Download - Stephen Marshak’s Essentials of Geology - TEST BANK 360testbank360.eu/sample/test-bank-essentials-of...The Third Edition of Stephen Marshak’s Essentials of Geology includes a pair

Stephen Marshak’s

Essentials of GeologyTHIRD EDITION

INSTRUCTOR’S MANUAL AND TEST BANK

Full file at http://testbank360.eu/test-bank-essentials-of-geology-3rd-edition-stephen-marshak

Stephen Marshak’s

Essentials of GeologyTHIRD EDITION

John WernerSEMINOLE COMMUNITY COLLEGE

W. W. NORTON & COMPANY • NEW YORK • LONDON

INSTRUCTOR’S MANUAL AND TEST BANK


Copyright © 2009, 2007 by W. W. Norton & Company, Inc.

All rights reserved

Printed in the United States of America

Third Edition

Composition and Layout by Roberta Flechner Graphics

ISBN 978-0-393-93314-7

W. W. Norton & Company, Inc., 500 Fifth Avenue, New York, NY 10110

www.wwnorton.com

W. W. Norton & Company Ltd., Castle House, 75/76 Wells Street, London W1T 3QT

1 2 3 4 5 6 7 8 9 0


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CONTENTS

New Features for the Third Edition of Essentials of Geology:On Further Thought and Geotours vii

Acknowledgments viii

Chapter 1 | The Earth in Context 1

Chapter 2 | The Way the Earth Works: Plate Tectonics 12

Chapter 3 | Patterns in Nature: Minerals 25

Chapter 4 | Up from the Inferno: Magma and Igneous Rocks 32

Chapter 5 | The Wrath of Vulcan: Volcanic Eruptions 39

Chapter 6 | Pages of Earth’s Past: Sedimentary Rocks 47

Chapter 7 | Metamorphism: A Process of Change 56

Chapter 8 | A Violent Pulse: Earthquakes 66

Chapter 9 | Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building 75

Chapter 10 | Deep Time: How Old Is Old? 83

Chapter 11 | A Biography of Earth 94

Chapter 12 | Riches in Rock: Energy and Mineral Resources 104

Chapter 13 | Unsafe Ground: Landslides and Other Mass Movements 114

Chapter 14 | Running Water: The Geology of Streams and Floods 119

Chapter 15 | Restless Realm: Oceans and Coasts 128

Chapter 16 | A Hidden Reserve: Groundwater 138

Chapter 17 | Dry Regions: The Geology of Deserts 146

Chapter 18 | Amazing Ice: Glaciers and Ice Ages 154

Chapter 19 | Global Change in the Earth System 161

Answers to Multiple-Choice Questions 169


New Features for the Third Edition ofEssentials of Geology:On Further Thought and Geotours

vii

The Third Edition of Stephen Marshak’s Essentials of Geology includes a pair of featuresthat are new and that should be of great value to both the student and the educator ofintroductory geology. The “On Further Thought” section at the end of each chapter includesnew questions that go beyond the chapter synopsis, giving the student a chance to improvehis or her critical thinking, reference, and basic math skills. Since the majority of studentstaking introductory geology will not be continuing further in the field, the questions in thisnew section are of great value in reinforcing skill sets needed for success in the rest of adegree program and beyond.

The “Geotours” section that appears near the end of the book (after the Appendix)provides a guided tour of Earth’s varied landscape using the free Internet application GoogleEarth. Available for download at http://earth.google.com/download-earth.html, GoogleEarth offers detailed mosaic satellite imagery of the continents and a coarser physiographicview of the ocean basins. The program may well be the best free application on the Internet.From my experience, students find the program engaging whether used in lectures topinpoint a geologic feature of interest (its renderings of the Grand Canyon and Mount St.Helens are outstanding) or in the context of an exercise where students have to identify andexplain physiographic features related to streams, coasts, plate tectonics, or glacialgeomorphology (among other topics). In assembling the Geotours, Stephen Marshak hasdone a remarkable job of compiling some of the most interesting examples of geologyvisible at the scale of satellite surveillance and accessible from any well-equipped computer.The Geotours are an exciting addition to the text, and instructors with Internet capability inthe classroom are encouraged to use them in their teaching.


Acknowledgments

Thanks to Stephen Marshak, who recommended my involvement in constructing the Instructor’s Manual and Test Bank, and to Jack Repcheck and Matthew Freeman at Norton.My dissertation advisor, Dan Blake, provided as good a role model as possible as to how a scientist thinks, acts, and writes. I thank my parents, Ed and Pinky Werner, who have beenthere for me since day one. Lastly, and most of all, I wish to thank my dear and loving wife,Melissa Wilder, for everything she has done to make my life a better place.

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1

Learning objectives

1. Students should be aware of the Big Bang and the major evidence supporting it.Distant galaxies are uniformly red-shifted, rather than blue-shifted; this implies that they areall moving away from us. The farthest galaxies are those that are most strongly red-shifted,meaning that they are receding fastest. Extrapolation of velocities and trajectories into thepast suggests that all matter in the Universe was contained in a single point, approximately13.7 billion years ago. At that time, the Universe came into existence explosively (hence thename Big Bang); radiation from the Big Bang still can be perceived in all directions in thesky (even apparently empty space) with a radio telescope.

2. Stars, including our Sun, are nuclear fusion reactors. For most of stars’ life histories(on the order of billions of years), hydrogen atoms are fused together to form helium. Laterstages in stellar evolution include fusion of helium atoms and other, heavier elements;ultimately, iron is the heaviest element that can be produced through fusion reactions withinstars.

3. After their cycles of fusion are complete, large stars violently explode, formingelements heavier than iron and leaving behind a residue of diffuse nebulae, which may berecycled to form a new star at some point in the future. These explosive events are termednovas and supernovas because some have been bright enough to be seen as “new stars” inthe night sky. Historically, a few supernovae have even been bright enough to be seen duringdaylight.

4. Our Sun is approximately 5 billion years old and is expected to continue fusinghelium as it does today for about another 5 billion years. All planetary orbits are coplanar,and all planets orbit in the same direction (counterclockwise as viewed from above Earth’snorth pole). These facts imply simultaneous planetary formation from a swirling nebulasurrounding the Sun (the similarities in orbits would then be a natural result of conservationof angular momentum). The planets accreted from this nebula through gravitationalattraction and haphazard collisions. Pluto, long considered the “ninth planet,” has recentlyseen its status demoted; astronomers now recognize only eight major planets in our Solar

The Earth in ContextCHAPTER 1


2 | Chapter 1

System. Pluto belongs to a group of icy and rocky bodies beyond Neptune’s orbit termed theKuiper Belt, the origination site for numerous comets.

5. The terrestrial planets (Mercury, Venus, Earth, and Mars) are relatively small, dense,and rocky worlds because solar winds from the nearby Sun expelled most of thesuperabundant (but very light) elements, hydrogen and helium. The gas giant planets(Jupiter, Saturn, Uranus, and Neptune) retained these elements and are thus much larger andmuch less dense (Saturn is less dense than water).

6. Our Moon, responsible for Earth’s tides, has a composition similar to Earth’smantle; the Moon is thought to have originated from debris accumulated when a Mars-sizedbody impacted the Earth very early in Earth’s history.

7. Students should be aware of the presence of Earth’s magnetic dipole, how themagnetic field arises, and its important consequences for life on Earth.

8. Earth is composed of a variety of materials with disparate physical properties(minerals, organics, gases, and melts). This has led to a complex physical chemistry andbiochemistry, allowing both Earth’s surface and its constituent life to evolve dramaticallyover time.

9. Earth is chemically divided into a thin, rocky crust dominated by silicate minerals, athick mantle dominated by iron- and magnesium-rich silicates (subject locally to partialmelting), and a thick, metallic core which is primarily iron (the outer portion of which isliquid). Students should know how seismic waves tell us that the outer core must be liquid.

10. Physically, the uppermost layers of Earth are the rigid lithosphere (crust anduppermost mantle) and the asthenosphere, which is softer and flows plastically. The “plates”of plate tectonic theory are discrete slabs of the lithosphere, which move with respect to oneanother atop the weaker asthenosphere.

Answers to review questions

1. Contrast the geocentric and heliocentric concepts of the Universe.The geocentric concept placed Earth at the center of the Universe, with the Sun and the

other planets revolving around it. The heliocentric concept placed the Sun as the center, withEarth and the other planets revolving around it.

2. Describe how the Doppler effect works.Sound waves (and light waves as well) emanating from an approaching source object

arrive at a higher frequency than they would if the source object were stationary. Thisfrequency shift arises because each successive sound wave emanates from a closer distancethan the previous wave. These high frequencies are interpreted by our brains (aftertransmission through our ears) as a higher pitch. Once a wave source passes an observer, itssound waves have a reduced frequency, as each wave is emitted from a slightly more distantpoint.

3. What does the red shift of the galaxies tell us about their motion with respect to theEarth?

All distant galaxies are moving away from our own, with the farthest galaxies movingfastest.


The Earth in Context | 3

4. Briefly describe the steps in the formation of the Universe according to the Big Bangtheory.

The Universe formed from the big bang, an explosion of matter and space from aninfinitely dense point source (singularity). It is thought that only hydrogen and helium,among the known elements, were produced in the big bang. Other elements are the result ofstellar fusion and explosive supernovas. Our Sun and the surrounding solar system condensedfrom a mostly gaseous nebula, which itself contained material from previous supernovas (asevidenced by the diversity of heavier elements present in the Sun and solar system).

5. How does the composition of the Solar System, in terms of the elements making up theSun and the planets, differ from that of the nebulae that existed a million years afterthe birth of the Universe? Explain the difference.

Our Solar System has proportionally less hydrogen, helium and other light elements,but proportionally more heavier elements, than the early nebulae. Only very light elements(primarily hydrogen and helium) were present immediately after the big bang. Fusionwithin stars and supernovae produced the heavier elements over time.

6. Why isn’t the Earth homogeneous?Chemical layers in the Earth are related to Earth’s formation. Due to the great initial

heat within the Earth, the planet formed in a molten state, primarily a mixture of metal ironand rocky silicates. The great density of the iron caused most of it to be pulled to the center,forming the core. Through cycles of melting and solidification, lighter silicate mineralsmigrated to the outermost part of the solid Earth, forming a thin crust, in contrast to thesomewhat denser mantle. Physical layers in the Earth have formed due to both this chemicallayering and to the changes in physical properties that occur as temperature and pressureboth increase with depth.

7. Describe how the Moon was formed.The Moon formed when a Mars-sized body impacted Earth early in the history of the

solar system. The force of impact ejected material similar in composition to Earth’s mantle.This mantle-like mass cooled and hardened, resulting in our Moon.

8. Why is the Earth round?Self-gravity forces objects the size of Earth to be nearly spherical (the most compact

shape, minimizing the distances of points from the center).

9. What is the Earth’s magnetic field? Draw a representation of the field on a piece of paper. A region of space affected by the magnetic force of Earth (student’s sketch should

show a dipole field stretched into a teardrop trailing away from the Sun; see Figure 1.11C).

10. What is the Earth’s atmosphere composed of? Why would you die of suffocation if youwere to eject from a fighter plane at an elevation of 12 km without taking an oxygen tankwith you?

Earth’s atmosphere is mostly nitrogen and oxygen (with minor amounts of argon,carbon dioxide, and other gases). The atmosphere becomes less and less dense with altitude;at 12 km oxygen molecules are too sparse to support human life.


4 | Chapter 1

11. Describe the major categories of materials constituting the Earth.Organic chemicals, which make up the majority of living matter, are carbon- and

hydrogen-based compounds (including oil and natural gas), many of which are quitecomplex, sometimes incorporating oxygen (as in sugars, starches, and fats), sometimesadditionally nitrogen (as in proteins), and occasionally some phosphorus and sulfur as well.

Minerals are solid, inorganic materials in which there is a fixed arrangement of atoms(this arrangement is often termed a crystalline lattice). Quartz and calcite are important,familiar examples. Mineral crystals are commonly weathered to produce fragments withrough or rounded surfaces, which are termed grains.

Rocks are cohesive aggregates of crystals or grains. Igneous rocks crystallize frommolten (liquid) rock. Sedimentary rocks arise from the cementation of loose grains (sand,mud, pebbles, etc.) and through chemical precipitation (from the ocean or continentalbodies of water). Metamorphic rocks arise from heat- and pressure-induced alteration ofpreexistent rock (without melting).

Glasses are physically solid structures in which the atoms are internally disordered (asin liquids, but without the tendency to flow rapidly). Commercial glass is produced whenquartz is melted and then rapidly cooled (quenched in cool water), so that atoms cannot alignthemselves into the quartz crystalline arrangement before the rigidity of cooling sets in.

Metals are solids made up only (to a strong approximation) of metallic elements, suchas gold, iron, and copper. (Naturally occurring metals are a subset of minerals.)

Melts are hot liquids that crystallize at surface temperatures to form igneous rocks.Melts within Earth are termed magma; melts extruded on the surface are termed lava.

Sediments are accumulated, loose mineral grains.

12. What are the principal layers of the Earth?The three principal layers are the crust, the mantle, and the core. The former two are

rocky layers, the core is mostly metallic.

13. How do temperature and pressure change with increasing depth in the Earth?Both temperature and pressure increase with increasing depth.

14. What is the Moho? Describe the differences between continental crust and oceaniccrust.

The Moho is the crust/mantle boundary, first recognized by an abrupt change inseismic wave velocities. Crustal thickness is variable (and is thicker under continents thanoceans) but is generally less than 1% of the Earth’s total diameter. Continental crust isvariable in composition, but mostly granitic, and less dense than the basaltic oceanic crust.

15. What is the mantle composed of? What are the three sublayers within the mantle? Isthere any melt within the mantle?

The mantle is mostly made of an ultramafic silicate rock termed peridotite. Its layersare the upper mantle, transition zone, and lower mantle. There is a small amount of melt inthe upper mantle.



16. What is the core composed of? How do the inner core and outer core differ from eachother?

The core is mostly iron; the inner part is solid, whereas the outer part is liquid.

17. What is the difference between the lithosphere and the asthenosphere? At what depthdoes the lithosphere/asthenosphere boundary occur? Is this above or below the Moho?

The lithosphere is relatively cool and rigid compared to the hot, soft asthenosphere,which flows more readily. The lithosphere consists of the crust (oceanic basalt and gabbro,or continental granite) plus the uppermost mantle (peridotite) down to a depth of 100 to 150 km. This boundary lies below the Moho.

On Further Thought

1. Recent observations suggest that the Moon has a very small, solid core that is less than3% of its mass. In comparison, Earth’s core is about 33% of its mass. Explain why thisdifference might exist.

The Moon is thought to have formed when a Mars-sized body collided with the Earthearly in the planet’s history. The impact hurled a portion of the Earth, mostly mantlematerial with no contribution from the core, into orbit about our Earth, where it solidified toform our Moon. The Moon differentiated (as the Earth had earlier), but with a minuteamount of iron compared to the Earth (which had already seen most of its iron descend intoits larger core).

2. There is hardly any hydrogen or helium in the Earth’s atmosphere, yet most of thenebula from which the Solar System formed consisted of hydrogen and helium. Where didall this gas go?

The Earth is not massive enough to keep the light gases hydrogen and helium fromescaping to space.

3. The popular media sometimes imply that the crust floats on a “sea of magma.” Is this acorrect image of the mantle just below the Moho? Explain your answer.

No, the mantle just beneath the crust is not only solid, but also rigid and, along withthe crust, forms the lithosphere. Even the asthenosphere is (mostly) solid, though ductile.

4. As you will see later in this book, emplacement of a huge weight (e.g., a continentalice sheet) causes the surface of lithosphere to sink, just as your weight causes the surface ofa trampoline to sink. Emplacement of such a weight does not, however, cause a change inthe thickness of the lithosphere. How is this possible? (Hint: Think about the nature of theasthenosphere.)

The asthenosphere below the burdened lithosphere is ductile, and will flow away fromthe sagging lithosphere, much as water is displaced by ships afloat at sea.


6 | Chapter 1

Test bank

1. The geocentric model was developed during the time of the ancient Greeks. This model____________.

A. was abandoned during the time of the Roman Empire and would never be widelyheld again

B. was held to be true by thinkers throughout the Middle Ages, up until theRenaissance

C. was rediscovered by the Polish astronomer Copernicus and has been the acceptedmodel of the Universe ever since

D. has been proven by NASA space photos

2. In the heliocentric model ____________.A. Earth orbits around the SunB. the Sun orbits around EarthC. Earth is a stationary planetD. Mercury and Venus orbit around the Sun, but all other planets orbit around Earth

3. In our current understanding of the big bang, ____________.A. Earth is much older than the rest of the UniverseB. the Universe is considerably older than EarthC. Earth and the Universe formed at about the same timeD. there is no way of knowing how old the Universe might be

4. As the Universe has evolved, ____________.A. hydrogen has been lost through fusion to form helium within starsB. hydrogen concentration has increased through the fission of helium atomsC. hydrogen concentration has increased through the fusion of helium atomsD. the number of hydrogen atoms has likely remained constant

5. Among the choices below, the best estimate of the age of the Universe is____________ years old.

A. 5 million C. 14 billionB. 6 billion D. 100 billion

6. The big bang theory states that ____________.A. all stars will end their lives explosively as supernovasB. Earth formed through a series of violent collisionsC. meteors were responsible for the extinction of the dinosaursD. all matter in the Universe was once confined to a single point

7. Strong evidence that the Universe is expanding comes from the fact that the lightemitted from distant galaxies appears to be ____________.

A. red-shifted C. green-shiftedB. blue-shifted D. none of the above



8. Italian Renaissance astronomer Galileo was the first person to deduce that planets weredistinct entities from stars.

A. true B. false

9. Since the initiation of the Big Bang, the temperature of the Universe has____________.

A. increasedB. decreasedC. stayed about the same

10. Atoms that are heavier than iron are generally produced by ____________.A. fission reactions within starsB. fusion reactions within starsC. the explosion of supernovaeD. the big bang

11. By far the most common elements in the Universe and in our Solar System are____________.

A. nitrogen and oxygen C. hydrogen and heliumB. iron and manganese D. hydrogen and oxygen

12. Which of the following bodies is the smallest?A. planet C. protoplanetB. star D. planetesimal

13. Aside from Earth, the terrestrial planets are ____________.A. Mars, Mercury, and VenusB. Mars, Venus, and JupiterC. Jupiter, Saturn, Uranus, and NeptuneD. Mars and Saturn

14. The gas-giant, or Jovian, planets are ____________.A. Mars, Mercury, and VenusB. Mars, Venus, and JupiterC. Jupiter, Saturn, Uranus, and NeptuneD. Mars and Saturn

15. The branch of science that studies the structure and history of the Universe is____________.

A. cosmetology C. cosmologyB. scientology D. universalism

16. All objects in the Solar System are in orbit around ____________.A. Earth C. the SunB. Jupiter D. the Kuiper Belt


8 | Chapter 1

17. An ancient Greek philosopher concluded (correctly) that ____________.A. the Earth was spherical (round)B. the Sun was the center of the whole UniverseC. the Sun was the center of the Earth’s orbitD. the Earth was the center of the Universe

18. The circumference of Earth is most nearly ____________.A. 400 km C. 40,000 kmB. 4,000 km D. 4,000,000 km

19. A light year is a unit that measures ____________.A. time C. distanceB. mass D. luminous intensity

20. Our Sun belongs to a galaxy known as ____________.A. Andromeda C. the Milky WayB. Cepheus D. the Stratosphere

21. In agreement with the Big Bang theory, our Universe is ____________.A. expanding C. static (unchanging)B. contracting

22. The stream of charged particles given off by the Sun, which prevented theaccumulation of hydrogen and helium during the formation of the terrestrial planets, iscalled ____________.

A. the aurora borealis C. the Sun’s coronaB. solar wind D. the Van Allen belts

23. Chemically, the Moon is quite similar to ____________.A. seawater C. Earth’s mantleB. Earth’s crust D. Earth’s core

24. Foucault’s experiment with a pendulum proved that ____________.A. Earth is the center of the UniverseB. Earth revolves around the SunC. Earth rotates about an internal axisD. the Sun revolves around Earth

25. Humans first realized that the Earth was spherical ____________.A. as a result of the voyages of Christopher ColumbusB. when Magellan’s crew was able to sail completely around the worldC. during the RenaissanceD. during the time of Aristotle in ancient Greece



26. Differentiation of the core from the mantle early in Earth’s history was possiblebecause the planet was ____________ at the time.

A. very cold C. very smallB. very hot D. the only planet in the Solar System

27. The metal alloy that makes up the core of Earth is ____________ as compared to therocky mantle.

A. less denseB. denserC. very similar in chemistry and densityD. distinct in chemistry but of very similar density

28. Earth’s surface is protected from solar wind and cosmic radiation by ____________.A. Earth’s gravitational fieldB. Earth’s magnetic fieldC. a large metallic shield launched into orbit by NASA in the 1960sD. a powerful stream of ions emitted by the Sun

29. The shape of Earth’s magnetic field is approximately that of a ____________.A. monopoleB. dipole (such as that produced by a bar magnet)C. torus, a donut-shaped ring parallel to Earth’s equator

30. Presently, Earth’s atmosphere is dominated by which two gases?A. hydrogen and oxygen C. nitrogen and oxygenB. carbon dioxide and methane D. nitrous oxide and sulfur dioxide

31. In the whole Earth, the four most common elements are oxygen, silicon, magnesium,and ____________.

A. copper C. ironB. lead D. zinc

32. As compared to ultramafic rocks, mafic rocks have a ____________.A. greater proportion of silicaB. lesser proportion of silicaC. greater proportion of iron and magnesium atoms

33. By mass, the four most abundant elements in the Earth are oxygen, silicon,magnesium, and ____________.

A. hydrogen C. heliumB. carbon D. iron

34. Hot, liquid rock beneath the surface of the Earth is termed ____________.A. lava C. volatilesB. magma D. brimstone


10 | Chapter 1

35. A fracture in the crust, where rocks slide past one another, is termed a ____________.A. fold C. flying layerB. fault D. frictional discontinuity

36. The boundary between the crust and mantle is marked by a seismic-velocitydiscontinuity called ____________.

A. the Edsel C. Lyell’s surfaceB. the Moho D. the crantle

37. As seismic (earthquake-generated) waves travel downward and reach the Moho, they____________.

A. speed up C. continue at the same velocityB. slow down D. are all reflected directly back toward the

surface

38. Earth’s magnetic field is generated by ____________.A. the flow of the liquid inner coreB. the flow of the liquid outer coreC. the convective flow of the mantleD. magnetic minerals within the crust

39. The lithosphere is composed of the ____________.A. crust onlyB. crust, mantle, and outer coreC. top 100 m of sediments and sedimentary rocksD. crust and the uppermost part of the mantle

40. Moving into the interior of Earth, temperature ____________.A. and pressure both increaseB. and pressure both decreaseC. increases, but pressure stays nearly the sameD. remains remarkably constant, but pressure increases

41. The thickness of Earth’s crust varies from ____________.A. 100 to 500 m C. 5 to 500 kmB. 1 to 10 km D. 7 to 70 km

42. Of the three primary chemical layers of the Earth (crust, mantle, core), which is thethickest layer?

A. crustB. mantleC. core

43. Which of Earth’s layers has the greatest density?A. crustB. mantleC. core



44. With increasing altitude, the concentration of gases in our atmosphere ____________.A. becomes denserB. becomes less denseC. remains the sameD. increases for the first 10 km, then starts to decline

45. The two most common elements in the crust of Earth are ____________.A. iron and calcium C. oxygen and hydrogenB. magnesium and manganese D. oxygen and silicon

46. The metallic content of Earth’s core is ____________.A. likely similar to what has been found in metallic meteoritesB. partly liquid and partly solidC. an iron alloy (mostly iron with a few other elements mixed in)D. all of the above

47. As compared to continental crust, the rocks that make up oceanic crust are____________.

A. denserB. made of minerals that contain more silicaC. thickerD. all of the above

48. The Moho ____________.A. lies at uniform depth everywhere it is found in EarthB. is found deeper underneath continents than under oceansC. is found deeper underneath oceans than under continentsD. is found well below the crust/mantle boundary

49. The lithosphere lies directly above the ____________.A. transition zone C. asthenosphereB. crust D. lower mantle

50. As compared to the asthenosphere, the lithosphere is ____________.A. cooler and more able to flow C. cooler and less able to flowB. hotter and more able to flow D. hotter and less able to flow


12

The Way the Earth Works: Plate Tectonics

CHAPTER 2

Learning objectives

1. Students should be aware of Wegener’s amassed evidence for continental drift. Thefit of coastal outlines and the distributions of rocks, fossils, and ancient climatic belts allstrongly suggest that the continents were once aligned to form a supercontinent namedPangaea. Wegener’s ideas had few supporters during his lifetime because he could notprovide a workable mechanism through which continents could move with respect to oneanother.

2. During the twentieth century, paleomagnetic data showed that continents must havedrifted, because the rocks of isolated continents produce unequal apparent polar-wanderpaths for the magnetic north pole. Additionally, within the rocks of a single continent,magnetic inclination angles may change over time; this is only readily explained by thecontinent having drifted in a northward or southward direction.

3. Continental rocks cannot plow through oceanic crust (as suggested by Wegener).Rather, the continents are passively pushed by the activity of sea-floor spreading, in whichmolten rock rises at mid-ocean ridges, cools to form new oceanic crust, and spreadslaterally. Simultaneously, crust is pulled downward and engulfed at deep-ocean trenches (asrequired by a nonexpanding Earth).

4. Sea-floor spreading was proven in the late 1960s by examination of marinemagnetic anomalies, which are symmetric about the mid-ocean ridges. Combined withradiometric dates, these patterns clearly show that oceanic crust is created at the ridges andspreads outward, with crustal age increasing with distance from the ridge axis in eitherdirection. Areas of positive anomaly (including the ridges themselves) arise from rocks thatcrystallized and cooled during times when Earth’s magnetic polarity was normal (the sameas today’s); rocks producing negative anomalies cooled during times of reversed polarity.

5. Together, the evidence for sea-floor spreading and continental drift form the basis ofour modern understanding of plate tectonics, the unifying theory of geology that explainsthe links between earthquakes, volcanism, and mountain building.


6. The “plates” of plate tectonics are discrete slabs of lithosphere (crust and rigidportion of the mantle) that move with respect to one another. They glide over a ductile layerof the mantle termed the asthenosphere. Boundaries between plates are either convergent(where plates move toward one another, with material from one of the plates subducted intothe mantle), divergent (where plates are pushed apart at a mid-ocean ridge), or transform(where plates slide past one another). Relative plate motions are on the order of a fewcentimeters a year (a common analogy is that these rates approximate the rate of humanfingernail growth).

7. Plate motion at all three boundary types triggers earthquakes. Plate boundaries aredelineated by belts of high historical earthquake frequency.

8. Volcanism is associated with both convergent (island and continental volcanic arcs)and divergent (mid-ocean ridges), but not transform boundaries.

9. Only oceanic lithosphere is dense enough to be subducted at convergent boundaries.When continental lithosphere is pushed (by a ridge) and pulled (by a leading edge ofsubducting oceanic lithosphere) into another continent, a mountainous collision zone isformed, and the two plates involved become sutured together. Conversely, a single largeplate can become rifted apart when its lithosphere is stretched, thinned, and broken apart bya new mid-ocean ridge.

Answers to review questions

1. What was Wegener’s continental drift hypothesis? What was his evidence?Wegener stated that the continents had once been contiguous, forming a

supercontinent (which he termed Pangaea), and that they later moved apart to form theirpresent configuration. Wegener’s evidence included the fit of continental coastlines acrossthe Atlantic, the distributions of animals and plants within the fossil record, the matchup ofrock units found on continents that are now widely separated, and the interpretation ofancient climatic belts that are discordant with the modern positions of the continents.

2. How do apparent polar-wander paths show that the continents, rather than the poles,have moved?

Apparent polar-wander paths constructed for different continents provide differentindications of the position of the geomagnetic north pole. Although it is possible for themagnetic pole to wander, it is not possible for it to be in two places at the same time.

3. Describe the basic characteristics of mid-ocean ridges, deep-ocean trenches, andseamount chains.

Mid-ocean ridges are elongate, relatively narrow chains of basaltic volcanoes thattraverse the ocean floor, rising to a relief of 2 to 2.5 km above the surrounding abyssal plain.In some cases, a narrow axial trough runs down the center of the ridge.

Deep-ocean trenches are elongate arcs where ocean depths reach down to as much as12 km. The trenches border chains of volcanoes at subduction zones.

A seamount chain is an elongate series of former volcanic islands that have subsidedbelow sea level.

The Way the Earth Works: Plate Tectonics | 13


4. Describe the hypothesis of sea-floor spreading.Sea-floor spreading is the idea that new oceanic basalt is produced at mid-ocean ridges

and spreads laterally to either side. It is the push of these oceanic basalts that causes thecontinents to drift over the surface of Earth.

5. How did the observations of heat flow and seismicity support the hypothesis of sea-floor spreading?

Heat flow and seismicity are both anomalously high at mid-ocean ridges, suggestingextensive rising magma and crustal movement at these sea-floor spreading centers.

6. What is a marine magnetic anomaly? How is it detected?A marine magnetic anomaly is the finding of a region of oceanic crust where Earth’s

magnetic field appears to be either slightly stronger or weaker than expected. Anomalies aredetected by a device which measures magnetic field strength (a magnetometer) and which istowed behind a ship.

7. Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How isthis pattern explained?

Over the ridge crest, Earth’s magnetic field is anomalously strong. This elongate belt ofpositive anomaly is flanked on either side by belts of anomalously weak magnetic fieldstrength (negative anomaly). The alternating sequence of positive and negative anomaliescontinues in either direction outward from the ridge, forming a pattern that possessesmirror-image symmetry about the ridge axis.

The explanation for this is that iron atoms in crystals formed in the most recent pasthave remnant magnetism in concert with today’s global magnetic field (and are said to havenormal polarity). Extra field strength derives from the alignment of all these mini-magnetswith the modern dipole. The same is true for all positive anomalies; they representcrystallization and cooling that took place during times when Earth’s magnetic polarity wasthe same as it is today.

Negative anomalies are derived from bodies of rock that crystallized and cooled duringtimes when Earth’s magnetic field had a polarity opposite to today’s. The iron atoms ofthese rocks destructively interfere with the modern dipole, weakening the observedmagnetic strength.

8. Did drilling into the sea floor contribute further proof of sea-floor spreading? If so,how?

Sediments atop oceanic basalts become thicker with increasing distance from mid-ocean ridges, and the lower-most (oldest) layers become progressively older with increasingdistance from the ridge as well.

9. What are the characteristics of a lithosphere plate?The lithosphere is the rocky portion of Earth, relatively cool and rigid as compared to

underlying mantle material (the ductile asthenosphere). The lithosphere is composed of thecrust and the uppermost portion of the mantle.

14 | Chapter 2


10. How does oceanic lithosphere differ from continental lithosphere in thickness,composition, and density?

Oceanic crust is thinner, more mafic (largely basalt, whereas continental crust isgranitic), and more dense. The mantle is essentially identical beneath each type of crust.

11. What are the basic premises of plate tectonics?The lithosphere is divided into discrete plates that move with respect to one another;

this motion is facilitated by the contrast in rigidity between the plates and the weak, flexibleasthenosphere immediately below them. Plate motion is driven by mid-ocean ridges, wherenew oceanic crust is created and pushed to either side, and also by subduction zones, whereolder oceanic crust descends into the mantle below. The continents are more passive players,mutually colliding, sliding past one another, or drifting apart depending on the distributionof ridges and subduction zones.

12. How do we identify a plate boundary?Plate boundaries are marked by linear or arcoid segments of relatively high earthquake

frequency (earthquake belts).

13. Describe the three types of plate boundaries.Divergent plate boundaries exist where lithosphere on either side is moving away from

the boundary. At convergent plate boundaries, lithosphere to either side comes together,bringing subduction (if oceanic lithosphere is involved) or collision (of two continentalplates). At transform plate boundaries, plates slide past one another.

14. How does crust form along a mid-ocean ridge?The high-heat flux at the ridge melts mantle material to form magma, which is

relatively light and rises to the surface. Some of the magma crystallizes beneath the surface(as gabbro or in thin basaltic dikes), and some erupts to form volcanic lava, which flows andultimately solidifies to form pillow basalt.

15. Why is subduction necessary on a nonexpanding Earth with spreading ridges?The introduction of new crust at the mantle causes subduction to be a topological

necessity. Unless the Earth expands (or its shape is dramatically altered), surface arearemains constant. Gravity prevents rocks from breaking off at the surface and floating intospace, so any new surficial material must be balanced by a loss of surficial material throughsubduction.

16. Describe the major features of a convergent boundary.The boundary is marked by a deep trench where the subducting oceanic plate bends

downward in opposition to the horizontal overriding plate. Sediments scrape off thesubducting plate to form an accretionary prism at the edge of the overriding plate. Behindthe prism, melting associated with the subducting plate produces either a volcaniccontinental arc or a volcanic island arc.



17. Why are transform plate boundaries required on an Earth with spreading andsubducting plate boundaries?

The mid-ocean ridge is segmented, with adjacent segments offset laterally andconnected by fractured segments of crust. In the region between two offset segments, thedirection of motion on either side of a fracture is mutually opposed (each side beingdominated by the push of volcanism emanating from the ridge segment on its side of thefracture).

18. What is a triple junction?A point at which three plate boundaries meet.

19. How is a hot-spot track produced, and how can hot-spot tracks be used to track the pastmotions of a plate?

Very hot rock from deep in the mantle rises at the hot spot and produces abundantvolcanic material. Hot spots are relatively stable points, whereas the plates that overliethem, and which bear the associated volcanoes, are moving. Over periods of millions ofyears, as the plate slides over the hot spot, extinct volcanoes are ferried in the direction ofplate motion, while new volcanoes are formed at the hot spot.

20. Describe the characteristics of a continental rift, and give examples of where thisprocess is occurring today.

Continental rifts appear as elongate valleys bounded on either side by faults. Volcanismoccurs along the rift as asthenosphere rises to accommodate the thinning lithosphere andmelts. Rifts can be found in East Africa and in the Great Basin of the western United States.

21. Describe the process of continental collision, and give examples of where this processhas occurred.

Continental rock is not dense enough to subduct beneath an overriding, opposedcontinental plate and will thus collide, suturing together with the adjacent plate, folding therocks in the zone of collision, and thickening the crust locally, to form a nonvolcanicmountain range.

22. Discuss the major forces that move lithosphere plates.Plates are pushed by mid-ocean ridges, as elevated lithosphere directly over the ridge

pushes downward on less-elevated lithosphere to either side. Plates are pulled by descendingslabs at subduction zones, because old oceanic lithosphere is generally denser than theasthenosphere into which the slabs sink.

23. Explain the difference between relative plate velocity and absolute plate velocity.Relative plate velocity describes rates of motion calculated for the movement of

material on one plate with respect to material from an adjacent plate (or with respect to aplate boundary). Absolute plate velocity is calculated using age and distance data from thematerial on a plate, with the distance calculated from a hot spot or other fixed point ofreference on Earth’s surface.

16 | Chapter 2


On Further Thought

1. Why are the marine magnetic anomalies bordering the East Pacific Rise in thesoutheastern Pacific Ocean wider than marine magnetic anomalies bordering the Mid-Atlantic Ridge in the South Atlantic Ocean?

The East Pacific Rise is spreading faster, so it produces a greater width of basalt in thetime intervals between polarity reversals.

2. The Pacific Plate moves north relative to the North American Plate at a rate of 6 cm peryear. How long will it take Los Angeles (a city on the Pacific Plate) to move northwards by480 km, the present distance between Los Angeles and San Francisco?

Assuming no future change in plate velocity, 8 million years.

3. Look at a map of the western Pacific Ocean, and examine the position of Japan withrespect to mainland Asia. Japan’s older crust contains rocks similar to those of eastern Asia.Presently, there are many active volcanoes along the length of Japan. With these facts inmind, explain how the Japan Sea (the region between Japan and the mainland) formed.

East Asia rifted, with Japan drifting toward the east. The seafloor of the Japan Seaformed from basalt eruptions associated with the rifting and spreading. When this occursbehind a volcanic arc, such as Japan, the phenomenon is often termed backarc spreading.

Test bank

1. Wegener proposed continental drift after he observed evidence from fossils, glacialdeposits, and the fit of the continents that suggested all of the continents were once____________.

A. aligned north to south along the prime meridian during the late CenozoicB. aligned east to west along the equator during the late Mesozoic through the

CenozoicC. combined to form a supercontinent (he termed Rodinia) in the ProterozoicD. combined to form a supercontinent (he termed Pangaea) in the late Paleozoic

through the Mesozoic

2. Late Paleozoic glacial deposits are NOT found in which of the following places?A. India C. North AmericaB. southern Africa D. South America

3. Abundant swamps led to the formation of coal during the Late Paleozoic in which ofthe following places?

A. India C. North AmericaB. southern Africa D. Antarctica



4. Which plant genus dominated glaciated regions during the late Paleozoic and earlyMesozoic?

A. Ginkgo C. NeuropterisB. Glossopteris D. Quercas

5. Wegener’s idea of continental drift was rejected by American geologists because____________.

A. his English was too poor to be understood by themB. he could not conceive of a valid mechanism that would cause continents to shift

positionsC. he had relatively little evidence supporting the existence of a supercontinentD. the apparent fit of continental coastlines is blurred when the margins are defined by

the edges of continental shelves rather than at sea level

6. Currently, most geologists ____________.A. continue to reject continental driftB. agree that continental drift occurs, but they still do not understand why it occursC. agree that continental drift occurs; the mechanisms that drive drift are at work in the

ocean basins and upper mantle and were unknown in Wegener’s timeD. agree that continental drift occurs; the mechanisms that drive drift are at work in the

lower mantle and outer core and were unknown in Wegener’s time

7. The magnetic field of Earth in the geologic past is ____________.A. unknown, but it is assumed to have been identical to today’sB. known to have been constant through geologic time, due to remnant magnetization

of iron-rich minerals in rocksC. known to have experienced numerous polarity reversals, due to remnant

magnetization of iron-rich minerals in rocksD. known to have been constant through time, on the basis of theoretical calculations

8. The apparent tendency of the north (or south) magnetic pole to vary in position overtime is termed ____________.

A. dipole C. magnetic inclinationB. magnetic declination D. polar wander

9. The apparent polar-wander paths for continents that were not connected over somespan of geologic history will likely ____________ concerning the positions of the ancientmagnetic pole.

A. agree B. disagree

10. Sea-floor spreading is driven by volcanic activity ____________.A. in the middle of abyssal plains C. at the edges of continental shelvesB. along mid-ocean ridges D. along fracture zones

18 | Chapter 2


11. Within the sea floor, the rate of heat flow is greatest ____________.A. along mid-ocean ridges C. at the edges of ocean basinsB. along fracture zones D. in the center of abyssal plains

12. Regions of the sea floor with positive magnetic anomalies were formed during timeswhen Earth’s magnetic field ____________.

A. was exceptionally strong C. had normal polarityB. was exceptionally weak D. had reversed polarity

13. Regions of the sea floor with negative magnetic anomalies were formed during timeswhen Earth’s magnetic field ____________.

A. was exceptionally strong C. had normal polarityB. was exceptionally weak D. had reversed polarity

14. Marine magnetic anomaly belts run parallel to ____________.A. mid-ocean ridges C. continental coastlinesB. fracture zones D. continental shelves

15. Marine magnetic anomaly belts are widest when and where ____________.A. continents are joined to form supercontinentsB. sea-floor spreading rates are relatively rapidC. sea-floor spreading rates are relatively slow

16. The age of oceanic crust ____________ with increasing distance from a mid-oceanridge.

A. increases B. decreases

17. Wegener’s evidence for a united Pangaea was so compelling that virtually allgeologists agreed with the idea of continental drift during his lifetime.

A. true B. false

18. Distinctive rock sequences on South America terminate at the Atlantic Ocean butreappear on the continent of ____________.

A. Africa C. North AmericaB. Europe D. Australia

19. If we mentally align the continents to fit Wegener’s concept of Pangaea, evidence oflate Paleozoic glacial deposits ____________.

A. is more difficult to explain than in the modern continental configurationB. is much more readily explained than in the modern continental configurationC. makes very little sense in either the Pangaea configuration or the modern

configuration



20. The apparent polar-wander path obtained from magnetite crystals in basalts on theNorth American continent is now interpreted to be the result of ____________.

A. wandering of the geomagnetic north poleB. drifting of the North American continent

21. The deep ocean floor is flat and nearly featureless.A. true B. false

22. Beneath a blanket of sediments, oceanic crust is primarily composed of two rocks,____________.

A. granite and diorite C. sandstone and shaleB. gabbro and basalt D. slate and gneiss

23. All basalts younger than 700,000 years old ____________.A. have normal magnetic polarityB. have reverse magnetic polarityC. are found on the ocean floor very far from mid-ocean ridgesD. are found on the continents

24. Marine magnetic anomalies result from sea-floor spreading in conjunction with____________.

A. global warmingB. magnetic storms on the surface of the SunC. magnetic polarity reversalsD. apparent wander of the magnetic poles

25. The oldest sediments on the ocean floor are about ____________ years old.A. 50 thousand C. 200 millionB. 4 billion D. 2.5 million

26. The primary difference between lithospheric and asthenospheric mantle that gives riseto numerous divergent patterns of physical behavior, is ____________.

A. physical state (the lithosphere is solid, and the asthenosphere is liquid)B. chemical composition (the lithosphere is mafic, and the asthenosphere is felsic)C. temperature (the lithosphere is cooler than the asthenosphere)D. chemical composition (the lithosphere is felsic, and the asthenosphere is mafic)

27. The theory of plate tectonics ____________.A. incorporates continental drift but not sea-floor spreadingB. incorporates sea-floor spreading but not continental driftC. incorporates and explains both sea-floor spreading and continental driftD. does not incorporate sea-floor spreading or continental drift

20 | Chapter 2


28. Unlike the lithosphere, the asthenosphere ____________.A. is relatively weak and flows readilyB. has a density similar to the coreC. varies in thickness from place to placeD. is relatively cool

29. Continental lithosphere ____________.A. is thicker than oceanic lithosphereB. contains more mafic rocks than oceanic lithosphereC. is denser than oceanic lithosphereD. contains no crustal material, consisting solely of lithified upper mantle

30. The average thickness of continental lithosphere is about ____________.A. 30 km C. 150 kmB. 60 km D. 10,000 km

31. The thickness of oceanic lithosphere is ____________.A. uniformly 100 kmB. greatest at the geographic poles and least near the equatorC. greatest near the mid-ocean ridges and thins out away from the ridgesD. least near the mid-ocean ridges and thickens away from the ridges

32. Under the theory of plate tectonics, the plates themselves are ____________.A. discrete pieces of lithosphere at the surface of the solid Earth that move with

respect to one anotherB. discrete layers of lithosphere that are vertically stacked one atop the otherC. composed only of continental rocks, which plow through the weaker oceanic rocksD. very thick (approximately one-quarter of Earth’s radius)

33. In the terminology of plate tectonics, an active margin is ____________.A. synonymous with “subduction zone”B. a 5-mile radius surrounding an active volcanoC. a continental coastline that coincides with a plate boundaryD. anywhere on Earth where earthquakes are especially frequent

34. Continental coastlines that occur within the interior of a tectonic plate are called____________.

A. internal margins C. active marginsB. passive margins D. inert margins

35. Broad, sediment-covered continental shelves are found along ____________.A. active margins B. passive margins



22 | Chapter 2

36. Tectonic plates might consist of ____________.A. continental lithosphere onlyB. oceanic lithosphere onlyC. oceanic or continental lithosphere or a combination of bothD. either oceanic or continental lithosphere, but not both

37. Deformed (bent, stretched, or cracked) lithosphere occurs ____________.A. randomly over the surface of EarthB. primarily within the interiors of tectonic platesC. primarily on the margins of tectonic plates

38. Every plate boundary can be recognized by ____________.A. the presence of active volcanoesB. the presence of an earthquake beltC. a deep chasm which can be seen from spaceD. none of the above

39. Tectonic plates move at rates that are approximately ____________.A. 1 to 5 cm every 1,000 years C. 1 to 15 m/yearB. 1 to 15 cm/year D. 10 to 100 m/year

40. At a divergent plate boundary, two opposed plates ____________.A. move toward one anotherB. move away from one anotherC. slide past one another

41. At a convergent plate boundary, two opposed plates ____________.A. move toward one anotherB. move away from one anotherC. slide past one another

42. At a transform plate boundary, two opposed plates ____________.A. move toward one anotherB. move away from one anotherC. slide past one another

43. Mid-ocean ridges are ____________.A. convergent plate boundariesB. divergent plate boundariesC. transform plate boundaries

44. As compared to a slowly spreading mid-ocean ridge, a rapidly spreading ridge is____________.

A. widerB. narrowerC. more silicic in lava composition



45. All lithospheric plates are approximately the same size and contain a combination ofoceanic and continental crust.

A. true B. false

46. The youngest sea floor occurs ____________.A. along passive margins C. along mid-ocean ridgesB. along active margins D. randomly over the entire ocean basin

47. Oceanic lithosphere thickens away from the mid-ocean ridge primarily due to____________.

A. the addition of new crust due to hot-spot volcanismB. the addition of new crust due to sedimentationC. the addition of new lithospheric mantle as a result of coolingD. reasons that geologists cannot determine at present

48. Subduction zones are ____________.A. convergent plate boundariesB. divergent plate boundariesC. transform plate boundaries

49. At a subduction zone, the overriding plate ____________.A. is always composed of continental lithosphereB. is always composed of oceanic lithosphereC. may be composed of either oceanic or continental lithosphere

50. At a subduction zone, the downgoing (subducting) plate ____________.A. is always composed of continental lithosphereB. is always composed of oceanic lithosphereC. may be composed or either oceanic or continental lithosphere

51. The Wadati-Benioff zone is a belt of earthquakes found ____________.A. within an otherwise stable continental interiorB. within an overriding plate at a subduction zoneC. within a downgoing plate at a subduction zoneD. along mid-ocean ridges

52. The Wadati-Benioff zone extends down within the mantle to a maximum depth of____________.

A. 30 km C. 670 kmB. 150 km D. 990 km

53. At transform plate boundaries ____________.A. earthquakes are common, but volcanoes are absentB. volcanoes are common, but earthquakes do not occurC. both earthquakes and volcanoes are common


24 | Chapter 2

54. A triple junction is a place on Earth’s surface where ____________.A. three volcanoes form a tight, triangular clusterB. glacial ice, continental rocks, and the ocean can be found togetherC. the boundaries of three lithospheric plates meet at a single pointD. the boundaries of three lithospheric plates meet to form an elongate surface

55. The mid-ocean ridges are elevated above the surrounding sea floor because____________.

A. ridge rocks are hot and therefore of relatively low densityB. the lithospheric plates are thickest at the ridges so they stand up tallerC. rising ocean currents leave a vacuum above the ridgeD. ridge rocks are mafic, whereas the ocean basin crust consists of ultramafic rock

56. Hawaii is an example of ____________.A. hot-spot volcanism C. a volcanic island arcB. mid-ocean ridge volcanism D. a transform margin

57. Segments of the mid-ocean ridge system are offset. Between the offset segments weobserve ____________.

A. a second series of ridges, perpendicular to the main setB. deep-ocean trenchesC. transform faultsD. None of the above is correct.

58. When two bodies of continental lithosphere are pushed together at a convergentboundary, the result is ____________.

A. subduction B. collision and mountain formation

59. Most of the pushing force that drives plate motion is produced ____________.A. at mid-ocean ridges C. at collision zonesB. at subduction zones D. in the interiors of continental plates

60. Most of the pulling force that drives plate motion is produced ____________.A. at mid-ocean ridges C. at collision zonesB. at subduction zones D. in the interiors of continental plates


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