Download Preview Sample 2

INSTRUCTOR’S MANUAL AND TEST BANK
Stephen Marshak’s
Essentials of Geology
THIRD EDITION
INSTRUCTOR’S MANUAL AND TEST BANK
Stephen Marshak’s
Essentials of Geology
THIRD EDITION
John Werner
SEMINOLE COMMUNITY COLLEGE
W. W. NORTON & COMPANY • NEW YORK • LONDON
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
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
v
New Features for the Third Edition of
Essentials of Geology:
On Further Thought and Geotours
The Third Edition of Stephen Marshak’s Essentials of Geology includes a pair of features
that are new and that should be of great value to both the student and the educator of
introductory geology. The “On Further Thought” section at the end of each chapter includes
new questions that go beyond the chapter synopsis, giving the student a chance to improve
his or her critical thinking, reference, and basic math skills. Since the majority of students
taking introductory geology will not be continuing further in the field, the questions in this
new section are of great value in reinforcing skill sets needed for success in the rest of a
degree 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 Google
Earth. Available for download at http://earth.google.com/download-earth.html, Google
Earth offers detailed mosaic satellite imagery of the continents and a coarser physiographic
view 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 to
pinpoint 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 and
explain physiographic features related to streams, coasts, plate tectonics, or glacial
geomorphology (among other topics). In assembling the Geotours, Stephen Marshak has
done a remarkable job of compiling some of the most interesting examples of geology
visible 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 in
the classroom are encouraged to use them in their teaching.
vii
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 been
there 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.
viii
CHAPTER 1
The Earth in Context
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 are
all 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 the
past suggests that all matter in the Universe was contained in a single point, approximately
13.7 billion years ago. At that time, the Universe came into existence explosively (hence the
name Big Bang); radiation from the Big Bang still can be perceived in all directions in the
sky (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. Later
stages 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 within
stars.
3. After their cycles of fusion are complete, large stars violently explode, forming
elements heavier than iron and leaving behind a residue of diffuse nebulae, which may be
recycled to form a new star at some point in the future. These explosive events are termed
novas and supernovas because some have been bright enough to be seen as “new stars” in
the night sky. Historically, a few supernovae have even been bright enough to be seen during
daylight.
4. Our Sun is approximately 5 billion years old and is expected to continue fusing
helium 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’s
north pole). These facts imply simultaneous planetary formation from a swirling nebula
surrounding the Sun (the similarities in orbits would then be a natural result of conservation
of angular momentum). The planets accreted from this nebula through gravitational
attraction and haphazard collisions. Pluto, long considered the “ninth planet,” has recently
seen its status demoted; astronomers now recognize only eight major planets in our Solar
1
2 | Chapter 1
System. Pluto belongs to a group of icy and rocky bodies beyond Neptune’s orbit termed the
Kuiper 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 the
superabundant (but very light) elements, hydrogen and helium. The gas giant planets
(Jupiter, Saturn, Uranus, and Neptune) retained these elements and are thus much larger and
much less dense (Saturn is less dense than water).
6. Our Moon, responsible for Earth’s tides, has a composition similar to Earth’s
mantle; the Moon is thought to have originated from debris accumulated when a Mars-sized
body impacted the Earth very early in Earth’s history.
7. Students should be aware of the presence of Earth’s magnetic dipole, how the
magnetic 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 and
biochemistry, allowing both Earth’s surface and its constituent life to evolve dramatically
over time.
9. Earth is chemically divided into a thin, rocky crust dominated by silicate minerals, a
thick mantle dominated by iron- and magnesium-rich silicates (subject locally to partial
melting), and a thick, metallic core which is primarily iron (the outer portion of which is
liquid). 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 and
uppermost 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 one
another 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, with
Earth 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. This
frequency shift arises because each successive sound wave emanates from a closer distance
than the previous wave. These high frequencies are interpreted by our brains (after
transmission through our ears) as a higher pitch. Once a wave source passes an observer, its
sound waves have a reduced frequency, as each wave is emitted from a slightly more distant
point.
3. What does the red shift of the galaxies tell us about their motion with respect to the
Earth?
All distant galaxies are moving away from our own, with the farthest galaxies moving
fastest.
The Earth in Context | 3
4. Briefly describe the steps in the formation of the Universe according to the Big Bang
theory.
The Universe formed from the big bang, an explosion of matter and space from an
infinitely 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 of
stellar fusion and explosive supernovas. Our Sun and the surrounding solar system condensed
from a mostly gaseous nebula, which itself contained material from previous supernovas (as
evidenced 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 the
Sun and the planets, differ from that of the nebulae that existed a million years after
the 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. Fusion
within 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 iron
and 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 minerals
migrated to the outermost part of the solid Earth, forming a thin crust, in contrast to the
somewhat denser mantle. Physical layers in the Earth have formed due to both this chemical
layering and to the changes in physical properties that occur as temperature and pressure
both 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 you
were to eject from a fighter plane at an elevation of 12 km without taking an oxygen tank
with 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 quite
complex, sometimes incorporating oxygen (as in sugars, starches, and fats), sometimes
additionally 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 with
rough or rounded surfaces, which are termed grains.
Rocks are cohesive aggregates of crystals or grains. Igneous rocks crystallize from
molten (liquid) rock. Sedimentary rocks arise from the cementation of loose grains (sand,
mud, pebbles, etc.) and through chemical precipitation (from the ocean or continental
bodies of water). Metamorphic rocks arise from heat- and pressure-induced alteration of
preexistent rock (without melting).
Glasses are physically solid structures in which the atoms are internally disordered (as
in liquids, but without the tendency to flow rapidly). Commercial glass is produced when
quartz is melted and then rapidly cooled (quenched in cool water), so that atoms cannot align
themselves 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, such
as 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 oceanic
crust.
The Moho is the crust/mantle boundary, first recognized by an abrupt change in
seismic wave velocities. Crustal thickness is variable (and is thicker under continents than
oceans) but is generally less than 1% of the Earth’s total diameter. Continental crust is
variable 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? Is
there any melt within the mantle?
The mantle is mostly made of an ultramafic silicate rock termed peridotite. Its layers
are the upper mantle, transition zone, and lower mantle. There is a small amount of melt in
the upper mantle.
The Earth in Context | 5
16. What is the core composed of? How do the inner core and outer core differ from each
other?
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 depth
does 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 than
3% of its mass. In comparison, Earth’s core is about 33% of its mass. Explain why this
difference might exist.
The Moon is thought to have formed when a Mars-sized body collided with the Earth
early in the planet’s history. The impact hurled a portion of the Earth, mostly mantle
material with no contribution from the core, into orbit about our Earth, where it solidified to
form our Moon. The Moon differentiated (as the Earth had earlier), but with a minute
amount of iron compared to the Earth (which had already seen most of its iron descend into
its larger core).
2. There is hardly any hydrogen or helium in the Earth’s atmosphere, yet most of the
nebula from which the Solar System formed consisted of hydrogen and helium. Where did
all this gas go?
The Earth is not massive enough to keep the light gases hydrogen and helium from
escaping to space.
3. The popular media sometimes imply that the crust floats on a “sea of magma.” Is this a
correct 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 with
the 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 continental
ice sheet) causes the surface of lithosphere to sink, just as your weight causes the surface of
a trampoline to sink. Emplacement of such a weight does not, however, cause a change in
the thickness of the lithosphere. How is this possible? (Hint: Think about the nature of the
asthenosphere.)
The asthenosphere below the burdened lithosphere is ductile, and will flow away from
the 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 widely
held again
B. was held to be true by thinkers throughout the Middle Ages, up until the
Renaissance
C. was rediscovered by the Polish astronomer Copernicus and has been the accepted
model of the Universe ever since
D. has been proven by NASA space photos
2. In the heliocentric model ____________.
A. Earth orbits around the Sun
B. the Sun orbits around Earth
C. Earth is a stationary planet
D. 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 Universe
B. the Universe is considerably older than Earth
C. Earth and the Universe formed at about the same time
D. 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 stars
B. hydrogen concentration has increased through the fission of helium atoms
C. hydrogen concentration has increased through the fusion of helium atoms
D. 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 billion
B. 6 billion
D. 100 billion
6. The big bang theory states that ____________.
A. all stars will end their lives explosively as supernovas
B. Earth formed through a series of violent collisions
C. meteors were responsible for the extinction of the dinosaurs
D. 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 light
emitted from distant galaxies appears to be ____________.
A. red-shifted
C. green-shifted
B. blue-shifted
D. none of the above
The Earth in Context | 7
8. Italian Renaissance astronomer Galileo was the first person to deduce that planets were
distinct entities from stars.
A. true
B. false
9. Since the initiation of the Big Bang, the temperature of the Universe has
____________.
A. increased
B. decreased
C. stayed about the same
10. Atoms that are heavier than iron are generally produced by ____________.
A. fission reactions within stars
B. fusion reactions within stars
C. the explosion of supernovae
D. 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 helium
B. iron and manganese
D. hydrogen and oxygen
12. Which of the following bodies is the smallest?
A. planet
C. protoplanet
B. star
D. planetesimal
13. Aside from Earth, the terrestrial planets are ____________.
A. Mars, Mercury, and Venus
B. Mars, Venus, and Jupiter
C. Jupiter, Saturn, Uranus, and Neptune
D. Mars and Saturn
14. The gas-giant, or Jovian, planets are ____________.
A. Mars, Mercury, and Venus
B. Mars, Venus, and Jupiter
C. Jupiter, Saturn, Uranus, and Neptune
D. Mars and Saturn
15. The branch of science that studies the structure and history of the Universe is
____________.
A. cosmetology
C. cosmology
B. scientology
D. universalism
16. All objects in the Solar System are in orbit around ____________.
A. Earth
C. the Sun
B. 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 Universe
C. the Sun was the center of the Earth’s orbit
D. the Earth was the center of the Universe
18. The circumference of Earth is most nearly ____________.
A. 400 km
C. 40,000 km
B. 4,000 km
D. 4,000,000 km
19. A light year is a unit that measures ____________.
A. time
C. distance
B. mass
D. luminous intensity
20. Our Sun belongs to a galaxy known as ____________.
A. Andromeda
C. the Milky Way
B. 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 the
accumulation of hydrogen and helium during the formation of the terrestrial planets, is
called ____________.
A. the aurora borealis
C. the Sun’s corona
B. solar wind
D. the Van Allen belts
23. Chemically, the Moon is quite similar to ____________.
A. seawater
C. Earth’s mantle
B. Earth’s crust
D. Earth’s core
24. Foucault’s experiment with a pendulum proved that ____________.
A. Earth is the center of the Universe
B. Earth revolves around the Sun
C. Earth rotates about an internal axis
D. the Sun revolves around Earth
25. Humans first realized that the Earth was spherical ____________.
A. as a result of the voyages of Christopher Columbus
B. when Magellan’s crew was able to sail completely around the world
C. during the Renaissance
D. during the time of Aristotle in ancient Greece
The Earth in Context | 9
26. Differentiation of the core from the mantle early in Earth’s history was possible
because the planet was ____________ at the time.
A. very cold
C. very small
B. 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 the
rocky mantle.
A. less dense
B. denser
C. very similar in chemistry and density
D. 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 field
B. Earth’s magnetic field
C. a large metallic shield launched into orbit by NASA in the 1960s
D. a powerful stream of ions emitted by the Sun
29. The shape of Earth’s magnetic field is approximately that of a ____________.
A. monopole
B. 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 oxygen
B. 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. iron
B. lead
D. zinc
32. As compared to ultramafic rocks, mafic rocks have a ____________.
A. greater proportion of silica
B. lesser proportion of silica
C. 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. helium
B. carbon
D. iron
34. Hot, liquid rock beneath the surface of the Earth is termed ____________.
A. lava
C. volatiles
B. 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 layer
B. fault
D. frictional discontinuity
36. The boundary between the crust and mantle is marked by a seismic-velocity
discontinuity called ____________.
A. the Edsel
C. Lyell’s surface
B. 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 velocity
B. 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 core
B. the flow of the liquid outer core
C. the convective flow of the mantle
D. magnetic minerals within the crust
39. The lithosphere is composed of the ____________.
A. crust only
B. crust, mantle, and outer core
C. top 100 m of sediments and sedimentary rocks
D. crust and the uppermost part of the mantle
40. Moving into the interior of Earth, temperature ____________.
A. and pressure both increase
B. and pressure both decrease
C. increases, but pressure stays nearly the same
D. remains remarkably constant, but pressure increases
41. The thickness of Earth’s crust varies from ____________.
A. 100 to 500 m
C. 5 to 500 km
B. 1 to 10 km
D. 7 to 70 km
42. Of the three primary chemical layers of the Earth (crust, mantle, core), which is the
thickest layer?
A. crust
B. mantle
C. core
43. Which of Earth’s layers has the greatest density?
A. crust
B. mantle
C. core
The Earth in Context | 11
44. With increasing altitude, the concentration of gases in our atmosphere ____________.
A. becomes denser
B. becomes less dense
C. remains the same
D. 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 hydrogen
B. 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 meteorites
B. partly liquid and partly solid
C. 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. denser
B. made of minerals that contain more silica
C. thicker
D. all of the above
48. The Moho ____________.
A. lies at uniform depth everywhere it is found in Earth
B. is found deeper underneath continents than under oceans
C. is found deeper underneath oceans than under continents
D. is found well below the crust/mantle boundary
49. The lithosphere lies directly above the ____________.
A. transition zone
C. asthenosphere
B. 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 flow
B. hotter and more able to flow
D. hotter and less able to flow
CHAPTER 2
The Way the Earth Works:
Plate Tectonics
Learning objectives
1. Students should be aware of Wegener’s amassed evidence for continental drift. The
fit of coastal outlines and the distributions of rocks, fossils, and ancient climatic belts all
strongly suggest that the continents were once aligned to form a supercontinent named
Pangaea. Wegener’s ideas had few supporters during his lifetime because he could not
provide a workable mechanism through which continents could move with respect to one
another.
2. During the twentieth century, paleomagnetic data showed that continents must have
drifted, because the rocks of isolated continents produce unequal apparent polar-wander
paths 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 the
continent 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 which
molten rock rises at mid-ocean ridges, cools to form new oceanic crust, and spreads
laterally. Simultaneously, crust is pulled downward and engulfed at deep-ocean trenches (as
required by a nonexpanding Earth).
4. Sea-floor spreading was proven in the late 1960s by examination of marine
magnetic anomalies, which are symmetric about the mid-ocean ridges. Combined with
radiometric dates, these patterns clearly show that oceanic crust is created at the ridges and
spreads outward, with crustal age increasing with distance from the ridge axis in either
direction. Areas of positive anomaly (including the ridges themselves) arise from rocks that
crystallized and cooled during times when Earth’s magnetic polarity was normal (the same
as 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 of
our modern understanding of plate tectonics, the unifying theory of geology that explains
the links between earthquakes, volcanism, and mountain building.
12
The Way the Earth Works: Plate Tectonics | 13
6. The “plates” of plate tectonics are discrete slabs of lithosphere (crust and rigid
portion of the mantle) that move with respect to one another. They glide over a ductile layer
of 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 into
the 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 few
centimeters a year (a common analogy is that these rates approximate the rate of human
fingernail growth).
7. Plate motion at all three boundary types triggers earthquakes. Plate boundaries are
delineated 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 of
subducting oceanic lithosphere) into another continent, a mountainous collision zone is
formed, and the two plates involved become sutured together. Conversely, a single large
plate can become rifted apart when its lithosphere is stretched, thinned, and broken apart by
a 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 their
present configuration. Wegener’s evidence included the fit of continental coastlines across
the Atlantic, the distributions of animals and plants within the fossil record, the matchup of
rock units found on continents that are now widely separated, and the interpretation of
ancient 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 different
indications of the position of the geomagnetic north pole. Although it is possible for the
magnetic 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, and
seamount chains.
Mid-ocean ridges are elongate, relatively narrow chains of basaltic volcanoes that
traverse 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 as
12 km. The trenches border chains of volcanoes at subduction zones.
A seamount chain is an elongate series of former volcanic islands that have subsided
below sea level.
14 | Chapter 2
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 the
continents to drift over the surface of Earth.
5. How did the observations of heat flow and seismicity support the hypothesis of seafloor spreading?
Heat flow and seismicity are both anomalously high at mid-ocean ridges, suggesting
extensive 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 are
detected by a device which measures magnetic field strength (a magnetometer) and which is
towed behind a ship.
7. Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How is
this pattern explained?
Over the ridge crest, Earth’s magnetic field is anomalously strong. This elongate belt of
positive anomaly is flanked on either side by belts of anomalously weak magnetic field
strength (negative anomaly). The alternating sequence of positive and negative anomalies
continues in either direction outward from the ridge, forming a pattern that possesses
mirror-image symmetry about the ridge axis.
The explanation for this is that iron atoms in crystals formed in the most recent past
have remnant magnetism in concert with today’s global magnetic field (and are said to have
normal polarity). Extra field strength derives from the alignment of all these mini-magnets
with the modern dipole. The same is true for all positive anomalies; they represent
crystallization and cooling that took place during times when Earth’s magnetic polarity was
the same as it is today.
Negative anomalies are derived from bodies of rock that crystallized and cooled during
times when Earth’s magnetic field had a polarity opposite to today’s. The iron atoms of
these rocks destructively interfere with the modern dipole, weakening the observed
magnetic 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 midocean ridges, and the lower-most (oldest) layers become progressively older with increasing
distance 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 the
crust and the uppermost portion of the mantle.
The Way the Earth Works: Plate Tectonics | 15
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 is
granitic), 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, flexible
asthenosphere immediately below them. Plate motion is driven by mid-ocean ridges, where
new oceanic crust is created and pushed to either side, and also by subduction zones, where
older 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 distribution
of 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 continental
plates). 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 and
ultimately 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 area
remains constant. Gravity prevents rocks from breaking off at the surface and floating into
space, so any new surficial material must be balanced by a loss of surficial material through
subduction.
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 the
subducting plate to form an accretionary prism at the edge of the overriding plate. Behind
the prism, melting associated with the subducting plate produces either a volcanic
continental arc or a volcanic island arc.
16 | Chapter 2
17. Why are transform plate boundaries required on an Earth with spreading and
subducting plate boundaries?
The mid-ocean ridge is segmented, with adjacent segments offset laterally and
connected by fractured segments of crust. In the region between two offset segments, the
direction of motion on either side of a fracture is mutually opposed (each side being
dominated by the push of volcanism emanating from the ridge segment on its side of the
fracture).
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 past
motions of a plate?
Very hot rock from deep in the mantle rises at the hot spot and produces abundant
volcanic material. Hot spots are relatively stable points, whereas the plates that overlie
them, and which bear the associated volcanoes, are moving. Over periods of millions of
years, as the plate slides over the hot spot, extinct volcanoes are ferried in the direction of
plate motion, while new volcanoes are formed at the hot spot.
20. Describe the characteristics of a continental rift, and give examples of where this
process is occurring today.
Continental rifts appear as elongate valleys bounded on either side by faults. Volcanism
occurs along the rift as asthenosphere rises to accommodate the thinning lithosphere and
melts. 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 process
has occurred.
Continental rock is not dense enough to subduct beneath an overriding, opposed
continental plate and will thus collide, suturing together with the adjacent plate, folding the
rocks in the zone of collision, and thickening the crust locally, to form a nonvolcanic
mountain 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 descending
slabs at subduction zones, because old oceanic lithosphere is generally denser than the
asthenosphere 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 a
plate boundary). Absolute plate velocity is calculated using age and distance data from the
material on a plate, with the distance calculated from a hot spot or other fixed point of
reference on Earth’s surface.
The Way the Earth Works: Plate Tectonics | 17
On Further Thought
1. Why are the marine magnetic anomalies bordering the East Pacific Rise in the
southeastern Pacific Ocean wider than marine magnetic anomalies bordering the MidAtlantic Ridge in the South Atlantic Ocean?
The East Pacific Rise is spreading faster, so it produces a greater width of basalt in the
time intervals between polarity reversals.
2. The Pacific Plate moves north relative to the North American Plate at a rate of 6 cm per
year. How long will it take Los Angeles (a city on the Pacific Plate) to move northwards by
480 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 with
respect 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 in
mind, 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 Sea
formed from basalt eruptions associated with the rifting and spreading. When this occurs
behind 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, glacial
deposits, 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 Cenozoic
B. aligned east to west along the equator during the late Mesozoic through the
Cenozoic
C. combined to form a supercontinent (he termed Rodinia) in the Proterozoic
D. 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 America
B. southern Africa
D. South America
3. Abundant swamps led to the formation of coal during the Late Paleozoic in which of
the following places?
A. India
C. North America
B. southern Africa
D. Antarctica
18 | Chapter 2
4. Which plant genus dominated glaciated regions during the late Paleozoic and early
Mesozoic?
A. Ginkgo
C. Neuropteris
B. 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 them
B. he could not conceive of a valid mechanism that would cause continents to shift
positions
C. he had relatively little evidence supporting the existence of a supercontinent
D. 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 drift
B. agree that continental drift occurs, but they still do not understand why it occurs
C. 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 time
D. 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’s
B. known to have been constant through geologic time, due to remnant magnetization
of iron-rich minerals in rocks
C. known to have experienced numerous polarity reversals, due to remnant
magnetization of iron-rich minerals in rocks
D. 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 over
time is termed ____________.
A. dipole
C. magnetic inclination
B. magnetic declination
D. polar wander
9. The apparent polar-wander paths for continents that were not connected over some
span of geologic history will likely ____________ concerning the positions of the ancient
magnetic 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 shelves
B. along mid-ocean ridges
D. along fracture zones
The Way the Earth Works: Plate Tectonics | 19
11. Within the sea floor, the rate of heat flow is greatest ____________.
A. along mid-ocean ridges
C. at the edges of ocean basins
B. along fracture zones
D. in the center of abyssal plains
12. Regions of the sea floor with positive magnetic anomalies were formed during times
when Earth’s magnetic field ____________.
A. was exceptionally strong
C. had normal polarity
B. was exceptionally weak
D. had reversed polarity
13. Regions of the sea floor with negative magnetic anomalies were formed during times
when Earth’s magnetic field ____________.
A. was exceptionally strong
C. had normal polarity
B. was exceptionally weak
D. had reversed polarity
14. Marine magnetic anomaly belts run parallel to ____________.
A. mid-ocean ridges
C. continental coastlines
B. fracture zones
D. continental shelves
15. Marine magnetic anomaly belts are widest when and where ____________.
A. continents are joined to form supercontinents
B. sea-floor spreading rates are relatively rapid
C. sea-floor spreading rates are relatively slow
16. The age of oceanic crust ____________ with increasing distance from a mid-ocean
ridge.
A. increases
B. decreases
17. Wegener’s evidence for a united Pangaea was so compelling that virtually all
geologists 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 but
reappear on the continent of ____________.
A. Africa
C. North America
B. Europe
D. Australia
19. If we mentally align the continents to fit Wegener’s concept of Pangaea, evidence of
late Paleozoic glacial deposits ____________.
A. is more difficult to explain than in the modern continental configuration
B. is much more readily explained than in the modern continental configuration
C. makes very little sense in either the Pangaea configuration or the modern
configuration
20 | Chapter 2
20. The apparent polar-wander path obtained from magnetite crystals in basalts on the
North American continent is now interpreted to be the result of ____________.
A. wandering of the geomagnetic north pole
B. 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 shale
B. gabbro and basalt
D. slate and gneiss
23. All basalts younger than 700,000 years old ____________.
A. have normal magnetic polarity
B. have reverse magnetic polarity
C. are found on the ocean floor very far from mid-ocean ridges
D. are found on the continents
24. Marine magnetic anomalies result from sea-floor spreading in conjunction with
____________.
A. global warming
B. magnetic storms on the surface of the Sun
C. magnetic polarity reversals
D. apparent wander of the magnetic poles
25. The oldest sediments on the ocean floor are about ____________ years old.
A. 50 thousand
C. 200 million
B. 4 billion
D. 2.5 million
26. The primary difference between lithospheric and asthenospheric mantle that gives rise
to 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 spreading
B. incorporates sea-floor spreading but not continental drift
C. incorporates and explains both sea-floor spreading and continental drift
D. does not incorporate sea-floor spreading or continental drift
The Way the Earth Works: Plate Tectonics | 21
28. Unlike the lithosphere, the asthenosphere ____________.
A. is relatively weak and flows readily
B. has a density similar to the core
C. varies in thickness from place to place
D. is relatively cool
29. Continental lithosphere ____________.
A. is thicker than oceanic lithosphere
B. contains more mafic rocks than oceanic lithosphere
C. is denser than oceanic lithosphere
D. contains no crustal material, consisting solely of lithified upper mantle
30. The average thickness of continental lithosphere is about ____________.
A. 30 km
C. 150 km
B. 60 km
D. 10,000 km
31. The thickness of oceanic lithosphere is ____________.
A. uniformly 100 km
B. greatest at the geographic poles and least near the equator
C. greatest near the mid-ocean ridges and thins out away from the ridges
D. 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 another
B. discrete layers of lithosphere that are vertically stacked one atop the other
C. composed only of continental rocks, which plow through the weaker oceanic rocks
D. 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 volcano
C. a continental coastline that coincides with a plate boundary
D. 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 margins
B. 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 only
B. oceanic lithosphere only
C. oceanic or continental lithosphere or a combination of both
D. either oceanic or continental lithosphere, but not both
37. Deformed (bent, stretched, or cracked) lithosphere occurs ____________.
A. randomly over the surface of Earth
B. primarily within the interiors of tectonic plates
C. primarily on the margins of tectonic plates
38. Every plate boundary can be recognized by ____________.
A. the presence of active volcanoes
B. the presence of an earthquake belt
C. a deep chasm which can be seen from space
D. 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/year
B. 1 to 15 cm/year
D. 10 to 100 m/year
40. At a divergent plate boundary, two opposed plates ____________.
A. move toward one another
B. move away from one another
C. slide past one another
41. At a convergent plate boundary, two opposed plates ____________.
A. move toward one another
B. move away from one another
C. slide past one another
42. At a transform plate boundary, two opposed plates ____________.
A. move toward one another
B. move away from one another
C. slide past one another
43. Mid-ocean ridges are ____________.
A. convergent plate boundaries
B. divergent plate boundaries
C. transform plate boundaries
44. As compared to a slowly spreading mid-ocean ridge, a rapidly spreading ridge is
____________.
A. wider
B. narrower
C. more silicic in lava composition
The Way the Earth Works: Plate Tectonics | 23
45. All lithospheric plates are approximately the same size and contain a combination of
oceanic and continental crust.
A. true
B. false
46. The youngest sea floor occurs ____________.
A. along passive margins
C. along mid-ocean ridges
B. 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 volcanism
B. the addition of new crust due to sedimentation
C. the addition of new lithospheric mantle as a result of cooling
D. reasons that geologists cannot determine at present
48. Subduction zones are ____________.
A. convergent plate boundaries
B. divergent plate boundaries
C. transform plate boundaries
49. At a subduction zone, the overriding plate ____________.
A. is always composed of continental lithosphere
B. is always composed of oceanic lithosphere
C. may be composed of either oceanic or continental lithosphere
50. At a subduction zone, the downgoing (subducting) plate ____________.
A. is always composed of continental lithosphere
B. is always composed of oceanic lithosphere
C. 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 interior
B. within an overriding plate at a subduction zone
C. within a downgoing plate at a subduction zone
D. along mid-ocean ridges
52. The Wadati-Benioff zone extends down within the mantle to a maximum depth of
____________.
A. 30 km
C. 670 km
B. 150 km
D. 990 km
53. At transform plate boundaries ____________.
A. earthquakes are common, but volcanoes are absent
B. volcanoes are common, but earthquakes do not occur
C. 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 cluster
B. glacial ice, continental rocks, and the ocean can be found together
C. the boundaries of three lithospheric plates meet at a single point
D. 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 density
B. the lithospheric plates are thickest at the ridges so they stand up taller
C. rising ocean currents leave a vacuum above the ridge
D. 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 arc
B. mid-ocean ridge volcanism
D. a transform margin
57. Segments of the mid-ocean ridge system are offset. Between the offset segments we
observe ____________.
A. a second series of ridges, perpendicular to the main set
B. deep-ocean trenches
C. transform faults
D. None of the above is correct.
58. When two bodies of continental lithosphere are pushed together at a convergent
boundary, 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 zones
B. 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 zones
B. at subduction zones
D. in the interiors of continental plates