goals for this lecture. Take notes that will help you understand key landscape features such as the following. Glacial features f o r m e d by the erosive p o w e r of glacial ice (see E a r t h Issues 2 1 . 1 a n d F i g u r e s 2 1 . 1 6 and 21.17) • S t r i a t i o n s — s c r a t c h e s and g r o o v e s — c a r v e d in b e d r o c k over w h i c h t h e glacier flowed • Cirque • U - s h a p e d valley • H a n g i n g valley • Fjord • Arete Glacial features f o r m e d by deposition of r o c k material by glacial ice (see Figures 21.18 and 21.19): • •

Glacial m o r a i n e s — t h e different types are described in Table 21.1 Esker

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PART II

C H A P T E R 21

•

Kame

•

Glacial erratic

•

Kettle

•

Varves

F o r each feature, sketch its essentials and annotate w h a t m a k e s it u n i q u e . W r i t e the numbers of figures adapted from the text in the m a r g i n so that you can review t h e m later.

After Lecture Review Notes

Check Your Notes: Have y o u . . . • c a p t u r e d the glacial p r o c e s s e s ? Your notes s h o u l d say clearly (1) h o w glaciers form and m o v e and (2) h o w they e r o d e material. •

d e s c r i b e d t h e essential features of the glacial l a n d s c a p e ? R e f e r to D u r i n g L e c t u r e for a list of l a n d s c a p e features y o u s h o u l d have in y o u r n o t e s on the D u r i n g L e c t u r e section. Be sure to go b a c k to the text for features y o u m i s s e d .

•

a d d e d visual material? D o i n g simple sketches of the glacial l a n d s c a p e features is a great w a y to learn t h e m . It will be helpful to have all y o u r features on a single page in y o u r notes for easy review.

Intensive Study Session Study Tip T h e pictures in C h a p t e r 21 are d e s i g n e d as a virtual field trip. U s e the p i c t u r e s to master glacial l a n d s c a p e features. Study e a c h p i c t u r e until y o u u n d e r s t a n d w h a t t h e feature is and h o w it differs from other features. (For a list of the l a n d s c a p e features y o u n e e d to study, refer to D u r i n g L e c t u r e . )

Set p r i o r i t i e s for s t u d y i n g this chapter. G i v e h i g h e s t p r i o r i t y to a c t i v i t i e s that involve a n s w e r i n g 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. • Text. R e v i e w the key figures in the chapter. T h e y 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. F i g u r e s 2 1 . 9 and 2 1 . 1 1 will h e l p y o u u n d e r s t a n d h o w glaciers form and m o v e . F i g u r e 2 1 . 1 6 e x p l a i n s h o w glaciers e r o d e b e d r o c k . F i g u r e s 2 1 . 1 5 , 2 1 . 1 7 , 2 1 . 1 8 , and 2 1 . 1 9 a n d Table 21.1 p r o v i d e a virtual field trip of the distinctive features of the glacial l a n d s c a p e . E a r t h Issues 21.1 explains w h y ice shelves float. S o m e t i m e before the e x a m , a n s w e r the exercises at the e n d of the c h a p t e r in t h e text. T h e y 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 y o u k n o w the material. T h e y m a k e a n excellent review o f C h a p t e r 2 1 . • Practice Exercises a n d R e v i e w Questions. Start with Practice Exercise 1, which will help you learn the distinctive features of glacial landscape. T h e n go to the multiple-choice Review Questions to check your understanding of the lecture. Try to answer each question before you look at the answer. • 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 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 for the a n s w e r s .


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 . 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. 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 provides a helpful overview that should 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 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 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 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 t h e q u e s t i o n incorrectly.

Chapter Summary How do glaciers form and how do they move? • G l a c i e r s form in c o l d and s n o w y climates w h e r e snow a c c u m u l a t i o n e x c e e d s t h e ablation of ice d u e to m e l t i n g , sublimation, w i n d erosion, and iceberg calving. Glacial ice m o v e s by plastic flow and slip along the base, w h i c h m a y be lubricated by m e l t water. T h e rate of ice flow typically varies from m e t e r s p e r year to meters per week. • G l a c i e r s are d e s c r i b e d as a d v a n c i n g or retreating d e p e n d i n g on the b a l a n c e b e t w e e n s n o w a c c u m u l a t i o n and ablation. W h e n ablation e x c e e d s a c c u m u l a tion, t h e shrinking glacier " r e t r e a t s " as the toe or t e r m i n u s m o v e s u p s l o p e . W h e n a c c u m u l a t i o n e x c e e d s ablation, the e x p a n d i n g glacier " a d v a n c e s " as its toe or terminus moves downslope.

Why do ice shelves float? • E a r t h Issues 21.1 illustrates h o w isostasy, the d y n a m i c b a l a n c e b e t w e e n the forces of gravity a n d buoyancy, k e e p ice shelves and bergs afloat in water. T h e m a s s of t h e floating ice is e q u a l to the m a s s of the w a t e r t h e iceberg displaces. W h e n the ice m e l t s , it simply r e p l a c e s t h e water it displaced a n d therefore t h e r e is no c h a n g e in sea level.

How do glaciers erode bedrock, transport and deposit sediments, and shape the landscape? • G l a c i e r s are powerful agents of erosion and deposition. Glaciers e r o d e by scraping, p l u c k i n g , and grinding rock. • L a n d s c a p e s sculptured by ice have distinctive features that have provided geologists with evidence for reconstructing the position of ice sheets during the ice ages a n d for deciphering the existence of ice ages throughout E a r t h ' s history. U - s h a p e d and h a n g i n g valleys, m o r a i n e s , aretes, cirques, drumlins, k a m e s , eskers, striated rock, and other features characterize a glacial landscape. • Ice-laid deposits of r o c k material are called till and consist of a h e t e r o g e n e o u s m i x t u r e of rock, sand, a n d clay. A c c u m u l a t i o n s of till are called m o r a i n e s ; e a c h t y p e of m o r a i n e is n a m e d for its position relative to the glacier that f o r m e d it. A n c i e n t tills, called tillites, p r o v i d e e v i d e n c e for n u m e r o u s ancient glaciations d u r i n g E a r t h ' s history. • Water-laid deposits from glaciers are called o u t w a s h a n d consist of sand, gravel, and fine r o c k flour.

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230


What are the ice ages and what causes them? • T h e ice sheets of the last m a j o r a d v a n c e w e r e g o n e by a b o u t 10,000 y e a r s a g o , the b e g i n n i n g of the H o l o c e n e e p o c h . S t u d i e s of the g e o l o g i c a g e s of glacial d e p o s i t s on land and s e d i m e n t s of t h e seafloor s h o w that the P l e i s t o c e n e glacial e p o c h c o n s i s t e d of m u l t i p l e a d v a n c e s (glacial intervals) and retreats (interglacial intervals) of t h e c o n t i n e n t a l ice sheets. E a c h a d v a n c e c o r r e s p o n d e d to a global l o w e r i n g of the sea level, w h i c h e x p o s e d large a r e a s of c o n t i n e n t a l shelf; d u r i n g the interglacial intervals, the sea level r o s e and s u b m e r g e d the s h e l v e s . • A l t h o u g h the c a u s e s of the ice ages r e m a i n uncertain, the general c o o l i n g of the E a r t h l e a d i n g to glaciation a p p e a r s to have b e e n the result of plate t e c t o n ics that gradually m o v e d continents to p o s i t i o n s w h e r e they o b s t r u c t e d the general transport of heat from t h e e q u a t o r to the polar r e g i o n s . • A favored e x p l a n a t i o n for t h e a l t e r n a t i o n of glacial and i n t e r g l a c i a l intervals i s t h e effect o f a s t r o n o m i c a l c y c l e s b y w h i c h v e r y s m a l l p e r i o d i c c h a n g e s in E a r t h ' s orbit a n d axis of r o t a t i o n alter the a m o u n t of s u n l i g h t r e c e i v e d at t h e E a r t h ' s surface. T h e r e i s also e v i d e n c e that d e c r e a s e d levels o f c a r b o n d i o x i d e i n t h e a t m o s p h e r e d i m i n i s h e d t h e g r e e n h o u s e effect, w h i c h w o u l d contribute to global cooling.

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 Study G u i d e .

Exercise 1: The glacially sculpted landscape You are hired as a seasonal r a n g e r at a national p a r k with a spectacular glacially sculpted landscape. However, there is not o n e glacier in t h e p a r k today. Your j o b for t h e s u m m e r is to lead natural history hikes along w h i c h y o u interpret t h e e v i d e n c e for the p a s t glaciation that helped f o r m e d the p a r k l a n d s c a p e . Briefly d e s c r i b e five features a l o n g the n a t u r e trail that speak to the past glacial e p i s o d e in this region. R e m e m b e r that a picture or sketch is w o r t h a thousand w o r d s , so a g o o d w a y to p r o v i d e a brief description is with a w e l l - l a b e l e d d i a g r a m . F e a t u r e s f o r m e d by the erosive p o w e r of glacial ice

1.

2.

3.

F e a t u r e s f o r m e d by deposition of r o c k material by glacial ice 4.

5.

1 76

PART II C H A P T E R IS

15. G l o b a l w a r m i n g of a b o u t

during the twentieth c e n t u r y correlates with the sig-

nificant 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 c a u s e d b y fossil-fuel burn2

ing, deforestation, and other h u m a n activities. A. 0.6°C

C.

B.

D. 2.6°C

1.6°C

2.0°C

16. E a r t h ' s a l b e d o is not affected by a c h a n g e in the a m o u n t of A. sea ice.

C.

B . C 0 i n the a t m o s p h e r e .

D . cloud cover.

2

trees covering land m a s s e s .

17. M i l a n k o v i t c h cycles correlate strongly with Pleistocene glacial cycles (ice ages) b e c a u s e A. eccentricity, w o b b l e , and p r e c e s s i o n c a n all influence E a r t h ' s a l b e d o . B . solar o u t p u t c h a n g e s d u r i n g t h e s e cycles. C. ice cores show that the a m o u n t of C 0 in the a t m o s p h e r e is higher during the ice ages. 2

D. a high orbital eccentricity r e d u c e s the solar radiation r e a c h i n g the Earth. H i n t : Refer to F i g u r e 15.11 and the textbook section M i l a n k o v i t c h C y c l e s .

CHAPTER 16


Figure 16.3. As a rock mass breaks into smaller pieces, more of its surface becomes available for the chemical reactions of weathering.

Before Lecture Chapter Preview •

W h a t is w e a t h e r i n g ? Brief a n s w e r : W e a t h e r i n g is the b r e a k d o w n of r o c k s by c h e m i c a l a n d p h y s i c a l processes.

177

206


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 e n d of this S t u d y G u i d e .

Exercise 1: Stream velocity S t r e a m velocity is a d e p e n d e n t variable that g o v e r n s s t r e a m b e h a v i o r — w h e t h e r a stream is d o m i n a n t l y e r o d i n g a n d t r a n s p o r t i n g or d e p o s i t i n g s e d i m e n t s a l o n g a section of the channel. Various i n d e p e n d e n t variables (factors) influence s t r e a m velocity and therefore c a n affect the b e h a v i o r of a certain stretch of s t r e a m c h a n n e l . T h e m a j o r i n d e p e n d e n t variables affecting velocity are listed in the table. C o m p l e t e the table by d e s c r i b i n g h o w c h a n g e s in e a c h factor affect s t r e a m velocity.

Variable affecting stream velocity

Relationship of variable to stream velocity

Analogy

G r a d i e n t — t h e slope

You tend to walk faster d o w n a

of t h e s t r e a m c h a n n e l

steeper slope.

Discharge—the amount

W i l l y o u m o v e into a n e w h o u s e slower

of w a t e r in t h e s t r e a m c h a n n e l

or faster if y o u h a v e m o r e p e o p l e helping you?—faster.

Sediment load

Typically, will y o u travel faster or slower if y o u are c a r r y i n g m o r e in your backpack?—slower.

C h a n n e l characteristics • Channel roughness • Channel shape

As channel roughness increases,

Cross-country

velocity decreases.

tends to slow one down.

hiking

without

a

trail

The stream has more contact with the

When you have more contact with the

channel surface if the channel is very

ground surfaces

move

wide or very narrow. More contact

is

walking,

with

the

and

decreases

channel

;

increases

slower

than

slower.

drag

velocity.

Figure 18.14. A current flowing over a bed of sand, silt, and clay transports particles in two ways: as bed load, the material sliding and rolling along the bottom; and as suspended load, the material temporarily or permanently suspended in the flow itself. Saltation is an intermittent jumping motion of grains. In general, the smaller the particle, the higher it jumps and the farther it travels.

Crawling


Exercise 2: Relationship between stream flow and groundwater W h y do s t r e a m s in desert r e g i o n s typically flow intermittently and streams in m o r e t e m p e r ate r e g i o n s , like N e w E n g l a n d , flow y e a r - r o u n d ? D r a w i n g well-labeled d i a g r a m s illustrating each situation w i t h a brief d i s c u s s i o n is an excellent w a y to a n s w e r this question. Refer to Figure 17.9. Desert (ephemeral) stream

Temperate (perennial) stream

Exercise 3: How do rivers cut through mountain ranges? The Kali G a n d a k i River cuts o n e of the d e e p e s t gorges on Earth right t h r o u g h the H i m a l a y a n mountains. Briefly d e s c r i b e t w o w a y s that a river c a n cut t h r o u g h a m o u n t a i n r a n g e . Refer to Figures 18.8 and 18.9. A.

B.

207


Exercise 2: Your personal budget as a metaphor for a glacial budget Discuss h o w t h e c h a n g i n g b a l a n c e of cash in y o u r c h e c k i n g a c c o u n t is a g o o d m e t a p h o r for an a d v a n c i n g a n d retreating glacier.

Exercise 3: Glacial advances and retreats

A glacier a d v a n c e s , halts, and retreats. Will the glacier c o n t i n u e to deposit material at its snout while it is h a l t e d and even w h i l e it is retreating? D i s c u s s .

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 this Study G u i d e . 1. Glacial ice is m o s t like a(n) A . i g n e o u s rock. C . m e t a m o r p h i c rock.

B . s e d i m e n t a r y rock. D . sediment.

2. T h e force that m o v e s glaciers is A. recrystallization. B. melting.

C. lubrication. D . gravity.

3. As snow is transformed into glacial ice, a transitional p h a s e of densely p a c k e d granular snow is called A . p a c k ice. B. firn. C. alpine ice. D . crystalline ice. H i n t : Refer t o F i g u r e 2 1 . 7 . 4 . Glaciers retreat w h e n t h e A. a c c u m u l a t i o n of snow is less t h a n the ablation off the glacier. B. a c c u m u l a t i o n of snow e x c e e d s the ablation off the glacier. C. a c c u m u l a t i o n of s n o w is equal to the ablation. D. boundary between the zone of accumulation and ablation moves to lower elevations.

231

5. M o r a i n e s are A. erosional glacial features carved in b e d r o c k over w h i c h the ice flowed. B . m a d e from glacial o u t w a s h s e d i m e n t s . C. deposits of loess carried by w i n d from recently glaciated r e g i o n s . D. deposits of glacial till. 6. A d r u m l i n is a A. block of b e d r o c k not quarried a w a y by the b o t t o m of a glacier. B. sinuous ridge of water-deposited glacial debris. C. a small depression formed from the melting of a b l o c k of ice buried beneath till. D. streamlined hill c o n s t r u c t e d of glacial till. H i n t : Refer to F i g u r e 2 1 . 1 9 . 7. R a g g e d , knife-edged ridges are c o m m o n l y found in glaciated m o u n t a i n s . Such a ridge line is called a(n) A. horn.

C. col.

B . cirque. H i n t : Refer to F i g u r e 2 1 . 1 7 .

D . arete.

8. W h e n continental glaciers a d v a n c e over the land surface, A. B. C. D.

t h e sea level l o w e r s . the sea level rises. plate tectonic p r o c e s s e s are especially active. E u r o p e is significantly warmer.

9. M i l a n k o v i t c h calculated that t h e eccentricity of the E a r t h ' s orbit varies over a cycle of years. A. 10,000 B. 23,000 H i n t : Refer to F i g u r e 15.11.

C. 4 1 , 0 0 0 D . 100,000

10. Orbital factors that affect the E a r t h ' s heat b u d g e t primarily affect the A. reflectivity of the E a r t h ' s u p p e r a t m o s p h e r e . B. a m o u n t of solar energy r e a c h i n g our planet. C. c o m p o s i t i o n of the a t m o s p h e r i c g a s e s . D. distribution of heat over the g l o b e . H i n t : Refer to F i g u r e 15.11 and associated text. 1 1 . W h i c h of the following features of a glacial l a n d s c a p e is the p r o d u c t of depositional p r o c e s s rather than e r o s i o n ? A. fjord B. kames

C. h a n g i n g valley D . cirque

12. W h i c h of the following is N O T characteristic of glacial till? A. sand grains of virtually all quartz B. lack of clear stratification C. very p o o r sorting D. boulder-, c o b b l e - , pebble-, sand-, and clay-sized r o c k particles 13. A b l a t i o n refers to the A. melting, calving, sublimation, and erosion of glacial ice. B. erosion a n d deposition of glacial s e d i m e n t s . C. c h a n g e s in sea level associated with glacial and interglacial p e r i o d s . D. glacier a d v a n c e and retreat.


14. T h e glacial b u d g e t is b a l a n c e d w h e n the rate of a c c u m u l a t i o n is equal to the rate of A . ablation. B . deposition.

C. advance. D . retreat.

15. T h e last major a d v a n c e of continental ice over N o r t h A m e r i c a r e a c h e d A. t h e G u l f of M e x i c o . B . California. C. south of the G r e a t L a k e s . D . Florida. H i n t : Refer t o F i g u r e 2 1 . 2 4 . 16. W h i c h of the following m a y have influenced E a r t h ' s climate during ice a g e s ? A. variations in E a r t h orbital characteristics B. c h a n g e s in the c o m p o s i t i o n of the a t m o s p h e r e C. plate tectonic m o v e m e n t s of t h e continents D. all of the a b o v e 17. As a result of the collapse and m e l t i n g of ice shelves and bergs, like that of the L a r s o n Ice Shelf in 2 0 0 2 , t h e sea level A. rises. B. falls. C. fluctuates. D. remains unchanged. H i n t : D o e s the level of water in a glass rise w h e n the ice c u b e s in the water m e l t ?

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CHAPTER 22

Landscapes: Tectonic and Climate Interaction


Before Lecture Before y o u attend the lecture, be sure to s p e n d s o m e t i m e previewing t h e chapter. F o r an efficient preview, u s e t h e C h a p t e r P r e v i e w questions.

Chapter Preview • W h a t are the p r i n c i p a l c o m p o n e n t s of l a n d s c a p e s ? Brief a n s w e r : L a n d s c a p e s are described in t e r m s of their t o p o g r a p h y : elevation, the altitude of the surface of the E a r t h above sea level; and relief, the difference b e t w e e n t h e h i g h e s t a n d the l o w e s t spots in a region. L a n d s c a p e s also consist of the varied l a n d f o r m s p r o d u c e d by g e o l o g i c p r o c e s s e s such as erosion and s e d i m e n t a t i o n by rivers, glaciers, m a s s w a s t i n g , and wind. • H o w do t h e c l i m a t e a n d plate tectonic s y s t e m s i n t e r a c t to control l a n d s c a p e ? Brief a n s w e r : Tectonics affects the height and distribution of the crust and its c o m p o s i t i o n . C l i m a t e affects w e a t h e r i n g and erosion. Characteristics of the b e d r o c k influence w e a t h e r i n g and erosion rates. • H o w d o l a n d s c a p e s evolve? Brief a n s w e r : T h e evolution of l a n d s c a p e s d e p e n d s strongly on the c o m p e t i t i o n b e t w e e n uplift and erosion. F o r e x a m p l e , a l a n d s c a p e with high relief will form if tectonic activity is high, which in turn stimulates erosion. Erosion will at first e n h a n c e relief, but o v e r time, water, w i n d , and ice will w e a r d o w n t h e high spots and fill in the low spots with s e d i m e n t . • W h y don't mountains sink? Brief a n s w e r : Like icebergs, m o u n t a i n s float. T h e y float on Earth's mantle, w h i c h exerts a b u o y a n t force and c o u n t e r s the force of gravity. D u r i n g rapid erosion of m o u n t a i n r a n g e s , s u m m i t s m a y b e uplifted t o even greater heights b e c a u s e the m a s s of the m o u n t a i n is reduced d u r i n g erosion, resulting in isostatic uplift. Refer to F i g u r e 2 2 . 1 7 b and Earth Issues 2 1 . 1 .

Vital Information from Other Chapters Congratulations! You have reached the point in y o u r mastery of geology at w h i c h y o u will begin to m a k e sense of n e w landscapes y o u m a y encounter during travel or on a hike. C h a p t e r 22 d r a w s heavily on all you h a v e learned previously about tectonics and climate (note the chapter title, L a n d s c a p e s ) . F o r that reason it will be even m o r e important than usual to review other chapters in conjunction with y o u r study of C h a p t e r 2 2 . U s e the following quick reference list to expedite y o u r review of important information from previous chapters. N o t e that this list is organized a r o u n d the major t h e m e of C h a p t e r 22: Landforms are shaped by the interaction of tectonic uplift and climate. Tectonics • H o w c o n v e r g e n t m a r g i n s p r o d u c e uplift (Figure 2.6) • L a n d f o r m s a s s o c i a t e d with faults and folds (Figures 7.6, 7.8, 7.10, 7 . 1 1 , 7.12, 7 . 1 7 , 7 . 1 8 , and 7.19) • Plate tectonics and s e d i m e n t a r y basins (Figure 5.4) Climate • P h y s i c a l a n d c h e m i c a l w e a t h e r i n g (Figure 16.11) • H o w m a s s w a s t i n g p r o c e s s e s s h a p e t h e l a n d s c a p e (Figure 16.17) • T h e r e l a t i o n s h i p a m o n g m o u n t a i n s , a t m o s p h e r i c circulation, and deserts (Figures 17.3, 1 9 . 1 , and 19.15) • Glacial l a n d s c a p e s (Figures 2 1 . 1 7 and 2 1 . 1 8 )

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C o m p a r e d to ice and w i n d , r u n n i n g w a t e r p l a y s the largest role in sculpting the Earth's land surface today. A review of the h y d r o l o g i c cycle (Figure 17.2) and t h e material in the last half of C h a p t e r 18 (Figures 18.8, 18.9, 18.23, 18.24, and 18.25), will reinforce your understanding of the information p r e s e n t e d in C h a p t e r 2 2 . T h i s review will also h e l p in preparation for a final e x a m .

Web Site Preview http://www.whfreeman.com/understandingearth5e Identify

North America's

Landforms

and

Understanding

Landscape

Evolution

are

Interactive

E x e r c i s e s definitely worth c o m p l e t i n g before or right after y o u r first lecture on this topic.

During Lecture O n e g o a l for lecture should b e t o leave t h e r o o m with g o o d a n s w e r s t o the C h a p t e r Preview questions. • To avoid getting lost in details, k e e p the b i g picture in m i n d . C h a p t e r 22 tells the story of h o w l a n d s c a p e s form. "Uplift p r o p o s e s and e r o s i o n d i s p o s e s . " • C h a p t e r 22 p r o v i d e s a virtual t o u r of l a n d f o r m s created by g e o l o g i c a l p r o c e s s e s . D u r i n g lecture, be alert to tips that will h e l p y o u sort o u t links b e t w e e n a l a n d s c a p e and a certain set of g e o l o g i c a l c i r c u m s t a n c e s . • E v e n m o r e i m p o r t a n t than the l a n d s c a p e features t h e m s e l v e s are t h e p r o c e s s e s that form e a c h feature. You already k n o w the p r o c e s s e s . Your goal in this c h a p ter is to u n d e r s t a n d h o w g e o l o g i c p r o c e s s e s w o r k t o g e t h e r to f o r m a particular k i n d o f l a n d s c a p e . E x a m p l e : T h e Tibetan plateau and the H i m a l a y a n M o u n tains are supported by b u o y a n t continental crust, t h i c k e n e d during c o n t i n e n t a l collision. E r o s i o n a l u n l o a d i n g of the s o u t h e r n m a r g i n of the T i b e t a n plateau m a y c o n t r i b u t e to the height of the p e a k s in the H i m a l a y a n r a n g e .

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 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 . . . • clearly described each landform (mountains, plateaus, Appalachian Valley and Ridge, and so on) and the geological processes that shape t h e m ? Use the text to check your notes. •

a d d e d visual material? S u g g e s t i o n s : M a k e s o m e s i m p l e s k e t c h e s to h e l p you learn the features that identify landforms in the chapter. Work s o m e of the s k e t c h e s into a c o m p a r i s o n chart like the o n e in Practice E x e r c i s e 2. Tip: T h e a n s w e r s for Practice E x e r c i s e 2, available at the end of this Study G u i d e , s h o w s o m e g o o d e x a m p l e s of s i m p l e l a n d f o r m sketches.

• created a brief big picture overview of this lecture (using a sketch or written outline)? H i n t : S e e Practice Exercise 1.


Study Tip: Learn by drawing Puting simplified sketches of l a n d s c a p e features into y o u r notes is a helpful w a y to learn and r e m e m b e r b e c a u s e it activates both visual and kinesthetic learning modalities. Visual learners 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 clues. F o r kinesthetic learners, m e m o r y is activated by the act of d r a w i n g . You learn as you l o o k and draw.

Intensive Study Session • First, get the big picture in m i n d . Take a look at F i g u r e 2 2 . 1 6 , w h i c h shows h o w tectonics a n d c l i m a t e interact t o p r o d u c e l a n d s c a p e . • Next, r e v i e w t h e m a i n c o n c e p t s of t h e chapter: relief (Figure 2 2 . 3 ) ; d e v e l o p m e n t of r i d g e s and valleys in folded m o u n t a i n s (Figure 2 2 . 1 0 ) ; erosion driven b y t h e b a l a n c e b e t w e e n s t r e a m p o w e r and s e d i m e n t l o a d (Figure 2 2 . 1 2 ) ; the d y n a m i c s of uplift (Figure 2 2 . 1 7 ) ; and classic m o d e l s of l a n d s c a p e evolution (Figure 2 2 . 1 9 ) . 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 t h e exercises a n d r e v i e w q u e s t i o n s .

Study Tip for Figure 22.17 and Earth Issues box 2 2 . 1 . Study Figure 2 2 . 1 7 and Earth Issues 22.1 as a p a c k a g e . Both relate uplift to climate change. T h e y reinforce each other and m a k e better sense together than they do when read separately.

• Take the visual field tour of l a n d f o r m s . Be sure y o u u n d e r s t a n d these landforms and k n o w w h a t they l o o k like: aretes (Figure 22.6), m e s a s (Figure 22.8), valley a n d r i d g e t o p o g r a p h y (22.10 and 2 2 . 1 1 ) , cuestas (Figure 22.14), h o g backs (Figure 22.15). 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 end of this chapter. They are s h o r t - a n s w e r questions and will not take long to c o m p l e t e if you k n o w t h e material. 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 4, 6, 8, and 9. We also r e c o m m e n d w o r k i n g on T h o u g h t Q u e s t i o n s 1, 3, 4, 8. • C o m p l e t e Practice E x e r c i s e s 1 a n d 2. 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 these exercises b e c a u s e they will h e l p you r e m e m ber the m o s t i m p o r t a n t ideas in the chapter. • A n s w e r 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 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 g o , but try to a n s w e r the question before you l o o k at the answer. P a y attention to the test-taking tips. T h e y will h e l p y o u do better on y o u r 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 t h e 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 . 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 . T h e G e o l o g y in P r a c t i c e exercise C a n E r o s i o n M a k e M o u n t a i n s H i g h e r ? is a g o o d review and will reinforce information p r e s e n t e d in this chapter.

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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 exams. The 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 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 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 . 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 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 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 i m p o r t a n t , review any 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 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 principal components of landscapes? • L a n d s c a p e s are described in t e r m s of their t o p o g r a p h y : e l e v a t i o n , the altitude of the surface of the E a r t h a b o v e sea level; relief, t h e difference b e t w e e n the h i g h e s t and the lowest spots in a r e g i o n ; and the varied l a n d f o r m s p r o d u c e d b y e r o s i o n and s e d i m e n t a t i o n b y rivers, glaciers, m a s s w a s t i n g , and w i n d . E l e vation is a b a l a n c e b e t w e e n tectonic activity and e r o s i o n rate.

Figure 2 2 . 3 . Relief is the difference between the highest and lowest elevations in a region. Three types of relief can be defined for a typical mountainous area.

How do the climate and plate tectonic systems interact to control landscape? • Tectonics (uplift and s u b s i d e n c e ) , e r o s i o n , c l i m a t e , and the t y p e of b e d r o c k control the evolution of landscapes. Positive and negative feedback m e c h a n i s m s b e t w e e n tectonic p r o c e s s and c l i m a t e d y n a m i c a l l y adjust t o c h a n g e . Water, w i n d , and ice act to e r o d e and transport r o c k material from the h i g h spots and d e p o s i t it in t h e low spots. Refer to F i g u r e 2 2 . 1 7 and E a r t h Issues 2 2 . 1 .


How do landscapes evolve? • T h e formation of river valleys and b e d r o c k erosion are controlled by a b a l a n c e b e t w e e n s t r e a m p o w e r and s e d i m e n t load. Refer to Figure 2 2 . 1 2 . • L a n d s c a p e s go t h r o u g h different p h a s e s d e p e n d i n g on tectonic activity and clim a t e . F o r e x a m p l e , a l a n d s c a p e with high relief forms if tectonic activity is high, w h i c h in turn stimulates erosion. Erosion at first e n h a n c e s relief, but over time, water, w i n d , and ice w e a r d o w n the high spots and fill in t h e low spots with s e d i m e n t . • Current views of l a n d s c a p e evolution e m p h a s i z e the d y n a m i c e q u i l i b r i u m b e t w e e n e r o s i o n and tectonic uplift. Uplift c o m p e t e s w i t h erosion. If uplift is faster, the m o u n t a i n will rise; if erosion is faster, the m o u n t a i n s are lowered. W h e n tectonics d o m i n a t e s , m o u n t a i n s are high a n d steep, and they r e m a i n so as long as the b a l a n c e is in favor of tectonics. W h e n the rate of deformation w a n e s , rates of erosion p r e d o m i n a t e , resulting in a gradual d e c r e a s e in both relief and m e a n elevation.

Why don't mountains sink? • Like an iceberg floating in water, m o s t m o u n t a i n s are supported by a buoyant, low-density root of continental crust, w h i c h floats in the d e n s e r m a n t l e . Isostatic r e b o u n d of thick continental crust in r e s p o n s e to u n l o a d i n g d u e to differential erosion m a y c a u s e m o u n t a i n p e a k s to rise even higher.

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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 e n d of this S t u d y G u i d e .

Exercise 1: Landscapes: Tectonic and climate interaction flowchart To review the basic relationships between landscape relief, tectonic activity (uplift), and erosion, fill in the flowchart with the following words. Tectonic activity H i g h relief L o w relief Erosion Physical weathering Chemical weathering

Exercise 2: Comparison of Some of the Landforms C h a p t e r 22 p r o v i d e s a virtual field trip e x p e r i e n c e . To m a s t e r t h e l a n d f o r m s d i s c u s s e d in this chapter, it will be helpful to d e v e l o p a c o m p a r i s o n chart identifying the i m p o r t a n t features that distinguish e a c h landform. Fill in the m i s s i n g information in t h e c o l u m n " I m p o r t a n t features." H i n t : U s e the text figures, c a p t i o n s , and a c c o m p a n y i n g text to h e l p y o u . T h e n m a k e a very s i m p l e sketch of e a c h landform. S k e t c h i n g is b o t h a t h i n k i n g tool and a great kinesthetic l e a r n i n g tool. W h e n y o u draw, y o u tap the part of the b r a i n that learns by m o v i n g .


Sketch (Keep it very simple.)

Landform

Important feature(s)

Mesa (See F i g u r e 22.8.)

A small plateau with sides. H e l d up by

Cuesta (See F i g u r e 22.14.)

A structurally controlled cliff. B e d s are s o m e w h a t tilted and alternate b e t w e e n w e a k and resistant layers. T h e resistant layers get u n d e r c u t b e c a u s e the w e a k layers t e n d to e r o d e faster.

Hogback (See F i g u r e 22.15.)

A structurally controlled cliff w i t h b e d s that are . R i d g e is m o r e or less

Valley r i d g e topography (See Figures 2 2 . 1 0 and 22.11.)

) form In y o u n g m o u n t a i n s , upfolds (_ J form valleys. ridges and d o w n f o l d s ( As tectonic activity moderates and erosion digs d e e p e r into t h e structures, the may form valleys and syncline ridges.

. slopes on all

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 t h e e n d of this S t u d y G u i d e . 1. D u r i n g t h e earliest stages of d e v e l o p m e n t of a river valley, the valley w o u l d h a v e a A. s i m p l e V-shaped profile. B . s i m p l e U - s h a p e d profile. C. low s t r e a m gradient. D. w e l l - e s t a b l i s h e d floodplain. H i n t : Refer t o F i g u r e 2 2 . 1 2 . 2. Elevation is t h e result of A. tectonic activity. B. the b a l a n c e b e t w e e n tectonic activity and erosion. C. erosion and deposition. D . deposition. 3. Relief is t h e A. difference b e t w e e n the h i g h e s t point and the lowest point in a region. B. difference b e t w e e n t h e h i g h e s t point in a region a n d sea level. C. a v e r a g e h e i g h t of a l a n d s c a p e . D. steepness of t h e slopes.

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4. W h i c h of t h e following are i m p o r t a n t controls on l a n d s c a p e e v o l u t i o n ? A. tectonics

C.

B. c l i m a t e

D. all of t h e a b o v e

t y p e of b e d r o c k

5. T h e E a r t h has t w o f u n d a m e n t a l levels on its surface. T h e y are t h e A . land and o c e a n s . B . crust and m a n t l e . C . m o u n t a i n s and t r e n c h e s . D . c o n t i n e n t a l crust and o c e a n b a s i n s . 6. M o u n t a i n belts are c o m m o n l y found in association with A. h o t spots.

C.

B. convergent boundaries.

D. mid-ocean ridges.

transform faults.

7. An e x a m p l e of a relatively short-term positive-feedback process in landscape evolution is A. a m o u n t a i n p e a k b e c o m i n g h i g h e r as a result of erosion. B. uplift c a u s i n g e r o s i o n to slow d o w n . C. rivers w a s h i n g s e d i m e n t s out of subsiding b a s i n s . D. interactions at c o n v e r g e n t b o u n d a r i e s resulting in low relief. H i n t : Refer t o F i g u r e 2 2 . 1 7 . 8. T h e A p p a l a c h i a n Valley a n d R i d g e p r o v i n c e is c h a r a c t e r i z e d by a l a n d s c a p e controlled by A. a series of regional faults that w e r e active millions of years a g o . B. glacial e r o s i o n and deposition, since a continental ice sheet o n c e c o v e r e d the entire region. C . w i n d e r o s i o n and d e p o s i t i o n partly c o n s t r a i n e d b y z o n e s o f d e n s e vegetation. D. an intricate series of anticlines and synclines. H i n t : Refer t o F i g u r e s 2 2 . 1 0 and 2 2 . 1 1 . 9. G i v e n that erosion by s t r e a m s is c o n t r o l l e d by a b a l a n c e b e t w e e n s t r e a m p o w e r and sedi m e n t load, then in steep, wet terrain, s t r e a m p o w e r is A. h i g h and s e d i m e n t is transported away. B. low and s e d i m e n t is transported away. C. high and s e d i m e n t is deposited. D. low a n d s e d i m e n t is d e p o s i t e d . H i n t : Refer t o F i g u r e 2 2 . 1 2 .

Test-Taking Tip: Think logically during exams L e a r n i n g should result from t h i n k i n g rather than from m e m o r i z a t i o n . On e x a m s it is e a s y to b e c o m e n e r v o u s and e x p e r i e n c e a brief m e m o r y lapse. At such t i m e s logical t h i n k i n g c a n save the day. H a v e y o u noticed that s o m e test items are written so that y o u can d e d u c e the answer logically? C o n s i d e r item 10. T h e clue. T h e fact that A and B are very similar is a clue that o n e of t h e m is probably the correct answer. (This o c c u r s as a b y - p r o d u c t of test c o n s t r u c t i o n . As the instructor d e v e l o p s t h e item, she w a n t s to build in t w o alternatives and h a v e y o u d e c i d e w h i c h alternative is correct.) T h e logic. It stands to r e a s o n that a formation with steeply d i p p i n g b e d s w o u l d be susceptible to erosion. If the cuesta has steeply d i p p i n g b e d s (as stated in A ) , then it w o u l d also be likely to e r o d e faster (as stated in C ) . But y o u k n o w that there is only o n e correct answer, so A and C c a n n o t both be correct. H e n c e the correct a n s w e r is likely to be B. T h e r e is, of c o u r s e , a caveat. L o g i c is not a substitute for study. A l s o , w h e n a p p l y i n g logic it is easy to be trapped by s o m e h i d d e n m i s c o n c e p t i o n y o u harbor. So c o n s i d e r logic a tool y o u can use occasionally, w h e n y o u n e e d it. It is a l w a y s a g o o d idea to have a full t o o l b o x !


10. C u e s t a s a n d h o g b a c k s are b o t h long ridges of erosion-resistant rock. T h e difference b e t w e e n t h e m i s that A . m u c h m o r e steeply d i p p i n g b e d s form the cuesta. B . m u c h m o r e steeply d i p p i n g o r vertical b e d s form the h o g b a c k . C. t h e c u e s t a tends to e r o d e faster. D . h o g b a c k s are a s y m m e t r i c a l . 11. T h e d e b a t e a b o u t t h e d y n a m i c interactions b e t w e e n uplift and c l i m a t e is fueled by the o b s e r v a t i o n that g l o b a l c o o l i n g over the last few tens of millions of years coincides with t h e uplift of t h e Tibetan plateau. O n e side of the d e b a t e argues that there w a s negative f e e d b a c k b e t w e e n uplift and c l i m a t e b e c a u s e A. h i g h e r o s i o n rates led to the r e m o v a l of carbon d i o x i d e — a n important g r e e n h o u s e g a s — f r o m t h e a t m o s p h e r e , w h i c h in turn led to further c o o l i n g , increased precipitation, and erosion. B. h i g h e r erosion rates resulted in h i g h e r rates of uplift d u e to isostatic adjustments. C. c o o l e r c l i m a t e s a l l o w e d glaciers to g r o w on the m o u n t a i n p e a k s , and t h e w e i g h t of t h e ice isostatically d e p r e s s e d the m o u n t a i n s . D. none of the above. 12. B e r y l l i u m - 1 0 is u s e d as a m e t h o d to d a t e t h e a g e of river-terrace surfaces b e c a u s e beryllium-10 A. B. C. D.

is radioactive a n d its decay is d e t e r m i n e d by e x p o s u r e to sunlight. is slowly l e a c h e d out of r o c k o n c e it is e x p o s e d at t h e surface. is slowly r e l e a s e d from t h e r o c k s as they are u n c o v e r e d by erosion. a c c u m u l a t e s in t h e u p p e r surface of t h e r o c k the longer it is e x p o s e d at t h e surface t o c o s m i c ray b o m b a r d m e n t .

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CHAPTER 23

The Human Impact on Earth's Environment


Before Lecture Before y o u attend lecture, be sure to 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 an efficient preview, u s e t h e 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 o r i g i n of oil a n d natural g a s ? Brief a n s w e r : Oil and natural gas form from organic matter deposited in marine s e d i m e n t s . T h e organic matter is pressure c o o k e d as the s e d i m e n t s are buried, and the organic materials are transformed into liquid and g a s e o u s h y d r o c a r b o n s . Figure 23.6 illustrates h o w the organic fluids migrate and a c c u m u l a t e in geologic traps. • W h y is t h e r e c o n c e r n a b o u t the w o r l d ' s oil s u p p l y ? Brief a n s w e r : Oil is a n o n r e n e w a b l e r e s o u r c e and it is being used faster than natural p r o c e s s e s g e n e r a t e it. T h e key issue is not w h e n oil runs out but w h e n oil production stops rising. Refer to Figure 2 3 . 1 0 and the a c c o m p a n y i n g discussion of the controversy c o n c e r n i n g H u b b e r t ' s peak. • W h a t is the origin of coal a n d h o w big a resource is it? Brief a n s w e r : C o a l is formed by the c o m p a c t i o n and mild m e t a m o r p h i s m of buried terrestrial w e t l a n d vegetation. C o a l has supplied an increasing proportion of U . S . energy n e e d s since 1975. Refer to Figures 23.12 and 2 3 . 1 3 . • W h a t a r e t h e p r o s p e c t s for alternative e n e r g y s o u r c e s ? Brief a n s w e r : Alternative sources include nuclear, hydroelectric, wind, solar, b i o m a s s , and g c o t h c r m a l . Taken together, these sources currently provide only about 14 percent of w o r l d e n e r g y n e e d s . W i t h a d v a n c e s in t e c h n o l o g y and safety plus reductions in cost, r e n e w a b l e sources of e n e r g y and nuclear p o w e r can b e c o m e major sources of energy in this century. • W h a t is a c i d r a i n ? Brief a n s w e r : A c i d rain is rain that contains sulfuric a n d / o r nitric acid and is f o r m e d w h e n e m i s s i o n s from the burning of fossil fuels react with rainwater. • W h a t is o z o n e d e p l e t i o n , a n d w h y s h o u l d it c o n c e r n u s ? Brief a n s w e r : O z o n e a b s o r b s harmful ultraviolet radiation s t r e a m i n g in from the Sun. U s e d as refrigerants and in industrial p r o c e s s e s , C F C s are c h e m i c a l s that r e a c h the upper a t m o s p h e r e a n d react w i t h o z o n e faster than o z o n e is generated. F i g u r e 23.21 shows h o w levels of o z o n e in the u p p e r a t m o s p h e r e have d e c r e a s e d relative to 1980. • H o w m u c h g l o b a l w a r m i n g will there b e i n t h e twenty-first century, a n d w h a t will be t h e c o n s e q u e n c e s of it? T h e r a n g e accepted by m o s t experts is from 1.4 to 5.6 degrees centigrade. C o n s e q u e n c e s will i n c l u d e rising sea level and shrinking ice caps at t h e poles, as well as species extinction and shifts in habitat and e c o s y s t e m s . Refer to Figures 2 3 . 2 2 and 23.23 and Table 2 3 . 1 . • W h a t s h o u l d be t h e g o a l of e n e r g y a n d e n v i r o n m e n t a l policies? Brief a n s w e r : Policies should g u i d e the nations of t h e world t h r o u g h the transition from h y d r o c a r b o n fuels to less polluting, sustainable e n e r g y sources. In particular, carbon d i o x i d e e m i s s i o n s should be r e d u c e d to d e c r e a s e t h e i m p a c t of global c l i m a t e c h a n g e . Refer to E a r t h Policy 2 3 . 2 .

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Vital Information from Other Chapters T h e f o r m a t i o n of oil, natural gas, and coal involves s e d i m e n t a r y and m e t a m o r p h i c processes that y o u learned a b o u t in C h a p t e r s 5 and 6. T h e c a r b o n cycle, g r e e n h o u s e effect, and discussions of the c l i m a t e c h a n g e in C h a p t e r 15 are essential b a c k g r o u n d m a t e r i a l for Chapter 23. Toxic a n d nuclear w a s t e c o n t a m i n a t i o n b r i n g s us b a c k to issues related to groundwater and s t r e a m transport, d i s c u s s e d in C h a p t e r s 17 and 18.

During Lecture • K e e p the big picture in m i n d as y o u t a k e notes. C h a p t e r 23 tells the story of e n e r g y r e s o u r c e s that p o w e r o u r e c o n o m y and h o w the activities of o u r civilization affect o u r e n v i r o n m e n t . P r o b l e m s s u c h as the d e p l e t i o n of p e t r o l e u m as an energy r e s o u r c e and the potential for c l i m a t e c h a n g e that arises from e n e r g y c o n s u m p t i o n are explored. 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 the functions of E a r t h s y s t e m s , so m u c h so that s o m e scientists a r g u e that we are p l u n g i n g o u r w o r l d into a n e w g e o l o g i cal e p o c h , the A n t h r o p o c e n e . F o c u s on 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, a n d f e e d b a c k s within e a c h s y s t e m . • B e c a u s e this c h a p t e r covers the h u m a n i m p a c t on E a r t h ' s e n v i r o n m e n t , w h i c h is linked to cultural, social, and e c o n o m i c i s s u e s , there m a y be opportunities for discussion or for 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 t a k e part in these activities. • T h e instructor m a y m a k e use of a discussion or debate to address s o m e of these issues. S u m m a r i z e the important social issues and arguments in your notes. Pay particular attention to capturing arguments that contradict your own. Circle these arguments and return to t h e m later for study and consideration. True learning often involves changing our conceptions (and particularly our misconceptions).

We Americans

think we

are pretty good!

We want to build a house,

we cut down some trees.

We want to build a fire, we dig a little coal. But when we run out of all these things, then we will find out just how good we

really are.

— W I L L ROGERS

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 both 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 . . . • written a s u m m a r y of w h a t is c o v e r e d in this lecture? 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 global e n v i r o n m e n t and t h e potential for global 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 o n a n E a r t h 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 information in C h a p t e r 23 is relevant to the i s s u e . Try to d e v e l o p a p o s i t i o n that is b a s e d on r e a s o n and is consistent with existing s c i e n c e . • c a p t u r e d a r g u m e n t s a b o u t h o w we s h o u l d u s e o u r n o n r e n e w a b l e e n e r g y a n d materials that c a m e u p i n the lecture o r d u r i n g c l a s s r o o m d i s c u s s i o n s o r d e b a t e ? • created a brief big picture overview of this lecture and c h a p t e r ?


Intensive Study Session Set priorities for s t u d y i n g this chapter. Give h i g h e s t priority to activities that involve a n s w e r ing 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. • Text. This is a chapter with unusually powerful implications for our future as h u m a n beings. To gain an overview of those implications, read the sections called Energy Sources and Rise of the Fossil-Fuel E c o n o m y and examine Figures 23.2, 2 3 . 3 , 2 3 . 4 , 2 3 . 5 , 2 3 . 7 , 2 3 . 9 , 23.10, and 23.13. T h e n read the two Earth Policy features. Earth Policy 23.1 deals with toxic and nuclear waste contamination and Earth Policy 23.2 reviews the Kyoto Accords and the politics of climate change. T h i n k about the implications of these figures. It is time well spent. • R e v i e w t h e key figures on the formation of e n e r g y r e s o u r c e s . T h e y illustrate t h e f o r m a t i o n of oil ( F i g u r e 23.6) a n d coal (Figure 2 3 . 1 2 ) . You have to understand t h e s e figures to a n s w e r the review questions. • R e v i e w Q u e s t i o n s . A n s w e r the R e v i e w Questions to c h e c k your understanding of the lecture. C h e c k y o u r a n s w e r s as you go, but try to a n s w e r each question before y o u l o o k at the answer. N o t i c e the test-taking tips that are interspersed with the questions. T h e y are designed to help you do better on y o u r next e x a m . • 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 a n d W e b 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 . 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 Fossil Fuels Cycle will h e l p y o u inventory major c o m p o n e n t s of this cycle. T h e G e o l o g y in P r a c t i c e E x e r c i s e The Tempest in the Teapot—Finding Oil illustrates h o w oil reservoirs are discovered in t h e c o n t e x t of a bit of history. D i d you k n o w that it takes about t w o p o u n d s of coal to produce 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 the internet, and that this action releases four p o u n d s of 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 ? L e a r n all about it in t h e G e o l o g y in P r a c t i c e exercises.

Exam Prep Materials in this section are m o s t useful d u r i n g preparation for e x a m s .

Final Exam Prep Each semester, in o n e w e e k , y o u get to take an e x a m in e a c h of y o u r courses. M o s t of the exams are c o m p r e h e n s i v e finals that cover t h e entire semester. D e a l i n g with finals w e e k successfully can be a c h a l l e n g e . H e r e are s o m e tips that will e n s u r e that y o u do y o u r best w o r k during finals w e e k .

Tips for Surviving Finals Week • B e o r g a n i z e d and s y s t e m a t i c . U s e t h e P i n a l 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 ) t o h e l p y o u get o r g a n i z e d for finals. U s e 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 ) for e v e r y c o u r s e in w h i c h the final e x a m will be an i m p o r t a n t factor in d e t e r m i n i n g your grade. • Stick to priorities. Say no to distractions. • Build in m o m e n t s of relaxation. Take regular short b r e a k s , exercise, and be sure to get e n o u g h sleep. • Be confident. By now you have built up good study habits. You are a competent learner.

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T h e C h a p t e r S u m m a r y and the 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 . N e x t , w o r k on t h e R e v i e w Q u e s t i o n s . C o m p l e t e the q u e s t i o n s j u s t as you would 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 t h e questions, score t h e m . Finally, a n d m o s t i m p o r t a n t , review any 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 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.

Exam Prep Tip: Get Organized for Finals Week T h e e n d of s e m e s t e r is a p p r o a c h i n g . It's t i m e to get o r g a n i z e d for taking e x a m s in all y o u r c o u r s e s . Take a look at 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). There you will find m a n y useful ideas about h o w to be successful as you enter the h o m e stretch of y o u r semester.

Our entire

society

rests

our land,

our forests,

influences

our health,

is

dependent

upon—our

water,

and our minerals.

upon—and

How we

use

resources

security,

economy,

and

these

well-being.

— J O H N F. KENNEDY, FEBRUARY 2 3 , 1961

Chapter Summary What is the origin of oil and natural gas? • O i l a n d natural gas f o r m from organic m a t t e r d e p o s i t e d in m a r i n e s e d i m e n t s . T h e o r g a n i c materials are b u r i e d as the s e d i m e n t a r y layers g r o w in t h i c k n e s s . Heat, p r e s s u r e , and bacterial action transform the o r g a n i c m a t t e r into fluid h y d r o c a r b o n s . T h e fluid h y d r o c a r b o n s tend to m i g r a t e out of the s o u r c e r o c k and a c c u m u l a t e in g e o l o g i c traps that confine the fluids w i t h i n i m p e r m e a b l e barriers. Refer to F i g u r e 2 3 . 6 .

Why is there concern about the world's oil supply? • O i l is a n o n r e n e w a b l e r e s o u r c e : it will be depleted faster t h a n n a t u r e c a n r e p l e n i s h it. • P e t r o l e u m r e s o u r c e s will be significantly depleted within a b o u t a century.

What is the origin of coal, and how big a resource is it? • C o a l is f o r m e d by the c o m p a c t i o n and m i l d m e t a m o r p h i s m of b u r i e d w e t l a n d vegetation. T h e p r o c e s s b y w h i c h c o a l forms b e g i n s w i t h the d e p o s i t i o n o f p l a n t matter. P r o t e c t e d from c o m p l e t e d e c a y and o x i d a t i o n in a w e t l a n d envir o n m e n t , the d e p o s i t is buried and c o m p r e s s e d into peat. S u b j e c t e d to further burial, p e a t u n d e r g o e s mild m e t a m o r p h i s m , w h i c h transforms i t successively into lignite, s u b b i t u m i n o u s a n d b i t u m i n o u s (soft) coal, a n d anthracite (hard) c o a l . A s t h e deposit b e c o m e s m o r e d e e p l y buried, the t e m p e r a t u r e rises and structural d e f o r m a t i o n m a y occur. Refer t o F i g u r e 2 3 . 1 2 . • D o m e s t i c c o a l r e s o u r c e s in the U n i t e d States w o u l d last for a few h u n d r e d y e a r s at current rates of u s e — a b o u t a billion tons p e r year. • E n v i r o n m e n t a l c o n c e r n s a s s o c i a t e d with the u s e of c o a l i n c l u d e m i n e reclam a t i o n , pollution, acid e m i s s i o n s that are p r e c u r s o r s of acid rain, a n d c a r b o n d i o x i d e (a g r e e n h o u s e gas) e m i s s i o n s .


What are the prospects for alternative sources of energy? • Alternative e n e r g y sources include nuclear, g e o t h e r m a l , hydroelectric, w i n d , b i o m a s s , and solar s o u r c e s . • T a k e n together, t h e s e s o u r c e s currently p r o v i d e only 14 p e r c e n t of w o r l d energy needs. • As with fossil fuels, there are significant e c o n o m i c , technological, environmental, a n d political c o n c e r n s associated w i t h alternative e n e r g y r e s o u r c e s .

What is acid rain? • A c i d rain is p r o d u c e d m a i n l y from e m i s s i o n s of sulfur-containing g a s e s . A c i d rain can c a u s e n o t i c e a b l e d a m a g e to forest and lake ecology, fabrics, paints, metals, and building materials. Refer to F i g u r e s 2 3 . 1 8 and 2 3 . 1 9 .

What is ozone depletion, and why should it concern us? • A well-defined, l a r g e - s c a l e z o n e of o z o n e d e p l e t i o n h a s f o r m e d w i t h i n the stratosphere due to complex interactions with chlorofluorocarbon (CFC) c o m p o u n d s . R e f e r t o F i g u r e 2 3 . 2 0 . F i g u r e 2 3 . 2 1 s h o w s h o w levels o f o z o n e i n t h e u p p e r a t m o s p h e r e h a v e d e c r e a s e d relative t o 1 9 8 0 . S t r a t o s p h e r i c o z o n e shields E a r t h ' s surface from d a m a g i n g ultraviolet r a d i a t i o n a n d this shield i s b e i n g c o m p r o m i s e d d u e t o o z o n e depletion. T h e M o n t r e a l P r o t o c o l is an i n t e r n a t i o n a l treaty that a p p e a r s to h a v e dealt successfully w i t h this envir o n m e n t a l disaster.

How much global warming will there be in the twenty-first century, and what will be the consequences? • T h e r a n g e a c c e p t e d by m o s t experts is from 1.4 to 5.6 degrees c e n t i g r a d e . • C o n s e q u e n c e s will i n c l u d e a rising sea level and shrinking ice c a p s at the poles, as well as species extinction and shifts in habitat and e c o s y s t e m s . Refer t o F i g u r e s 2 3 . 2 2 a n d 2 3 . 2 3 and Table 2 3 . 1 .

What should be the goal of energy and environmental policies? • Policies should g u i d e the nations of the w o r l d t h r o u g h the transition from h y d r o c a r b o n fuels to less polluting, sustainable energy sources. In particular, carbon d i o x i d e e m i s s i o n s should be r e d u c e d to d e c r e a s e the i m p a c t of global climate c h a n g e . M o r e efficient u s e of energy, greater u s e of natural g a s , safer nuclear energy, and clean coal t e c h n o l o g y w o u l d facilitate this transition. Refer to E a r t h Policy 2 3 . 2 .

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 S t u d y G u i d e . 1. W h a t is t h e s e q u e n c e from low to h i g h g r a d e for t h e transformation of plant matter into hard coal? A. plants, peat, lignite B . peat, lignite, b i t u m i n o u s , anthracite C. b i t u m i n o u s , anthracite, peat, lignite D . anthracite, b i t u m i n o u s , lignite, peat H i n t : Refer t o F i g u r e 2 3 . 1 2 .

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Test-Taking Tip: Use what you know to guess what you don't know W h e n confronted with an e x a m item like Review Q u e s t i o n 1, y o u often d o n ' t have to r e m e m b e r the entire s e q u e n c e to a n s w e r the item correctly. S u p p o s e , for example, that y o u r e m e m b e r for sure that anthracite is the final h i g h - g r a d e p r o d u c t in the series In that c a s e you c a n c h e c k B with confidence even if y o u c a n ' t r e m e m b e r the rest of the sequence.

2. T h e following are all energy s o u r c e s . W h i c h o n e is N O T a fossil fuel? A . natural gas

C. uranium

B. coal

D . oil

3. W h i c h of the following is N O T a c o n s e q u e n c e of fossil fuel c o n s u m p t i o n ? A. mine reclamation B . o z o n e depletion i n o u r a t m o s p h e r e C. disposal of residual ash from the b u r n i n g of c o a l D . acid rain 4. Oil and natural gas are m o s t l y found in s e d i m e n t a r y r o c k s d e p o s i t e d in A . t h e d e e p ocean. B. river deltas and on the c o n t i n e n t a l shelf. C. wetlands. D . large lakes. 5. A l l of the following are effective oil traps E X C E P T A. faults. B . anticlines. C. salt d o m e s . D . h o r i z o n t a l s e d i m e n t a r y and volcanic b e d s . H i n t : Refer to F i g u r e 2 3 . 5 . 6. Oil p r o d u c t i o n in t h e U n i t e d States r e a c h e d a m a x i m u m in A . 1960.

C.

B . 1970.

D . 1990.

1980.

7. T h e m o s t i m p o r t a n t s o u r c e of U . S . e n e r g y is A. coal.

C. oil.

B . n u c l e a r power.

D . h y d r o e l e c t r i c power.

H i n t : Refer t o F i g u r e 2 3 . 3 . 8. H o w m a n y U . S . gallons are c o n t a i n e d in o n e barrel of oil? A. 16 gallons

C.

B . 2 5 gallons

D . 5 5 gallons

42 gallons

H i n t : Refer t o F i g u r e 2 3 . 5 . 9. T h e U n i t e d States r a n k s

in oil r e s e r v e s .

A. first

C.

B. second

D . tenth

eighth

H i n t : Refer to the text section Distribution of Oil R e s e r v e s and F i g u r e 23.7. 10. I m p o r t a n t factors contributing to the f o r m a t i o n of c o a l from v e g e t a t i o n are A . h e a t a n d oxidation. B . c o m p a c t i o n b y burial and heat. C. b i o l o g i c a l activity and dissolution. D . h y d r o t h e r m a l alteration and m e t a m o r p h i s m .


11. F r o m w h a t p r o c e s s is coal derived? A. t h e d e c a y of m a r i n e plants and a n i m a l m a t t e r B. t h e burial, c o m p r e s s i o n , and h e a t i n g of plant matter d e p o s i t e d in w e t l a n d s C. d e p o s i t i o n and m e t a m o r p h i s m of m a r i n e l i m e s t o n e s D. t r a n s p o r t of o r g a n i c matter by rivers to their delta H i n t : Refer to Figure 23.12. 12. A c i d rain forms w h e n

from t h e c o m b u s t i o n of coal and p e t r o l e u m c o m -

b i n e s w i t h rainwater. A. hydrogen

C.

B. sulfur d i o x i d e

D. nitrogen

oxygen

13. T h e potential for c h e m i c a l w e a t h e r i n g is

by acid rain.

A. not effected

C.

increased

B. decreased

D . neutralized

14. T h e o c c u r r e n c e of acid rain is m o s t influenced by the A. r e l e a s e of r a d i o i s o t o p e s by nuclear p o w e r plants. B. b u r n i n g of high-sulfur coals. C. b u r n i n g of low-sulfur c o a l s . D . w e a t h e r i n g o f feldspars.

Test-Taking Tip: Use what you know to guess what you don't know The p r e s e n c e of t w o alternatives d e a l i n g with the s a m e thing (in this c a s e "sulfur c o a l s " ) is a hint that o n e of t h e m is p r o b a b l y the correct answer.

15. Predict w h i c h well w o u l d give the best potential for oil and gas p r o d u c t i o n .

A. well A B. well B C.

wellC

D. well D H i n t : Refer t o F i g u r e 23.6.

251

252

PART II

C H A P T E R 23

16. W h y do scientists suspect that C F C s are the source of o z o n e depletion in the stratosphere? A . C F C s c o n t a i n c h l o r i n e , w h i c h reacts v i g o r o u s l y w i t h o z o n e , w h i l e measurements of o z o n e in t h e s t r a t o s p h e r e s h o w it d e c r e a s i n g at the s a m e t i m e that C F C s are increasing. B. C F C s f o r m a m i x t u r e with volcanic g a s e s in t h e l o w e r a t m o s p h e r e , w h i c h rises to t h e stratosphere a n d reacts with o z o n e . C . C F C s c o n c e n t r a t e U V radiation, splitting apart o z o n e m o l e c u l e s . D. C F C s i n c r e a s e the a l b e d o of the stratosphere, w h i c h r e d u c e s the solar radiation r e q u i r e d for the p r o d u c t i o n of o z o n e . 17. O z o n e is a very reactive g a s , so as a pollutant in t h e l o w e r a t m o s p h e r e , o z o n e presents a significant health hazard. W h y d o e s o z o n e exist in the s t r a t o s p h e r e ? A. It constantly is leaking from the t r o p o s p h e r e , w h e r e it is p r o d u c e d , up to the stratosphere. B. It is f o r m e d from the r e l e a s e of gases out of the m i c r o m e t e o r i c dusts that bombard the u p p e r a t m o s p h e r e . C. It is f o r m e d c o n t i n u o u s l y in the stratosphere by solar radiation, a n d it c a n n o t mix or react with other gases b e c a u s e of the thin a t m o s p h e r e at that altitude. D. It is p r o d u c e d constantly from the o c e a n s and rises t h r o u g h t h e t r o p o s p h e r e to the stratosphere. 18. 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 Earth 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. burning process generates C 0 . 2

D. s m o k e given off by b u r n i n g insulates the Earth 19. Toxic and radioactive c h e m i c a l s leaking from the H a n f o r d Superfund site h a v e A. r e m a i n e d confined to the H a n f o r d site. B. m i g r a t e d in the aquifer to the C o l u m b i a River. C. b e e n successfully c l e a n e d u p . D . r e a c h e d Seattle. H i n t : R e a d E a r t h Policy 2 3 . 1 . 2 0 . If we c o n t i n u e our r e l i a n c e on fossil fuels, projec tions of a t m o s p h e r i c c a r b o n d i o x i d e concentratioi in 2 1 0 0 are likely to be a b o u t

and the

average surface t e m p e r a t u r e will likely rise about m o r e than in 1990. A. 6 0 0 p p m and 3 ° C B . 9 5 0 p p m and 4 . 5 ° C C. 5 0 0 p p m and 2 ° C D . 7 5 0 p p m and 3 ° C H i n t : Refer t o Figure 2 3 . 2 2 .

Figure 2 3 . 2 2 . Projections of (a) atmospheric carbon dioxide concentrations and (b) average surface temperatures for the next 100 years.

APPENDIX A

Eight-Day Study Plan* (Make a copy for every e x a m you take.)

H e r e is a g u i d e y o u can use to p r e p a r e for y o u r e x a m s . E v e r y o n e develops their o w n a p p r o a c h to p r e p a r i n g for e x a m s ; feel free to adapt these ideas to y o u r particular n e e d s and situation. T h e basic idea is to c o n d u c t y o u r preparation in a systematic fashion with focus on t h e m o s t i m p o r t a n t m a t e r i a l . O u r plan a c c o m p l i s h e s this b y dividing the m a t e r i a l e q u a l l y and suggesting h o w to incorporate the E x a m P r e p materials p r o v i d e d in this Study G u i d e for e a c h text chapter.

8 Days Before the Exam: Get Organized! S t e p 1 : C l a r i f y t h e t a s k . D e t e r m i n e w h a t t y p e o f e x a m y o u ' l l b e taking b y briefly a n s w e r ing the following questions. 1. T h i s e x a m will cover (list each chapter to be c o v e r e d ) :

2. Material and k i n d s of skills to be particularly e m p h a s i z e d (list chapters/ideas/skills the instructor said w o u l d be particularly important):

3. Q u e s t i o n format will be (check one that applies): • • •

M u l t i p l e choice True-false Essay

• •

Thought problems O t h e r (specify)

'Adapted with permission from the University Learning Center, University of Arizona.

253

254

Appendix A: Eight-Day Study Plan

4. Review session is scheduled for (enter date here and be sure to attend): S t e p 2 : D i v i d e t h e m a t e r i a l y o u m u s t review into four e q u a l Parts: A , B , C , and D .

7 Days Before the Exam: Review Part A.

B e g i n y o u r review with the material

in Part A. Do the following for e a c h c h a p t e r y o u review. 1. C h a p t e r S u m m a r y . To get yourself started, read the C h a p t e r S u m m a r y ( E x a m Prep section of this g u i d e ) for the c h a p t e r y o u w a n t to review. 2. R e v i e w Q u e s t i o n s . A n s w e r the R e v i e w Q u e s t i o n s ( E x a m P r e p section of this guide) to see w h e r e y o u are with the material. F o r c e yourself to a n s w e r all questions for the chapter w i t h o u t referring to the a n s w e r key. C o r r e c t y o u r a n s w e r s only after y o u have tried all the questions. Be sure to review carefully any questions y o u m i s s e d . Correct the misc o n c e p t i o n that resulted in the error. 3 . L e c t u r e N o t e s . R e v i e w y o u r lecture notes and the a n n o t a t i o n s y o u m a d e i n the margins b y a s k i n g yourself q u e s t i o n s . 4. P r a c t i c e E x e r c i s e s . T h i s m a y be a g o o d t i m e to r e d o s o m e of t h e P r a c t i c e Exercises in this S t u d y G u i d e . 5. R e v i e w visual m a t e r i a l s a n d k e y figures. R e v i e w the visual m a t e r i a l in your notes. Test yourself by seeing if y o u c a n r e c o n s t r u c t k e y text figures from m e m o r y . 6. Self-Test. S p e n d as m u c h of y o u r study t i m e as p o s s i b l e q u i z z i n g yourself.

6 Days Before the Exam: Review Part B.

R e p e a t the instructions for D a y 7.

If y o u h a v e p r o b l e m s with t h e material, see the instructor at the next o p e n office hour.

5 Days Before the Exam: Review Part C .

R e p e a t the instructions for D a y 7.

If y o u h a v e p r o b l e m s with the material, see the instructor at t h e next o p e n office hour.

4 Days Before the Exam: Review Part D.

R e p e a t t h e instructions for D a y 7.

I f ' y o u h a v e p r o b l e m s with the material, see the instructor at the next o p e n office hour.

3 Days Before the Exam: Review all four parts.

R e v i e w Parts A , B , C, and

D fully. Prioritize y o u r t i m e . F o c u s on i m p o r t a n t material that will be c o v e r e d . W o r k h a r d e s t w h e r e y o u are least sure of yourself. If y o u h a v e p r o b l e m s with t h e material, see t h e instructor at the next o p e n office hour.

2 Days Before the Exam: Review all four parts.

R e v i e w Parts A, B , C, and

D fully. Prioritize y o u r t i m e . If y o u h a v e p r o b l e m s with the material, see the instructor at t h e next o p e n office hour.

Night Before the Exam:

B e sure y o u get the a m o u n t of sleep y o u n e e d to b e alert

and p e r f o r m at y o u r best. You d o n ' t n e e d to c r a m . Just stay focused.

Zero Hour:

You have p r e p a r e d well. A l l o w yourself to b e confident. Stay f o c u s e d

a n d confident during the e x a m . U s e y o u r best-test taking strategies.

APPENDIX B

Final Exam Prep* Worksheet (To be c o m p l e t e d 3 w e e k s p r i o r to final e x a m )

1. C o u r s e s h e e t s : In y o u r n o t e b o o k set up a separate sheet of p a p e r for e a c h c o u r s e you are taking. L a b e l t h e top of e a c h sheet with the c o u r s e and the g r a d e y o u presently h a v e (be realistic, not hopeful). 2 . D a t e : N o t e t h e date o f the f i n a l u n d e r e a c h c o u r s e n a m e . 3. C o m p r e h e n s i v e finals: M a r k with a " C " each c o u r s e with a c o m p r e h e n s i v e final. 4. E x a m format: Identify the format of the e x a m (multiple choice, essay, and so on) under the date of the final for each course. 5. Task: Identify t h e levels of thinking expected. H i n t : P r e v i o u s m i d t e r m s are y o u r ultim a t e r e s o u r c e . List all k i n d s of q u e s t i o n s . E s t i m a t e w h a t p e r c e n t of total points will be devoted to e a c h k i n d of thinking. • A p p l i c a t i o n to r e a l - w o r l d situations • P r o b l e m solving • Critical thinking • U n d e r s t a n d i n g principles • M e m o r y of basic facts 6. R a n k finals in i m p o r t a n c e : In the u p p e r r i g h t - h a n d c o r n e r of e a c h sheet, r a n k in order the m o s t critical and i m p o r t a n t final to the least i m p o r t a n t final—the final that will m a k e the least difference i n y o u r g r a d e . ( B e a w a r e o f h o w m u c h i m p a c t y o u r f i n a l e x a m has on y o u r overall class grade.)

'Adapted with permission from the University Learning Center, University of Arizona.

255

256

Appendix B: Final Exam Prep

7. L i s t W h a t t h e T e s t W i l l C o v e r : F o r e a c h c o u r s e on e a c h sheet, list everything the test will cover; r e m e m b e r w h i c h e x a m s are c o m p r e h e n s i v e . •

Handouts?

•

C h a p t e r s ? ( W h i c h ones?)

•

Lectures?

•

Discussions?

•

Other?

C h e c k y o u r syllabi to be sure y o u h a v e not left o u r any i m p o r t a n t material. 8. D r a w a line b e n e a t h this list. T h e n list w h a t y o u still h a v e left to do for that particular course. •

W h i c h chapters do y o u still h a v e to p r e v i e w ?

•

W h i c h lecture notes do y o u need to review and u p d a t e ?

•

W h i c h 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 do y o u n e e d to c o m p l e t e ?

•

W h i c h labs do y o u still have to finish?

•

W h a t p a p e r s do y o u still h a v e to w r i t e ?

9. D r a w a n o t h e r line. N o w list the test p r e p a r a t i o n strategies y o u will use to study for the e x a m — s t u d y g r o u p s or study patterns, self-questioning u s i n g the annotations, mapping, charting, q u e s t i o n s and a n s w e r s , c o n c e p t cards, g o i n g o v e r old tests and quizzes, and making up your own problems. 10. N o w fill in the c a l e n d a r by identifying e x a m s , finals, and d u e dates for p a p e r s . Each day y o u n e e d to do s o m e t h i n g from n o . 8, but y o u will also n e e d to study and review for the finals at least t w o h o u r s a day. Be sure to use all of y o u r available times—weekends, waiting t i m e , and so on.

Work Toward These Goals •

Finish all w o r k u n d e r n o . 8 (Chapter P r e v i e w s , Practice E x e r c i s e s , Review Questions, and so on) o n e w e e k prior to y o u r first final. R e v i e w all lecture notes by asking yourself the questions out loud or by h a v i n g s o m e o n e quiz y o u five days prior to y o u r first final (allow t w o to three hours).

•

D i v i d e the w o r k that r e m a i n s so that y o u do an E i g h t - D a y S t u d y Plan for each c o u r s e that y o u assigned a h i g h priority in n o . 6.

•

R e m o v e the distractions from y o u r life. T h i s is not the w e e k to be captured by TV or other addictions. Stick to y o u r priorities. Tell friends and family that you need to focus all y o u r energy on y o u r finals until they are over.

•

A v o i d b u r n o u t . B u i l d t i m e into y o u r s c h e d u l e for a d e q u a t e sleep, relaxation, and exercise.


CHAPTER 1

Answer to Practice Exercise A. T h e highest point on Earth is M o u n t Everest in the H i m a l a y a n m o u n t a i n s of Asia. B. T h e lowest point on Earth is the M a r i a n a s T r e n c h in the Pacific O c e a n south of Japan. C. T h e M a r i a n a s T r e n c h is 2 1 8 2 m e t e r s l o w e r relative to sea level than M o u n t E v e r e s t is high. D. T h e total t o p o g r a p h i c relief on Earth is the s u m of the highest and lowest p o i n t s : 11,030 m e t e r s d e e p + 8,848 m e t e r s high = 19,878 m e t e r s ( 6 5 , 2 1 6 feet).

Answers to Review Questions 1. B. A h y p o t h e s i s p r o v i d e s an opportunity for feedback a n d testing. 2. A. A h y p o t h e s i s is a tentative explanation that can h e l p focus attention on plausible features and relationships of a w o r k i n g m o d e l . If a h y p o t h e s i s is eventually confirmed by a large b o d y of data, it m a y be elevated to a theory. T h e o r i e s are a b a n d o n e d w h e n s u b sequent investigations show t h e m to be false. C o n f i d e n c e g r o w s in theories that withstand repeated tests a n d are able to predict the results of n e w e x p e r i m e n t s . 3. B. He m e a s u r e d the difference in the angle of sunlight hitting the E a r t h on the s u m m e r solstice at two different places in E g y p t . Refer to F i g u r e 1.2. 4. C. T h e present is the key to the past. G e o l o g i c p r o c e s s e s we see in action today have w o r k e d in m u c h t h e s a m e w a y t h r o u g h o u t geologic t i m e . T h e principle of uniformitari a n i s m d o e s not m e a n that all g e o l o g i c p h e n o m e n a are slow. 5. C. T h e m a n t l e represents 6 7 . 1 % of E a r t h ' s m a s s . Refer to F i g u r e 1.5. 6. A. T h e crust is thickest on the continents. T h e thickest crust stands the highest. Refer to F i g u r e 1.7. 7. D. Silicon is m o r e a b u n d a n t in the crust. Refer to Key F i g u r e 1.8. 257

258


8. 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 . Refer to Key Figure 1.11. It is i m p o r t a n t to distinguish b e t w e e n a result and a c a u s e . Plate tectonics is a result of c o n v e c t i o n in the m a n t l e , but density differences c a u s e d by heating and gravity are w h a t drive c o n v e c t i o n and c a u s e plate tectonic m o t i o n s . 9 . C . T h e E a r t h ' s m a g n e t i c field i s c a u s e d b y t h e g e o g d y n a m o , c r e a t e d b y convection in the outer liquid core and electrical currents. 10. A. E a r t h ' s m a g n e t i c field reverses (the p o l e s flip) at irregular intervals. 1 1 . 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 . 12. B. T h e Big B a n g occurred about 5 4 0 million years ago, and in an interval of perhaps less than 10 million years, eight entirely new branches of the animal k i n g d o m w e r e established.

CHAPTER 2

Answers to Practice Exercises Exercise 1: Characteristics of active tectonic plate boundaries

Divergent Ocean/ See Figures 2.5, Characteristics 2.6, 2.7, and 2.8. Ocean Examples

Convergent See Figures 2.5 and 2. 6. Ocean/ Continental

Collision

Transform See Figures 2.5, 2.6, and 2.9.

Mid-Atlantic Ridge,

J a p a n e s e islands

Peru-Chile Trench

H i m a l a y a s and

African Rift Valley,

Marinas Trench

Andes Mountains

Tibetan P l a t e a u

San A n d r e a s Fault

R e d Sea, and

Aleutian Trench

Cascade Range

trench,

trench, volcanic

very high

offset

arc, and high

plateau and

beds

mountains

mountains

topographic

G u l f of California Topography

oceanic rift

ridge,

valley,

ocean

basins,

ocean offset

island

floor by

arc

of creek and

features

features

cross

transforms,

other that

the fault

seamounts Volcanism

present

present

present

not characteristic

not characteristic


Exercise 2: Construct a conceptual flowchart or diagram illustrating how plate tectonics works Key e l e m e n t s in t h e flowchart or d i a g r a m to review and clearly u n d e r s t a n d should be the roles of differential h e a t i n g ( w h i c h creates density differences), b u o y a n c y (a force created by d e n sity differences), and gravity, all of w h i c h g e n e r a t e convection w i t h i n the Earth's m a n t l e .

Answers to Review Questions 1. A. V o l c a n i s m at the o c e a n i c r i d g e s builds the seafloor. 2 . A . R e f e r t o F i g u r e s 2.10 a n d 2 . 1 2 . 3. B. Figure 2.5 shows that the rate of plate motion today varies from a few centimeters to 15 centimeters per year. 4. D. T h e t y p i c a l plate m o t i o n a l o n g m o s t transform faults is horizontal slip (shearing). H o w e v e r , a l o n g c u r v e s in the transform fault, transextension (forming a d e p r e s s i o n ) and t r a n s c o m p r e s s i o n (forming m o u n t a i n s ) m a y b e generated. 5. A. R e f e r to F i g u r e 2.6b.

259

260


6. C. T h e H a w a i i a n islands f o r m e d over a hot spot in the m i d d l e of the Pacific ocean plate. S o m e hot spots, such as in Iceland, are located coincidentally adjacent to a spreading center. M o s t hot spots on E a r t h are not directly associated with plate b o u n d a r i e s . 7. A. T h e Atlantic coast of N o r t h A m e r i c a is a passive c o n t i n e n t a l m a r g i n , which is not associated with an active plate m a r g i n . 8 . D . P a n g a e a b e g a n t o b r e a k u p during t h e Jurassic Period. Refer t o F i g u r e 2 . 1 3 . 9. B. M i d - o c e a n i c r i d g e s or s p r e a d i n g centers are divergent b o u n d a r i e s w h e r e the crust is e x t e n d i n g (pulling apart) and mafic m a g m a s are intruding u p w a r d from the asthenos p h e r e to feed basaltic v o l c a n i s m that is building n e w o c e a n floor. 10. A. Refer to F i g u r e 2.6(e). 1 1 . C. Volcanism is not characteristic of transform plate boundaries. S o m e volcanism, typically m i n o r a m o u n t s , m a y occur in association with transform faults w h e r e transextension is occurring. 12. C. Refer to F i g u r e 2 . 1 3 . 13. D . S e e F i g u r e 2 . 1 3 . 14. A. R e f e r to F i g u r e 2 . 1 0 . 15. B. T h i s age b e c o m e s m o r e interesting w h e n y o u c o m p a r e it to the a g e of the oldest r o c k s on the c o n t i n e n t s , w h i c h is 4.0 billion y e a r s . Refer to F i g u r e 2.12 and pages 35 - 3 7 o n seafloor i s o c h r o n s . 16. B . Refer to F i g u r e 2.12. 17. A . Refer to F i g u r e 2.6. 18. B. Refer to F i g u r e s 2.5 and 2.6. 19. C. Refer to F i g u r e 2.8. 20. A. R e f e r to F i g u r e 2 . 1 0 . 2 1 . B . Refer t o p a g e s 3 7 ^ 1 0 . 22. D.

CHAPTER 3

Answers to Practice Exercises Exercise 1: Crystal structures of some common silicate minerals A. single c h a i n s of silica t e t r a h e d r a p y r o x e n e (augite) t w o g o o d c l e a v a g e p l a n e s intersecting a t a b o u t 9 0 ° . Refer t o F i g u r e 3.17. B . d o u b l e c h a i n s o f silica t e t r a h e d r a amphibole (hornblende) t w o g o o d c l e a v a g e p l a n e s intersecting at about 60° and 120°. Refer to F i g u r e 3.17. C. sheets of silica tetrahedra mica (muscovite) excellent c l e a v a g e in o n e direction. Refer to F i g u r e 3 . 1 5 .


Exercise 2: Major mineral classes 1. silicate

(double-chain)

13. native e l e m e n t (carbon)

2. o x i d e o f m a g n e s i u m and a l u m i n u m

14. iron sulfide

3. silicate (sheet)

15. silicate (sheet)

4. c a l c i u m c a r b o n a t e

16. c a l c i u m sulfate

5. h y d r a t e d c a l c i u m sulfate

17. silicate (single-chain)

6. silicate ( f r a m e w o r k )

18. iron sulfide

7. native e l e m e n t ( c a r b o n )

19. o x i d e of a l u m i n u m

8. c a l c i u m / m a g n e s i u m c a r b o n a t e

20. silicate (framework)

9. silicate ( f r a m e w o r k )

2 1 . native e l e m e n t (silver)

10. o x i d e of a l u m i n u m

22. silicate (framework)

11. s o d i u m halide

2 3 . silicate (isolated tetrahedra)

12. iron o x i d e

24. silicate (sheet)

Exercise 3: Identifying minerals by their physical properties A. muscovite

D. orthoclase

B. pyrite

E. calcite

C. m a l a c h i t e

F. g y p s u m

Exercise 4: Rock cycle review A. Plate tectonic settings for m a g m a g e n e r a t i o n are: hotspots/mantle

plumes

divergent b o u n d a r i e s convergent boundaries B. Types of i g n e o u s r o c k s E x t r u s i v e : Fast c o o l i n g , f i n e g r a i n e d Intrusive: Slow cooling, coarse grained C. R o c k a n d 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 r u n n i n g water, w i n d , and ice. D. T w o p r o c e s s e s that convert l o o s e s e d i m e n t s into r o c k are c o m p a c t i o n (burial) and cementation. E. T h e t w o m a i n t y p e s of s e d i m e n t a r y r o c k s and their constituents are: Siliclastic: R o c k and m i n e r a l fragments Chemical/biochemical: precipitation of minerals previously dissolved during weathering F. F o l l o w i n g are the four major conditions (geologic settings) that result in m e t a m o r p h i c rocks: C o n t a c t m e t a m o r p h i s m a s s o c i a t e d with intrusions o f m a g m a R e g i o n a l m e t a m o r p h i s m a s s o c i a t e d with plate collisions U l t r a - h i g h - p r e s s u r e m e t a m o r p h i s m d e e p w i t h i n the lithosphere H i g h - p r e s s u r e , l o w - t e m p e r a t u r e m e t a m o r p h i s m associated with s u b d u c t i o n z o n e s G. N o . T h e r o c k is n o t m e l t e d d u r i n g m e t a m o r p h i s m , a l t h o u g h a m i n o r a m o u n t of m e l t " s w e a t " m a y b e g e n e r a t e d during h i g h - g r a d e m e t a m o r p h i s m . I g n e o u s r o c k s are f o r m e d from t h e solidification of melts ( m a g m a s ) .

261

262


Answers to Review Questions 1. D. C h l o r i d e is an anion and easily gains an electron from s o d i u m , w h i c h loses an electron to b e c o m e a cation. W i t h o n e extra electron, c h l o r i n e has the electron configuration of the n o b l e gas argon. On losing an electron to chlorine, s o d i u m has the electron configuration of the n o b l e gas n e o n . 2. D. G r a p h i t e and d i a m o n d are both c o m p o s e d of p u r e c a r b o n but h a v e significantly different crystal structures. Refer to F i g u r e 3.11 in the textbook. 3. B. A r o c k is an aggregate of o n e or m o r e m i n e r a l s . 4. C. A l l m i n e r a l s are crystalline solids. 5. D. By definition, m i n e r a l s are inorganic. A r e g r a p h i t e and d i a m o n d m i n e r a l s ? Yes. They are b o t h m a d e from p u r e c a r b o n — a c o m m o n e l e m e n t i n o r g a n i c m a t e r i a l . However, g r a p h i t e and d i a m o n d are not p r o d u c e d by b i o l o g i c a l p r o c e s s e s . G r a p h i t e is typically found in m e t a m o r p h i c r o c k s , and d i a m o n d originates in the E a r t h ' s m a n t l e . 6. B. G l a s s is an a m o r p h o u s m a t e r i a l that lacks a crystal structure. N a t i v e copper, diam o n d , and w a t e r ice all fit the definition of a m i n e r a l . 7. B. Silicate m i n e r a l s are the m o s t c o m m o n m i n e r a l g r o u p in the E a r t h ' s crust and mantle. T h e E a r t h ' s c o r e is t h o u g h t to consist m o s t l y of an iron-nickel alloy. 8. B. Refer to F i g u r e 3.9(e). 9. C. M i c a , like m u s c o v i t e in F i g u r e 3.11(g), is a sheet silicate. 10. B. Refer to Table 3.1 and A p p e n d i x 4. 11. A C l a y m i n e r a l s h a v e a sheet silicate structure. 12. A Refer to F i g u r e 3.11 (e and f). 13. B. Q u a r t z is a silicate and calcite is a c a r b o n a t e . 14. B. C l e a v a g e is the t e n d e n c y for m i n e r a l s to b r e a k a l o n g p l a n e s of w e a k e r chemical b o n d s within their crystal structure. 15. A. T h e p h y s i c a l characteristics of a m i n e r a l are d e t e r m i n e d by its c o m p o s i t i o n , the n a t u r e of its c h e m i c a l b o n d s , and the crystal structure. A l t h o u g h g r a p h i t e and diamond are b o t h p u r e c a r b o n , their crystal structures and the c h e m i c a l b o n d s w i t h i n the crystal structures are significantly different. 16. B. C h e m i c a l b o n d s typically exhibit a m i x t u r e of i o n i c and covalent characteristics. B o n d s with a m o r e covalent character are stronger and the b o n d l e n g t h is shorter. Bonds with a m o r e ionic character are w e a k e r and the a t o m s tend to be farther apart. 17. D. C a t i o n s of similar sizes and c h a r g e s tend to substitute for o n e a n o t h e r and to form c o m p o u n d s h a v i n g t h e s a m e crystal structure but different c h e m i c a l c o m p o s i t i o n s . C a t i o n substitution is c o m m o n in silicate m i n e r a l s . 18. A . C o m p a r e d t o i o n i c b o n d s , c o v a l e n t b o n d l e n g t h i s s h o r t e r and c o v a l e n t bond strength is higher. 19. B. Plutonic rocks solidify from melts called m a g m a s . Refer to Figures 3.22 and Figure 3.26. 2 0 . C. M o s t m a g m a s are g e n e r a t e d from the m e l t i n g of silicate r o c k s w i t h i n the Earth's crust and m a n t l e . On rare o c c a s i o n , a m a g m a c o m p o s e d of c a r b o n a t e s erupts on the E a r t h ' s surface. On Io, the m o o n of Jupiter, sulfur m a g m a s erupt from a b o u t ten active v o l c a n o e s . I o ' s e r u p t i o n s c a n shoot fountains o f sulfur c o m p o u n d s 3 6 0 k m high. 2 1 . A. An i g n e o u s r o c k with a c o a r s e texture, w h e r e individual m i n e r a l g r a i n s (crystals) are visible w i t h o u t magnification, forms w h e n the r o c k crystallizes slowly b e n e a t h the E a r t h ' s surface. Solidification of a m a g m a b o d y m a y t a k e tens to h u n d r e d s of t h o u s a n d s of y e a r s within the crust, and millions of years m o r e to c o o l after c o m p l e t e crystallization. To e x p o s e a c o a r s e grain i g n e o u s (plutonic) r o c k at the E a r t h ' s surface requires significant uplift and erosion of the r o c k s that o n c e sat on t o p .


263

22. D . Refer t o p a g e s 6 5 - 6 6 i n the textbook. 2 3 . A. L a y e r i n g is characteristic of s e d i m e n t a r y r o c k s . S o m e volcanic r o c k s , like lava flows and v o l c a n i c ash a n d tuff, also f o r m layers. 24. D . C o n t a c t m e t a m o r p h i s m i s associated w i t h m a g m a intrusions. 2 5 . A . W e a t h e r i n g creates s e d i m e n t s . 26. D. M o u n t a i n s are a result of s u b d u c t i o n and c o n t i n e n t s colliding.

CHAPTER 4

Answers to Practice Exercises Exercise 1: Igneous rock textures Texture t e r m / s k e t c h d e s c r i p t i o n A. p h a n e r i t i c / c o a r s e - g r a i n e d , large interlocking crystals B. p h a n e r i t i c / v i s i b l e crystals but not as c o a r s e g r a i n e d as s a m p l e A C. p o r p h y r i t i c / m i x e d c o o l i n g history c a u s e s large and very small crystals D . a p h a n i t i c / f m e g r a i n e d — c r y s t a l s m a y not b e visible w i t h o u t magnification, even then l o o k i n g like tiny dots

Exercise 2: Distribution of igneous rocks within the Earth Major layer in the Earth

Example igneous rock


Continental crust

Granite

Felsic

(For c o n t i n e n t a l crust, there

M o r e Si, Na, K Less Fe, Mg, Ca

are t w o a p p r o p r i a t e a n s w e r s . )

Ocean crust

General chemical composition

Andesite/diorite

Intermediate

Intermediate

Basalt/gabbro

Mafic

More Fe, Mg, Ca Less Si, N a , K

Mantle

Peridotite

Ultramafic

Less Si, Na, K More Fe, Mg, Ca

Exercise 3: Predicting the change in composition in a crystallizing magma A. 1. Silica c o n t e n t i n c r e a s e d 2. Iron c o n t e n t d e c r e a s e d Explanation: Refer to the top of Figure 4.5, w h i c h illustrates h o w mafic (iron- and m a g n e s i u m - r i c h ) silicate m i n e r a l s crystallize first in a cooling m a g m a . As iron b e c o m e s tied up in the crystallizing solid phase (olivine and pyroxene), the remaining liquid b e c o m e s progressively enriched in silica. B. 1. Silica c o n t e n t i n c r e a s e d 2 . Iron c o n t e n t d e c r e a s e d Explanation: Refer to the top of Figure 4.5, which illustrates h o w mafic (iron- and m a g n e s i u m - r i c h ) silicate minerals crystallize first in a cooling m a g m a . As iron and calcium b e c o m e tied up in the crystallizing solid phase, the remaining liquid b e c o m e s progressively enriched in silica and sodium.

264


Exercise 4: Sequence of mineral crystallization in a solidifying magma Refer to F i g u r e s 4.4 and 4.5 and Table 4.2. A. L E S S . T h e p r e s e n c e of iron and m a g n e s i u m in the m a g m a greatly influences the c o m p l e x i t y of the silicate structure b e c a u s e the iron a n d m a g n e s i u m act to "poison" the p o l y m e r i z a t i o n of the silica tetrahedra, p r e v e n t i n g m o r e c o m p l e x silica tetrahedra crystalline structures. S i n c e m u c h of the iron and m a g n e s i u m is incorporated into the early-formed crystals, as illustrated in F i g u r e 4 . 5 , m i n e r a l s that crystallized later in the history of the solidification of the m a g m a tend to be depleted in iron and m a g n e s i u m , enriched in silica, and have m o r e c o m p l e x silicate structures. B. L A S T . Refer to the explanation for A. C. M O R E . Refer to the explanation for A.

Exercise 5: Partial melting and magma composition A . L O W E R . Refer t o F i g u r e 4 . 5 . B. D E P L E T E D . M u c h of the iron and m a g n e s i u m in a m a g m a is incorporated into the e a r l y - f o r m e d crystals, as illustrated in B o w e n ' s reaction series. M i n e r a l s that crystallized later in the history of the solidification of the m a g m a tend to be depleted in iron and m a g n e s i u m and e n r i c h e d in silica. C. E N R I C H E D . Refer to the explanation for B and F i g u r e 4 . 5 .

Exercise 6: Predicting the composition of magma generated in subduction zones A. M O R E . As illustrated by B o w e n ' s reaction series, silicate m i n e r a l s with more silica c o n t e n t have l o w e r melting t e m p e r a t u r e s . So a partial melt will be enriched in silica relative to the i g n e o u s r o c k from w h i c h it w a s g e n e r a t e d . B. L E S S . Silicate m i n e r a l s rich in iron and m a g n e s i u m have h i g h e r melting points and are t h e last to melt c o m p a r e d to m i n e r a l s l o w e r on the B o w e n ' s reaction series diag r a m . T h e r e f o r e a m a g m a g e n e r a t e d from a partial m e l t will be e n r i c h e d in silica and d e p l e t e d in iron and m a g n e s i u m relative to the bulk c o m p o s i t i o n of the original r o c k from w h i c h the m e l t w a s generated. C. M O R E . F i g u r e s 4.5 and 4.14 and Table 4.2 are very helpful. Na and K concentrate in m i n e r a l s with greater a m o u n t s of silica. So they are e n r i c h e d in that they have l o w e r m e l t i n g t e m p e r a t u r e s and crystallize late in the c o o l i n g history of the magma. D. L E S S . A partial m e l t will a l w a y s h a v e m o r e silica and less i r o n / m a g n e s i u m than the p a r e n t r o c k from w h i c h it is g e n e r a t e d . T h e r e f o r e it will be less mafic.

Answers to Review Questions 1. A. M a n y students m i s t a k e n l y c h o o s e B or C as the answer. H o w e v e r , grain size is just o n e of t w o criteria for the classification and n a m i n g of i g n e o u s r o c k s . Composition (mineral content) is the other criterion. 2. D. R h y o l i t e is an aphanitic volcanic r o c k and granite is a p h a n e r i t i c p l u t o n i c rock. Their c o o l i n g histories are different and therefore their textures are different. H o w e v e r , they have the s a m e general c o m p o s i t i o n (felsic). 3 . B . R h y o l i t e contains the m o s t silica o f the r o c k s listed. Fissure e r u p t i o n s m a y b e c o m p o s e d of a great variety of lavas, but they are typically basaltic. 4. A. G a b b r o is the intrusive equivalent of basalt, w h i c h is extrusive. 5. D. T h e distinction b e t w e e n intrusive (plutonic) and extrusive (volcanic) r o c k s is grain size, w h i c h is d e t e r m i n e d by the rate of crystallization.


6. A. P y r o x e n e s like augite are a c o m m o n m i n e r a l in basalts. T h e y are a f e r r o m a g n e s i u m m i n e r a l w i t h a s i n g l e - c h a i n silicate crystal structure. 7. A. Refer to F i g u r e s 4.4 and 4.5 and Table 4.2. 8. B. S l o w c o o l i n g typically p r o d u c e s larger crystals. R a p i d cooling p r o d u c e s finer c r y s tals. An i g n e o u s r o c k with a m i x t u r e of c o a r s e - and fine-grained m i n e r a l s formed u n d e r c o n d i t i o n s w h e r e the c o o l i n g history w a s m i x e d . B o t h volcanic and plutonic r o c k s exhibit p o r p h y r i t i c texture. A porphyritic volcanic r o c k erupts from a m a g m a that has b e g u n t o c r y s t a l l i z e — c r y s t a l s already exist within the m a g m a . E a r l y - f o r m e d crystals are literally carried to the surface by the r e m a i n i n g melt, w h i c h cools quickly on erupting at the E a r t h ' s surface. A porphyritic texture in a plutonic rock m a y be a result of its c o o l i n g history or c h a n g e s in pressure and other c o n d i t i o n s that influence crystal g r o w t h within t h e m a g m a c h a m b e r . 9. D. Table 4.1 is a g o o d reference for this question. 10. B. If no n e w m a g m a is ejected into the m a g m a c h a m b e r , the c o m p o s i t i o n of the m a g m a m a y c h a n g e o v e r time b e c a u s e of fractional crystallization and the segregation of the earlier-formed iron-rich m i n e r a l s b y settling from the melt. T h e r e m a i n i n g m a g m a b e c o m e s p r o g r e s s i v e l y e n r i c h e d in silica and depleted in iron and m a g n e s i u m . 1 1 . C. O l i v i n e and c a l c i u m - r i c h p l a g i o c l a s e are t h e first m i n e r a l s to crystallize from a m a g m a with a mafic c o m p o s i t i o n . Refer to F i g u r e s 4.4 and 4 . 5 . 12. B. A b a t h o l i t h is a very large b o d y of i g n e o u s rock. Batholiths are found in the c o r e s of m o u n t a i n belts. 13. D . Q u a r t z a n d olivine d o not crystallize from t h e s a m e m a g m a body. I f there w a s e n o u g h iron in t h e original m a g m a , the silica is c o n s u m e d in the formation of the ferr o m a g n e s i u m m i n e r a l s and p l a g i o c l a s e feldspars. T h e m a g m a c o m p l e t e l y solidifies before p u r e q u a r t z c a n crystallize. A p a r e n t m a g m a with e n o u g h silica to g e n e r a t e q u a r t z will not c o n t a i n e n o u g h iron and m a g n e s i u m to generate olivine. 14. A. Intrusive (plutonic) r o c k s are typically coarser g r a i n e d b e c a u s e of slower c o o l i n g . E x t r u s i v e (volcanic) r o c k s are finer grained b e c a u s e of quicker c o o l i n g rates. C o o l i n g rates are not the only factors that influence the grain size of i g n e o u s r o c k s . R a p i d c h a n g e s in p r e s s u r e within a m a g m a c h a m b e r c a n i n d u c e rapid crystallization and therefore fine-grained crystals within an intrusive r o c k . H i g h w a t e r c o n t e n t in residual fluids from a solidifying m a g m a c a n e n h a n c e crystal g r o w t h and size. 15. C. Refer to the d i s c u s s i o n on Plutons in the textbook. 16. D. R e f e r to the d i s c u s s i o n on Plutons in the textbook. 17. B. Refer to F i g u r e s 4.11 and 4 . 1 3 . 18. B. F i g u r e 4.5 illustrates h o w the residual liquid within a m a g m a c h a m b e r b e c o m e s p r o gressively e n r i c h e d in silica and depleted in iron as the crystallization of e a r l y - f o r m e d m i n e r a l s u s e s u p the available iron. 19. B. Refer to F i g u r e 4 . 1 3 . 20. D. S u b d u c t i o n at convergent plate m a r g i n s p r o d u c e s large a m o u n t s of andesites. In fact, andesites are n a m e d after the A n d e s M o u n t a i n s in South A m e r i c a b e c a u s e they are very a b u n d a n t there. M a g m a s generated within the u p p e r m a n t l e , such as those at divergent plate b o u n d a r i e s and hot spots, are typically mafic. Refer to Figures 4 . 1 1 , 4 . 1 3 , and 4.14. 2 1 . B . Refer t o F i g u r e 4 . 1 2 . 22. A . R e f e r t o F i g u r e 4 . 1 3 . 2 3 . B. This is the m o s t unlikely hypothesis b e c a u s e melting within a mantle p l u m e , like the hot spot H a w a i i , is thought to be caused by d e c o m p r e s s i o n of hot ultramafic rock, w h i c h generates basalt. 24. C. To m e l t a solid in a k i t c h e n y o u h e a t it up on t h e stove. R o c k s w i t h i n the E a r t h are already relatively h o t but m o s t are still solid. R e d u c t i o n in p r e s s u r e ( d e c o m p r e s s i o n ) and t h e addition of w a t e r (fluid induction) c a u s e r o c k s to melt within the Earth. Refer t o F i g u r e s 4 . 1 3 and 4 . 1 4 .

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CHAPTER 5

Answers to Practice Exercises Exercise 1: Common sedimentary environments A l p i n e or glacial river c h a n n e l : sand and gravel D u n e s in a desert: fine sand F l o o d plain a l o n g a b r o a d m e a n d e r b e n d : silt and clay ( m u d ) R i v e r delta a l o n g a m a r i n e shoreline: sand, m u d , calcified o r g a n i s m s (seashells) C o n t i n e n t a l shelf: sand and m u d D e e p sea adjacent to a continental shelf: m u d S h o r e l i n e b e a c h d u n e s : sand, gravel, calcified o r g a n i s m s Tidal flats: sand and m u d O r g a n i c reef: calcified o r g a n i s m s

Exercise 2: Grain sizes for clastic sedimentary rocks G r a i n size

C o m m o n Object

Sediment

Coarse

football or b u s

boulder

p l u m or lime

gravel

conglomerate

pea or bean

gravel

conglomerate

c o a r s e - g r o u n d p e p p e r or salt

sand

sandstone

fine-ground p e p p e r or salt

sand

sandstone

talcum powder or baby powder

m u d , silt, clay

siltstone, mudstone

4

Fine

Rock type gravel

conglomerate

shale, claystone

Exercise 3: Clastic and chemical sediments and sedimentary rocks Statement

Sediment type

Sedimentary rock example

C o m p o s e d largely of r o c k fragments

clastic

sandstone, graywacke, conglomerate

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 d e p o s i t i o n

chemical

evaporite, p h o s p h o r i t e

I m p o r t a n t s o u r c e of c o a l

biochemical

peat

Often f o r m e d b y d i a g e n e s i s

chemical

dolostone

F o r m e d from a b u n d a n t skeleton fragments

biochemical

limestone

Produced by physical weathering

clastic

c o n g l o m e r a t e , s a n d s t o n e , siltstone, shale

P r o d u c e d from rapidly e r o d i n g granitic and

clastic

arkose sandstone

and

phosphorite

of m a r i n e or lake o r g a n i s m s , such as coral, seashells, and foraminifers

gneissic terrains in an arid or s e m i - a r i d c l i m a t e

Answers to Review Questions 1. D. Shale, s a n d s t o n e and c o n g l o m e r a t e are clastic r o c k s . D o l o m i t e , chert, l i m e s t o n e , coal and g y p s u m are c h e m i c a l / b i o c h e m i c a l r o c k s . 2. D. S e e Table 5.3 3. D. S a n d is a t e r m that refers solely to a particular r a n g e of grain sizes. It d o e s not imply any specific c o m p o s i t i o n . Q u a r t z and feldspar are c o m m o n c o n s t i t u e n t s of sand grains

Answers to Practice Exercises and Review Questions 2 6 7

b e c a u s e they are c o m m o n m i n e r a l s in the E a r t h ' s continental crust. C a r b o n a t e s , like l i m e s t o n e , d o l o s t o n e , a n d fragments o f seashells, c a n also m a k e u p sand grains. 4. D. See Figure 5.1. 5 . B . T h e r e i s n o k n o w n m a r i n e life that precipitates d o l o m i t e . D o l o m i t e c a n b e p r e c i p i tated i n o r g a n i c a l l y in seawater, not fresh water. 6. A. R e f e r to F i g u r e 5.12. 7. A . 8. A . 9. D. Cross-bedding is a feature produced as sediment is deposited by currents of air or water. 10. D. C a r b o n a t e s e d i m e n t s are d e p o s i t e d in w a r m water. C a r b o n a t e s dissolve in cold w a t e r b e c a u s e their solubility is linked to the a m o u n t of c a r b o n d i o x i d e dissolved in t h e water. T h e solubility of c a r b o n d i o x i d e increases as the t e m p e r a t u r e of the w a t e r d e c r e a s e s . 11. A. R e f e r to F i g u r e 5.16. 12. B. S e e Table 16.2. 13. D. F i g u r e 5.16 will h e l p . 14. C. S e e Table 5 . 1 . 15. D. S e e Table 5 . 1 . Ultimately, the w e a t h e r i n g of m o s t silicate m i n e r a l s , e x c e p t quartz, p r o d u c e s c l a y s that b e c o m e c o m p a c t e d into shale. S e e F i g u r e 5.12. 16. B. Refer to F i g u r e 5 . 1 1 . 17. A. Refer to F i g u r e 5.18 and E a r t h Issues 5 . 1 . 18. D . 19. A .

CHAPTER 6

Answers to Practice Exercises Exercise 1: Classification of metamorphic rocks based on texture

Parent rock

Metamorphic rock

Texture (foliated/ granoblastic)

shale

slate

foliated

quartzite

granoblastic

granulite

granoblastic

granite

schist and g n e i s s

foliated

limestone

marble

granoblastic

carbonate-rich sedimentary rocks

hornfels

granoblastic

basalt (mafic volcanics)

amphibolites

igneous a n d m e t a m o r p h i c r o c k s

migmatite

quartz-rich

sandstone

shale, i m p u r e s a n d s t o n e , and many k i n d s of i g n e o u s r o c k s

and

greenstones

granoblastic foliated

268


Exercise 2: Comparing igneous, sedimentary, and metamorphic rocks


Texture

Rock name Rock type (igneous, sedimentary, (granite, sandstone, marble) metamorphic)

calcium

nonfoliated

metamorphic

marble

sedimentary

limestone

i g n e o u s (plutonic)

granite

sedimentary

m u d s t o n e , shale

i g n e o u s (volcanic)

basalt

nonfoliated

metamorphic

quartzite

clastic

sedimentary

cconglomerate

bioclastic,

sedimentary

limestone

metamorphic

schist

quartz,

carbonate

phaneritic

K and Na feldspar,

mica,

and

amphibole fine-grained

clay pyroxene, and

calcium feldspar,

aphanitic, porphyritic

olivine

quartz pebbles

and

variety

of rock

fragments

cobbles

of a

types

of seashells

and fine

mud

quartz,

muscovite,

and

clastic

biochemical foliated

chlorite,

garnet

Answers to Review Questions 1. D. If the r o c k is melted, it is an i g n e o u s rock. S o m e m e t a m o r p h i c r o c k s , such as m i g a m t i t e , get hot e n o u g h to " s w e a t " quartz. 2. B . 3 . B . Refer t o the section M e t a m o r p h i c Textures. 4. B. Refer to F i g u r e 6.4. 5. A. Refer to F i g u r e 6.4. 6. C. M e t a m o r p h i c r o c k s characteristically exhibit either foliated or g r a n o b l a s t i c textures. 7. D. A l t h o u g h m i n e r a l o g y m a y be altered by m e t a m o r p h i s m , typically there is little to no c h a n g e in bulk c o m p o s i t i o n of the rock. 8 . D . E v e n preexisting m e t a m o r p h i c r o c k s c a n b e m e t a m o r p h o s e d . 9. B. Slate -> p h y l l i t e -* schist

g n e i s s is the c o r r e c t s e q u e n c e f r o m f i n e - g r a i n e d to

p r o g r e s s i v e l y c o a r s e r g r a i n e d m e t a m o r p h i c r o c k s . A l l r o c k s listed i n this s e q u e n c e are foliated. 10. A. Refer to F i g u r e 6.7. 11. C. M e t e o r i t e i m p a c t of the lunar surface is an e x a m p l e of s h o c k m e t a m o r p h i s m . 12. D. Refer to F i g u r e 6.7.


269

13. B. G n e i s s h a s the s a m e m i n e r a l o g i c a l c o m p o s i t i o n as granite. Its distinctive characteristic is foliation. G r a n i t e is a c o m m o n p a r e n t r o c k for g n e i s s . 14. C. B o t h slate and g n e i s s are foliated m e t a m o r p h i c r o c k s . Slate is lower in g r a d e , and t h u s is m u c h finer grained. 15. D . R e f e r t o F i g u r e 6 . 3 . 16. A. R e f e r to t h e t o p of F i g u r e 4 . 5 . 17. B. T h e i n t r u s i o n of a hot m a g m a b o d y c a u s e s contact m e t a m o r p h i s m . 18. B. As illustrated in t h e d i a g r a m at the b o t t o m of F i g u r e 4 . 5 , quartz softens at relatively low t e m p e r a t u r e s . O v a l quartz p e b b l e s c o u l d have b e e n stretched into the cigar-shaped features b y l o w - g r a d e m e t a m o r p h i s m . 19. C. Refer to F i g u r e 6.7. 20. A. Refer to F i g u r e 6.10 and the C o n t i n e n t - C o n t i n e n t Collisions section in the text.

CHAPTER 7

Answers to Practice Exercises Exercise 1: Silly Putty Behavior of Silly Putty

Behavior of rock

Type of force

Geologic structure produced by this style of deformation

Snaps

brittle

tensional

fault or j o i n t s

Bends

ductile

compressional

folds

Bounces

elastic

compressional

into pieces

Exercise 2: Geologic structures A. reverse fault B.

compressional

C.

convergent

D . anticline E.

compressional

F.

convergent

G. n o r m a l fault Ft. t e n s i o n a l I.

divergent

270


J.

strike-slip fault

K. shearing L.

transform

M . syncline N. compressional O. c o n v e r g e n t

Exercise 3: Anticline versus syncline A. In an anticline the y o u n g e s t r o c k layer is on the outside of the fold, whereas in a syncline the y o u n g e s t r o c k layer is in the m i d d l e of the fold. B . Refer t o F i g u r e s 7.10 and 7 . 1 1 .

Exercise 4: Identifying geologic structures A. anticline B.

syncline

C.

thrust

D . reverse E.

normal

Answers to Review Questions 1. C. Solids that b r e a k are called brittle. W h e t h e r a solid b e h a v e s brittlely 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 a t u r e , and confining p r e s s u r e and t h e rate of a p p l i c a t i o n of directional forces. 2. A. Refer to F i g u r e 7 . 3 . 3. B. T h e S a n A n d r e a s is a right-lateral strike-slip fault. It also r e p r e s e n t s a transform plate boundary. 4. B. Refer to F i g u r e s 7.7, 7 . 1 5 , 7.16, and 7.19. 5. B. Joints are fractures a l o n g w h i c h there is no a p p r e c i a b l e m o v e m e n t . Faults are fractures a l o n g w h i c h there is a p p r e c i a b l e offset. 6. D. All other c o n d i t i o n s m e n t i o n e d w o u l d favor brittle behavior. 7. A. Basalt is an i g n e o u s r o c k with i n t e r l o c k i n g fine-grained silicate m i n e r a l s that have a high m e l t i n g point. T h e r e f o r e , basalt t e n d s to b e h a v e brittlely. R o c k s with h i g h clay or c a l c i u m - c a r b o n a t e content tend to b e h a v e ductilely. 8. C. C o l u m n a r jointing is fracturing caused by shrinkage during cooling of an igneous rock. 9. D. N o r m a l faults are a result of brittle b e h a v i o r in r e s p o n s e to tension. 10. B. T h r u s t faults are c a u s e d by c o m p r e s s i o n a l forces typical of w h a t is g e n e r a t e d at convergent b o u n d a r i e s like continental collisions. 1 1 . A . Refer t o F i g u r e 7 . 3 . 12. B . Refer t o F i g u r e s 7 . 1 1 .


CHAPTER 8

Answers to Practice Exercises Exercise 1: Determining the succession of geologic events A.

12. E v e n t h o u g h the b l a c k d i k e d o e s not cut all r o c k units in t h e d i a g r a m , it d o e s c u t t h e g r a n i t e a n d layer 1, w h i c h is a b o v e all o t h e r layered r o c k units. T h e r e f o r e , the dike is y o u n g e r than the granite and layer 1. If the dike was not s h o w n cutting the g r a n i t e , t h e relative a g e s o f the dike and t h e g r a n i t e c o u l d not b e r e s o l v e d . R a d i o m e t r i c d a t e s o n t h e t w o i g n e o u s r o c k b o d i e s w o u l d r e s o l v e their a g e s .

B.

10. L a y e r 10 is the oldest in the o u t c r o p . It is at t h e b o t t o m of a tilted s e q u e n c e of l a y e r e d r o c k s that go from layer 10 (the oldest) to layer 3 (the y o u n g e s t of t h e s e q u e n c e ) , a s s u m i n g that this w h o l e s e q u e n c e has not b e e n c o m p l e t e l y overturned.

C. Yes. L a y e r 3 is older than the d i k e (unit 12). E v e n t h o u g h the dike does not cut layer 3, it d o e s cut the u n c o n f o r m i t y that cuts layer 3, and it also cuts layers 2 and 1, w h i c h are a b o v e layer 3. D . N o t p o s s i b l e t o k n o w . B e c a u s e the b l a c k dike cuts b o t h the granite (unit 11) a n d l a y e r 1, we c a n say that layer 1 and the g r a n i t e are older than the d i k e . H o w e v e r , t h e g r a n i t e m a y h a v e i n t r u d e d into the r o c k layers b e f o r e or after t h e d e p o s i t i o n of l a y e r 1. T h e r e is no w a y of telling w h e t h e r t h e g r a n i t e is o l d e r or y o u n g e r than layers 4, 3, 2, or 1. A r a d i o m e t r i c d a t e on the granite a n d layer 1, w h i c h is a lava flow, w o u l d h e l p t o r e s o l v e this q u e s t i o n . E. 1. U n i t 1 is older than unit 12 but y o u n g e r than unit 1 1 . You do not even h a v e to calc u l a t e r a d i o m e t r i c ages to a n s w e r this question. Just look at the trend in the a b u n d a n c e o f t h e radioactive p a r e n t a t o m s o r a c c u m u l a t i o n o f the d a u g h t e r a t o m s . T h e r o c k with t h e greatest a m o u n t of p a r e n t a t o m s (and least a m o u n t of d a u g h t e r a t o m s ) i s the y o u n g e s t . T h e r o c k w i t h t h e lowest a m o u n t o f radioactive p a r e n t a t o m s and the largest a c c u m u l a t i o n of d a u g h t e r a t o m s is the oldest. B e c a u s e the a b u n d a n c e of radioactive p a r e n t a t o m s in unit 1 falls b e t w e e n units 11 and 12, the r a d i o m e t r i c age of unit 1 w o u l d be s o m e n u m b e r of y e a r s b e t w e e n the ages of units 11 and 12. U n i t 11 has t h e fewest radioactive p a r e n t a t o m s r e m a i n i n g ; therefore, it is the oldest.

Exercise 2: Ordering geologic events This is a clear illustration of the p r i n c i p l e of superposition and cross-cutting relationships at work. U s i n g t h e p r i n c i p l e o f s u p e r p o s i t i o n , w e see that the l i m e s t o n e m u s t h a v e b e e n d e posited before t h e shale and the shale before the s a n d s t o n e . L i k e w i s e , t h e s e b e d s m u s t have existed before b e i n g first cut by the d i k e and second by t h e fault. Youngest

Faulting Dike intrudes D e p o s i t i o n of s a n d s t o n e D e p o s i t i o n of shale

Oldest

D e p o s i t i o n of l i m e s t o n e

271

272


Exercise 3: Marker events for the Geologic Time Scale Eon

Era

evolve

Epoch

Quaternary

Holocene

Cenozoic

Tertiary

Pleistocene

Mesozoic

Cretaceous

Phanerozoic Humans

Period

Dinosaur extinction event

Jurassic Triassic Paleozoic

Permian

v

Pliocene

\

Miocene \

Oligocene \

Eocene ^ Paleocene

Pennsylvanian Mississippian Devonian Silurian Ordovician Evolutionary Big Bang

Cambrian

Proterozoic O x y g e n b u i l d u p in atmosphere First

nucleus-bearing

cells

develop Archeon Earliest e v i d e n c e of life M a j o r p h a s e of c o n t i n e n t formation completed Hadean E n d of heavy b o m b a r d m e n t M o o n forms Earth

accretion

begins

Exercise 4: Geologic Time Scale mnemonic T h e r e i s n o " c o r r e c t " a n s w e r t o this q u e s t i o n . A n y m n e m o n i c d e v i c e that y o u will find useful in r e m e m b e r i n g the different e p o c h s and p e r i o d s of t h e G e o l o g i c T i m e S c a l e is an acceptable answer.

Answers to Review Questions 1. B. T h e r o c k layer at the top of an u n d e f o r m e d s e q u e n c e of r o c k layers is the y o u n g e s t in the s e q u e n c e . 2. D. C e n o z o i c , M e s o z o i c , P a l e o z o i c . Refer to F i g u r e 8.14. 3. B. P a l e o c e n e , E o c e n e , O l i g o c e n e , and P l i o c e n e are the e p o c h s of the Tertiary p e r i o d in the G e o l o g i c T i m e Scale. R e m e m b e r that the P l e i s t o c e n e and H o l o c e n e are e p o c h s within the Q u a t e r n a r y period. Refer to F i g u r e 8.14. 4 . 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 . Refer to F i g u r e 8.14. 5. D. D e p o s i t i o n , d e f o r m a t i o n , erosion, d e p o s i t i o n . Refer to F i g u r e 8.8.


6. A. R o c k layer 3 is d e p o s i t e d on top of layer 2 and the 60-million-year-old pluton A. B a s e d on s u p e r p o s i t i o n , layer 3 is y o u n g e r than 60 million y e a r s . L a y e r 3 is also cut by t h e 3 4 - m i l l i o n - y e a r - o l d p l u t o n B. B a s e d on this cross-cutting relationship, layer 3 has t o b e o l d e r t h a n 3 4 million y e a r s . 7. C. Refer to the M e a s u r i n g A b s o l u t e T i m e with Radioactive Clocks section of Chapter 8. 8. C. Refer to the M e a s u r i n g A b s o l u t e T i m e with Radioactive Clocks section of C h a p t e r 8. 9. D. Refer to F i g u r e 8.14. 10. B . R a d i o c a r b o n c a n b e u s e d t o radiometrically date c a r b o n - c o n t a i n i n g materials like c h a r c o a l if t h e s a m p l e is y o u n g e r than a b o u t 7 0 , 0 0 0 years old. Refer to Table 8 . 1 . 11. B. B e c a u s e r a d i o c a r b o n has a short half-life ( 5 7 3 0 years), the level of r a d i o c a r b o n d e c a y s b e l o w the d e t e c t a b l e limit in s a m p l e s older than a b o u t 7 0 , 0 0 0 years. Refer to Table 8 . 1 . 12. C. G i v e n a half-life of 1 billion years, for every billion years that e l a p s e s , the n u m b e r of r a d i o a c t i v e a t o m s d e c r e a s e s by half. T h e r e f o r e after 1 billion years there are 5 0 0 left from the original 1000 a t o m s ; after 2 billion years there are 2 5 0 out of the 5 0 0 left; after 3 billion y e a r s there are 125 out of the 2 5 0 left. 13. D. To correctly interpret this question it is important to distinguish b e t w e e n observation (data) and inference. T h e radiocarbon age of 3 0 0 0 years is data derived from laboratory analysis of the charcoal sample. T h e result of the analysis represents the a p p r o x i m a t e time that has elapsed since the organism (in this case w o o d ) died and was no longer exchanging carbon dioxide with the environment. It is reasonable to infer that the charcoal formed w h e n inhabitants at the site burned w o o d for cooking and heating. It is also reasonable to a s s u m e that the w o o d w a s not dead for a long time before it w a s harvested for fuel. If these a s s u m p t i o n s are correct, then the radiocarbon date represents the approximate time for the o c c u p a t i o n of the site. However, the charcoal could have formed from a forest fire d e c a d e s or centuries earlier than h u m a n occupation. Or the w o o d harvested for fuel m a y have b e e n dead and lying on the ground for hundreds or even thousands of years. In arid regions w h e r e decay is very slow, dead w o o d can lie on the ground for h u n d r e d s to thousands of years. If " o l d " w o o d h a p p e n e d to be used to fuel the fires at this site, then the radiocarbon age could be significantly older than the time of occupation. 14. A . T h e G e o l o g i c T i m e S c a l e w a s c o n s t r u c t e d over a b o u t the last 2 0 0 years b y g e o l o gists u s i n g m a i n l y fossils, superposition, and cross-cutting relationships to establish the relative ages for t h o u s a n d s of rock o u t c r o p s a r o u n d the world. In a b o u t the last 50 years, t h e G e o l o g i c T i m e Scale 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 t o d a t e m o s t l y i g n e o u s and m e t a m o r p h i c r o c k s . 15. A. R e v i e w the section M e a s u r i n g A b s o l u t e T i m e with Radioactive Clocks in C h a p t e r 8. 16. A. R e f e r to F i g u r e 8.15. 17. A. L o s s of d a u g h t e r a t o m s , such as lead, generated from the d e c a y of radioactive parent a t o m s , s u c h as u r a n i u m , w o u l d result in a date that w a s y o u n g e r than the actual time that has elapsed since the r o c k solidified. O n e important a s s u m p t i o n that is m a d e w h e n interpreting r a d i o m e t r i c dates is that the mineral or rock has r e m a i n e d a closed s y s t e m — n o e l e m e n t s have b e e n added to or r e m o v e d from the mineral except by radioactive decay. If this a s s u m p t i o n h o l d s , then the radiometric date typically represents the time that has elapsed since the r o c k solidified from a m a g m a . 18. C. Weathering and alteration of rock samples by m e t a m o r p h i s m can cause a redistribution of radioactive parent atoms and the daughter atoms produced by decay of the parent. This redistribution usually causes uncertainty in the radiometric date. Therefore, geologists typically radiometrically date the freshest rock samples that have been the least affected by subsequent geologic events. S a m p l e C in the middle of the lava flow is the least likely to be affected by weathering, like sample A, or contact metamorphism, like sample B. 19. B. B e c a u s e the r a d i o m e t r i c a l l y dated c o b b l e is included in the c o n g l o m e r a t e layer, t h e layer h a s t o b e y o u n g e r than the c o b b l e — t h e c o b b l e had t o f o r m before i t could b e c o m e a part of t h e c o n g l o m e r a t e . T h e r a d i o m e t r i c date represents a m a x i m u m a g e for the c o n g l o m e r a t e , w h i c h h a s to be y o u n g e r than any included c o m p o n e n t . If geologists really w a n t e d to get a better e s t i m a t e for the age of the c o n g l o m e r a t e , they w o u l d need to do r a d i o m e t r i c dates for as m a n y c o b b l e s of different i g n e o u s r o c k s as possible to find the

273

274


d a t e of t h e y o u n g e s t c o b b l e i n c l u d e d in t h e layer. In this w a y , t h e y c o u l d focus on t h e a p p r o x i m a t e age of the s e d i m e n t a r y layer u s i n g the r a d i o m e t r i c dates on igneous inclusions and the principle of i n c l u d e d fragments. 2 0 . A. C r o s s - c u t t i n g relationships tell us that the dike and lava flow m u s t be the last event to have occurred. U s i n g the p r i n c i p l e of superposition we c a n see that the first events m u s t be t h e deposition of the l o w e r shale,

then the l i m e s t o n e - s h a l e - s a n d s t o n e

s e q u e n c e , so these events c a n n o t be the m o s t recent. C r o s s - c u t t i n g relationships again inform us that t h e fault and p l u t o n are t h e next e v e n t s , t h o u g h w h i c h is m o r e recent t h a n the other is inconclusive, as neither cuts the other. Finally, the dike cuts all the prec e d i n g r o c k units, e x t r u d i n g the lava flow at the s u r f a c e — t h e final r o c k unit. Refer to the sections on r e c o n s t r u c t i n g g e o l o g i c history from the stratigraphic r e c o r d in the text for a discussion of these c o n c e p t s . 2 1 . D. D is the best answer, although one might be inclined to answer C, based on superposition. Archeologists will c o m m o n l y use radiocarbon ages to bracket the age of an archeological site if no datable material is found in the site itself. Again, it is important to distinguish between observation (the data) and inferences (the interpretation). T h e radiocarbon date represents the time that has elapsed since the wood died. Perhaps the w o o d died during a forest fire, and the charcoal was washed away from the burned area and deposited in a layer that covered the fire pit. If this is the case, then the radiocarbon date would be y o u n g e r than the fire pit. Although charcoal is soft, it is chemically very stable and remains well preserved in sediments. It is also possible that the charcoal weathered out of some sedi m e n t a r y l a y e r s that are a c t u a l l y m u c h o l d e r t h a n t h e fire pit a n d e n d e d u p incorpor a t e d in t h e l a y e r that c o v e r s that pit. In this c a s e , t h e r a d i o c a r b o n d a t e w o u l d be m i s l e a d i n g because the charcoal was actually significantly older than the fire pit but ended up in a soil layer above the fire pit because it was recycled from eroding older sediments.


Exercise 2: Evidence of water on Mars Discuss four lines of e v i d e n c e revealed by recent p r o b e s for w a t e r on M a r s . Refer to F i g u r e s 9.17, 9.20, 9.22, 9 . 2 3 , 9.24, 9.25, and 9.26 and the text on p a g e s 2 0 2 - 2 1 0 in the textbook. E v i d e n c e of past and/or present w a t e r on M a r s includes Geomorphology Channel networks Meandering patterns L a n d f o r m s that s u g g e s t p a s t glacial p r o c e s s e s Mineral a n d r o c k c o m p o s i t i o n / c h e m i s t r y Iron o x i d e m i n e r a l s , e.g., h e m a t i t e ( p r o d u c e d by rusting of iron-rich m i n e r a l s , like pyrite) G y p s u m (a sulfate m i n e r a l d e p o s i t e d by evaporation of water) Jarosite (an iron-rich sulfate m i n e r a l ) S e d i m e n t o l o g y and stratigraphy Layered sedimentary rocks N o t e : L o o k for W e b sites that discuss evidence of water ice at the M a r i a n poles.

Answers to Review Questions 1. D. G r a v i t a t i o n a l attraction c a u s e d the dust and c o n d e n s i n g m a t e r i a l to collide a n d c l u m p t o g e t h e r (sticky collisions). Refer to F i g u r e 9.2. 2. B. T h e heat generated by the decay of radioactive elements continues to heat the Earth today. 3. C. Volcanic d e g a s s i n g a c c o u n t s for all major gases in t h e E a r t h ' s a t m o s p h e r e e x c e p t o x y g e n . O x y g e n is 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 to F i g u r e 1.8. 4. B. F i g u r e s 1.5 a n d 1.12 p r o v i d e t i m e lines for the early history of t h e Earth. 5 . B . T h e E a r t h ' s core, m a n t l e , and crust are t h o u g h t t o h a v e f o r m e d w h e n m u c h o f the E a r t h m e l t e d . M e l t i n g a l l o w e d materials w i t h i n the earth to segregate (differentiate) a c c o r d i n g to their d e n s i t y — h e a v i e r m a t t e r sinks t o w a r d the center. Refer to F i g u r e 1.6. 6. D. Refer to F i g u r e 9.15. 7 . A . T h e inner terrestrial o r r o c k y planets are distinctly m o r e c o m p a c t — d e n s e r — t h a n the outer g a s e o u s p l a n e t s . T h e d e n s i t y of Saturn is less t h a n that of liquid water. If an o c e a n w e r e b i g e n o u g h , Saturn w o u l d float. 8. D. R e f e r to F i g u r e 9.4. 9. B . 10. A . Refer t o p a g e s 1 9 0 - 1 9 3 . 11. C. R e f e r to Table 9.2. 12. D . S o m e a t m o s p h e r i c o x y g e n m o l e c u l e s diffuse i n t o t h e u p p e r a t m o s p h e r e , w h e r e solar r a d i a t i o n t r a n s f o r m s t h e m into o z o n e . O z o n e a b s o r b s U V r a d i a t i o n before i t r e a c h e s t h e surface w h e r e it can d a m a g e plant and a n i m a l cells. O x y g e n is not essential to all life. In fact, s o m e life f o r m s find o x y g e n t o x i c . B e c a u s e o x y g e n gas is very r e a c tive, its p r e s e n c e p r o b a b l y w o u l d h a v e r e d u c e d the l i k e l i h o o d for the c h e m i c a l evolut i o n of life in t h e earliest h i s t o r y of E a r t h . L u c k y for u s , the E a r t h b e g a n as an o x y g e n free p l a n e t . O x y g e n r e a d i l y reacts w i t h h y d r o g e n to f o r m water, but the a m o u n t of free h y d r o g e n in E a r t h ' s a t m o s p h e r e is m i n u s c u l e ; therefore, this r e a c t i o n is n o t significant. Refer to Chapter 11.

275

276


CHAPTER 10

Answers to Practice Exercises Exercise 1: Evolution of the continents


277

Exercise 2: Ocean crust versus continental crust Characteristics

Ocean crust

Continental crust

Composition

Malic

Felsic to i n t e r m e d i a t e

Rock type(s)

H o m o g e n e o u s basalt, g a b b r o ,

Very

and pelagic s e d i m e n t s

but

3

Density

3.0

g/cm

Thickness

10 km

heterogeneous—can dominantly

with

a

2.7

g/cm

granitic

gneissic

15-65 km The 4

Topographic

Abyssal

c r a t o n or shield

features

Ridge with axial rift

stable p l a t f o r m

Trenches

continental m a r g i n

Sea

rock

3

175 million y e a r s or y o u n g e r w i t h older floor

Mounts

Hot-spot

and

any

cover of sediments

f r a g m e n t s c a u g h t u p within continents

Age

contain

island

ages

of continental

billion

crust

spans

years.

coastal plain chains

Plateaus

continental shelf a n d slope m o u n t a i n belts sedimentary basins

Structure/ Architecture

A m o d e l for the structure of the o c e a n

The

crust is t h e ophiolite suite: d e e p - s e a

complex.

architecture

sediments, basaltic pillow lavas and dikes,

accreted

a n d g a b b r o . ( N o t e : peridotites are part of

arcs,

suture zones,

the mantle lithosphere, not the ocean crust.)

belts

representing

It

microplates,

Sediments

continents

is

of preexisting

cratons,

island

volcanic

arcs,

ophiolite suites, ancient

of the

and

orogenic

cover basement rock

interior platform Origin

of the

consists

in

zones. the

continent,

mafic m a g m a t i s m and v o l c a n i s m at the

orogenic processes

and

accretion

ocean ridge system •

preexisting

crustal

blocks

convergent

plate

boundaries

of

along

Answers to Review Questions 1. C. R e f e r to p a g e s 2 3 4 - 2 3 8 . 2 . A . C o n v e r g e n t plate m a r g i n s . Refer t o F i g u r e 10.12. 3. C. P a s s i v e c o n t i n e n t a l m a r g i n s are located far from active plate b o u n d a r i e s . 4. C. T h e oldest k n o w n continental r o c k is r a d i o m e t r i c a l l y dated at 4 billion y e a r s old. It is the A c a s t a g n e i s s f r o m the Slave P r o v i n c e in C a n a d a . Refer to F i g u r e 10.22. 5 . D . R e f e r t o the section H o w C o n t i n e n t s G r o w starting o n p a g e 2 2 4 . 6. C. Refer to F i g u r e 10.18. 7. A. Refer to Figure 10.11. 8. B. R e f e r to F i g u r e s 10.1 and 10.8. 9. C. Y o u n g o r o g e n i c belts are c h a r a c t e r i z e d by hotter and thicker crust. 10. A . R e f e r t o F i g u r e 10.11 and the section T h e N o r t h A m e r i c a n Cordilleran. 11. B. T h e A n d e s M o u n t a i n s along the west coast of South A m e r i c a are an active orogenic system associated with the subduction of ocean crust beneath the South A m e r i c a n continent. 12. A. O r o g e n y refers to the collection of p r o c e s s e s , typically active at c o n v e r g e n t plate b o u n d a r i e s , that f o r m m o u n t a i n s . Rifting leads to the f o r m a t i o n of a n e w o c e a n basin. 13. C. Refer to t h e section T h e A p p a l a c h i a n Fold Belt. 14. B . S e e t h e section T h e N o r t h A m e r i c a n Cordilleran.

278


15. A. T h e C o r d i l l e r a is a y o u n g e r m o u n t a i n belt c o m p a r e d to the A p p a l a c h i a n s . In fact, portions of the C o r d i l l e r a are still involved in o r o g e n i c p r o c e s s e s . T h e western North A m e r i c a n crust sits h i g h e r than the A p p a l a c h i a n s b e c a u s e the Cordillera crust is thicker and hotter, and tectonic p r o c e s s e s r e m a i n very active. 16. 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 s , sandstones, and shale are characteristic of a passive continental m a r g i n or a s e d i m e n t a r y basin within a continental platform. 17. A. T h i c k e r continental crust typically stands higher. H o t t e r crust also stands higher b e c a u s e heat lowers the density of the r o c k m a t e r i a l . 18. B. In a sense, m o u n t a i n s are m a d e from m o u n t a i n s . M o s t of t h e s e d i m e n t s eroded from p r e v i o u s o r o g e n i c s y s t e m s ultimately end up a l o n g a continental m a r g i n w h e r e it eventually is d e f o r m e d and uplifted by the evolution of an active c o n v e r g e n t boundary. Some s e d i m e n t s are p r o b a b l y s u b d u c t e d , but m o s t are d e f o r m e d and e n t a n g l e d in thrusting and m e t a m o r p h i s m d u r i n g orogeny. S o m e s e d i m e n t s m a y b e carried t o d e p t h s where they melt. M e l t s derived from s e d i m e n t s are typically felsic, not mafic, and crystallize within the crust to f o r m granitic r o c k s . E x c e p t for r e g i o n s adjacent to t h e continental m a r g i n , m o s t o c e a n floor is surprisingly lacking in s e d i m e n t . T h i s is partly because the o c e a n floor is y o u n g and continually b e i n g recycled by plate t e c t o n i c s and b e c a u s e sedi m e n t a t i o n rates in the d e e p o c e a n are very slow. 19. B. Rifting b r e a k s up a c o n t i n e n t and forms a new passive m a r g i n on w h i c h sediments a c c u m u l a t e . Eventually, with the d e v e l o p m e n t of s u b d u c t i o n adjacent to the passive m a r g i n , o r o g e n y is initiated. O r o g e n i c p r o c e s s e s t h i c k e n the o v e r r i d i n g crust, which then b e g i n s to rise d u e to a b u o y a n t b a l a n c e b e t w e e n the m o u n t a i n root and the denser m a n t l e in w h i c h it sits. C a n y o u think of a g o o d r e a s o n w h y s u b d u c t i o n is likely to eventually o c c u r adjacent to an old passive continental m a r g i n ? 2 0 . C. Refer to F i g u r e 10.8. 2 1 . B . Refer t o the section H o w C o n t i n e n t s G r o w . 22. A. T h e A n d e s M o u n t a i n s along the west c o a s t of S o u t h A m e r i c a are in a m o r e youthful stage of o r o g e n y than the N o r t h A m e r i c a n C o r d i l l e r a and exhibits a great n u m b e r of similarities to past g e o l o g i c c i r c u m s t a n c e s in w e s t e r n N o r t h A m e r i c a . T o d a y geologists actively study the A n d e s as a w a y of traveling b a c k in t i m e to the earlier C e n o z o i c history of the N o r t h A m e r i c a n Cordillera. 2 3 . B . Refer t o F i g u r e 10.20. 2 4 . A. Refer to F i g u r e 10.24. 2 5 . D. Refer to F i g u r e 10.6.


279

CHAPTER 11

Answers to Practice Exercises Exercise 1: How organisms and the Earth interact Life's impact on Earth

Life process(es) generating the impact

Description of the interaction and impact

0 i n Earth's

B y p r o d u c t of p h o t o s y n t h e s i s

G r e e n plants and a l g a e u s e e n e r g y in t h e a t m o s p h e r e

2

atmosphere

from sunlight to convert w a t e r and c a r b o n d i o x i d e to c a r b o h y d r a t e s , such as sugar, and o x y g e n . O x y g e n is released to the a t m o s p h e r e .

Greenhouse

Extraction

effect: C o o l i n g

oceans

of carbon from

and

atmosphere

by

T h e extraction of carbon dioxide from the a t m o s p h e r e and oceans by photosynthetic organisms and shell-producing

shell-producing

and

organisms, like c l a m s and coral, d r a w s d o w n the a m o u n t

photo synthetic

organisms

o f C 0 i n the E a r t h ' s a t m o s p h e r e and o c e a n s . C a r b o n 2

dioxide is a heat-trapping molecule. R e d u c i n g the a m o u n t of C 0 Greenhouse

Respiration and metabolism

effect: W a r m i n g

of anaerobic microbes. Refer to Table 11.2 and textbook page 246.

2

i n the a t m o s p h e r e reduces the g r e e n h o u s e effect.

T h e m e t a b o l i s m of s o m e o r g a n i s m s p r o d u c e s c a r b o n d i o x i d e and m e t h a n e , w h i c h are b o t h h e a t - t r a p p i n g g a s e s that contribute to global w a r m i n g .

Mineral

M i c r o b i a l precipitation of pyrite

M i c r o b i a l precipitation of pyrite o c c u r s in the a n a e r o b i c

precipitation

o c c u r s in the a n a e r o b i c z o n e of

z o n e of s e d i m e n t s c o n t a i n i n g i r o n - b e a r i n g m i n e r a l s .

s e d i m e n t s c o n t a i n i n g iron-bearing

S o m e m i c r o b e s , insect, and a n i m a l s precipitate m a g n e t i t e

m i n e r a l s . S o m e m i c r o b e s , insect

(iron o x i d e ) . P h o s p h a t e c a n a c c u m u l a t e w i t h i n m i c r o o r -

and a n i m a l s precipitate m a g n e t i t e

g a n i s m s within s e d i m e n t s .

(iron o x i d e ) . P h o s p h a t e can a c c u m u late in m i c r o o r g a n i s m s in s e d i m e n t s . Mineral

"Sulfate eating " microbes

dissolution

Hydrogen, methane

hydrogen gases

may

sulfide,

Answers to Review Questions 1. A. In contrast, h e t e r o t r o p h s get food by feeding directly or indirectly on p r o d u c e r s . 2. A. You are w h a t y o u eat. T h i s is true for all o r g a n i s m s w h e t h e r they eat s i m p l e c a r b o n b e a r i n g m o l e c u l e s like c a r b o n d i o x i d e and m e t h a n e o r large c a r b o h y d r a t e m o l e c u l e s . 3. C. R e f e r to Table 11.2. 4. B. T h e alternative a n s w e r s are characteristic of bacteria and archaea. 5. C. Refer to F i g u r e s 8.14 a n d 11.6 and t e x t b o o k p a g e s 2 5 6 - 2 5 7 . 6 . D . S t r o m a t o l i t e s are s e d i m e n t a r y structures p r o d u c e d b y m i c r o b i a l m a t s . T h e outer layer o f the m i c r o b i a l m a t c o n t a i n s p h o t o s y n t h e t i c a u t o t r o p h s , w h i c h release o x y g e n gas as a b y p r o d u c t of their m e t a b o l i s m . M i c r o b i a l m a t s f o r m stomatolitic structures e v e n today, but the a b u n d a n c e of stromatolite structures today is limited by the p r e s e n c e of m o l l u s k s that feed on m i c r o b i a l m a t s and by other e n v i r o n m e n t a l factors. 7. D. B a n d e d iron f o r m a t i o n s f o r m e d over an interval of g e o l o g i c t i m e d u r i n g w h i c h o x y g e n w a s released by p h o t o s y n t h e t i c a l g a e into an o x y g e n - p o o r o c e a n that w a s rich in d i s s o l v e d iron. 8. A. Refer to F i g u r e 11.12 and the related text on p a g e 2 5 3 in the textbook. 9. D. Refer to Table 11.3. 10. B. Refer to the t e x t b o o k section Origin of Life and the O l d e s t Fossils.

and

be produced.

280


1 1 . D. Refer to F i g u r e 11.14 and the related text on p a g e 2 5 5 in the textbook. 12. B. Refer to t e x t b o o k F i g u r e s 11.18 and 11.19. 13. B. A b u n d a n t o x y g e n gas in the a t m o s p h e r e w o u l d be e v i d e n c e for p h o t o s y n t h e t i c life on the planet. T h e r e is no other p r o c e s s k n o w n that c a n a c c o u n t for a b u n d a n t oxygen gas in a planetary a t m o s p h e r e b e c a u s e o x y g e n is very reactive and n e e d s to be continually r e p l e n i s h e d . E v e n t h o u g h m e t h a n e w o u l d react rapidly w i t h o x y g e n in the atmosp h e r e , m i n o r a m o u n t s w o u l d be e v i d e n c e for life p r o c e s s e s , especially m i c r o b i a l metab o l i c p r o c e s s e s . It w o u l d be even m o r e exciting to find e v i d e n c e for the p r e s e n c e of liquid water, but the next best thing is the existence of w a t e r ice. W h e r e there is water ice there m a y also be s o m e liquid water, if not at the surface, m a y b e in the shallow subsurface w h e r e at least microbial life m a y flourish.

CHAPTER 12

Answers to Practice Exercises Exercise 1: Lava types—their properties, eruption styles, deposits, landforms, association with plate tectonics, and hazards Characteristics

Basalt (mafic)

Andesite (intermediate)

eruption temperature

1000 to 1 2 0 0 ° C

intermediate

800 to 1000°C

silica c o n t e n t

low ( - 5 0 % )

intermediate

high

gas c o n t e n t

low,

variable

high (up to ~ 15%)

viscosity

low-fluid

intermediate

high—very viscous

typical flow velocity

0.7 to 30 m/minute

9

less than 9 m/day

typical flow length

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

explosive

typically very

Rhyolite (felsic)

Properties

E r u p t i o n styles

up to a few percent magma

typically

not

very

m/day

explosive Deposits

flood

basalt

explosive lava

flow

fissure flow

dome

p a h o e h o e and a a flows

pyroclastic

pillow lava

(>-77%)

dome obsidian flow—

tuff and

welded

dome

pyroclastic f l o w — tuff

tuff and w e l d e d tuff

cinder Landforms

shield v o l c a n o

composite

lava plateau

summit

cinder

cone

caldera

small

caldera

cinder

volcano crater

composite volcano large c a l d e r a s u m m i t crate

cone

A s s o c i a t i o n with

divergent b o u n d a r i e s

plate t e c t o n i c s

hot

Hazards

lava flow

lava

explosive in c o n t a c t

pyroclastic/ash

convergent boundaries

convergent boundaries

spots

with w a t e r hot gases

flow

explosive hot

gases

mudflow

explosive blast blast

flow

pyroclastic/ash hot

gases

m u d f l o w s (lahars)


281

Exercise 2: Volcanoes at plate tectonic boundaries

Volcano or volcanic area

Type of volcano (shield, composite, caldera)

Magma type (mafic, intermediate, felsic)

Magmatic (plate tectonic) setting divergent, convergent, hot spot hotspot/mantle plume

Hawaii

shield

mafic

T o n g a Islands

composite

intermediate

Columbia Plateau

flood basalts

mafic

hot

Santorini ( T h e r a ) , G r e e c e

caldera

intermediate/felsic

convergent

Mayon, Philippines

composite

intermediate/felsic

convergent

Iceland

shields/fissures

m o s t l y mafic

divergent and

Yellowstone

caldera

intermediate/felsic

continental hot spot

Krakatoa, Indonesia

composite/caldera

intermediate/felsic

convergent

N o r t h Island, N e w Z e a l a n d

composite

intermediate/felsic

convergent

Crater L a k e , O r e g o n

composite/caldera

intermediate/felsic

convergent

Japan

composite

intermediate/felsic

convergent

Aleutian Islands, Alaska

composite

intermediate/felsic

convergent

M a r i a n a Islands

composite

intermediate/felsic

convergent

K i l i m a n j a r o , Africa

composite

intermediate/felsic

continental rift/hot spot

Pinatubo, Philippines

composite

intermediate/felsic

convergent

Katmai, Alaska

composite

intermediate/felsic

convergent

M o u n t Rainier, W a s h i n g t o n

composite

intermediate/felsic

convergent

Tambora, Indonesia

composite

intermediate/felsic

convergent

Vesuvius, Italy

composite

intermediate/felsic

convergent

and

caldera

and

caldera

and felsic

Answers to Review Questions 1. D. C o o l i n g rate d e t e r m i n e s the size of the m i n e r a l grains within lava flows. If m i n e r a l s a l r e a d y exist in the m a g m a c h a m b e r before an eruption o c c u r s , t h e lava will erupt with larger crystals floating in it and solidify into a v o l c a n i c r o c k with a m i x t u r e of grain sizes, k n o w n as a p o r p h y r y . 2 . B . Table 4 . 2 p r o v i d e s b a s i c c o m p o s i t i o n a l t r e n d s for i g n e o u s r o c k s . R h y o l i t e contains up to a b o u t 77 p e r c e n t silica. B a s a l t is mafic with a b o u t 50 p e r c e n t silica. 3. B. W a t e r m a k e s up 7 0 - 9 5 p e r c e n t of t h e v a p o r c o n t e n t of a m a g m a , followed by c a r b o n d i o x i d e and sulfur d i o x i d e . 4 . B . R e f e r t o F i g u r e 12.10. 5. B. H a w a i i h a s m a n y classic e x a m p l e s of shield v o l c a n o s . Refer to F i g u r e 12.11a. 6. C. Refer to F i g u r e 12.1 I d . 7. B. T h e h i g h e r silica content of rhyolitic lavas results in a m u c h h i g h e r viscosity, w h i c h favors very thick lava flows or d o m e s . Refer to F i g u r e 12.1 l b . 8. D. Refer to F i g u r e 1 2 . 1 1 . 9 . B . A n d e s i t i c v o l c a n o e s are c o m m o n l y associated with c o n v e r g e n t plate b o u n d a r i e s . Partial m e l t i n g of the s u b d u c t i n g slab or m a t e r i a l s a b o v e the s u b d u c t i n g slab typically generates i n t e r m e d i a t e m a g m a s . 10. B. T h e H a w a i i a n Islands are a chain of v o l c a n o e s g e n e r a t e d as the Pacific plate m o v e s o v e r a h o t spot in the m a n t l e . 1 1 . D . M a g m a s rich i n silica and dissolved g a s e s c a u s e the m o s t explosive volcanic e r u p t i o n s . I n c r e a s i n g silica c o n t e n t progressively increases the viscosity of the m a g m a .

convergent/subduction spot

hot

spot

282


M o r e v i s c o u s m a g m a s are m o r e likely to p l u g the throat of a v o l c a n o until pressure builds up high e n o u g h to c a u s e an explosive eruption. 12. A. Shield v o l c a n o e s are f o r m e d p r e d o m i n a n t l y by basaltic lavas, w h i c h are more fluid and therefore tend to spread out. C o m p o s i t e v o l c a n o e s are c o n s t r u c t e d predominantly from i n t e r m e d i a t e and felsic lava flows, d o m e s , and pyroclastic flows. Intermediate and felsic lavas are m o r e v i s c o u s and form thicker flows than basalt. 13. D. Refer to F i g u r e 12.12. 14. C. Refer to t e x t b o o k sections on flood basalts and fissure e r u p t i o n s . 15. B. Refer to F i g u r e s 12.10 and 1 2 . 1 1 . 16. D. Refer to F i g u r e 12.20. 17. B. Refer to the textbook section R e d u c i n g the R i s k s of H a z a r d o u s Volcanoes. 18. A. Basaltic eruptions o c c u r a l o n g the o c e a n i c r i d g e s y s t e m and form the o c e a n crust. 19. A. H o t spots are surface expressions of m a g m a p l u m e s c o m i n g up from the ultramafic mantle. T h e s e p l u m e s are basaltic in composition. T h e composition of hot-spot volcanism m a y b e c o m e m o r e felsic if the m a g m a p l u m e rises through continental crust, as it does under Yellowstone. Hot spots like Hawaii and Yellowstone occur within the middle of crustal plates. H o t spots are also located along divergent plate b o u n d a r i e s as in Iceland. 2 0 . B. A n d e s i t e s (intermediate m a g m a s ) erupt c o m m o n l y at c o n v e r g e n t plate boundaries w h e r e o c e a n i c crust is s u b d u c t i n g b e n e a t h continental crust. O n e e x a m p l e of where this p r o c e s s is h a p p e n i n g today is b e n e a t h the C a s c a d e R a n g e of v o l c a n o e s , w h i c h extends from n o r t h e r n California t h r o u g h W a s h i n g t o n . M o u n t St. H e l e n s is w i t h i n the Cascades. 2 1 . C . Refer t o F i g u r e 1 2 . 1 1 . 2 2 . B. If a v o l c a n o is fed by o n e large m a g m a c h a m b e r w i t h o u t any additional injections of m a g m a from below, then the r e m a i n i n g m e l t i n the m a g m a c h a m b e r b e c o m e s progressively e n r i c h e d in silica and depleted in iron and m a g n e s i u m as the m a g m a solidifies. Y o u n g e r lava flows are enriched in silica relative to flows that erupted w h e n m o r e of the m a g m a w a s m o l t e n . With a h i g h e r silica content and a l o w e r iron content, younger lava flows are less mafic and m o r e v i s c o u s (less fluid). 2 3 . B. Refer to the section L a v a s and O t h e r Volcanic D e p o s i t s in the textbook. 24. B.


Exercise 2: Characteristics of seismic waves F i g u r e 13.5 and t h e t e x t b o o k section S e i s m i c W a v e s will be helpful.

Characteristic

P (primary) waves

S (secondary) waves

Surface waves

Relative speed

fastest

second

slowest

Motion of material through

compressional—push/pull

shearing

fastest

rolling/elliptical sideways

which wave propagates Medium through which wave

solids

will p r o p a g a t e

liquid

confined to

solids only

the

surface

gas A n a l o g y with c o m m o n

sound waves

w a v e forms

S

waves propagation

is

difficult to visualize.

It is

somewhat

to

analogous

ocean waves the

way cards in a deck of playing cards slide

over each

other

as you shuffle the deck.

Exercise 3: Factors that amplify the damage caused by an earthquake F a c t o r s that amplify the d a m a g e c a u s e d by an e a r t h q u a k e follow. • T h e m a g n i t u d e of the e a r t h q u a k e . O b v i o u s l y , a larger m a g n i t u d e e a r t h q u a k e t e n d s t o b e m o r e destructive, but m a n y other factors b e s i d e s m a g n i t u d e c a n c o n t r i b u t e to the potential for destruction. • T h e duration of the e a r t h q u a k e . E a r t h q u a k e s that last longer usually cause m o r e d a m a g e . C o n s i d e r this a n a l o g y : T h e longer you p u s h s o m e o n e on a swing, the greater the swing b a c k and forward. L i k e w i s e , if structures experience a longer duration of p u s h e s and pulls by seismic w a v e s , the m o r e they will sway and the greater the potential for d a m a g e . • T h e g r o u n d acceleration caused by the earthquake. Faster ground acceleration is likely to cause m o r e d a m a g e . Rapid ground acceleration m a y have contributed to the destructiveness of the Northridge, California, earthquake in 1994. • D e p t h to t h e focus of the e a r t h q u a k e . T h e energy of seismic w a v e s dissipates with d i s t a n c e from the focus. • P r o x i m i t y to a c o a s t l i n e w h e r e a t s u n a m i m a y hit. Refer to E a r t h I s s u e 1 3 . 1 . • F i r e s ignited by r u p t u r e d gas and p o w e r lines. T h e fire started d u r i n g the 1906 S a n F r a n c i s c o e a r t h q u a k e c a u s e d far m o r e d a m a g e t o the city t h a n did the s e i s m i c waves themselves. • L i q u e f a c t i o n of water-rich soils and s e d i m e n t s that are typically associated with coastal r e g i o n s and d e v e l o p m e n t s . Liquefaction w a s a big factor in the d a m a g e c a u s e d t o A n c h o r a g e , A l a s k a , d u r i n g the 1964 e a r t h q u a k e . • P r e e x i s t i n g b u i l d i n g d e s i g n s and c o d e s for the c o m m u n i t y . • L a n d s l i d e s and u n s t a b l e slopes. You m a y c o m e u p with other r e a s o n a b l e factors.

and

motions Earth's

283

284


Answers to Review Questions 1. B. Refer to F i g u r e 1 3 . 1 . 2. B. An e a r t h q u a k e p r o p a g a t e s from the focus of the e a r t h q u a k e . T h e epicenter is the location on the surface directly a b o v e the focus. 3. A. P w a v e s travel fastest and surface w a v e s travel slowest. 4. C. T h r e e . Refer to F i g u r e 13.6. 5. C. T h e R i c h t e r scale m e a s u r e s the a m o u n t of g r o u n d m o t i o n . Refer to F i g u r e 13.7. 6. D. T h e Richter Scale for e a r t h q u a k e m a g n i t u d e is e x p o n e n t i a l . W i t h every unit of i n c r e a s e in m a g n i t u d e , such as from 4 m a g n i t u d e to 5 m a g n i t u d e , t h e g r o u n d motion increases by a factor of 10. 7. C. P w a v e s , like sound w a v e s , travel t h r o u g h solids, liquids, and g a s e s . 8. B. S w a v e s travel only t h o u g h solids. 9. B. E a r t h q u a k e s are strongly associated with all active plate tectonic b o u n d a r i e s . Refer to F i g u r e 13.12 and the text section T h e B i g Picture: E a r t h q u a k e s and Plate Tectonics. 10. B. P r o d u c i n g a seismic risk m a p w o u l d be a first step in e a r t h q u a k e risk assessment. 1 1 . D. Water-saturated stream delta s e d i m e n t s exhibit liquefaction in r e s p o n s e to the shaking from an e a r t h q u a k e . 12. C. 3 0 0 0 k i l o m e t e r s b a s e d on the seismic travel-time c u r v e s in F i g u r e 13.6. 13. C. D e e p - f o c u s e a r t h q u a k e s do not o c c u r in association with divergent b o u n d a r i e s . How do y o u e x p l a i n this o b s e r v a t i o n ? 14. C. Texas has the l o w e s t s e i s m i c risk c o m p a r e d to the other r e g i o n s listed. S e e Figure 13.18. 15. B. Refer to illustration below. N o t i c e that the arrows are p o i n t i n g w e s t and east to reflect t h e p u s h i n g and p u l l i n g of the land.

16. Left-lateral strike slip fault


Exercise 3: Evidence for the asthenosphere and its significance A. L i n e s of e v i d e n c e for the a s t h e n o s p h e r e : •

L o w - v e l o c i t y z o n e — S e i s m i c w a v e s travel slower t h r o u g h the a s t h e n o s p h e r e t h a n they do in shallower layers, suggesting that this z o n e m a y be closer to the m e l t ing t e m p e r a t u r e of the r o c k and therefore softer. Refer to F i g u r e s 14.6 and 14.8.

•

P o s t - g l a c i a l isostatic r e b o u n d — I s o s t a t i c adjustments of t h e crust r e q u i r e plastic flow in s o m e part of the m a n t l e to a c c o u n t for the d i s p l a c e m e n t of the crust by the m a n t l e as the crust is p u s h e d d o w n by the w e i g h t of the ice. Refer to E a r t h I s s u e s 14.1 and 2 1 . 1 .

•

R e d u c t i o n in the frequency and m a g n i t u d e of e a r t h q u a k e s b e t w e e n d e p t h s of 175 and 3 5 0 km w o u l d be explained by a m o r e ductile z o n e that is less likely to b r e a k , and, if it d o e s b r e a k , it d o e s so with less b u i l d u p of elastic strain energy.

•

W h e n t h e projected g e o t h e r m a l gradient is lotted with the m e l t i n g - p o i n t c u r v e for the r o c k t h o u g h t t o b e i n the m a n t l e , the t w o curves converge and a l m o s t touch at a d e p t h of 100 to 150 k m . This suggests that the r o c k s at this depth, d u e to the c o m b i n e d effects of t e m p e r a t u r e and p r e s s u r e , are at or very c l o s e to m e l t i n g .

B. T h e p r e s e n c e of an a s t h e n o s p h e r e m a y be a prerequisite for plate tectonics. T h e soft, ductile a s t h e n o s p h e r e allows the rigid lithosphere to d e c o u p l e and m o v e i n d e p e n d ently from the m a n t l e . Refer to y o u r w o r k on Practice E x e r c i s e 2 for C h a p t e r 2.

Answers to Review Questions 1. C. E a r t h ' s c o r e is a b o u t one-half the r a d i u s of the planet. S e e F i g u r e 14.5: In the illustration, E a r t h ' s center is at 6,000 km depth, and the outer c o r e is at 3,000 km depth. 2. B. Refer to F i g u r e 1.7. T h e o c e a n b a s i n s exist on E a r t h b e c a u s e the o c e a n crust is thinner a n d d e n s e r than t h e c o n t i n e n t a l crust. T h e r e f o r e , the o c e a n crust sits lower. 3. C. Ultramafic or peridotite. 4. B. E a r t h ' s tectonics plates are large fragments of the lithosphere, a rigid layer consisting of t h e crust and u p p e r m o s t m a n t l e . 5. C. C o n t i n e n t a l crust is very h e t e r o g e n e o u s in c o m p o s i t i o n . N e v e r t h e l e s s , its average c o m p o s i t i o n is felsic to i n t e r m e d i a t e . 6. D. T h e lithosphere is thought to be a rigid layer that rests on a plastic, w e a k asthenosphere. 7. B. S w a v e s do n o t pass t h r o u g h the outer core. 8. C. T h e metallic c o r e h a s the h i g h e s t density. 9. C. E a r t h ' s north m a g n e t i c p o l e is located b e t w e e n G r e e n l a n d and Baffin Island, not at the E a r t h ' s p o l e of r o t a t i o n — t h e N o r t h Pole. Refer to F i g u r e 14.10. 10. C. T h e e l e c t r o - d y n a m o theory explains the E a r t h ' s magnetic field as a c o n s e q u e n c e of electrical currents m o v i n g in the fluid, metallic outer core. T h e silicate minerals in the asthenosphere are not nearly as g o o d conductors of electricity. Therefore, the flow of electrical currents and the associated magnetic field are m u c h w e a k e r in the asthenosphere. 1 1 . B. T h e a s t h e n o s p h e r e is t h o u g h t to be a w e a k , plastic z o n e within the u p p e r m a n t l e that m a y b e partially m e l t e d . 12. B. Refer to F i g u r e 14.14. 13. A. R e f e r to F i g u r e 14.8. 14. C. T h e m a n t l e - c o r e b o u n d a r y is m a r k e d by t h e m o s t significant c h a n g e in c o m p o s i t i o n . 15. A. T h e thickest c o n t i n e n t a l crust on E a r t h lies b e n e a t h the Tibetan plateau, H i m a l a y a n m o u n t a i n s , and p o r t i o n s of the A n d e s m o u n t a i n s . Refer to F i g u r e 14.7. 16. B. Refer to F i g u r e 14.6. 17. D .

287

288


18. D. If a significant portion of the l o w e r m a n t l e w e r e m o l t e n , we should see lots m o r e volc a n i c activity on the E a r t h ' s surface in addition to tectonic plate b o u n d a r y v o l c a n i s m . 19. A. S e i s m i c w a v e velocities d e p e n d on the elasticity, rigidity, and density of materials. T h e s e p r o p e r t i e s d e p e n d on the c o m p o s i t i o n , physical state, and c o m p a c t n e s s of the a t o m i c structure of the material. 2 0 . C. R e f e r to F i g u r e s 1.12 and 14.13. 2 1 . B. T h e M o h o w a s first detected by an abrupt increase in seismic w a v e velocities d u e to the c h a n g e from lower-density crustal rocks to the m o r e dense ultramafic rocks within the mantle. 2 2 . A. R e f e r to F i g u r e 14.9 and E a r t h I s s u e s 14.2. 2 3 . D . R e f e r t o F i g u r e 14.15. 2 4 . A. R e f e r to F i g u r e s 14.1 and 14.2. 2 5 . D . R e f e r t o F i g u r e 14.7. 2 6 . D. Plates are located in the lithosphere. Their motion is driven in part by the force of gravity as cold, heavy crust sinks, dragging the plate behind it. As the plates sink at convergent b o u n d a r i e s , an o p e n i n g is created for hot m a g m a to rise through at the diverging boundary. T h e driving forces of plate tectonics are manifestations of mantle convection in the sense that they involve hot matter rising in o n e place and cold matter sinking in another. See C h a p t e r 2, " W h e r e do the plate-driving forces originate? Refer to pages 3 7 - 4 0 .

CHAPTER 15

Answers to Practice Exercises Exercise 1: Conceptual map/Flowchart of a climate factor C l i m a t e is a c o m p l e x s y s t e m with m a n y factors potentially influencing it. You m i g h t c h o o s e o n e of t h e following as a focus for y o u r c o n c e p t u a l flowchart: H i g h m o u n t a i n s -» rain s h a d o w effect El N i n o -> wet w e a t h e r in the eastern equatorial Pacific Gulf S t r e a m -> transfers a t r e m e n d o u s a m o u n t of heat to n o r t h e r n E u r o p e E a r t h ' s Orbital ( M i l a n k o v i t c h ) characteristics - * cyclical c h a n g e s i n the a m o u n t of energy reaching Earth Volcanic e r u p t i o n injects dust a n d aerosols into a t m o s p h e r e -> i n c r e a s e in a l b e d o ->• E a r t h cools Volcanic eruption r e l e a s e s t r e m e n d o u s a m o u n t s of c a r b o n d i o x i d e gas -> e n h a n c e d g r e e n h o u s e effect -*• E a r t h ' s surface h e a t s up T h e r e are m a n y other factors.


289

Exercise 2: Release of carbon dioxide from the burning of fossil fuels A . A n increase i n C 0 enhances the g r e e n h o u s e effect and results i n w a r m e r surface condi2

tions. W a r m e r surface conditions increase evaporation rates and m o r e water vapor goes into the a t m o s p h e r e . M o r e water vapor in the a t m o s p h e r e m a y result in m o r e cloud cover. B . A n i n c r e a s e i n c l o u d cover m a y p r o d u c e a negative f e e d b a c k b e c a u s e c l o u d cover t y p ically h a s a c o o l i n g effect on surface c o n d i t i o n s . So the i m p a c t of increased c l o u d cover i s o p p o s i t e t o that o f increasing C 0 i n the a t m o s p h e r e . 2

Exercise 3: Flow of carbon through Earth's systems and reservoirs

Carbon fluxes

Brief description of flux

Photosynthesis

Carbon is fixed in living

Carbon flows from

and p r e c i p i t a t i o n

organisms,

atmosphere

of c a r b o n a t e s Sedimentation

which

contribute

to

sediments,

coal,

ultimately

organic

matter

Calcium

and

to produce

carbonate

carbonate,

which

and collect

to form

cement

in

and

rock—the

Less g r e e n h o u s e gas in

the

oceans

into

a t m o s p h e r e results in a cooling.

lithosphere.

and oil.

combine

help

Climatic impact/ implications

Direction of flux

ions

C a r b o n is d r a w n out to the

Climate

cools.

calcium

a t m o s p h e r e and o c e a n and

Carbon

dioxide

precipitated as s e d i m e n t .

of the

can

precipitate

limestone

other

or

The

rock particles.

oceans

loss

is drawn and

out

atmosphere.

of CO from

the

2

oceans will result in a reduction of CO in the atmosphere. 2

Volcanism

CO

2

is

typically

abundant

gas

volcanic

the

second most

released

C0

2

is

released from

lithosphere

during

and

eruptions.

into

the

An increase in C0 in the

the

2

atmosphere

oceans.

atmosphere

enhances

greenhouse

effect

the Chemical

C0

weathering

minerals

2

in

rainwater in

combines

rock to form

with calcium

Heating,

atmosphere the

carbonate.

Metamorphism

Carbon flows from

recystallization,

decomposition

of rocks

metamorphism

can

amounts

C0 .

of

and

release

and

oceans

into

from the rocks

into the atmosphere

cools. environments

Uplift of

high

plateaus

and

mountains

may

enhance

this flux.

Climate

warms.

Increased levels greenhouse

2

and

stored in the crust.

atmosphere

oceans.

warms

surface.

Carbon is being drawn out of surface

flows

(the crust)

large

Climate

the

crust.

Carbon

during

and

Earth's

the

and

to

of C0

enhances effect,

2

in

the

the

which

acts

trap heat energy and slow

down the loss of heat to space. H u m a n activities:

The burning of fossil fuels

C o m b u s t i o n of

releases

fossil fuels

into

the

large

amounts

atmosphere.

of C0

Carbon 2

flows

sphere (coal, the

oceans

from the lithooil, and

A n i n c r e a s e i n C 0 i n the 2

and gas) into

a t m o s p h e r e e n h a n c e s the

atmosphere.

g r e e n h o u s e effect and w a r m the E a r t h ' s surface.

290


Answers to Review Questions 1. A. T h e E a r t h ' s a t m o s p h e r e is a b o u t 7 8 % nitrogen g a s . Refer to the t e x t b o o k section C o m p o n e n t s o f the C l i m a t e S y s t e m o n p a g e 3 4 8 . 2. A. C a r b o n d i o x i d e is transparent to visible light and a b s o r b s h e a t (infrared radiation). Refer to F i g u r e 15.7. 3. A. Refer to F i g u r e 15.16. 4. B. Refer to the section C a r b o n B u d g e t in y o u r t e x t b o o k . 5. B. Volcanic e r u p t i o n s are not associated with El N i n o events. Refer to Earth Issues 15.2. 6 . A . A s the o c e a n s b e c o m e w a r m e r , c a r b o n d i o x i d e solubility d e c r e a s e s . Therefore, more c a r b o n d i o x i d e will be released, w h i c h is a positive effect. A positive effect adds carbon d i o x i d e to the a t m o s p h e r e ; a negative effect subtracts it. Refer to F i g u r e 2 0 . 2 4 on page 5 0 1 for m o r e details on the solubility of c a l c i u m c a r b o n a t e . 7. C. C a r b o n d i o x i d e is a g r e e n h o u s e g a s . 8. A. T h e g r e e n h o u s e effect significantly influences surface t e m p e r a t u r e s for planets with atmospheres. 9. C. L i v i n g o r g a n i s m s . K e e p in m i n d the definition of flux: the flow of c h e m i c a l s from o n e reservoir to another. Refer to F i g u r e 15.14. 10. D . A t p r e s e n t b u r n i n g o f fossil fuel b y h u m a n s c o n t r i b u t e s far m o r e a t m o s p h e r i c carb o n t h a n d o e s d e f o r e s t a t i o n . T h e o t h e r a l t e r n a t i v e s (plant u p t a k e a n d o c e a n gas e x c h a n g e ) r e p r e s e n t in p a r t a r e s p o n s e to h u m a n activity. N o r m a l l y t h e r e is a b a l a n c e d c a r b o n flow b e t w e e n p l a n t s a n d a t m o s p h e r e a n d b e t w e e n o c e a n s a n d a t m o s p h e r e . See F i g u r e 1 5 . 1 4 . H u m a n activities are d i s r u p t i n g this b a l a n c e , s o t h e c l i m a t e system r e s p o n d s b y a b s o r b i n g m o r e c a r b o n i n t o t h e o c e a n s a n d i n c r e a s i n g p l a n t production on land. Refer to Figure 15.15. 1 1 . A. T r o p o s p h e r e . Refer to F i g u r e 15.2. 12. D . Refer t o p a g e s 3 5 0 - 3 5 1 i n the textbook. 13. B . Refer t o F i g u r e 15.3. 14. A. Refer to F i g u r e 15.13 and the section C a r b o n a t e W e a t h e r i n g and Precipitation in the textbook. 15. A. 0 . 6 ° C . 16. B. Refer to F i g u r e 15.7 and the text on p a g e 3 5 4 . 17. D. Refer to F i g u r e 15.11 a n d the t e x t b o o k section M i l a n k o v i t c h C y c l e s .


Chapter 16 Answers to Practice Exercises Exercise 1: Physical and chemical weathering R e f e r t o Table 16.2 a n d F i g u r e 1 6 . 1 1 .

291

292


Exercise 2: Inventory of the different kinds of mass wasting

Kind of mass wasting

Composition of slope (consolidated vs. unconsolidated and wet vs. dry) Characteristics

Rock avalanche

L a r g e m a s s e s o f rocky materials

Speed: R u n n i n g or a s p e e d i n g a u t o Slope angle: S t e e p slopes Triggering event(s): E a r t h q u a k e s Notes: O c c u r in m o u n t a i n o u s regions w h e r e rock is w e a k e n e d by weathering, structural deformation, weak bedding, or cleavage planes

Creep

Speed: Walking

Soil

Slope angle: Any angle T r i g g e r i n g event(s): N o n e N o t e s : Influenced by the k i n d of soil, c l i m a t e , steepness of slope, and density of v e g e t a t i o n Earthflows

Soils and fine-grained r o c k materials, s u c h as shales and clay-rich r o c k s

S p e e d : W a l k i n g or r u n n i n g Slope a n g l e : Any angle T r i g g e r i n g event(s): Intense rainfall Notes:

D e b r i s flow

Fluid-like

movement

S p e e d : R u n n i n g o r s p e e d i n g auto

R o c k fragments s u p p o r t e d by a

S l o p e a n g l e : Any angle

muddy matrix

Triggering event(s): Intense rainfall Notes: Contains coarser rock materials compared to earthflows. Mudflow

Mostly finer coarser of

rock

materials

rock debris

with

with large

some amounts

Speed:

Speeding

auto

Slope angle: Any angle T r i g g e r i n g event(s): I n t e n s e rainfall or c a t a s t r o p h i c

water.

melting of ice and snow by a v o l c a n i c e r u p t i o n Notes: Debris avalanche

Water-saturated

soil

and

rock

Contains

large

amounts

of water

S p e e d : S p e e d i n g auto Slope angle: Steep T r i g g e r i n g event(s): E a r t h q u a k e s Notes: Occurs in humid, mountainous regions

Slump

U n c o n s o l i d a t e d r o c k material

Speed:

Walking or running

S l o p e a n g l e : Any slope T r i g g e r i n g event(s): Rainfall N o t e s : D e b r i s slide m o v e s faster than a s l u m p Solifluction

Surface

layers

of soil

Speed:

Walking

S l o p e a n g l e : Any angle T r i g g e r i n g event(s): F r e e z e - t h a w Notes: O c c u r s only in cold r e g i o n s w h e n w a t e r in the surface layers of the soil alternately freezes and t h a w s ; water c a n n o t seep into the g r o u n d b e c a u s e d e e p e r layers are frozen.


Exercise 3: Water's Role in Mass Wasting Some sample reasonable answers 1. W a t e r lubricates, especially if the g r o u n d is saturated (all p o r e spaces are filled with w a t e r ) , by r e d u c i n g the internal friction b e t w e e n r o c k particles. In u n c o n s o l i d a t e d r o c k m a t e r i a l a s m a l l a m o u n t of w a t e r increases surface tension, w h i c h actually helps to w e a k l y " g l u e " t h e d a m p , l o o s e material together. Too m u c h w a t e r k e e p s the particles apart a n d a l l o w s t h e m to m o v e freely over o n e another. T h e r e f o r e , saturated sand, in w h i c h all p o r e spaces are o c c u p i e d by water, runs like a fluid and collapses to a flat p a n c a k e s h a p e (refer to F i g u r e 16.13). 2. W a t e r (hydrostatic) pressure m a y b e c o m e great e n o u g h to separate the grains or p r o m o t e the slippage of b e d s past o n e another. Refer to the textbook section Water Content. 3. W a t e r is a m a j o r a g e n t in 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 , w h i c h p r o m o t e s m a s s wasting. W e a t h e r i n g p r o m o t e s m a s s w a s t i n g b y c h e m i c a l l y and physically w e a k e n i n g rock. 4. F r e e z i n g and t h a w i n g are t w o specific roles that w a t e r plays in c a u s i n g solifluction, a k i n d o f m a s s m o v e m e n t . Refer t o t h e t e x t b o o k section U n c o n s o l i d a t e d M a s s M o v e m e n t s . U n d e r c u t t i n g and o v e r s t e e p e n i n g of hillslopes by e r o s i o n is another w a y w a t e r e n h a n c e s m a s s w a s t i n g . F l o w i n g w a t e r in rivers and frozen w a t e r in glaciers are p o w erful a g e n t s of erosion. Rivers typically e r o d e on the outside of river b e n d s . E r o s i o n can u n d e r c u t and o v e r s t e e p e n the riverbank and adjacent hillslope. 5 . W a t e r - s a t u r a t e d r o c k m a t e r i a l s rich i n c l a y s o r l o o s e s a n d m a y b e t r a n s f o r m e d into fluid slurries by a p r o c e s s called liquefaction. R e f e r to t h e t e x t b o o k section T r i g g e r s for M a s s M o v e m e n t s .

Exercise 4: Evaluation of slope stability A. P o s s i b l e factors that e n h a n c e the potential for mass m o v e m e n t at the homesite shown •

T h e h o u s e is built on a cut-and-fill foundation: the h o u s e is built partly on a cut into t h e b e d r o c k of t h e slope and partly on the gravel fill derived from the cut. A cut-andfill f o u n d a t i o n is particularly susceptible to slope failure b e c a u s e the material that h a s b e e n b u l l d o z e d into p l a c e is very l o o s e .

•

T h e h o u s e and a s s o c i a t e d p o s s e s s i o n s add w e i g h t to the slope.

•

W a t e r i n g t h e l a w n will e n h a n c e the potential for slope failure.

•

T h e p r e s e n c e of a spring indicates that the slope b e n e a t h the h o u s e is saturated with water. W a t e r adds w e i g h t to the slope and acts as a lubricant.

•

Traffic on t h e r o a d b e l o w the h o u s e adds w e i g h t and creates vibrations in the g r o u n d , w h i c h m a y c o m p r o m i s e slope stability.

•

T h e s l o p e consists of s e d i m e n t a r y r o c k s that h a v e a dip parallel to t h e slope. B e d d i n g p l a n e s are z o n e s of w e a k n e s s within t h e s e layers increasing the potential for slope failure.

•

T h e orientation of t h e r o c k fabric w i t h i n the slope e n h a n c e s the potential for slope failure. S h a l e is an especially soft and w e a k s e d i m e n t a r y rock. S l o p e failure is likely to o c c u r a l o n g t h e shale layer that dips parallel to the slope. E v e n the foliation within t h e g n e i s s i c b e d r o c k parallels the hillslope. P l a n e s of w e a k n e s s w i t h i n a r o c k typically o c c u r parallel to t h e r o c k ' s textural fabric.

•

If the slope w a s undercut by the road builders, the slope stability will be c o m p r o m i s e d .

B. T h i s is an inherently u n s t a b l e slope. A slope o r d i n a n c e p r o b a b l y should have restricted b u i l d i n g on this slope. H o w e v e r , given that the h o u s e is already there, h e r e are s o m e r e a s o n a b l e a p p r o a c h e s to d e c r e a s i n g the risk for slope failure. •

D o n ' t w a t e r the lawn, or r e p l a c e the l a w n with a g r o u n d c o v e r that r e q u i r e s no w a t e r i n g and sends d o w n d e e p r o o t s .

•

D r a i n w a t e r from the slope a b o v e and from t h e roof a w a y from the gravel fill b e n e a t h the h o u s e .

293

294


•

At s o m e e x p e n s e , r o c k bolts c o u l d be installed to h e l p stabilize r o c k layers.

•

M a i n t a i n a g o o d c o v e r of vegetation on the slope a b o v e and b e l o w the h o u s e .

•

Put in a retaining wall along the road b e l o w the h o u s e and be sure that the wall does not restrict the d r a i n a g e of water out of the slope.

Answers to Review Questions 1. D. O x y g e n is the principal c h e m i c a l a g e n t for oxidation r e a c t i o n s . W a t e r is the universal solvent, and c a r b o n d i o x i d e c o m b i n e s with w a t e r to form c a r b o n i c acid. All three play an i m p o r t a n t r o l e in c h e m i c a l w e a t h e r i n g . N i t r o g e n gas o c c u r s as a m o l e c u l e of t w o a t o m s of nitrogen. T h e nitrogen m o l e c u l e is relatively n o n r e a c t i v e . 2. A. Silica in solution, iron oxides, and clay minerals are all products of c h e m i c a l weathering. F e l d s p a r is a f r a m e w o r k silicate m i n e r a l , w h i c h crystallized from m a g m a . 3. D. T h e w e a t h e r i n g p r o d u c t s of m a n y silicate m i n e r a l s i n c l u d e clay. H o w e v e r , quartz is p u r e silicon d i o x i d e and is relatively resistant to w e a t h e r i n g . W h e r e c h e m i c a l weathering is i n t e n s e , as in the tropics, even quartz will dissolve. 4. C. L i m e s t o n e and the calcium carbonate c e m e n t of the sandstone are very susceptible to dissolution reactions in w e t climates but are m o r e resistant to weathering in dry climates. Shale is a soft sedimentary rock m a d e from c o m p a c t e d clay and typically weathers rapidly. A silica-cemented quartz sandstone might be as resistant to weathering as granite. 5. A. R u s t y streaks r e p r e s e n t the oxidation of i r o n - b e a r i n g m i n e r a l s . T h e other a n s w e r s are e x a m p l e s of physical w e a t h e r i n g . 6. B. Rainfall is naturally acidic due to carbonic acid, w h i c h forms w h e n c a r b o n dioxide from the a t m o s p h e r e dissolves in water. Water passing through soils rich in organic matter dissolves additional carbon dioxide from the decay of the organic matter and b e c o m e s m o r e acidic. 7. A. Refer to F i g u r e 16.4 in y o u r textbook. 8. D. Table 16.2 s h o w s that calcite, the major m i n e r a l in l i m e s t o n e , is very soluble in wet conditions. 9. A. Refer to F i g u r e s 16.5 and 16.6. 10. A. Refer to T h e R o l e of Water in W e a t h e r i n g : F e l d s p a r a n d O t h e r Silicates on textbook pages 3 7 4 - 3 7 5 . 1 1 . C. C h e m i c a l w e a t h e r i n g is m o s t intense in hot and w e t r e g i o n s . 12. B. Soil gases can be rich in c a r b o n d i o x i d e released by the d e c a y of o r g a n i c m a t t e r and life p r o c e s s e s . W a t e r percolating d o w n t h r o u g h soils rich in o r g a n i c m a t t e r b e c o m e s m o r e acidic as it dissolves the carbon d i o x i d e in soil g a s . I n c r e a s e d acidity of the water e n h a n c e s c h e m i c a l w e a t h e r i n g reactions. 13. D. Relative to other silicate minerals, olivine is very susceptible to chemical weathering. Its simple (less polymerized) silicate crystal structure of single tetrahedra bonded with cations of iron and/or m a g n e s i u m is easily attacked by chemical agents. Refer to Table 16.1. 14. C. Pollution and gases from volcanic e r u p t i o n s c o m b i n e with rainfall to e n h a n c e the r a i n ' s acidity. Before air pollution controls, rainfall in s o m e r e g i o n s of the U n i t e d States w a s a s acid a s l e m o n j u i c e . A c i d s are powerful c h e m i c a l a g e n t s . C h e m i c a l w e a t h e r i n g is greatly e n h a n c e d by increasing the acidity of water.


15. D. As t h e c a r b o n a t e c e m e n t dissolves, the quartz sand grains will fall away from the r o c k surface, but w e a t h e r i n g will have little i m p a c t on the quartz grains t h e m s e l v e s . Q u a r t z is a very stable m i n e r a l on the E a r t h ' s surface. Refer to Tables 16.1 and 16.2. 16. C. Soil fertility d e p e n d s on the availability of mineral n u t r i e n t s , w h i c h are r e l e a s e d b y c h e m i c a l w e a t h e r i n g from r o c k - f o r m i n g m i n e r a l s . W h e n c h e m i c a l w e a t h e r i n g i s very slow, nutrients r e m a i n tied up in the silicate minerals within the r o c k and are not easily e x t r a c t e d by plant roots. In regions w h e r e c h e m i c a l w e a t h e r i n g is intense, m o s t of the m i n e r a l nutrients are w a s h e d out of t h e soil. T h e r e f o r e , fertile soils form w h e r e w e a t h e r i n g o c c u r s a t m o d e r a t e rates. T h e p a r e n t r o c k i s a n o t h e r i m p o r t a n t influence o n soil fertility. W e a k soils d e v e l o p e d on quartz s a n d s t o n e are likely to be n u t r i e n t - p o o r b e c a u s e quartz is p u r e silicon dioxide and lacks vital mineral nutrients such as potassium, c a l c i u m , iron, or m a g n e s i u m for plant g r o w t h . 17. D. G r a n i t e is c o m p o s e d mostly of feldspars and quartz. C l a y m i n e r a l s are a major p r o d uct of t h e c h e m i c a l w e a t h e r i n g of feldspar. Q u a r t z is very resistant to w e a t h e r i n g . 18. T h e s a n d s t o n e o b e l i s k deteriorated so q u i c k l y after b e i n g m o v e d from E g y p t to N e w York City b e c a u s e the c l i m a t e in N e w York is significantly wetter than the c l i m a t e in E g y p t . As naturally acidic rain dissolves the c a l c i u m c a r b o n a t e c e m e n t , the surface of the s a n d s t o n e deteriorates. O t h e r factors that m a y h a v e c o n t r i b u t e d to the e n h a n c e d w e a t h e r i n g of the obelisk include acid rain g e n e r a t e d by air pollution and frost w e d g ing, s i n c e freeze/thaw c o n d i t i o n s are c o m m o n in N e w York during the winter. 19. A. Site D m a y h a v e t h e best view of the shoreline and site B the b e s t o c e a n view, but site A is on t h e m o s t stable g r o u n d . 2 0 . B. M a s s m o v e m e n t s o c c u r w h e n the force of gravity e x c e e d s the strength of the slope materials. 2 1 . A. U n d e r c u t t i n g by a river or w a v e s will o v e r s t e e p e n a hillslope and e n h a n c e the p o t e n tial for s l o p e failure. S i n c e w a t e r c a n act as a lubricant and also adds w e i g h t to the slope m a t e r i a l s , d r a i n i n g the slope will r e d u c e the w e i g h t of the slope material and increase friction, t h e r e b y r e d u c i n g the potential for slope failure. 2 2 . C. Talus refers to the b l o c k s of r o c k that collect at the b a s e of a steep slope or cliff. 2 3 . D. T h e a n g l e of r e p o s e for m o s t l o o s e sands is a b o u t 3 5 ° . T h e a n g l e of r e p o s e varies significantly w i t h a n u m b e r of factors, o n e of w h i c h is the size of the particles (refer to F i g u r e 16.13). 2 4 . B. B e d d i n g , j o i n t p l a n e s , or a foliation fabric all are potential p l a n e s of w e a k n e s s within r o c k . T h e orientation of any of t h e s e fabrics parallel to the hillslope c o m p r o m i s e s s l o p e stability (refer to F i g u r e 16.13). 25. C. Refer to Figure 16.13. 2 6 . A. Solifluction is a result of r e p e a t e d freezing and t h a w i n g . 2 7 . C. D r a i n i n g the w a t e r from the l a n d s l i d e area w o u l d h e l p to r e d u c e the w e i g h t of the s l o p e m a t e r i a l s and r e d u c e the potential lubricating effects of water. 2 8 . D. B e d d i n g planes, j o i n t planes, and textural fabrics like foliation are zones of w e a k n e s s w i t h i n r o c k . If they parallel the hillslope, the potential for slope failure is e n h a n c e d . 2 9 . B . Refer t o the section U n c o n s o l i d a t e d M a s s M o v e m e n t s o n p a g e 3 8 6 i n y o u r textbook. 30. D . T h e b a r r e n s l o p e s left b y t h e w i l d f i r e s will e n h a n c e t h e p o t e n t i a l for all t h e h a z a r d s listed.

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CHAPTER 17

Answers to Practice Exercises Exercise 1: Evaluating rock materials as potential aquifers

Rock material

Potential as an aquifer (good, moderate, poor)

Porosity (high, medium, low)

L o o s e , well-sorted, c o a r s e sand

High

Good

Silt and clay

Low

Poor

Granite and gneiss

L o w — I n t e r l o c k i n g grains

Poor

of silicate minerals provide for little p o r e s p a c e . H i g h l y fractured granite

Medium—Fracturing

can

Moderate

significantly increase p o r e s p a c e and i m p r o v e Permeability. Sandstone

Medium—The

cement

that

Moderate to good

h o l d s the sand grains together r e d u c e s p o r e s p a c e . Nevertheless, sandstones are typically g o o d aquifers. L o w — F r a c t u r i n g will i n c r e a s e

Shale

Poor

p o r e s p a c e but p e r m e a b i l i t y m a y still r e m a i n low. Shales are typically a q u i c l u d e s . Highly jointed limestone

M e d i u m — F r a c t u r i n g and the

Moderate

to

good

formation of a cavern s y s t e m w i t h i n the l i m e s t o n e c a n greatly e n h a n c e the porosity and p e r m e a b i l i t y of l i m e s t o n e s . C a v e r n s serve as an o p e n p l u m b i n g s y s t e m for g r o u n d w a t e r .

Exercise 2: Evaluating groundwater wells Well A — p o t e n t i a l for 1. p o l l u t i o n — H i g h , b e c a u s e t h e w a t e r table slopes t o w a r d w e l l A, and u p s l o p e is the outh o u s e . T h e c o n e of d e p r e s s i o n p r o d u c e d by p u m p i n g well A e n h a n c e s the potential for p o l l u t i o n by creating a larger gradient in the w a t e r table b e t w e e n well A and the outhouse. 2. artesian f l o w — N o n e . Well A will exhibit no artesian flow b e c a u s e the aquifer is neither confined n o r sloping. 3. d i s c h a r g e — H i g h , b e c a u s e the aquifer is a p o r o u s and p e r m e a b l e s a n d s t o n e of large volu m e and w e l l A is drilled d e e p into t h e aquifer. Well B — p o t e n t i a l for 4 . p o l l u t i o n — L o w , a s s u m i n g that the h o u s e d o e s not r e l e a s e p o l l u t a n t s , b e c a u s e the water table slopes a w a y from well B and the o u t h o u s e is lower. T h e c o n e of d e p r e s s i o n around w e l l B is small and not an issue. 5. artesian f l o w — N o n e . Well B will exhibit no artesian flow b e c a u s e the aquifer is neither confined n o r sloping. 6. d i s c h a r g e — P o t e n t i a l l y h i g h , as l o n g as the w a t e r table d o e s not lower.


7. l o n g - t e r m s u p p l y — L o w . B e c a u s e well B is shallow, a lowering of the w a t e r table, d u e to a c h a n g e in c l i m a t e or d u e to p u m p i n g from wells A and B at rates that e x c e e d r e c h a r g e , c o u l d c o m p r o m i s e the productivity o f well B . Well C — p o t e n t i a l for 8. p o l l u t i o n — L o w , b e c a u s e there is no s o u r c e of pollution s h o w n for the confined sands t o n e aquifer. 9. a r t e s i a n f l o w — H i g h , b e c a u s e the aquifer for w e l l C is a tilted and confined layer of s a n d s t o n e and the r e c h a r g e area for the aquifer appears to be h i g h e r in elevation than the t o p of the well. 10. d i s c h a r g e — H i g h , b e c a u s e with h i g h m o u n t a i n s t o the east there s h o u l d b e g o o d r e c h a r g e of this artesian aquifer. T h e shale layers a b o v e and b e l o w the sandstone confine the p o r o u s and p e r m e a b l e s a n d s t o n e and thereby allow for the d e v e l o p m e n t of significant w a t e r p r e s s u r e within the aquifer. Refer to F i g u r e 17.10.

Answers to Review Question L B . E v a p o r a t i o n from t h e o c e a n s i s m o r e than six times that from the land surface. Refer to F i g u r e 17.2. 2. D. P o l a r ice c a p s and glaciers c o n t a i n a b o u t 2 . 9 7 % of all w a t e r on Earth. T h i s is t h e seco n d largest reservoir for water. Refer to F i g u r e 1 7 . 1 . 3. A. Refer to Figure 17.1. 4. C. R o c k particles typically c a n c o m p a c t m o r e efficiently as grain size d e c r e a s e s , w h i c h r e d u c e s porosity and permeability. Refer to F i g u r e 17.7 and Table 17.2. 5 . B . R e f e r t o F i g u r e 17.8. 6. C. P e r m e a b i l i t y is t h e ability of a solid to allow fluids to p a s s t h r o u g h it. Generally, but not a l w a y s , p e r m e a b i l i t y i n c r e a s e s as porosity increases. P e r m e a b i l i t y also d e p e n d s on t h e sizes of the p o r e s , h o w w e l l they are c o n n e c t e d , and h o w tortuous a path the w a t e r m u s t travel to p a s s t h r o u g h the material. 7. A. A stalactite h o l d s " t i g h t " to the ceiling.

Memory Tip You can easily r e m e m b e r the difference b e t w e e n a stalactite and a stalagmite b e c a u s e stalactites hold " t i g h t " to the ceiling and stalagmites " m i g h t " reach the ceiling of a cave.

8. C. C a r b o n a t e r o c k s , like l i m e s t o n e , are susceptible to dissolution. A w e t c l i m a t e favors cave formation. 9. D. An aquifer is a r o c k that will yield a g o o d flow of g r o u n d w a t e r . 10. C. Refer to t h e section W a t e r D e e p in the C r u s t in C h a p t e r 17. 1 1 . D . S h a l e s are typically a q u i c l u d e s . 12. D . P o r o s i t y and p e r m e a b i l i t y b o t h influence h o w m u c h w a t e r a n aquifer c a n p r o d u c e . P o r o s i t y r e p r e s e n t s the total a m o u n t of p o r e space in w h i c h g r o u n d w a t e r c a n be stored. P e r m e a b i l i t y is linked to porosity and r e p r e s e n t s the ability of w a t e r to flow t h r o u g h the rock. S o m e shales and lava flows have moderate to high porosity but very low permeability b e c a u s e the pore spaces are not interconnected. S o m e rocks, like granite, are m o d e r ately p e r m e a b l e b e c a u s e of extensive fracturing. H o w e v e r , their porosity m a y be low b e c a u s e the fracturing is very tight. 13. B . L o o s e sand typically exhibits h i g h porosity and p e r m e a b i l i t y b e c a u s e t h e r o u n d sand g r a i n s c a n n o t p a c k t o g e t h e r very efficiently. Refer to F i g u r e 17.7. 14. A. R e f e r to F i g u r e 1 7 . 1 1 . 15. B. R i v e r s that flow c o n t i n u o u s l y are fed by springs from a shallow g r o u n d w a t e r table that intersects the stream c h a n n e l . Refer to F i g u r e 1 7 . 1 1 . 16. A. R e f e r to F i g u r e 17.10.

297

298


17. D. Refer to F i g u r e 17.10. 18. B. S a n d a n d gravel typically exhibit a high porosity and permeability. 19. D. S a n d s t o n e s are typically g o o d aquifers. S i n c e w a t e r flows d o w n h i l l , it t e n d s to collect and r e c h a r g e aquifers located b e n e a t h t o p o g r a p h i c a l l y low spots like valleys. 2 0 . B. T h e c o n e of depression a r o u n d well B has l o w e r e d the w a t e r table e n o u g h that effluent l e a c h e d from the o u t h o u s e will flow t o w a r d w e l l B even t h o u g h the t o p o g r a p h y slopes t o w a r d the river. Refer to F i g u r e s 17.12 and 17.20. 2 1 . B. Well B will be the m o s t p r o d u c t i v e w a t e r well b e c a u s e it taps into a larger s a n d s t o n e aquifer. Well A is not likely to be p r o d u c t i v e since it b o t t o m s out in a shale that typically has very low permeability. Well C will yield w a t e r since it b o t t o m s out in the sandstone a b o v e a shale unit that creates a p e r c h e d w a t e r table. T h e v o l u m e of w a t e r from w e l l C will be limited. 2 2 . C. Refer to F i g u r e 17.14. 2 3 . A. T h e spring is least likely to be c o n t a m i n a t e d by t h e o u t h o u s e b e c a u s e it is p r o d u c e d from a p e r c h e d w a t e r table well a b o v e and i n d e p e n d e n t of the aquifer that is affected by the pollutants from the o u t h o u s e . 24. C. h/1 for A = 0.04, B = 0 . 0 3 , C = 0 . 0 5 , and D = 0 . 0 0 6 .

CHAPTER 18

Answers to Practice Exercises Exercise 1: Stream velocity Variable affecting stream velocity

Relationship of variable to stream velocity

G r a d i e n t — t h e slope

A s g r a d i e n t d e c r e a s e s , stream velocity d e c r e a s e s .

You tend to walk faster down a

of the s t r e a m c h a n n e l

Velocity is p r o p o r t i o n a l to the gradient.

steeper

Analogy

slope.

N o t e : In h e a d w a t e r s of s t r e a m s (in the m o u n t a i n s w h e r e gradient is h i g h e s t ) , other factors ( d e c r e a s e d d i s c h a r g e and i n c r e a s e d c h a n n e l r o u g h n e s s ) will c o u n t e r t h e effect of the h i g h gradient. Discharge—the amount

A s d i s c h a r g e increases, s t r e a m velocity

Will you move into a new house slower

of w a t e r in the s t r e a m

i n c r e a s e s . Velocity is p r o p o r t i o n a l to the

or faster

discharge.

helping

you?—faster.

As the availability of s e d i m e n t i n c r e a s e s , the

Typically,

will you

s t r e a m velocity will d e c r e a s e .

slower if you

N o t e : Various factors, s u c h as the b e d r o c k , a landslide, erosion of soil from a b u r n e d area, and c o n s t r u c t i o n , c a n influence the availability of s e d i m e n t load.

your

channel

if you

have

more people

N o t e : Surprisingly large objects c a n m o v e a river d u r i n g a flood. S e d i m e n t load

are

travel faster or

carrying

more

in

backpack?—slower.

C h a n n e l characteristics • Channel roughness

As will

• Channel shape

channel

roughness

increases,

velocity

Cross-country

hiking

without

a

trail

tends to slow one down.

decrease.

The stream has more contact with the channel

When you have more contact with

surface if the channel shape is very wide or

ground surface,

very narrow.

More

contact with the

Crawling

will

drag

and decrease

increase

channel

velocity.

is

you

slower

move than

slower. walking.

the


299

Exercise 2: Relationship between stream flow and groundwater S t r e a m s in desert regions typically flow intermittently, and s t r e a m s in m o r e t e m p e r a t e regions, like N e w E n g l a n d , flow year-round, b e c a u s e effluent streams are fed by springs from a water table that intersects the stream channel. T h e water table is well b e l o w the c h a n n e l of an influent stream, w h i c h flows in r e s p o n s e to rainfall but quickly dries up as runoff from a storm decreases. Infiltration of stream flow into the channel b o t t o m sediments m a y help to recharge the groundwater table beneath the channel.

Exercise 3: How do rivers cut through mountain ranges? T w o w a y s a river can cut t h r o u g h a t o p o g r a p h y obstruction like a m o u n t a i n r a n g e : A . A n t e c e d e n c e , w h e r e the river existed before the p r e s e n t t o p o g r a p h y w a s c r e a t e d and m a i n t a i n e d its original c o u r s e despite c h a n g e s in t h e u n d e r l y i n g r o c k s and topography B. S u p e r p o s i t i o n , w h e r e the river w a s established at a h i g h e r level on a u n i f o r m surface before e r o d i n g d o w n and s u p e r i m p o s i n g itself on a b u r i e d g e o l o g i c structure, like an anticline

Answers to Review Questions 1. B. G r a v i t y is t h e force that drives w a t e r d o w n hill. 2. B. D i s c h a r g e is the v o l u m e (typically m e a s u r e d in c u b i c feet or m e t e r s ) of w a t e r flowing p a s t a p o i n t on the stream c h a n n e l for a given interval of t i m e (typically s e c o n d s ) . 3. A. Velocity is directly p r o p o r t i o n a l to gradient. An increase in slope increases velocity if all other factors do not c o u n t e r the c h a n g e in slope. 4. B. S t r e a m velocity is the m o s t i m p o r t a n t variable d e t e r m i n i n g the b e h a v i o r of a stream. 5. A. T h e c h a n g e in elevation w a s 2 0 0 feet over the 2 0 0 m i l e s c a n o e d . 2 0 0 feet / 2 0 0 miles = 1 foot per m i l e . 6. B. Refer to F i g u r e 18.7 (3). 7. A. S t r e a m c o m p e t e n c y is a m e a s u r e of the size of particles a s t r e a m c a n transport. S t r e a m capacity is a m e a s u r e of the a m o u n t of s e d i m e n t load a s t r e a m c a n transport. 8. B. C o a r s e r particles, like p e b b l e s , settle out before sand and silt. 9. B. T h e outer b a n k a r o u n d a m e a n d e r is m u c h m o r e likely to be e r o d e d b e c a u s e s t r e a m velocity is fastest a r o u n d the outside of the m e a n d e r i n g c h a n n e l w h e r e w a t e r d e p t h is greater. On the inside of a m e a n d e r , water d e p t h is lower, velocity slows, and t h e s t r e a m is m o r e likely to deposit s e d i m e n t and form a p o i n t bar. Refer to F i g u r e 18.3. 10. C. An increase in rainfall or in stream gradient or a l o w e r i n g in b a s e level i n c r e a s e the s t r e a m ' s velocity and give the stream r e n e w e d ability to d o w n c u t . Typically, the stream e n t r e n c h e s its c h a n n e l along its preexisting m e a n d e r i n g c o u r s e . 1 1 . C. Refer to F i g u r e 18.24. 12. C. R e g u l a r addition of s e d i m e n t load is likely to c a u s e the stream velocity in t h e d i s turbed stretch to d e c r e a s e d u e to the i n c r e a s e d availability of l o a d and a r e d u c e d g r a d i ent. A r e d u c t i o n in velocity m a y i n d u c e deposition. 13. D. T h e net effect of straightening out the river c h a n n e l by cutting off a m e a n d e r b e n d is to i n c r e a s e the gradient of the stream c h a n n e l in the cutoff section. An i n c r e a s e in gradient will result in an increase in velocity and i n c r e a s e d potential for c h a n n e l erosion. 14. D. A 50-year flood event has a 2% chance of occurring in any year over a 50-year period. 15. D . T h i s m a y surprise y o u , but stream velocity typically increases d o w n s t r e a m . T h e reduction of s t r e a m gradient is c o u n t e r e d by the increase in d i s c h a r g e f r o m tributary c h a n n e l s . T h e lazy old Mississippi River r u n s faster p a s t V i c k s b u r g , M i s s i s s i p p i , t h a n u p s t r e a m at St. L o u i s , M i s s o u r i .

300


16. C. A l o w e r i n g of b a s e level increases the stream gradient, giving the s t r e a m r e n e w e d ability to e r o d e and transport r o c k material. E r o s i o n will b e g i n in the vicinity of the change. 17. B. Refer to F i g u r e 18.25 and associated text. A n s w e r s A a n d C are incorrectly stated b e c a u s e velocity w o u l d d e c r e a s e with a d e c r e a s e in gradient, and an i n c r e a s e in disc h a r g e w o u l d result in erosion, not deposition. 18. D. C h o i c e s A - C are all possible results of regional uplift, w h i c h w o u l d raise the headw a t e r s o f the d r a i n a g e s y s t e m , w h i c h w o u l d increase s t r e a m g r a d i e n t and e n h a n c e the potential for s t r e a m erosion. S e e F i g u r e s 18.2 and 18.26. 19. A. H o u s e A is located on the outside of a m e a n d e r b e n d , w h i c h is m o s t susceptible to b a n k erosion. Refer to F i g u r e 18.3.

CHAPTER 19

Answer to Practice Exercise Exercise: Sand dune types Dune type

Characteristics

Sand supply

Wind direction/strength

Barchan

C r e s c e n t - s h a p e d with a r m s p o i n t i n g

Limited

Unidirectional/strong

Abundant

Unidirectional/strong

d o w n w i n d . Slip face is a c o n c a v e curve advancing downwind. Transverse

L o n g , w a v y ridges oriented at right a n g l e s to the prevailing w i n d

Blowout

C r e s c e n t - s h a p e d with a r m s p o i n t i n g

Limited to

moderate

Unidirectional/gusty

u p w i n d (into the w i n d ) . Slip face forms a convex curve advancing downwind. Linear

L o n g , straight ridges m o r e or less parallel

( S e e F i g u r e 19.8.)

to g e n e r a l direction of wind.

Moderate

Variable direction/ m o d e r a t e t o strong

Answers to Review Questions 1. D. Refer to F i g u r e 19.1 and the text section W i n d Belts. 2. A. R i s i n g air is characteristic of equatorial latitudes w h e r e t h e s u n ' s radiation is m o r e c o n c e n t r a t e d . As the air rises, it cools and releases a b u n d a n t rainfall typical of tropical r e g i o n s . T h e other three a n s w e r s all contribute to desert c o n d i t i o n s . 3. D. Refer to F i g u r e 1 9 . 1 1 . 4 . D . Refer t o F i g u r e 19.11 and the text section H o w S a n d D u n e s F o r m and M o v e . 5. C. Refer to F i g u r e 19.18. B e c a u s e it is so flat and s m o o t h , a p l a y a (dried lake b e d ) in the M o j a v e D e s e r t east of L o s A n g e l e s is a regular l a n d i n g site for s p a c e shuttles. 6. A. L o e s s is fine w i n d - b l o w n dust. 7. B. Refer to F i g u r e 19.7. 8. B. Silt a n d clay are c a r r i e d in s u s p e n s i o n . S a n d is t y p i c a l l y t r a n s p o r t e d by saltation, s l i d i n g , and r o l l i n g . 9. B. Refer to F i g u r e 19.1 and the text section W i n d B e l t s .


10. D. T h e a r m s of the barchan point d o w n w i n d . Since the wind is blowing from south to north, the a r m s are pointing north and the town is in the opposite direction. 1 1 . B (south). T h e a r m s of blowout d u n e s point into the w i n d (upwind). Since the wind b l o w s inland from t h e coast, the a r m s of the d u n e point to the b e a c h . 12. D. All the a n s w e r s are a p p r o p r i a t e . 13. desert varnish. Refer to F i g u r e 19.16.

CHAPTER 2 0

Answers to Practice Exercises Exercise 1: Profile from the shoreline to the ocean floor

Exercise 2: Passive versus active continental margins A passive m a r g i n is a continental b o r d e r l a n d far from an active plate boundary. In contrast, active m a r g i n s are a s s o c i a t e d with s u b d u c t i o n z o n e s and transform faults. T h e volcanic activity and frequent e a r t h q u a k e s give these n a r r o w and tectonically d e f o r m e d c o n t i n e n t a l m a r g i n s their n a m e . C o n t i n e n t a l s h e l v e s are b r o a d and relatively flat at passive continental m a r g i n s a n d are n a r r o w a n d u n e v e n at active m a r g i n s . A. passive B. passive C . active ( s u b d u c t i o n / c o n v e r g e n t plate b o u n d a r y ) D . passive E.

active

F.

passive

Answers to Review Questions 1. C. T h e w i n d g e n e r a t e s m o s t of the w a v e s in the o c e a n s . 2. C. T h e e d g e of the c o n t i n e n t is rarely if ever the shoreline. T h e e d g e is the continental s l o p e a n d rise, w h i c h m a y be h u n d r e d s of k i l o m e t e r s from the shoreline. 3. A. M a f i c v o l c a n i s m at the o c e a n i c spreading centers generates n e w o c e a n crust.

301

302


4. D. Wave refraction is the tendency of a w a v e to b e n d into the shoreline it approaches. W a v e s b e n d a s t h e w a v e b o t t o m " d r a g s " o n the s h a l l o w i n g b o t t o m before t h e shoreline, s l o w i n g that portion o f the w a v e d o w n . S e e F i g u r e 2 0 . 2 . 5 . A . R e f e r t o F i g u r e 20. I d and p a g e s 4 8 7 ^ 1 8 8 . 6. A. Mafic v o l c a n i s m at divergent plate b o u n d a r i e s g e n e r a t e s the basaltic seafloor crust, w h i c h is, over t i m e , b u r i e d in p e l a g i c s e d i m e n t s . 7. A. S e a m o u n t s are extinct s u b m a r i n e v o l c a n o e s m a d e m o s t l y of basalt. 8. B. Refer to F i g u r e 2 0 . I d . 9. D. 4 0 0 0 to 6 0 0 0 m e t e r s is the depth of the abyssal o c e a n floor. 10. A. T h e sea level a p p e a r s to be rising a b o u t 4 m i l l i m e t e r s p e r year. 1 1 . A . Turbidity c u r r e n t s , often triggered b y e a r t h q u a k e s , p l u n g e d o w n t h e continental slopes. T h e y are flowing m a s s e s of turbid m u d s s u s p e n d e d in water, d e n s e r than the clear w a t e r a b o v e t h e m . S e e F i g u r e 2 0 . 1 6 , 12. A. Refer to F i g u r e 2 0 . 3 . 13. C. T h e groin will b l o c k t h e drift of sand p a s t y o u r b e a c h in the surf z o n e . R o b b e d of a c o n t i n u o u s supply of sand, l o n g s h o r e currents will carry sand from y o u r property, w h i c h will not be replenished by the sand supply up current. Your s a n d y b e a c h is likely to d i s a p p e a r in a few y e a r s . 14. B. W a v e e n e r g y is focused on p o i n t s and h e a d l a n d s by w a v e refraction. 15. C . Refer t o F i g u r e 2 0 . 1 2 illustrating the sand b u d g e t — t h e d y n a m i c b a l a n c e b e t w e e n i n p u t s a n d outputs of sand a l o n g the shoreline. 16. A. R i p currents are g e n e r a t e d by rapidly m o v i n g b a c k f l o w s and r u n p e r p e n d i c u l a r to the b e a c h . T h e r e f o r e , the best w a y to e s c a p e f r o m the grip of a rip tide is to s w i m parallel t o t h e b e a c h , u n l e s s y o u are u p for s o m e l o n g - d i s t a n c e s w i m m i n g . 17. C. Reef-forming corals require sunlight to grow and do not thrive in water m u c h deeper t h a n 20 m e t e r s . T h e r e f o r e atolls begin as fringing reefs a r o u n d a v o l c a n i c island. O v e r t i m e , as the volcanic island slowly subsides, the coral reef g r o w s u p w a r d , m a i n t a i n i n g a shallow m a r i n e e n v i r o n m e n t . Refer to F i g u r e 5.18 and E a r t h I s s u e s 5 . 1 . 18. C . Refer t o F i g u r e 2 0 . 2 . 19. A. Refer to F i g u r e 2 0 . 2 . 20. C. Turbidity currents (flows) are a g e n t s of e r o s i o n and d e p o s i t i o n of s e d i m e n t a l o n g the c o n t i n e n t a l slope a n d adjacent o c e a n floor. Refer to F i g u r e 2 0 . 1 6 . 2 1 . T h e l o n g s h o r e current will flow parallel to the b e a c h in the surf z o n e and to t h e left, since the w a v e s are c o m i n g in at an a n g l e from t h e l o w e r right. T h e best a n s w e r has the arrow c l o s e to t h e beachfront in the surface z o n e . S e e F i g u r e 2 0 . 2 . 22. A. Refer to F i g u r e 2 0 . 2 4 .

CHAPTER 21

Answers to Practice Exercises Exercise 1: The glacially sculpted landscape T h e text section Glacial L a n d s c a p e s and F i g u r e s 2 1 . 1 7 , 2 1 . 1 8 , and 2 1 . 1 9 will b e very helpful for c o m p l e t i n g t h e brief descriptions of glacial features. A list of p o s s i b l e glacial features that m i g h t be f o u n d a n d interpreted by a seasonal r a n g e r i n c l u d e t h e following. F e a t u r e s f o r m e d b y t h e erosive p o w e r o f glacial ice • S t r i a t i o n s — s c r a t c h e s and g r o o v e s — c a r v e d in b e d r o c k over w h i c h t h e glacier flowed. • Cirque • U - s h a p e d valley


• H a n g i n g valley • Fjord • Arete Glacial features f o r m e d by deposition of r o c k material by glacial ice • G l a c i a l m o r a i n e s — t h e different types are d e s c r i b e d in Table 2 1 . 1 . • Drumlins • Esker • Kame • Kettle • Varve • Kettle A l l of t h e s e glacial features are described in the t e x t b o o k and are a p p r o p r i a t e a n s w e r s .

Exercise 2: Your personal budget as a metaphor for a glacial budget S e e t h e text section Glacial B u d g e t s : A c c u m u l a t i o n M i n u s A b l a t i o n . A p e r s o n a l c h e c k b o o k is a g o o d m e t a p h o r for the d y n a m i c b a l a n c e b e t w e e n a c c u m u l a t i o n and ablation on a glacier. For e x a m p l e , if y o u deposit m o n e y into y o u r c h e c k i n g a c c o u n t faster than y o u w i t h d r a w it, the cash b a l a n c e g r o w s . Similarly, as snow a c c u m u l a t e s faster than the rate of ablation (loss), the glacier e x p a n d s and advances. If you w i t h d r a w m o n e y from your c h e c k i n g a c c o u n t faster than y o u deposit it, y o u r account shrinks. Similarly, if a glacier's rate of ablation e x c e e d s the rate of a c c u m u l a t i o n , the glacier will shrink and retreat u p s l o p e . It m i g h t d i s a p p e a r altogether if ablation e x c e e d s a c c u m u l a t i o n for an extended period.

Exercise 3: Glacial advances and retreats Rarely does a glacier actually r e m a i n stationary. Driven by the force of gravity, glacial ice and the rock material that it carries are m o v i n g downhill. T h e words advance, retreat, and halt are u s e d to describe the m o v e m e n t , or location, of the toe, or terminus, of the glacier; they do not refer to the actual m o v e m e n t of glacial ice within the glacier. T h e terminus of the glacier will r e m a i n stationary (halt), retreat up the valley, or a d v a n c e d o w n the valley d e p e n d i n g on the glacial b u d g e t . Refer to F i g u r e 2 1 . 9 . F o r e x a m p l e , if the s n o w a c c u m u l a t i n g in the u p p e r reaches of the glacier equals the loss (ablation) of glacial ice from the lower and w a r m e r reaches of the glacier, the size of the glacier will r e m a i n constant and the glacial t e r m i n u s will r e m a i n stationary. N e v e r t h e l e s s , the glacial ice is still flowing d o w n s l o p e with rock material and m a y pile up a sizeable e n d m o r a i n e . Refer to Table 2 1 . 1 .

Answers to Review Questions 1. C. G l a c i a l ice b e s t fits the general definition of a m e t a m o r p h i c rock. Refer to the section Ice as a R o c k in y o u r textbook. 2. D. G r a v i t y is t h e force that pulls glacial ice d o w n h i l l . In r e s p o n s e to gravity, glacial ice m o v e s by plastic flow and basal slip. 3. B. Refer to Figure 21.7. 4. A. Refer to F i g u r e 2 1 . 9 . 5. D. M o r a i n e s are d e p o s i t s of till. Refer to text section Glacial S e d i m e n t a t i o n and Sedim e n t a r y L a n d f o r m s and Table 2 1 . 1 . 6. D. D r u m l i n s are s t r e a m l i n e d hills of till and b e d r o c k . B e c a u s e d r u m l i n s parallel the d i r e c t i o n of ice m o v e m e n t , they c a n be used to reconstruct the direction of m o v e m e n t for the ice sheet. 7. D . 8. A. W a t e r is tied up as glacial ice on land. T h e r e f o r e , the sea level d r o p s . 9. D. Refer to Figure 15.11.

303

304


10. B . Refer t o 1 5 . 1 1 . 1 1 . B. K a m e s are small hills of sand and gravel d u m p e d n e a r or at the e d g e of t h e ice. Refer to F i g u r e 2 1 . 1 9 . 12. A. S i n c e glacial ice is c a p a b l e of c a r r y i n g r o c k particles of a great variety of sizes, and since p h y s i c a l w e a t h e r i n g d o m i n a t e s in glacial e n v i r o n m e n t s , it is very unlikely for a glacial till to consist of p u r e quartz sand. 13. A . 14. A. Refer to F i g u r e 2 1 . 9 . 15. C. Refer to F i g u r e 2 1 . 2 4 . 16. D. E a c h factor listed p l a y s a role in c l i m a t e c h a n g e and n e e d s to be c o n s i d e r e d as a c o m p o n e n t of a m o d e l that a t t e m p t s to describe the c a u s e of the ice a g e s . 17. D. T h e m a s s of t h e floating ice is e q u a l to the m a s s of the w a t e r t h e iceberg d i s p l a c e s . W h e n the ice m e l t s , it simply replaces the water it d i s p l a c e s , so there is no c h a n g e in t h e sea level.

CHAPTER 2 2

Answers to Practice Exercises Exercise 1: Landscapes: Tectonic and climate interaction flowchart


Exercise 2: Comparison of some landforms Landform

Important feature(s)

Mesa

A small plateau with steep slopes on

(See F i g u r e 22.8.)

!Sketch (Hint: Keep it very simple)

all sides. H e l d up by u n d e r f o r m e d S e d i m e n t a r y layers or lava flows.

Cuesta

A structurally controlled cliff. S o m e

( S e e F i g u r e 22.14.)

w h a t tilted beds alternating w e a k and resistance to erosion. Typically u n d e r c u t and a s y m m e t r i c a l .

Hogback

A structurally controlled cliff with b e d s

(See F i g u r e 22.15.)

that are steeply d i p p i n g to vertical. A r i d g e is m o r e or less s y m m e t r i c a l .

Valley r i d g e

In y o u n g m o u n t a i n s , upfolds (anticlines)

topography

form ridges and d o w n f o l d s (synclines)

(See Figures 22.10

form valleys. A s tectonic activity

and 22.11.)

m o d e r a t e s and e r o s i o n digs d e e p e r into the structures, the anticlines m a y f o r m valleys v a u e y s and a n a syncline s y n c n n e ridges. nages.

Answers to Review Questions 1. A. River valleys begin with a V-shaped profile. As the sides of the steep valley retreat and the floor of the valley w i d e n s , a floodplain and relatively flat-floored valley will e v o l v e . T h e g r a d i e n t in a youthful river s y s t e m is steep but d e c r e a s e s t h o u g h t i m e unless other g e o l o g i c events are s u p e r i m p o s e d on the history of the d r a i n a g e . Glaciers carve U - s h a p e d valleys. Refer to F i g u r e 2 2 . 1 2 . 2. B. Elevation is the result of the b a l a n c e b e t w e e n tectonic activity (uplift and subsid e n c e ) a n d erosion. 3 . A . Refer t o F i g u r e 2 2 . 3 . 4. D. T e c t o n i c activity, erosion, c l i m a t e , and type of b e d r o c k are i m p o r t a n t controls on l a n d s c a p e evolution. 5. D. E a r t h ' s surface has t w o f u n d a m e n t a l levels, the continents, w h i c h on a v e r a g e are a b o u t half a mile a b o v e sea level, and the o c e a n floor, w h i c h on average is a b o u t 2.5 m i l e s b e l o w sea level. 6. B. A l t h o u g h m o u n t a i n s form at all four of t h e s e tectonic settings, the longest a n d highest m o u n t a i n r a n g e s form at convergent plate b o u n d a r i e s . 7 . A . R e f e r t o F i g u r e 2 2 . 1 7 and p a g e s 5 4 1 - 5 4 3 . 8 . D . R e f e r t o F i g u r e s 2 2 . 1 0 , 2 2 . 1 1 , and 2 2 . 1 9 . 9. A. R e f e r to F i g u r e 2 2 . 1 2 . 10. B . R e f e r t o F i g u r e 2 2 . 1 5 . 1 1 . A . R e f e r t o Earth Issues 2 2 . 1 . 12. D . R e f e r t o text p a g e s 5 4 7 - 5 4 8 .

305

306


CHAPTER 2 3

Answers to Review Questions l . B . Refer t o F i g u r e 2 3 . 1 2 . 2. C. U r a n i u m is not a fossil fuel. 3. B. O z o n e depletion in our a t m o s p h e r e is largely influenced by the release of chlorofluoroh y d r o c a r b o n s ( C F C s ) , synthetic c h o r i n e , and fluorine c o m p o u n d s u s e d w i d e l y i n aerosols and refrigerants t h r o u g h m u c h of the last century. 4. B. O i l a n d gas form from o r g a n i c m a t t e r derived from m a r i n e o r g a n i s m that thrive in shallow coastal w a t e r s . 5. D. Refer to F i g u r e 2 3 . 6 . 6. B. U . S . oil p r o d u c t i o n r e a c h e d a m a x i m u m in 1970 a n d is n o w in decline. 7. C. Refer to F i g u r e 2 3 . 3 . 8. C. Refer to F i g u r e 2 3 . 5 . T h e r e are 42 gallons to a barrel of oil, of w h i c h a b o u t half is refined into g a s o l i n e for a u t o m o b i l e s . F r o m t h e rest c o m e j e t fuel, diesel and other fuels, solvents, lubricants, g r e a s e s , and asphalt. 9. D. S e e the section Distribution of Oil R e s e r v e s and F i g u r e 2 3 . 7 . 10. B . Refer t o F i g u r e 2 3 . 1 2 . 1 1 . B . Refer t o F i g u r e 2 3 . 1 2 . 12. B. Sulfur d i o x i d e c o m b i n e s with r a i n w a t e r to form sulfuric acid, a m a j o r c o m p o n e n t of acid rain. For this reason, low sulfur coals are e n v i r o n m e n t a l l y m o r e favorable to burn. 13. C. Refer to F i g u r e 2 3 . 1 8 . 14. B. Sulfur d i o x i d e g e n e r a t e d by b u r n i n g fossil fuels high in sulfur c o n t e n t is t h e major s o u r c e of acid in rain. T h e sulfur d i o x i d e c o m b i n e s with rainfall, w h i c h is already slightly acidic from the dissolved c a r b o n d i o x i d e to p r o d u c e sulfuric acid. 15. B. Well B is m o s t likely to p r o d u c e oil b e c a u s e it is drilled into a tilted sand layer, w h i c h is confined by the salt d o m e . H y d r o c a r b o n s are likely to a c c u m u l a t e in sand b e c a u s e of its high porosity and permeability. B e c a u s e salt d e p o s i t s are essentially n o n p o r o u s and i m p e r m e a b l e , the d o m e w o u l d seal off the sand layer and prevent the fluid h y d r o c a r b o n s from e s c a p i n g to the surface. Well A is drilled in shale, w h i c h c a n be a g o o d s o u r c e r o c k for h y d r o c a r b o n s , but its low p e r m e a b i l i t y m a k e s it a very p o o r reservoir r o c k . Well D is drilled into a sand layer that is not sealed off by the salt d o m e . T h e r e f o r e , h y d r o c a r b o n s w o u l d m i g r a t e a l o n g the well D sand layer to the surface and be lost. Refer to F i g u r e 2 3 . 6 . 16. A. Refer to the text section Stratospheric O z o n e D e p l e t i o n . 17. C. Refer to the text section Stratospheric O z o n e D e p l e t i o n . 18. C. B u r n i n g fossil fuels releases C 0 , a g r e e n h o u s e g a s , into the a t m o s p h e r e . 2

19. B . Refer t o E a r t h Policy 2 3 . 1 . 2 0 . B . Refer t o F i g u r e 2 3 . 2 2 .

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