Plate Tectonics A Scientific Reolution Unfolds

C H A P T E R F I F T E E N
15
Plate Tectonics: A Scientific
Revolution Unfolds
CONCEPTS
To assist you in learning
P LATE TECTONICS IS THE the important concepts in this

chapter, focus on the following
questions:
FIRST THEORY TO PROVIDE
What evidence was used to
A COMPREHENSIVE VIEW support the continental drift
hypothesis?
of the processes that produced What was one of the main
objections to the continental
drift hypothesis?
Earth’s major surface features, What is the theory of plate
tectonics?
including the continents and ocean In what major way does the plate
tectonics theory depart from the
basins. Within the framework of continental drift hypothesis?
What are the three types of plate
this theory, geologists have found boundaries?
Where does new lithosphere
explanations for the basic causes form?
How do mountain systems such
ON
and distribution of earthquakes, as the Himalayas form?
What type of plate motion
volcanoes, and mountain belts. occurs along a transform fault
boundary?
Further, we are now better able to What evidence is used to
support the plate tectonics
explain the distribution of plants theory?
What are the major driving
and animals in the geologic past, as forces for plate tectonics?
What models have been
FOCUS
well as the distribution of proposed to explain the driving
mechanism for plate motion?
economically significant mineral

deposits.
This hiker is walking across glacial debris in Pakistan’s

rugged and remote Karakoram Range, a part of the
Himalayas. (Photo by Bill Stevenson)
362 CHAPTER 15 Plate Tectonics: A Scientific Revolution Unfolds
From Continental chains (FIGURE 15.1). Furthermore,

landmasses occasionally split apart. As the
early in the 20th century as a relatively
straightforward proposal called continental
Drift To Plate continental blocks separate, a new ocean
basin emerges between them. Meanwhile,
drift. For more than 50 years the idea that
continents were capable of movement was
Tectonics other portions of the seafloor plunge into categorically rejected by the scientific estab-
Prior to the 1960s most geologists held the the mantle. In short, a dramatically lishment. Continental drift was particularly
view that the ocean basins and continents different model of Earth’s tectonic distasteful to North American geologists,
had fixed geographic positions and were of processes emerged.* perhaps because much of the supporting
great antiquity. Less than a decade later This profound reversal in scientific evidence had been gathered from the
researchers came to realize that Earth’s con- thought has been appropriately described as continents of Africa, South America, and
tinents are not static, instead they gradually a scientific revolution. The revolution began Australia, with which most North American
migrate across the globe. Because of these geologists were unfamiliar.
movements, blocks of continental material *Tectonic processes are those that deform Earth’s Following World War II, modern
collide, deforming the intervening crust, crust to create major structural features such as instruments replaced rock hammers as the
thereby creating Earth’s great mountain mountains, continents, and ocean basins. tools of choice for many researchers.
FIGURE 15.1 Climbers camping on a sheer rock face of a mountain known as K7 in Pakistan’s
Karakoram, a part of the Himalayas. These mountains formed as India collided with Eurasia.
(Photo by Jimmy Chin/National Geographic Stock)
Continental Drift: An Idea before Its Time 363
Armed with these more advanced tools, FIGURE 15.2 Reconstructions of

geologists and a new breed of researchers, Pangaea as it is thought
including geophysicists and geochemists, to have appeared 200
million years ago.
made several surprising discoveries that A. Modern Tethys Sea
began to rekindle interest in the drift
P A
reconstruction.
hypothesis. By 1968 these developments B. Wegener’s
led to the unfolding of a far more encom- reconstruction
N
passing explanation known as the theory of redrawn from
G
plate tectonics. his book
A
In this chapter, we will examine the published in
E
1915. A
events that led to this dramatic reversal of
scientific opinion in an attempt to provide
insight into how science works. We will
also briefly trace the development of the A. Modern reconstruction of Pangaea
continental drift hypothesis, examine why it
was first rejected, and consider the evi-
dence that finally led to the acceptance of
its direct descendant—the theory of plate
tectonics.
B. Wegener’s Pangaea
CONCEPT CHECK 15.1
200 million years ago, during the early part
1 Briefly contrast the view held by most of the Mesozoic era, this supercontinent DID YOU KNOW?
geologists regarding ocean basins and began to fragment into smaller landmasses. Although Alfred Wegener is rightfully
continents prior to the 1960s with their These continental blocks then “drifted” to credited with formulating the
perspective a decade later. continental drift hypothesis, he was not
their present positions over a span of
the first to suggest continental mobility.
millions of years. The inspiration for
An American geologist, F. B. Taylor,
continental drift is believed to have come published the first paper to outline this
Continental Drift: to Wegener when he observed the break-up
of sea ice during a Danish-led expedition
important idea. However, Taylor’s paper
provided little supporting evidence,
An Idea before to Greenland.
Wegener and others who advocated the
whereas Wegener spent much of his
professional life trying to substantiate
Its Time continental drift hypothesis collected sub- his views.
The idea that continents, particularly stantial evidence to support their point of
South America and Africa, fit together view. The fit of South America and Africa
like pieces of a jigsaw puzzle came and the geographic distribution of fossils and ancient climates all seemed to buttress the
about during the 1600s as better world idea that these now separate landmasses were once joined. Let us examine some of this
maps became available. However, little evidence.
significance was given to this notion until
1915, when Alfred Wegener (1880–1930), Evidence: The Continental Jigsaw Puzzle
a German meteorologist and geophysicist, Like a few others before him, Wegener suspected that the continents might once have been
wrote The Origin of Continents and joined when he noticed the remarkable similarity between the coastlines on opposite sides
Oceans. This book, published in several of the Atlantic Ocean. However, Wegener’s use of present-day shorelines to fit these conti-
editions, set forth the basic outline of nents together was challenged immediately by other Earth scientists. These opponents
Wegener’s hypothesis called continental correctly argued that shorelines are continually modified by wave erosion and depositional
drift—which dared to challenge the long- processes. Even if continental displacement had taken place, a good fit today would be
held assumption that the continents unlikely. Because Wegener’s original jigsaw fit of the continents was crude, it is assumed
and ocean basins had fixed geographic that he was aware of this problem (see FIGURE 15.2B).
positions. Scientists later determined that a much better approximation of the outer boundary of a
Wegener suggested that a single continent is the seaward edge of its continental shelf, which lies submerged a few hundred
supercontinent consisting of all Earth’s
landmasses once existed.* He named this
giant landmass Pangaea (pronounced Pan- *Wegener was not the first person to conceive of a long-vanished supercontinent. Edward Suess
jee-ah; meaning “all lands”) (FIGURE 15.2). (1831–1914), a distinguished 19th-century geologist, pieced together evidence for a giant landmass consisting
of the continents of South America, Africa, India, and Australia.
Wegener further hypothesized that about
meters below sea level. In the early 1960s, that identical fossil organisms had been discovered in rocks from both South America
Sir Edward Bullard and two associates and Africa that his pursuit of continental drift became more focused. Through a review of
constructed a map that pieced together the the literature, Wegener learned that most paleontologists (scientists who study the fossilized
edges of the continental shelves of South remains of ancient organisms) were in agreement that some type of land connection was
America and Africa at a depth of about needed to explain the existence of similar Mesozoic age life forms on widely separated land-
900 meters (FIGURE 15.3). The remarkable masses. Just as modern life forms native to North America are quite different from those of
fit that was obtained was more precise than Africa and Australia, one would expect that during the Mesozoic era, organisms on widely
even these researchers had expected. As separated continents would be distinct.
shown in Figure 15.3 there are a few places
MESOSAURUS. To add credibility to his argument, Wegener documented cases of several
where the continents overlap. Some of
fossil organisms that were found on different landmasses despite the unlikely possibility
these overlaps are related to the process of
that their living forms could have crossed the vast ocean presently separating them (FIGURE
stretching and thinning of the continental
15.4). A classic example is Mesosaurus, an aquatic fish-catching reptile whose fossil remains
margins as they drifted apart. Others can be
are limited to black shales of the Permian period (about 260 million years ago) in eastern
explained by the work of major river sys-
South America and southwestern Africa. If Mesosaurus had been able to make the long
tems. For example, since the break-up of
journey across the South Atlantic, its remains would likely be more widely distributed.
Pangaea the Niger River has built an exten-
As this is not the case, Wegener asserted that South America and Africa must have been
sive delta that enlarged the continental
joined during that period of Earth history.
shelf of Africa.
How did opponents of continental drift explain the existence of identical fossil organ-
isms in places separated by thousands of kilometers of open ocean? Rafting, transoceanic
land bridges (isthmian links), and island stepping stones were the most widely invoked
Evidence: Fossils Match explanations for these migrations (FIGURE 15.5). We know, for example, that during the Ice
across the Seas Age that ended about 8,000 years ago the lowering of sea level allowed mammals (includ-
Although the seed for Wegener’s hypothesis ing humans) to cross the narrow Bering Strait that separates Russia and Alaska. Was it pos-
came from the remarkable similarities of sible that land bridges once connected Africa and South America but later subsided below
the continental margins on opposite sides sea level? Modern maps of the seafloor substantiate Wegener’s contention that if land
of the Atlantic, it was when he learned bridges of this magnitude once existed, their remnants would still lie below sea level.
Continental
shelf
C. Lystrosaurus
r AFRICA
Equato
Modern
Mo Mesosaurus Lystrosaurus
der
nE
SOUTH AMERICA qua
tor
Glossopteris
A. Mesosaurus B. Glossopteris
Overlap
FIGURE 15.4 Fossil evidence supporting continental drift. A. Fossils of Mesosaurus are
found only in nonmarine deposits in eastern South America and western Africa.
Mesosaurus was a freshwater reptile incapable of swimming the 5000 kilometers of
open ocean that now separate these continents. B. Remains of Glossopteris and
FIGURE 15.3 Drawing that shows the best fit of South America and related flora are found in Australia, Africa, South America, Antarctica, and India,
Africa along the continental slope at a depth of 500 fathoms (about landmasses which currently have quite varied climates. However, when Glossopteris
900 meters). The areas where continental blocks overlap appear in inhabited these regions during the late Paleozoic era, their climates were all subpolar.
orange. (After A. G. Smith, “Continental Drift,” in Understanding the C. Fossils of Lystrosaurus, a land-dwelling reptile, are also found on three of these
Earth, edited by I. G. Gass.) landmasses.
Continental Drift: An Idea before Its Time 365
DID YOU KNOW?

Alfred Wegener, best known for his
continental drift hypothesis, also wrote
numerous scientific papers on weather
and climate. Because of his interest in
meteorology, Wegener made four
extended trips to the Greenland ice
sheet in order to study its harsh winter
weather. In November of 1930, while
making a month-long trek across the ice
sheet, Wegener and a companion
perished.
GLOSSOPTERIS. Wegener also cited the

distribution of the fossil “seed fern”
Glossopteris as evidence for the existence
of Pangaea (see Figure 15.4). This plant, FIGURE 15.5 These sketches by John Holden illustrate various explanations for the
identified by its tongue-shaped leaves occurrence of similar species on landmasses that are presently separated by vast
and seeds that were too large to be carried oceans. (Reprinted with permission of John Holden)
by the wind, was known to be widely
dispersed among Africa, Australia, India, continent. Wegener found evidence of tain belt that includes the Appalachians
and South America. Later, fossil remains 2.2-billion-year-old igneous rocks in Brazil trends northeastward through the
of Glossopteris were also discovered in that closely resembled similarly aged eastern United States and disappears off
Antarctica.* Wegener also learned that rocks in Africa. the coast of Newfoundland (FIGURE 15.6A).
these seed ferns and associated flora grew Similar evidence can be found in Mountains of comparable age and structure
only in a subpolar climate. Therefore, he mountain belts that terminate at one are found in the British Isles, and
concluded that when these landmasses coastline, only to reappear on landmasses Scandinavia. When these landmasses are
were joined, they were located much across the ocean. For instance, the moun- reassembled, as in FIGURE 15.6B, the
closer to the South Pole. mountain chains form a nearly
Caledonian continuous belt.
Mountains Wegener described how the
Evidence: Rock Types similarities in geologic features
and Geologic Features North British Scandinavia on both sides of the Atlantic
Anyone who has worked a jigsaw puzzle America Isles
knows that its successful completion
requires that you fit the pieces together Appalachian
Mountains
while maintaining the continuity of the Africa
picture. The “picture” that must match in
the “continental drift puzzle” is one of
rock types and geologic features such as Greenland
South Europe
mountain belts. If the continents were once America
together, the rocks found in a particular
region on one continent should closely
match in age and type those found in A.
adjacent positions on the once adjoining FIGURE 15.6 Matching mountain ranges across
North
the North Atlantic. A. The Appalachian America
Africa
Mountains trend along the eastern flank of
*In 1912 Captain Robert Scott and two companions North America and disappear off the coast of
froze to death lying beside 35 pounds of rock on Newfoundland. Mountains of comparable age South
their return from a failed attempt to be the first to and structure are found in the British Isles and America
B.
reach the South Pole. These samples, collected on Scandinavia. B. When these landmasses are
the moraines of Beardmore Glacier, contained fossil placed in their pre-drift locations, these ancient
remains of Glossopteris. mountain chains form a nearly continuous belt.
Evidence: Ancient Climates swamps existed in several locations in the

DID YOU KNOW? Because Alfred Wegener was a student of Northern Hemisphere. The lush vegetation
A group of scientists proposed an
world climates, he suspected that paleocli- in these swamps was eventually buried and
interesting although incorrect
explanation for the cause of continental matic (paleo = ancient, climatic = climate) converted to coal. Today these deposits
drift. Their proposal suggested that data might also support the idea of comprise major coal fields in the eastern
early in Earth’s history, our planet was mobile continents. His assertion was United States, Northern Europe, and Asia.
only about half its current diameter and bolstered when he learned that evidence Many of the fossils found in these coal-
completely covered by continental for a glacial period that dated to the late bearing rocks were produced by tree ferns
crust. Through time Earth expanded, Paleozoic had been discovered in southern that possessed large fronds; a fact consis-
causing the continents to split into their
Africa, South America, Australia, and tent with a warm, moist climate. Further-
current configurations, while new
seafloor “filled in” the spaces as they India (FIGURE 15.7A). This meant that more, these fern trees lacked growth rings,
drifted apart. about 300 million years ago, vast ice a characteristic of tropical plants that grow
sheets covered extensive portions of the in regions having minimal yearly fluctua-
Southern Hemisphere as well as India tions in temperature. By contrast, trees
(FIGURE 15.7B). Much of the land area that inhabit the middle latitudes, like
linked these landmasses when he said, “It is that contains evidence of this period of those found in most of the United States,
just as if we were to refit the torn pieces of Paleozoic glaciation presently lies within develop multiple tree rings—one for each
a newspaper by matching their edges and 30 degrees of the equator in subtropical or growing season.
then check whether the lines of print run tropical climates. Wegener suggested that a more plausi-
smoothly across. If they do, there is noth- How could extensive ice sheets form ble explanation for the late Paleozoic glacia-
ing left but to conclude that the pieces near the Equator? One proposal suggested tion was provided by the supercontinent of
were in fact joined in this way.”* that our planet experienced a period of Pangaea. In this configuration the southern
*Alfred Wegener, The Origin of Continents and extreme global cooling. Wegener rejected continents are joined together and located
Oceans, translated from the 4th revised German ed. this explanation because during the same near the South Pole (FIGURE 15.7C). This
of 1929 by J. Birman (London: Methuen, 1966). span of geologic time, large tropical would account for the conditions necessary
FIGURE 15.7 Paleoclimatic evidence for continental drift. A. Glacial

striations (scratches) and grooves like these are produced as glaciers
drag rock debris across the underlying bedrock. The direction of
glacial movement can be deduced from the distinctive patterns of
aligned scratches and grooves. (Photo by Gregory S. Springer) B. Near
the end of the Paleozoic era (about 300 million years ago) ice sheets
covered extensive areas of the Southern Hemisphere and India.
Arrows show the direction of ice movement that can be inferred from
the pattern of glacial striations and grooves found in the bedrock.
C. The continents restored to their pre-drift positions when they
were part of Pangaea. This configuration accounts for the conditions
necessary to generate a vast ice sheet and also explains the
directions of ice movement that radiated away from an area near
the present position of the South Pole.
A.
India
Africa India
Africa
South
America Australia
Australia
Antarctica
South
America
Antarctica
B. C.
The Great Debate 367
to generate extensive expanses of glacial ice over much of these landmasses. At the same
time, this geography would place today’s northern continents nearer the equator and The Great Debate
account for the tropical swamps that generated the vast coal deposits. Wegener was so con- Wegener’s proposal did not attract much
vinced that his explanation was correct that he wrote, “This evidence is so compelling that open criticism until 1924, when his book
by comparison all other criteria must take a back seat.” was translated into English, French, Span-
How does a glacier develop in hot, arid central Australia? How do land animals migrate ish, and Russian. From that point until his
across wide expanses of the ocean? As compelling as this evidence may have been, 50 years death in 1930, the drift hypothesis encoun-
passed before most of the scientific community accepted the concept of continental drift tered a great deal of hostile criticism. The
and the logical conclusions to which it led. respected American geologist R.T. Cham-
berlain stated, “Wegener’s hypothesis in
CONCEPT CHECK 15.2 general is of the foot-loose type, in that it
takes considerable liberty with our globe,
1 What was the first line of evidence that led early investigators to suspect the continents were and is less bound by restrictions or tied
once connected? down by awkward, ugly facts than most of
2 Describe the four kinds of evidence that Wegener and his supporters gathered to substantiate its rival theories. Its appeal seems to lie in
the continental drift hypothesis. the fact that it plays a game in which there
3 Explain why the discovery of the fossil remains of Mesosaurus in both South America and are few restrictive rules and no sharply
Africa, but nowhere else, supports the continental drift hypothesis. drawn code of conduct.”
4 Early in the 20th century, what was the prevailing view of how land animals migrated across One of the main objections to
vast expanses of open ocean? Wegener’s hypothesis stemmed from his
5 How did Wegener account for the existence of glaciers in the southern landmasses at a time inability to identify a credible mechanism
when areas in North America, Europe, and Asia supported lush tropical swamps? for continental drift. Wegener proposed
The movement of Earth’s tectonic plates is the cause of our most destructive earthquakes. Pisco, Peru,
following a powerful earthquake on August 16, 2007. (Sergio Erday/epa/Corbis)
FIGURE 15.8 Illustration of Earth’s major CONCEPT CHECK 15.3

Eurasian
plate lithospheric plates.
North 1
American To which two aspects of Wegener’s conti-
plate Arabian nental drift hypothesis did most Earth
plate scientists object?
Caribbean African
plate plate
Cocos
plate Plate Tectonics
GEODe
North PLATE TECTONICS

South American plate ESSENTIALS Introduction
American OF GEOLOGY
plate
Nazca Eurasian Following World War II, oceanographers
plate plate
equipped with new marine tools and ample
funding from the U.S. Office of Naval
Research embarked on an unprecedented
Philippine
plate period of oceanographic exploration. Over
Scotia plate Pacific the next two decades a much better picture
Antarctic plate plate
of large expanses of the seafloor slowly and
painstakingly began to emerge. From this
work came the discovery of a global
that gravitational forces of the oceanic ridge system that winds through
Moon and Sun that produce Australian-Indian all of the major oceans in a manner similar
plate
Earth’s tides were also capable of to the seams on a baseball.
gradually moving the continents In other parts of the ocean, new
across the globe. However, the promi- discoveries were also being made. Earth-
nent physicist Harold Jeffreys correctly quake studies conducted in the western
countered that tidal forces of the Antarctic Pacific demonstrated that tectonic
plate
magnitude needed to displace the activity was occurring at great depths
continents would bring Earth’s rotation beneath deep-ocean trenches. Of equal
to a halt in a matter of a few years. importance was the fact that dredging of
Wegener also incorrectly suggested that the larger and sturdier continents broke the seafloor did not bring up any oceanic
through thinner oceanic crust, much like ice breakers cut through ice. However, no crust that was older than 180 million years.
evidence existed to suggest that the ocean floor was weak enough to permit passage of Further, sediment accumulations in the
the continents without the continents being appreciably deformed in the process. deep-ocean basins were found to be thin,
In 1930 Wegener made his fourth and final trip to the Greenland ice sheet. Although not the thousands of meters that were
the primary focus of this expedition was to study the harsh winter polar climate on the predicted.
ice-covered island, Wegener continued to test his continental drift hypothesis. As in earlier By 1968, these developments, among
expeditions, he used astronomical methods in an attempt to verify that Greenland had others, led to the unfolding of a far more
drifted westward with respect to Europe. While returning from Eismitte (an experimental encompassing theory than continental
station located in the center of Greenland), Wegener perished along with a companion. drift, known as plate tectonics. According
His intriguing idea, however, did not die. to the plate tectonics model, the
What went wrong? Why was Wegener unable to overturn the established scientific uppermost mantle and the overlying crust
views of his day? Foremost was the fact that, although the central theme of Wegener’s drift behave as a strong, rigid layer, known as
hypothesis was correct, it contained some incorrect details. For example, continents do not the lithosphere, which is broken into
break through the ocean floor, and tidal energy is much too weak to cause continents to be segments commonly referred to as plates
displaced. Moreover, in order for any comprehensive scientific theory to gain wide accept- (FIGURE 15.8). The lithosphere is thinnest
ance, it must stand up to critical testing from all areas of science. Wegener’s great contribu- in the oceans where it varies from as little
tion to our understanding of Earth notwithstanding, not all of the evidence supported the as a few kilometers along the axis of the
continental drift hypothesis as he had formulated it. oceanic ridge system to about 100 kilome-
Although many of Wegener’s contemporaries opposed his views, even to the point of ters (60 miles) in the deep-ocean basins.
open ridicule, some considered his ideas plausible. For those geologists who continued the By contrast, continental lithosphere is
search, the exciting concept of continents adrift held their interest. Others viewed continen- generally thicker than 100 kilometers
tal drift as a solution to previously unexplainable observations. Nevertheless, most of the and may extend to a depth of 200 to
scientific community, particularly in North America, either categorically rejected continen- 300 kilometers beneath stable continental
tal drift or at least treated it with considerable skepticism. cratons.
Divergent Boundaries 369
The lithosphere, in turn, overlies a between two locations on different plates,

weak region in the mantle known as the such as New York and London, gradually Divergent Boundaries
asthenosphere. The temperatures and changes whereas the distance between GEODe
PLATE TECTONICS
pressures in the upper asthenosphere sites on the same plate—New York and ESSENTIALS Divergent Boundaries
OF GEOLOGY
(100 to 200 kilometers in depth) are such Denver, for example—remains relatively
Most divergent boundaries are located
that the rocks there are very near their constant.
along the crests of oceanic ridges and can
melting temperatures and, hence, Because plates are in constant
be thought of as constructive plate margins
respond to stress by flowing. As a result, motion relative to each other, most major
because this is where new ocean floor is
Earth’s rigid outer shell is effectively interactions among them (and, therefore,
generated (FIGURE 15.10). Divergent bound-
detached from the layers below, which most deformation) occur along their
aries are also called spreading centers,
permits it to move independently. boundaries. In fact, plate boundaries were
because seafloor spreading occurs at these
first established by plotting the locations
boundaries. Here, two adjacent plates are
of earthquakes and volcanoes. Plates
moving away from each other, producing
Earth’s Major Plates are bounded by three distinct types of
long, narrow fractures in the ocean crust.
The lithosphere is composed of about boundaries, which are differentiated by
As a result, hot rock from the mantle below
two dozen segments having irregular the type of movement they exhibit.
migrates upward to fill the voids left as the
sizes and shapes called lithospheric plates These boundaries are depicted at the
crust is being ripped apart. This molten
or tectonic plates that are in constant bottom of Figure 15.9 and are briefly
material gradually cools to produce new
motion with respect to one another. As described here:
slivers of seafloor. In a slow, yet unending
shown in FIGURE 15.9, seven major litho- 1. Divergent boundaries (constructive manner, adjacent plates spread apart and
spheric plates are recognized. These margins)—where two plates move new oceanic lithosphere forms between
plates, which account for 94 percent of apart, resulting in upwelling of hot them.
Earth’s surface area, include the North material from the mantle to create
American, South American, Pacific, African, new seafloor (FIGURE 15.9A).
Eurasian, Australian-Indian, and Antarctic
2. Convergent boundaries (destructive
Oceanic Ridges
plates. The largest is the Pacific plate, and Seafloor Spreading
margins)—where two plates move
which encompasses a significant portion Most divergent plate boundaries are associ-
together, resulting in oceanic
of the Pacific Ocean basin. The six other lithosphere descending beneath an ated with oceanic ridges: elevated areas of
large plates include an entire continent overriding plate, eventually to be the seafloor that are characterized by high
plus a significant amount of ocean floor. reabsorbed into the mantle or possibly heat flow and volcanism. The global ridge
Notice in Figure 15.9 that the South in the collision of two continental system is the longest topographic feature
American plate encompasses almost all of blocks to create a mountain system on Earth’s surface, exceeding 70,000 kilo-
South America and about one half of the (FIGURE 15.9B). meters (43,000 miles) in length. As shown
floor of the South Atlantic. This is a 3. Transform fault boundaries in Figure 15.9 various segments of the
major departure from Wegener’s conti- (conservative margins)—where two global ridge system have been named,
nental drift hypothesis, which proposed plates grind past each other without including the Mid-Atlantic Ridge, East
that the continents moved through the the production or destruction of Pacific Rise, and Mid-Indian Ridge.
ocean floor, not with it. Note also that lithosphere (FIGURE 15.9C). Representing 20 percent of Earth’s
none of the plates are defined entirely by surface, the oceanic ridge system winds
the margins of a single continent. Divergent and convergent plate bound- through all major ocean basins like the
Intermediate-sized plates include the aries each account for about 40 percent of seam on a baseball. Although the crest
Caribbean, Nazca, Philippine, Arabian, Cocos, all plate boundaries. Transform faults of the oceanic ridge is commonly 2 to
Scotia, and Juan de Fuca plates. These plates, account for the remaining 20 percent. In 3 kilometers higher than the adjacent ocean
with the exception of the Arabian plate, are the following sections we will summarize
composed mostly of oceanic lithosphere. In the nature of the three types of plate
addition, there are several smaller plates boundaries.
(microplates) that have been identified but DID YOU KNOW?
are not shown in Figure 15.9. CONCEPT CHECK 15.4 An observer on another planet would
notice, after only a few million years,
1 Compare and contrast the lithosphere and that all the continents and ocean basins
on Earth are indeed moving. The
Plate Boundaries the asthenosphere.
Moon, on the other hand, is tectonically
2 List the three types of plate boundaries,
One of the main tenets of the plate dead, so it would look virtually
tectonics theory is that plates move as and describe the relative motion at each
unchanged millions of years into the
of them.
semicoherent units relative to all other future.
plates. As plates move, the distance
FIGURE 15.9 A mosaic of rigid

plates constitutes Earth’s outer
shell. (After W. B. Hamilton, U.S.
Geological Survey)
North American
basins, the term “ridge” plate
Urals
may be misleading Aleutian Arc
because this feature is not Eurasian plate
narrow but has widths Baikal Rift
that vary from 1000 to
more than 4000 kilome- Japan Arc
ters. Further, along the
axis of some ridge seg- Himalayas
ments is a deep down- Em
per
or-H
faulted structure called a awa
iian
Arabian Cha
rift valley. This structure is plate India in
Philippine Mariana Arc
evidence that tensional plate
forces are actively pulling M
the ocean crust apart at East id Pacific
African plate
-In
the ridge crest. Rift

dia
The mechanism that

n Ridge
operates along the oceanic

ridge system to create new African plate
seafloor is appropriately
called seafloor spreading.
Typical rates of spreading Australian-Indian plate Tonga
ou
S
average around 5 centime- ge t Arc

id he
ters (2 inches) per year. R ast
This is roughly the same d ian Ind Kermadec
n
rate at which human fin- st I ian Arc
w e Ridg
gernails grow. Compara- th e
ou
tively slow spreading rates
S
Alpine Fault
of 2 centimeters per year
are found along the Mid-
Atlantic Ridge, whereas Antarctic plate
spreading rates exceeding
15 centimeters (6 inches)
per year have been meas-
ured along sections of the
East Pacific Rise. Although
these rates of seafloor
production are slow on a
human time scale, they are nevertheless rapid enough to have generated all of
Earth’s ocean basins within the last 200 million years. In fact, none of the ocean Oceanic
floor that has been dated thus far exceeds 180 million years in age. lithosphere
Melting
The primary reason for the elevated position of the oceanic ridge is that newly
created oceanic crust is hot, making it less dense than cooler rocks found away Asthenosphere
from the ridge axis. As soon as new lithosphere forms, it is slowly yet continually
displaced away from the zone of upwelling. Thus, it begins to cool and contract, A. Divergent boundary
thereby increasing in density. This thermal contraction accounts for the increase in
ocean depths away from the ridge crest. It takes about 80 million years for the tem-
perature of the crust to stabilize and contraction to cease. By this time, rock that was once In addition, cooling strengthens the hot
part of the elevated oceanic ridge system is located in the deep-ocean basin, where it may material directly below the oceanic crust,
be buried by substantial accumulations of sediment. thereby adding to the plate’s thickness.
Divergent Boundaries 371
North American
plate
Iceland Eurasian plate
Canadian Shield Continental
Ro
Rifting
ck
y
Alps
M
ts. Divergent boundaries can

ou
Basin
nM
nt
and
ia also develop within a
ai n
Range
ach
s
pa
l continent, in which case
Juan de Fuca Ap the landmass may split
plate
San Andreas into two or more smaller
Fault segments separated by an
ocean basin. Continental
African plate rifting occurs where
Mi
Caribbean plate
d-
At opposing tectonic forces
Cocos lan
tic act to pull the litho-
plate Antilles Rid
ge sphere apart. The initial
Arc
Pacific plate Galapagos stage of rifting tends to
Ridge include mantle upwelling
An South American
de that is associated with
plate
East Pacific Rise
sM
o broad upwarping of the
un
Nazca overlying lithosphere

tains
plate (FIGURE 15.11A). As a

result, the lithosphere is
stretched, causing the
Chile Ridge
brittle crustal rocks to
break into large slabs. As
the tectonic forces con-
tinue to pull the crust
apart, these crustal frag-
Scotia plate ments sink, generating an
elongated depression
called a continental rift
Continental
Antarctic plate (FIGURE 15.11B).
Trench
volcanic arc A modern example of
an active continental rift is
the East African Rift (see
Oceanic
Figure 15.9, left). Whether
crust this rift will eventually
Oceanic
Subducting oc
Continental lithosphere result in the break-up of
ea lithosphere
nic Africa is a topic of contin-
lith
os
ph
ued research. Nevertheless,
ere
the East African Rift is an excel-
lent model of the initial stage in the
▲
break-up of a continent. Here, ten-
▲ C. Transform fault boundary
B. Convergent boundary sional forces have stretched and
thinned the crust, allowing molten
rock to ascend from the mantle. Evidence
Stated another way, the thickness of oceanic Oceanic lithosphere that exceeds 80 million for recent volcanic activity includes several
lithosphere is age-dependent. The older years in age is about 100 kilometers thick: large volcanic mountains including Mount
(cooler) it is, the greater its thickness. about its maximum thickness. Kilimanjaro and Mount Kenya, the tallest
Rift asthenosphere. In general, oceanic litho-

valley
sphere is more dense than the astheno-
sphere, whereas continental lithosphere is
less dense and resists subduction. As a
consequence, only oceanic lithosphere
will subduct to great depths.
Oceanic crust
Deep-ocean trenches are the surface
manifestations produced as oceanic litho-
sphere descends into the mantle. These
Partial
melting large linear depressions are remarkably
Asthenosphere long and deep. The Peru–Chile trench
along the west coast of South America is
North
America e
Europe more than 4500 kilometers (3000 miles) in
g
R id length, and its base is as much as 8 kilome-
ters (5 miles) below sea level. The trenches
ic
nt
tla
Spreading in the western Pacific, including the Mari-

d-A
center ana and Tonga trenches, tend to be even

Mi
Africa
deeper than those of the eastern Pacific.
Slabs of oceanic lithosphere descend
Lith
Asth
osp
here into the mantle at angles that vary from a
Upwelling eno
sph few degrees to nearly vertical (90 degrees).
ere
The angle at which oceanic lithosphere
descends depends largely on its density.
FIGURE 15.10 Most divergent plate boundaries are situated along the crests of oceanic ridges. For example, when a spreading center is
located near a subduction zone, as is the
case along the Peru–Chile trench, the
peaks in Africa. Research suggests that if rifting continues, the rift valley will lengthen and subducting lithosphere is young and, there-
deepen, eventually extending out to the margin of the landmass (FIGURE 15.11C). At this fore, warm and buoyant. Because of this, the
point, the rift will become a narrow sea with an outlet to the ocean. The Red Sea, which angle of descent is small, which results in
formed when the Arabian Peninsula split from Africa, is a modern example of such a feature. considerable interaction between the
Consequently, the Red Sea provides us with a view of how the Atlantic Ocean may have descending slab and the overriding plate.
looked in its infancy (FIGURE 15.11D). Consequently, the region around the
Peru–Chile trench experiences great
CONCEPT CHECK 15.5 earthquakes, including the 2010 Chilean
earthquake—one of the 10 largest on record.
1 Sketch or describe how two plates move in relation to each other along divergent As oceanic lithosphere ages (gets far-
plate boundaries. ther from the spreading center), it gradually
2 List four facts that characterize the oceanic ridge system. cools, which causes it to thicken and
3
increase in density. In parts of the western
Briefly describe the process of continental rifting. Where is it occurring today?
Pacific, some oceanic lithosphere is 180
million years old. This is the thickest and
densest in today’s oceans. The very dense
Convergent Boundaries slabs in this region typically plunge into the
GEODe
mantle at angles approaching 90 degrees.
PLATE TECTONICS
This largely explains the fact that most
ESSENTIALS Convergent Boundaries
OF GEOLOGY trenches in the western Pacific are deeper
New lithosphere is constantly being produced at the oceanic ridges; however, our planet is than trenches in the eastern Pacific.
not growing larger—its total surface area remains constant. A balance is maintained because Although all convergent zones have the
older, denser portions of oceanic lithosphere descend into the mantle at a rate equal to same basic characteristics, they are highly
seafloor production. This activity occurs along convergent boundaries, where two plates variable features. Each is controlled by the
move toward each other and the leading edge of one is bent downward, as it slides beneath type of crustal material involved and the
the other. tectonic setting. Convergent boundaries
Convergent boundaries are also called subduction zones, because they are sites where can form between two oceanic plates, one
lithosphere is descending (being subducted) into the mantle. Subduction occurs because oceanic and one continental plate, or two
the density of the descending tectonic plate is greater than the density of the underlying continental plates.
Convergent Boundaries 373
Upwarping DID YOU KNOW?

The remains of some of the earliest humans, Homo habilis and
Homo erectus, were discovered by Louis and Mary Leakey in the
East African Rift. Scientists consider this region to be the
“birthplace” of the human race.
al crust
Continent
Upwelling melting is triggered within the wedge of hot asthenosphere that lies
above it. But how does the subduction of a cool slab of oceanic
A. lithosphere cause mantle rock to melt? The answer lies in the fact
Continental rift
that water contained in the descending plates acts like salt does to
melt ice. That is, “wet” rock in a high-pressure environment
melts at substantially lower temperatures than does “dry” rock of
the same composition.
T Sediments and oceanic crust contain a large amount of water,
I which is carried to great depths by a subducting plate. As the plate
M
Upwelling plunges downward, heat and pressure drive water from the voids in
E
the rock. At a depth of roughly 100 kilometers, the wedge of mantle
B. rock is sufficiently hot that the introduction of water from the
Linear sea
Alfred Wegener
Upwelling
C.
Mid-ocean ridge
Rift valley
Continental
crust
Oceanic crust
Upwelling
FIGURE 15.11 Continental rifting and the formation of a new ocean basin. A. The initial
stage of continental rifting tends to include upwelling in the mantle that is associated
with broad upwarping of the lithosphere. Tensional forces and buoyant uplifting of the
heated lithosphere cause the crust to be broken into large slabs. B. As the crust is
pulled apart, these large blocks sink, generating a continental rift valley. C. Further
spreading generates a narrow sea similar to the present-day Red Sea. D. Eventually, an
expansive deep-ocean basin and oceanic ridge are created.
Oceanic–Continental
Convergence
Whenever the leading edge of a plate Alfred Wegener shown
capped with continental crust converges waiting out the 1912–13 Arctic
with a slab of oceanic lithosphere, the winter during an expedition to
Greenland, where he made a
buoyant continental block remains
1200-kilometer traverse across
“floating,” while the denser oceanic slab the widest part of the island’s
sinks into the mantle (FIGURE 15.12A). ice sheet. (Photo courtesy of
When a descending oceanic slab reaches a Archive of Alfred Wegener
depth of about 100 kilometers (60 miles), Institute)
Continental eventually build a chain of volcanic

volcanic arc
structures large enough to emerge
SOUTH as islands. The volcanic islands tend
Trench AMERICA
Pacific
to be spaced at intervals of about 80
kilometers (50 miles). This newly
E
Ocean
C H IL
Oceanic crust
Atlantic formed land consisting of an arc-
Continental Ocean
crust
Osorno shaped chain of volcanic islands is
Subducting oc Volcano
ea
nic
Continental called a volcanic island arc, or sim-
100 km lithosphere
lith ply an island arc (FIGURE 15.13B).
os
ph
Asthenosphere ere The Aleutian, Mariana, and
200 km Melting
Tonga islands are examples of rela-
tively young volcanic island arcs.
A.
Island arcs are generally located 100
to 300 kilometers (60 to 200 miles)
FIGURE 15.12 Oceanic–continental
from a deep-ocean trench. Located
convergent plate boundary. A. Illustration of
dense, oceanic lithosphere subducting
adjacent to the island arcs just men-
beneath a buoyant continental block. Melting in tioned are the Aleutian trench, the
the asthenosphere generates molten rock that rises Mariana trench, and the Tonga trench.
toward the surface. This activity produces a chain of
B.
Most volcanic island arcs are located in
structures built on the overriding landmass, called a the western Pacific. Only two are located in
continental volcanic arc. B. Osorno Volcano is one of the most active volcanoes of the southern Chilean the Atlantic—the Lesser Antilles arc, on the
Andes, having erupted 11 times between 1575 and 1869. Located on the shore of Lake Llanquihue,
eastern margin of the Caribbean Sea and
Osorno is similar in appearance to Mount Fuji, Japan. (Photo by Michael Collier)
the Sandwich Islands located off the tip of
South America. The Lesser Antilles are a
slab below leads to some melting. This are produced in part by volcanic activity product of the subduction of the Atlantic
process, called partial melting, is thought to associated with the subduction of oceanic seafloor beneath the Caribbean plate.
generate about 10 percent molten material, lithosphere, are called continental volcanic Located within this volcanic arc are the
which is intermixed with unmelted mantle arcs. The Cascade Range in Washington,
United States and British Virgin Islands as
rock. Being less dense than the surrounding Oregon, and California is another that well as the island of Martinique, where
mantle, this hot mobile material gradually consists of several well-known volcanic Mount Pelée erupted in 1902, destroying
rises toward the surface. Depending on the mountains, including Mount Rainier, the town of St. Pierre and killing an esti-
environment, these mantle-derived masses of Mount Shasta, and Mount St. Helens. mated 28,000 people. This chain of islands
molten rock may ascend through the crust This active volcanic arc also extends into also includes Montserrat, where there has
and give rise to a volcanic eruption. However, Canada, where it includes Mount Garibaldi, been recent volcanic activity.*
much of this material never reaches the sur- Mount Silverthrone, and others. Relatively young island arcs are fairly
face; rather, it solidifies at depth—a process simple structures made of numerous volcanic
that thickens the crust. cones that are underlain by oceanic crust
The volcanoes of the towering Andes are Oceanic–Oceanic that is generally less than 20 kilometers
the product of molten rock generated by the Convergence (12 miles) thick. By contrast, older island
subduction of the Nazca plate beneath the An oceanic–oceanic convergent boundary has arcs are more complex and are underlain by
South American continent (FIGURE 15.12B). many features in common with highly deformed crust that may reach
Mountain systems, such as the Andes, which oceanic–continental plate margins. Where 35 kilometers in thickness. Examples include
two oceanic slabs converge, one descends the islands that make up the countries of
beneath the other, initiating volcanic activity Japan, Indonesia, and the Philippines. These
DID YOU KNOW? by the same mechanism that operates at all island arcs are built upon material generated
Many of the tropical islands in the subduction zones (FIGURE 15.13A). Water by earlier episodes of subduction or on small
Caribbean, where Americans dream of squeezed from the subducting slab of slivers of continental crust.
taking winter holidays, are of volcanic oceanic lithosphere triggers melting in the
origin. Located within this volcanic
hot wedge of mantle rock above. In this
island arc is the island of Martinique,
where Mount Pelée erupted in 1902, setting, volcanoes grow up from the ocean Continental–Continental
killing about 28,000 people, and the floor, rather than upon a continental plat- Convergence
island of Montserrat, where recent form. When subduction is sustained, it will The third type of convergent boundary
volcanism required the entire island to results when one landmass moves toward
be evacuated. *More on these volcanic events is found in the margin of another because of sub-
Chapter 4. duction of the intervening seafloor
Convergent Boundaries 375
Volcanic island arc (FIGURE 15.14A). Whereas oceanic

Trench lithosphere tends to be dense and
sink into the mantle, the buoyancy
Oceanic
of continental material inhibits it
crust from being subducted.
Continental
A LA S KA Consequently, a collision between
crust two converging continental frag-
Oceanic lithosphere re
he
o sp ments ensues (FIGURE 15.14C). This
100 km Melting lith Pacific
n ic Cleveland Ocean event folds and deforms the accu-
ea Volcano
Asthenosphere oc mulation of sediments and sedimen-
ti ng
u c tary rocks along the continental
bd
200 km Su margins as if they had been placed
A. in a gigantic vise. The result is the
FIGURE 15.13 Oceanic–oceanic formation of a new mountain range
convergent plate boundaries. A. When composed of deformed sedimen-
oceanic plates converge, one descends tary and metamorphic rocks that
beneath the other, initiating volcanic
often contain slivers of
activity in the overriding plate. In this
setting a volcanic island arc is built.
Cleveland oceanic crust.
Volcano
B. These four volcanic structures are part of Such a collision began about 50
the Aleutian Islands, a string of both active million years ago when the subconti-
and dormant volcanoes fed by magma created nent of India “rammed” into Asia,
by the subduction of the Pacific Plate. Steam
B.
producing the Himalayas—the most
plumes have recently been observed emanating from
Cleveland Volcano (center) evidence of recent activity.
(Photo courtesy of NASA)
Continental volcanic arc
Continental
shelf
India deposits Eurasia
Ocean basin
A.
Subduct
ing o
cea
nic
l ith Melting
osp
h ere
Asthenosphere
FIGURE 15.14 The ongoing

collision of India and Asia began Himalayas
about 50 million years ago— India
producing the majestic today
Himalayas. A. As India migrated
northward the intervening Ganges
Plain Tibetan
ocean closed as the seafloor 10 million India Plateau
was subducted beneath Eurasia. years ago
B. Position of India in relation 38 million

to Eurasia at various times. years ago Indian
(Modified after Peter Molnar) plate Suture
C. Eventually the two Lithosphere
landmasses collided, deforming 55 million
years ago
and elevating the sediments
that had been deposited along 71 million
their continental margins. years ago
In addition, slices of crustal Asthenosphere
rocks were thrust onto the
colliding plates. B. C.
spectacular mountain range on Earth

(Figure 15.14C). During this collision, the Transform Fault shape of the original rifting that caused the
break-up of the supercontinent of Pangaea.
continental crust buckled and fractured and
was generally shortened and thickened. In
Boundaries (Compare the shapes of the continental
GEODe margins of the landmasses on both sides
addition to the Himalayas, several other PLATE TECTONICS
of the Atlantic with the shape of the Mid-
ESSENTIALS
major mountain systems, including the OF GEOLOGY
Transform Fault Boundaries
Atlantic Ridge.)
Alps, Appalachians, and Urals, formed as Along a transform fault boundary, Typically, transform faults are part of
continental fragments collided. (This topic plates slide horizontally past one another prominent linear breaks in the seafloor
will be considered further in Chapter 17.) without the production or destruction of known as fracture zones, which include
lithosphere (conservative plate margins). The both the active transform faults as well as
CONCEPT CHECK 15.6 nature of transform faults was discovered their inactive extensions into the plate
in 1965 by Canadian geologist J. Tuzo interior (FIGURE 15.15B). Active transform
1 Why does the rate of lithosphere produc- Wilson, who proposed that these large faults lie only between the two offset ridge
tion roughly balance with the rate at which faults connected two spreading centers segments and are generally defined by
it is destroyed?
(divergent boundaries) or less commonly weak, shallow earthquakes. Here seafloor
2 Compare a continental volcanic arc and a two trenches (convergent boundaries). produced at one ridge axis moves in the
volcanic island arc.
Most transform faults are found on the opposite direction of seafloor produced at
3 Why does oceanic lithosphere subduct ocean floor (FIGURE 15.15A). Here they an opposing ridge segment. Thus, between
while continental lithosphere does not?
offset segments of the oceanic ridge system, the ridge segments these adjacent slabs of
4 Briefly describe how mountain systems producing a steplike plate margin. Notice oceanic crust are grinding past each other
such as the Himalayas form.
that the zigzag shape of the Mid-Atlantic along a transform fault. Beyond the ridge
Ridge in Figure 15.9 roughly reflects the crests are inactive zones, where the frac-
The Alps were created by the collision of the

African and Eurasian plates. (Photo by Gareth
McCormack/Alamy)
Transform Fault Boundaries 377
Fracture zone FIGURE 15.15 Transform fault boundaries.

A. Most transform faults offset segments of a
Inactive Transform fault Inactive spreading center, producing a steplike plate
zone (active) zone
margin. The zigzag shape of the Mid-Atlantic
Ridge roughly reflects the shape of the zone of
rifting that produced the break-up of Pangaea.
B. Fracture zones are long, narrow fractures in
the seafloor that are nearly perpendicular to the
offset ridge segments. They include both the
active transform fault and its “fossilized” trace,
where oceanic crust of different ages is
juxtaposed. The offsets between ridge
Oceanic segments (transform faults) do not change in
crust length over time.
Like the Mendocino Fault, most

M
id
-A transform fault boundaries are located
tlan
tic
Rid within the ocean basins; however, a few
g Africa B. cut through continental crust. Two exam-
e
ples are the earthquake-prone San

South Andreas Fault of California and New
America
Zealand’s Alpine Fault. Notice in Figure
15.16 that the San Andreas Fault connects
a spreading center located in the Gulf of
California to the Cascadia subduction
zone and the Mendocino Fault located
KEY along the northwest coast of the United
Spreading centers States. Along the San Andreas Fault, the
Fracture zones Pacific plate is moving toward the north-
A. Transform faults west, past the North American plate
tures are preserved as linear topographic

Transform
depressions. The trend of these fracture
e
fault
zon
zones roughly parallels the direction of Juan

Juan
ion
de Fuca
plate motion at the time of their formation. Ridge de Fuca Oregon
uct
Thus, these structures are useful in plate

Juan Cascadia
ubd
California
mapping the direction of plate motion Pacific de Fuca subduction
ia s
plate plate zone

in the geologic past. Mendocino
cad
Fault
In another role, transform faults pro-
Cas
vide the means by which the oceanic crust

created at ridge crests can be transported NORTH
to a site of destruction—the deep-ocean AMERICAN
Mendocino PLATE
trenches. FIGURE 15.16 illustrates this Fault
Sa
situation. Notice that the Juan de Fuca San Francisco

nA
plate moves in a southeasterly direction,

n dr
eventually being subducted under the west PACIFIC ea

PLATE sF
Los au
coast of the United States. The southern Angeles l
t
end of this plate is bounded by a transform

fault called the Mendocino Fault. This FIGURE 15.16 The Mendocino transform fault
transform boundary connects the Juan de permits seafloor generated at the Juan de Fuca Relative motion of
ridge to move southeastward past the Pacific plate Pacific plate
Fuca ridge to the Cascadia subduction Gulf of
and beneath the North American plate. Thus, this California
zone. Therefore, it facilitates the movement transform fault connects a spreading center
of the crustal material created at the Juan (divergent boundary) to a subduction zone
de Fuca ridge to its destination beneath the (convergent boundary). Furthermore, the San Andreas Fault, also a transform fault, connects two
North American continent. spreading centers: the Juan de Fuca ridge and a divergent zone located in the Gulf of California.
(FIGURE 15.17). If this movement continues, that part of California west of the fault zone,
DID YOU KNOW? including the Baja Peninsula of Mexico, will become an island off the West Coast of the
Olympus Mons is a huge volcano on United States and Canada. It could eventually reach Alaska. However, a more immediate
Mars that strongly resembles the concern is the earthquake activity triggered by movements along this fault system.
Hawaiian shield volcanoes. Rising
25 kilometers above the surrounding
CONCEPT CHECK 15.7
plains, Olympus Mons owes its
massiveness to the fact that plate
1 Sketch or describe how two plates move in relation to each other along a transform
tectonics does not operate on Mars.
Consequently, instead of being carried plate boundary.
away from the hot spot by plate 2 Differentiate between transform faults and the two other types of plate boundaries.
motion, as occurred with the Hawaiian
volcanoes, Olympus Mons remained
fixed and grew to a gigantic size.
Testing the Plate Tectonics Model

Some of the evidence supporting continental drift and seafloor spreading has already been
presented. With the development of the theory of plate tectonics, researchers began testing
this new model of how Earth works. Although new supporting data were obtained, it was
often new interpretations of already existing data that swayed the tide of opinion.
FIGURE 15.17 Along the San Andreas Fault, the Pacific plate is moving
toward the northwest, relative to the North American plate. This aerial
view shows the offset in the dry channel of Wallace Creek near Taft,
California. (Photo by Michael Collier)
Testing the Plate Tectonics Model 379
Evidence: Ocean Drilling Thickness of sediments increase

with distance from the ridge crest
Some of the most convincing evidence for
seafloor spreading came from the Deep Sea
Drilling Project, which operated from 1968
until 1983. One of the early goals was to
gather samples of the ocean floor in order Age of seafloor
to establish its age. To accomplish this, the
Glomar Challenger, a drilling ship capable of
Drilling ship collects core samples of
working in water thousands of meters seafloor sediments and basaltic crust
deep, was built. Hundreds of holes were
drilled through the layers of sediments that
blanket the ocean crust, as well as into the
asalt)
basaltic rocks below. Rather than radiomet- Ocean crust (b
rically dating the crustal rocks, researchers
used the fossil remains of microorganisms
found in the sediments resting directly on
the crust to date the seafloor at each site.* FIGURE 15.18 Since 1968 drilling ships have gathered core samples of seafloor
hot-spot volcanism
sediment and crustal rocks at hundreds of sites. Results from these efforts showed
When the oldest sediment from each
that the ocean floor is indeed youngest at the ridge axis. This was the first direct
drill site was plotted against its distance evidence supporting the seafloor spreading hypothesis and the broader theory of
from the ridge crest, the plot showed that plate tectonics.
the sediments increased in age with
increasing distance from the ridge drilling ship, the JOIDES Resolution, to
(FIGURE 15.18). This finding supported continue the work of the Glomar Challenger.
the seafloor-spreading hypothesis, which While the Deep Sea Drilling Project vali-
predicted that the youngest oceanic crust dated many of the major tenets of the theory Hot-spot volcanism, Kilauea, Hawaii.
would be found at the ridge crest, the site of plate tectonics, the JOIDES Resolution was (U.S. Geological Survey)
of seafloor production, and the oldest able to probe deeper into the oceanic crust.
oceanic crust would be located adjacent to This allowed for the study of earthquake-
the continents. generating zones at convergent plate
The data collected by the Deep Sea margins and for the direct examination of
Drilling Project also reinforced the idea that oceanic plateaus and seamounts. Sediment
the ocean basins are geologically young cores from the Ocean Drilling Program
because no seafloor with an age in excess of have also extended our knowledge of
180 million years was ever found. By com- long- and short-term climatic changes.
parison, most continental crust exceeds In October 2003, the JOIDES
several hundred million years in age and Resolution became part of a new program,
some has been located that exceeds the Integrated Ocean Drilling Program
4 billion years in age. (IODP). This new international effort
The thickness of ocean-floor sediments uses multiple vessels for exploration,
provided additional verification of seafloor including the massive 210-meter-long
spreading. Drill cores from the Glomar (nearly 770-foot-long) Chikyu, (meaning
Challenger revealed that sediments are “planet Earth” in Japanese) which began
almost entirely absent on the ridge crest operations in 2007. One of the goals
and that sediment thickness increases with of the IODP is to recover a complete
increasing distance from the ridge (Figure section of the ocean crust, from top
15.18). This pattern of sediment distribu- to bottom.
tion should be expected if the seafloor-
spreading hypothesis is correct.
The Ocean Drilling Program, the Evidence: Hot Spots
successor to the Deep Sea Drilling Project, Mapping volcanic islands and seamounts
employed a more technologically advanced (submarine volcanoes) in the Pacific
Ocean revealed several linear chains
*Radiometric dates of the ocean crust itself are of volcanic structures. One of the
unreliable because of the alteration of basalt by most studied chains consists of at
seawater. least 129 volcanoes that extend
from the Hawaiian Islands to Midway Chapter 3.) The surface manifestation of jagged peaks and vast canyons. By contrast,
Island and continue northward toward the this activity is a hot-spot, an area of the relatively young island of Hawaii
Aleutian trench (FIGURE 15.19). Radiometric volcanism, high heat flow, and crustal exhibits many fresh lava flows, and one
dating of this structure, called the Hawaiian uplifting that is a few hundred kilometers of its five major volcanoes, Kilauea,
Island–Emperor Seamount chain, showed across. As the Pacific plate moved over the remains active today.
that the volcanoes increase in age with hot spot, a chain of volcanic structures Research suggests that at least some
increasing distance from the “big island” known as a hot-spot track was built. As mantle plumes originate at great depth, per-
of Hawaii. The youngest volcanic island shown in Figure 15.19, the age of each haps at the core–mantle boundary. Others,
in the chain (Hawaii) rose from the ocean volcano indicates how much time has however, may have a much shallower origin.
floor less than a million years ago, whereas elapsed since it was situated over the Of the 40 or so hot spots that have been
Midway Island is 27 million years old, and mantle plume. identified worldwide, more than a dozen are
Suiko Seamount, near the Aleutian Taking a closer look at the five largest located near spreading centers. For example,
trench, is about 65 million years old Hawaiian Islands, we see a similar pattern the mantle plume located beneath Iceland is
(Figure 15.19). of ages from the volcanically active island responsible for the vast accumulation of vol-
Most researchers are in agreement that of Hawaii, to the inactive volcanoes that canic rocks found along this exposed section
a cylindrically shaped upwelling of hot make up the oldest island, Kauai (see of the Mid-Atlantic Ridge.
rock, called a mantle plume, is located Figure 15.19). Five million years ago,
beneath the island of Hawaii. As the hot, when Kauai was positioned over the hot Evidence: Paleomagnetism
rocky plume ascends through the mantle, spot, it was the only Hawaiian Island in Anyone who has used a compass to find
the confining pressure drops, which trig- existence. Visible evidence of the age of direction knows that Earth’s magnetic field
gers partial melting. (This process, called Kauai can be seen by examining its extinct has a north and south magnetic pole. Today
decompression melting, is discussed in volcanoes, which have been eroded into these magnetic poles align closely, but not
exactly, with the geo-
graphic poles. (The geo-
FIGURE 15.19 The chain of islands and seamounts that extends from Hawaii to the Plate motion graphic poles are located
Aleutian trench was generated as the Pacific plate moved over a mantle plume
Extinct
Hot spot where Earth’s rotational
(hot spot). Radiometric dating of the Hawaiian Islands shows that the volcanic volcanism
activity increases in age moving away from the “big island” of Hawaii.
volcano axis intersects the sur-
face.) Earth’s magnetic
field is similar to that
Kauai produced by a simple bar
3.8—5.6
magnet. Invisible lines of
Rising force pass through the
mantle
Oahu plume planet and extend from
2.2–3.3 one magnetic pole to the
other (FIGURE 15.20). A
Molokai compass needle, itself a
1.3–1.8
small magnet free to
Maui rotate on an axis,
less than 1.0 becomes aligned with the
magnetic lines of force
and points to the
magnetic poles.
Hawaii Unlike the pull of
0.7 to present gravity, we cannot feel
Earth’s magnetic field, yet
Suiko Emperor its presence is revealed
65 my Seamount chain
because it deflects a com-
Hawaiian chain pass needle. In addition,
some naturally occurring
Midway
Island Hawaii minerals are magnetic
27 my Ages given and hence are influenced
in millions of
years by Earth’s magnetic field.
One of the most common
is the iron-rich mineral
magnetite, which is abun-
dant in lava flows of basaltic

composition.* Basaltic lavas DID YOU KNOW?
erupt at the surface at When all of the continents were joined
Geographic Magnetic temperatures greater than to form Pangaea, the rest of Earth’s
north north surface was covered with a huge ocean
1000 °C, exceeding a thresh-
called Panthalassa (pan = all,
old temperature for magnet- thalassa = sea). Today all that remains
ism known as the Curie point of Panthalassa is the Pacific Ocean,
(about 585 °C). Conse- which has been decreasing in size since
quently, the magnetite grains the break-up of Pangaea.
in molten lava are nonmag-
netic. However, as the lava
cools these iron-rich grains become magnetized and align themselves in
the direction of the existing magnetic lines of force. Once the minerals
solidify, the magnetism they possess will usually remain “frozen” in this
position. Thus, they act like a compass needle because they “point”
toward the position of the magnetic poles at the time of their formation.
Rocks that formed thousands or millions of years ago and contain a
*Some sediments and sedimentary rocks contain enough iron-bearing mineral grains
FIGURE 15.20 Earth’s magnetic field consists of to acquire a measurable amount of magnetization.
lines of force much like those a giant bar magnet
would produce if placed at the center of Earth.
The JOIDES Resolution, one of the drilling ships

of the Integrated Ocean Drilling Program. (Ocean
Drilling Program) Inset photo shows the newly
launched Japanese vessel Chikyu, the world’s
most advanced drilling ship. (AP/Wide World)
“record” of the direction of the magnetic when the Atlantic began to open. From Apparent polar Apparent polar
poles at the time of their formation are said this time forward, these continents contin- wandering wandering path for
path for Eurasia North America
to possess fossil magnetism, or uously moved apart. When North America
500 Ma
paleomagnetism. During the 1950s and Europe are moved back to their pre- 400 Ma
paleomagnetic data was collected from drift positions, as shown in FIGURE 15.21B, 400 Ma
lava flows around the globe. these apparent wandering paths coincide. 500 Ma 300 Ma
This is evidence that North America and 200 Ma 300 Ma

Europe were once joined and moved
Apparent Polar Wandering relative to the poles as part of the same North 100 Ma 200 Ma
A study of rock magnetism conducted in continent. America
100 Ma
Europe led to an interesting discovery. The
Eurasia
magnetic alignment of iron-rich minerals in
lava flows of different ages indicated that Magnetic Reversals
the position of the paleomagnetic poles had and Seafloor Spreading A.
changed through time. A plot of the Another discovery came when geophysi-
Africa
location of the magnetic north pole with cists learned that over periods of hundreds
respect to Europe revealed that during the of thousands of years, Earth’s magnetic field
past 500 million years, the pole had gradu- periodically reverses polarity. During a Apparent polar Apparent polar
ally wandered from a location near Hawaii magnetic reversal the north magnetic pole wandering path wandering
for North America path for Eurasia
northward to its present location near the becomes the south magnetic pole, and vice
North Pole (FIGURE 15.21A). This was versa. Lava solidifying during a period of
strong evidence that either the magnetic reverse polarity will be magnetized with the
poles had migrated, an idea known as polarity opposite that of volcanic rocks
polar wandering, or that the lava flows being formed today. When rocks exhibit
moved—in other words, Europe had the same magnetism as the present mag-
Eurasia
drifted in relation to the poles. netic field, they are said to possess normal
Although the magnetic poles are polarity, whereas rocks exhibiting the
known to move in an erratic path around opposite magnetism are said to have North
the geographic poles, studies of paleo- reverse polarity. America
magnetism from numerous locations Once the concept of magnetic reversals
show that the positions of the magnetic was confirmed, researchers set out to Africa
B.
poles, averaged over thousands of years, establish a time scale for these occurrences.
correspond closely to the positions of The task was to measure the magnetic
the geographic poles. Therefore, a more polarity of hundreds of lava flows and use
acceptable explanation for the apparent radiometric dating techniques to establish
polar wandering paths was provided by the age of each flow. FIGURE 15.22 shows FIGURE 15.21 Simplified apparent polar-
Wegener’s hypothesis. If the magnetic wandering paths as established from North
the magnetic time scale established
American and Eurasian paleomagnetic data.
poles remain stationary, their apparent for the past few million years. The major A. The more westerly path determined from
movement is produced by continental divisions of the magnetic time scale are North American data was caused by the westward
drift. called chrons and last for roughly 1 million movement of North America from Eurasia during
Further evidence for continental years. As more measurements became the break-up of Pangaea. B. The positions of the
drift came a few years later when a polar- available, researchers realized that several, wandering paths when the landmasses are
wandering path was constructed for North short-lived reversals (less than 200,000 reassembled.
America (Figure 15.21A). For the first years long) often occurred during a
300 million years or so, the paths for North single chron. the magnetic properties of the underlying
America and Europe were found to be Meanwhile, oceanographers had begun crustal rocks.
similar in direction, but separated by about to do magnetic surveys of the ocean floor in The first comprehensive study of this
5000 kilometers (3000 miles). Then, conjunction with their efforts to construct type was carried out off the Pacific Coast of
during the middle of the Mesozoic era detailed maps of seafloor topography. These North America and had an unexpected out-
(180 million years ago) they began to magnetic surveys were accomplished by come. Researchers discovered alternating
converge on the present North Pole. towing very sensitive instruments, called stripes of high- and low-intensity magnetism
Because the North Atlantic Ocean is about magnetometers, behind research vessels. as shown in FIGURE 15.23. This relatively
5000 kilometers wide, the explanation for The goal of these geophysical surveys was simple pattern of magnetic variation defied
these curves was that North America and to map variations in the strength of Earth’s explanation until 1963, when Fred Vine
Europe were joined until the Mesozoic, magnetic field that arise from differences in and D. H. Matthews demonstrated that the
Age Polarity of
high- and low-intensity stripes supported the concept of seafloor spreading.
Magnetic Time Scale Millions Dated Lavas Vine and Matthews suggested that the stripes of high-intensity magnetism are
of years Normal Reverse regions where the paleomagnetism of the ocean crust exhibits normal polarity
0 •
•••
(FIGURE 15.24). Consequently, these rocks enhance (reinforce) Earth’s magnetic
Brunhes • field. Conversely, the low-intensity stripes are regions where the ocean crust is
normal chron •
• polarized in the reverse direction and therefore weaken the existing magnetic
••
• field. But how do parallel stripes of normally and reversely magnetized rock
Jaramillo normal •• become distributed across the ocean floor?
subchron 1 ••
•• Vine and Matthews reasoned that as magma solidifies along narrow rifts at
•••
•• the crest of an oceanic ridge, it is magnetized with the polarity of the existing
Matuyama •
reversed chron • magnetic field (FIGURE 15.25). Because of seafloor spreading, this strip of mag-
• netized crust would gradually increase in width. When Earth’s magnetic field
••
Olduvai normal
subchron 2 ••
•
reverses polarity, any newly formed seafloor (having the opposite polarity)
••• would form in the middle of the old strip. Gradually, the two halves of the old
•
•• strip are carried in opposite directions away from the ridge crest. Subsequent
• reversals would build a pattern of normal and reverse magnetic stripes as
Gauss •
normal chron ••• shown in Figure 15.25. Because new rock is added in equal amounts to both
••
3 •• trailing edges of the spreading ocean floor, we should expect the pattern of
Mammoth reversed •
subchron •• stripes (size and polarity) found on one side of an oceanic ridge to be a mirror
• image of the other side. A few years later a survey across the Mid-Atlantic Ridge
Gilbert ••
••• just south of Iceland revealed a pattern of magnetic stripes exhibiting a remark-
reversed chron ••
• able degree of symmetry to the ridge axis.
4 •
FIGURE 15.22 Time scale of Earth’s magnetic field in the recent

past. This time scale was developed by establishing the
magnetic polarity for lava flows of known age. (Data from Allen Ridge
Research vessel axis
Cox and G. B. Dalrymple) towing magnetometer
across ridge crest
CA
NA
DA
50°
High
Normal intensity
polarity
Reverse
U N I T E D S TAT E S
polarity
Axis of
Juan de Fuca
Ridge
45° Magnetometer record Low intensity
showing symmetrical
magnetic field across ridge
PACIFIC FIGURE 15.24 The ocean floor as a magnetic tape recorder. Magnetic
intensities are recorded as a magnetometer is towed across a segment of the
OCEAN oceanic ridge. Notice the symmetrical stripes of low- and high-intensity
magnetism that parallel the ridge crest. Vine and Matthews suggested that the
stripes of high-intensity magnetism occur where normally magnetized oceanic
135° 130° 125°
rocks enhanced the existing magnetic field. Conversely, the low-intensity stripes
FIGURE 15.23 Pattern of alternating stripes of high- and low-intensity are regions where the crust is polarized in the reverse direction, which weakens
magnetism discovered off the Pacific Coast of North America. the existing magnetic field.
A. Normal polarity nearly 200 million years ago. From this work, the dates
when individual crustal fragments separated from one
another and their relative motions have become well
established (FIGURE 15.26).
An important consequence of the break-up of Pangaea
was the creation of a “new” ocean basin: the Atlantic. As
Magma you can see in FIGURE 15.26B, splitting of the superconti-
nent did not occur simultaneously along the margins of
B. Reverse polarity the Atlantic. The first split developed between North
T
America and Africa.
I
M
Here, the continental crust was highly fractured, pro-
E
viding pathways for huge quantities of fluid lavas to reach
the surface. Remnants of these lavas are found along the
Eastern Seaboard of the United States—primarily buried
Magma beneath younger sedimentary rocks that form the conti-
nental shelf. Radiometric dating of these solidified lavas
C. Normal polarity indicates that rifting began in various stages between
180 million and 165 million years ago. This time span can
be used as the “birth date” for this section of the North
Atlantic.
About 130 million years ago, the South Atlantic began
to open near the tip of what is now South Africa. As this
zone of rifting migrated northward, it gradually opened
Magma
the South Atlantic (compare Figure 15.26, parts B and C).
Continued break-up in the Southern Hemisphere led to
FIGURE 15.25 As new basalt is added to the ocean floor at mid-ocean ridges, it is
the separation of Africa and Antarctica and sent India on a
magnetized according to Earth’s existing magnetic field. Hence, oceanic crust behaves northward journey. By the early Cenozoic, about 50 mil-
much like a tape recorder as it records each reversal of our planet’s magnetic field. lion years ago, Australia had separated from Antarctica,
and the South Atlantic had emerged as a full-fledged ocean
(FIGURE 15.26D).
CONCEPT CHECK 15.8 A modern map ( FIGURE 15.26F ) shows that India eventually collided with Asia, an event
that began about 50 million years ago and created the Himalayas as well as the Tibetan
1 What is the age of the oldest sediments Highlands. About the same time, the separation of Greenland from Eurasia completed the
recovered by deep-ocean drilling? How do break-up of the northern landmass. During the last 20 million years or so of Earth history,
the ages of these sediments compare to Arabia rifted from Africa to form the Red Sea, and Baja California separated from Mexico to
the ages of the oldest continental rocks? form the Gulf of California (FIGURE 15.26E). Meanwhile, a sliver of land (now known as
2 Assuming hot spots remain fixed, in what Central America) was trapped between North America and South America to produce our
direction was the Pacific plate moving globe’s familiar, modern appearance.
while the Hawaiian Islands were forming?
When Suiko Seamount was forming?
3
CONCEPT CHECK 15.9
Describe how Fred Vine and D. H.
Matthews related the seafloor-spreading
1 When did the supercontinent of Pangaea begin to break apart?
hypothesis to magnetic reversals.
2 What two continents were the first to separate?
3 During the break-up of Pangaea, which continent was actually growing in size through the
accretion of other landmasses?
The Breakup
of Pangaea
Wegener used evidence from fossils, rock How Is Plate Motion Measured?
types, and ancient climates to create a GEODe
PLATE TECTONICS
jigsaw-puzzle fit of the continents—thereby
ESSENTIALS Formation and Break-up of Pangaea
creating his supercontinent of Pangaea. In a OF GEOLOGY
similar manner, but employing modern A number of methods have been employed to establish the direction and rate of plate
tools not available to Wegener, geologists motion. Paleomagnetism stored in the rocks of the ocean floor is one method used to deter-
have recreated the steps in the break-up of mine the speeds at which plates move away from the ridge axes where they were generated.
this supercontinent, an event that began In addition, hot spot tracks, such as the Hawaiian Island–Emperor Seamount chain, trace
How Is Plate Motion Measured? 385
the speed and direction of plate movement Hot spot tracks can also be useful Measuring Plate Motion
relative to the hot plume embedded in the when establishing the direction a plate is
mantle below. moving. Notice in Figure 15.19 that there from Space
is a bend in the Hawaiian Island–Emperor Plates are not flat surfaces; instead they are
Mantle Plumes Seamount chain. This bend occurred about curved sections of a sphere, which greatly
50 million years ago when the motion of complicates how plate motion is
and Plate Motions the Pacific plate changed from one that was described. In addition, plates usually
By measuring the length of a hot spot track nearly due north to a more northwesterly exhibit some degree of rotational motion,
and the time interval between the formation path. Similarly, hot spots found on the floor which can cause two locations on the
of its oldest and youngest volcanic struc- of the Atlantic have increased our under- same plate to move at different speeds and
tures, an average rate of plate motion can be standing of the migration of landmasses in different directions. The latter fact can
calculated. For example, that portion of the following the break-up of Pangaea. be illustrated by rotating your dinner plate
Hawaiian Island–Emperor Seamount chain The existence of mantle plumes and in a clockwise matter. When doing so you
that extends from Hawaii to Suiko Seamount their association with hot spots is well will notice that the items on the left side
is roughly 6000 kilometers in length and documented. Most mantle plumes are long- of the plate move away from you (diver-
formed over the past 65 million years. Thus, lived features that appear to maintain rela- gence) as the items on the right side move
the average rate of movement of the Pacific tively fixed positions within the mantle. toward you (convergence). Items in the
plate, relative to the mantle plume, was However, recent evidence has shown that center will rotate, but their position rela-
about 9 centimeters (4 inches) per year. some hot spots may slowly migrate. Prelimi- tive to yours will not change. The complex
nary results suggest that the Hawaiian nature of plate motion makes the task of
hotspot may have migrated southward by as describing plate motions more difficult
much as 20 degrees latitude. If this is the than simply establishing the relative
case, models of past plate motion that were motion between two plates along the
Tethys
Sea based on a “fixed hot spot” frame of boundary that separates them. Fortunately,
P
reference will need to be reevaluated. using space-age technology, researchers

A
N
G have recently been able to accurately cal-
A
E culate the absolute motion of hundreds of
A North Eurasia S.E.
America Asia locations across the globe.
You may be familiar with the Global
A. 200 Million Years Ago
Africa
Positioning System (GPS), which is part of
(Late Triassic Period) Tibet
South
America
the navigation system used in automobiles
to locate one’s position and to provide
North India
America Eurasia directions to some other location. The
Antarctica Australia
Global Positioning System employs two
B. 150 Million Years Ago dozen satellites that send radio signals that
Africa
Tibet (Late Jurassic Period) are intercepted by GPS receivers located at
Tethys
South Ocean
America Earth’s surface. The exact position of the
India North
America
Eurasia
receiver is determined by simultaneously
Australia
Antarctica
establishing the distance from the receiver
C. 90 Million Years Ago Africa to four or more satellites. Researchers use
India
(Cretaceous Period) South specifically designed equipment that is able
America
to locate the position of a point on Earth to
North Eurasia Australia within a few millimeters (about the
America
Hi
malaya
s
diameter of a small pea). To establish plate
Antarctica
Africa
India
D. 50 Million Years Ago
motion, a particular site is surveyed repeat-
(Early Tertiary/Paleogene) edly over a number of years.
South
America Data obtained from these and other
Australia similar techniques are shown in FIGURE
Gulf of
Arabia 15.27. Calculations show that Hawaii is
California
Antarctica
Red
moving in a northwesterly direction and
E. 20 Million Years Ago
(Late Tertiary/Neogene) Panama
Sea approaching Japan at 8.3 centimeters per
year. A site located in Maryland is retreating
from one in England at a speed of 1.7 cen-
timeters per year—a value that is close to
FIGURE 15.26 Several views of the break-up the 2.0-centimeters-per-year spreading rate
of Pangaea over a period of 200 million years. F. Present that was established from paleomagnetic
Mohns
Ridge
es
jan
Eurasian plate North American 1.8 yk ge
Re i d
▲
▲ plate R
▲ ▲
▲
▲
▲ ▲ h
▲ ▲ ▲
n
▲
Trenc 2.3
▲ Aleutia
▲
▲ ▲ ▲
▲ ▲
▲
▲ ▲
Juan de Fuca
▲
▲
▲ ▲ ▲ ▲ A zo
▲
▲ ▲ ▲ ▲
▲
plate 2.3 res
▲
▲
▲ ▲
Caribbean F.Z.
▲
▲ ▲
▲
▲
Ma
▲ ▲ ▲ ▲ ▲ ▲ ▲
plate
▲
▲
▲
Arabian
rian
▲
2.5
▲
Philippine
▲
▲
African plate plate ▲
a Tren
▲ ▲
Mi
▲ ▲
▲
plate
▲ ▲
Pacific plate
d-
10.8 ▲
tla
▲ ▲ ▲
▲
▲ ▲ ▲ ▲
A
▲
▲ ch
nti
▲
▲
▲ ▲ c Rid
Cocos 7.0 ge
▲ ▲
▲
2.7 ▲ Caroline
plate
▲
Somalia
▲
plate
▲ ▲ ▲
▲
Mid-Indian
plate 3.0 ▲
Ja
13.4 5.0 South American
▲
▲
▲
Ridge
▲ ▲
v a
▲ ▲ ▲ ▲
3.5 Tre ▲ ▲
nc h ▲ ▲ ▲ ▲ ▲▲ ▲
▲
plate
▲
▲
Pe
▲
▲
Nazca
cific Rise
ru
▲
▲
▲
-C
4.4
15.6 plate
▲ ▲ ▲ ▲
▲
hi
▲
le Trench
3.5
▲
ge Australian-Indian plate
Rid
East Pa
▲
▲
n ou 5.9
S
dia the
▲
In as
▲
1.4 .
t eF▲
▲ ▲ ▲
t ▲ Chi
es Ind in
hw ian Alp le R
1.4 o ut Ridge 9.4 idg
e Sandwich
▲
1.5 7.2 plate
S
▲
▲
▲
7.5
▲
▲
Scotia plate
▲
▲
▲
5 centimeters Antarctic plate

per year
FIGURE 15.27 This map illustrates directions and rates of plate motion in centimeters per year. The red arrows
show plate motion at selected locations based on GPS data. The small black arrows and labels show seafloor
spreading velocities. (Seafloor data from DeMets and others; GPS data from Jet Propulsion Laboratory)
evidence. Techniques using GPS devices Plate–Mantle Convection

have also been useful in establishing small- From geophysical evidence, we have learned that although the mantle consists almost
scale crustal movements such as those that entirely of solid rock, it is hot and weak enough to exhibit fluidlike convective flow. The
occur along faults in regions known to be simplest type of convection is analogous to heating a pot of water on a stove (FIGURE 15.28).
tectonically active. Heating the base causes the material to rise in relatively thin sheets or blobs that spread
out at the surface and cool. Eventually, the surface layer thickens (increases in
CONCEPT CHECK 15.10 density) and sinks back to the bottom where it is reheated until it
achieves enough buoyancy to rise again.
1 Briefly describe how hot-spot tracks can be Mantle convection is considerably more complex than
used to determine the rate of plate
the model just described. The shape of the mantle does not
motion.
resemble that of a cooking pot. Rather it is a spherically
2 Refer to Figure 15.27 and determine which
shaped zone with a much larger upper boundary (Earth’s
three plates appear to exhibit the highest Cooler
rates of motion. surface) than lower boundary (core–mantle boundary). water
Furthermore, mantle convection is driven by a combination sinks
of three thermal processes: heating at the bottom by heat
loss from Earth’s core; heating from within by the decay of
radioactive isotopes; and cooling from the top that creates
What Drives Plate thick, cold lithospheric slabs that sink into the mantle. Warm
water
When seafloor spreading was first introduced, geolo-
Motions? gists proposed that the main driving force for plate
rises
The plate tectonics theory describes plate motion was upwelling that came from deep in the man-
motion and the role that this motion plays tle. Upon reaching the base of the lithosphere, this flow FIGURE 15.28 Convection is a type of
in generating and modifying the major was thought to spread laterally and drag the plates heat transfer that involves the actual
features of Earth’s crust. Therefore, accept- along. Thus, plates were viewed as being carried pas- movement of a substance. Here the
stove warms the water in the bottom
ance of plate tectonics does not rely on sively by convective flow in the mantle. Based on physi-
of a cooking pot. The heated water
knowing precisely what drives plate cal evidence, however, it became clear that upwelling expands, becomes less dense (more
motion. This is fortunate, because none of beneath oceanic ridges is quite shallow and not related buoyant), and rises. Simultaneously,
the models yet proposed can account for all to deep circulation in the lower mantle. It is the hori- the cooler, denser water near the
major facets of plate tectonics. zontal movement of lithospheric plates away from the top sinks.
What Drives Plate Motions? 387
ridge that causes mantle upwelling, not the Ridge push

other way around. Thus, modern models is a gravity driven force that
results from the elevated
have plates being an integral part of mantle position of the ridge
convection and perhaps even its most
active component. Ocean ridge
Although convection in the mantle is
still poorly understood, researchers gener-
Trench
ally agree on the following:
Frictional resistance
1. Convective flow in the rocky 2900- between the
overriding plate
kilometer-thick mantle—in which and subducing
warm, buoyant rock rises and cooler, slab
denser material sinks—is the underly-
ll
ing driving force for plate movement.
pu
Mantle drag Mantle drag
ab
2. Mantle convection and plate tectonics resists enhances plate motion
Sl
subduction when the velocity of
are part of the same system. Subduct- Slab pull the asthenosphere
ing oceanic plates drive the cold results from the exceeds that of the plate
sinking of a cold,
downward-moving portion of convec- dense slab of
tive flow while shallow upwelling of lithosphere
hot rock along the oceanic ridge and
buoyant mantle plumes are the FIGURE 15.29 Illustration of some of the forces that act on tectonic plates.
upward-flowing arms of the convective
mechanism. pull. The primary evidence for this comes Models of Plate–Mantle
3. Convective flow in the mantle is the from comparing rates of seafloor spreading Convection
primary mechanism for transporting along ridge segments having different eleva- Any acceptable model for plate–mantle con-
heat away from Earth’s interior to the tions. For example, despite its greater aver- vection must explain compositional varia-
surface where it is eventually radiated age height above the seafloor, spreading tions known to exist in the mantle. For
into space. rates along the Mid-Atlantic Ridge are con- example, the basaltic lavas that erupt along
siderably less than spreading rates along the oceanic ridges, as well as those that are gen-
What is not known with any high degree of
less steep East Pacific Rise (see Figure erated by hot-spot volcanism, such as those
certainty is the exact structure of this con-
15.27). In addition, fast-moving plates are found in Hawaii, have mantle sources. Yet
vective flow. First, we will look at some of
being subducted along a larger percentage ocean ridge basalts are very uniform in com-
the forces that contribute to plate motion,
of their margins than slow-moving plates. position and depleted in certain elements.
and then we will examine two models
This fact supports the notion that slab pull Hot-spot eruptions, on the other hand, have
that have been proposed to describe
is a more significant driving force than ridge high concentrations of these elements and
plate–mantle convection.
push. Examples of fast-moving plates that tend to have varied compositions. Because
have extensive subduction zones along their basaltic lavas that arise from different tec-
Forces That Drive Plate margins include the Pacific, Nazca, and tonic settings have different compositions,
Motion Cocos plates. they are assumed to be derived from chemi-
There is general agreement that the subduc- Although slab pull and ridge push cally distinct mantle reservoirs.
tion of cold, dense slabs of oceanic litho- appear to be the dominant forces acting on
sphere is a major driving force of plate plates, they are not the only forces that LAYERING AT 660 KILOMETERS. Some
motion (FIGURE 15.29). As these slabs sink influence plate motion. Beneath plates, researchers argue that the mantle
into the asthenosphere, they “pull” the convective flow in the mantle exerts a force, resembles a “giant layer cake” divided at a
trailing plate along. This phenomenon, perhaps best described as “mantle drag” depth of 660 kilometers. As shown in
called slab pull, occurs because cold slabs (Figure 15.29). When flow in the astheno- FIGURE 15.30A, this layered model has two
of oceanic lithosphere are more dense than sphere is moving at a velocity that exceeds zones of convection—a thin, dynamic layer
the underlying asthenosphere and hence that of the plate, mantle drag enhances plate in the upper mantle and a thick, sluggish
“sink like a rock.” motion. However, if the asthenosphere is one located below. This model successfully
Another important driving force is moving more slowly than the plate, or in explains why basaltic lavas that erupt along
ridge push (Figure 15.29). This gravity- the opposite direction, this force tends to the oceanic ridges have a different chemical
driven mechanism results from the elevated resist plate motion. Another type of resist- make-up than those that erupt in Hawaii as
position of the oceanic ridge, which causes ance to plate motion occurs along subduc- a result of hot-spot activity. The mid-ocean
slabs of lithosphere to “slide” down the tion zones. Here friction between the ridge basalts come from the upper convec-
flanks of the ridge. Ridge push appears to overriding plate and the descending slab tive layer, which is well mixed, whereas
contribute far less to plate motions than slab generates significant earthquake activity. the mantle plume that feeds the Hawaiian
Oceanic Ridge push volcanoes taps a deeper, more primitive magma source that resides in the
lithosphere
lower convective layer.
However, data gathered from the study of earthquake waves have shown
that at least some subducting oceanic slabs penetrate the 660-kilometer
Slab-pull boundary and descend deep into the mantle. The subducting litho-
at trench Upper mantle
sphere should serve to mix the upper and lower layers together,
thereby destroying the layered structure proposed in this model.
Lower mantle WHOLE-MANTLE CONVECTION. Other researchers favor

some type of whole-mantle convection in which cold oceanic
lithosphere sinks to great depths and stirs the entire mantle
(FIGURE 15.30B). One whole-mantle model suggests that the
burial ground for subducting slabs is the core-mantle
boundary. Over time, this material melts and buoyantly
rises toward the surface as a mantle plume, thereby trans-
Core
porting hot material toward the surface (Figure 15.30B).
A. Layering at 660 kilometers Recent work has predicted that whole-mantle convec-
tion would cause the entire mantle to completely mix in a
matter of a few hundred million years. This, in turn, would
Oceanic ridge eliminate chemically distinct magma sources—those that
Hot
Cool spot are observed in hot-spot volcanism and those associated
descending with volcanic activity along oceanic ridges. Thus, the
oceanic
plate Upper mantle whole-mantle model also has shortcomings.
Although there is still much to be learned about the
Hot
rising mechanisms that cause Earth’s tectonic plates to migrate
mantle across the globe, one thing is clear. The unequal distribution
plume
of heat in Earth’s interior generates some type of thermal
convection that ultimately drives plate–mantle motion.
Hot
rising CONCEPT CHECK 15.11
mega-plume
1 Describe slab pull and ridge push. Which of these forces
660 km
Core
appears to contribute more to plate motion?
2 What role are mantle plumes thought to play in the
B. Whole mantle convection Cool descending
oceanic plate convective flow of the mantle?
3 Briefly describe the two models proposed for mantle–
FIGURE 15.30 Proposed models for mantle convection. A. The “layer cake”
plate convection. What is lacking in each of these models?
model consists of two convection layers—a thin, convective layer above 660
kilometers and a thick one below. B. In this whole-mantle
convection model, cold oceanic lithosphere
descends into the lowermost mantle, while hot
mantle plumes transport heat toward the surface.
Plate Tectonics in the Future
Geologists have extrapolated present-day plate movements into the future. FIGURE 15.31
illustrates where Earth’s landmasses may be 50 million years from now if present plate
DID YOU KNOW? movements persist during this time span.
Because plate tectonic processes are In North America we see that the Baja Peninsula and the portion of southern California
powered by heat from Earth’s interior, that lies west of the San Andreas Fault will have slid past the North American plate. If this
the forces that drive plate motion will northward migration takes place, Los Angeles and San Francisco will pass each other in
cease sometime in the distant future.
about 10 million years, and in about 60 million years Los Angeles will begin to descend
The work of external forces (wind,
water, and ice), however, will continue into the Aleutian Trench.
to erode Earth’s surface. Eventually, If Africa continues on a northward path, it will collide with Eurasia, closing the
landmasses will be nearly flat. What a Mediterranean and initiating a major mountain-building episode (Figure 15.31). In other
different world it will be—an Earth with parts of the world, Australia will be astride the equator and, along with New Guinea, will
no earthquakes, no volcanoes, and no be on a collision course with Asia. Meanwhile, North and South America will begin to
mountains. separate, while the Atlantic and Indian Oceans continue to grow at the expense of the
Pacific Ocean.
Chapter in Review 389
▲ ▲
▲
▲
▲
DID YOU KNOW?
North Researchers have estimated that the continents join to
▲
▲
Eurasia ▲
▲
America ▲
form supercontinents roughly every 500 million years.
▲ ▲
Since it has been about 200 million years since Pangaea
▲
▲ ▲ ▲
▲
▲
Africa broke up, we have only 300 million years to wait before
▲
▲
▲
▲
▲ ▲ ▲
▲ ▲
▲
▲
▲ ▲ ▲
▲
the next supercontinent is completed.
▲
▲
▲
▲
▲
▲
▲
▲
▲
South
▲
Australia
▲
▲
America
▲
▲
▲
A few geologists have even speculated on the nature of

▲
▲
the globe 250 million years into the future. As shown in

▲
▲ ▲
▲
▲
FIGURE 15.32, the next supercontinent may form as a result of

subduction of the floor of the Atlantic Ocean, resulting in the
Antarctica
collision of the Americas with the Eurasian–African landmass.
FIGURE 15.31 The world as it may look 50 million years from now. (Modified Support for the possible closing of the Atlantic comes from a similar
after Robert S. Dietz, John C. Holden, C. Scotese, and others)
event when the proto-Atlantic closed to form the Appalachian and
Caledonian mountains. During the next 250 million years, Australia
is also projected to collide with Southeast Asia. If this scenario is
accurate, the dispersal of Pangaea will end when the continents
Eurasia reorganize into the next supercontinent.
North
America Such projections, although interesting, must be viewed
Africa with considerable skepticism because many assumptions
must be correct for these events to unfold as just
South Australia described. Nevertheless, changes in the shapes and posi-
America
tions of continents that are equally profound will undoubt-
edly occur for many hundreds of millions of years to come.
Only after much more of Earth’s internal heat has been lost
will the engine that drives plate motions cease.
Antarctica
CONCEPT CHECK 15.12
FIGURE 15.32 Reconstruction of Earth as it may
appear 250 million years into the future. 1 Briefly describe some major changes to the globe when we extrapolate present-day plate
movements 50 million years into the future.
C H A P T E R F I F T E E N
Plate Tectonics
in Review
In the early 1900s Alfred Wegener set forth his continental region called the asthenosphere. Further, the lithosphere is broken
drift hypothesis. One of its major tenets was that a supercontinent into several large and numerous smaller segments, called plates,
called Pangaea began breaking apart about 200 million years ago. that are in motion and continually changing in shape and size.
The rifted continental fragments then “drifted” to their present Plates move as relatively coherent units and are deformed mainly
positions. To support his hypothesis, Wegener used the fit of South along their boundaries.
America and Africa, fossil evidence, rock types and structures, and
Divergent plate boundaries occur where plates move apart,
ancient climates. One of the main objections to the continental
resulting in upwelling of material from the mantle to create new
drift hypothesis was its inability to provide an acceptable
seafloor. Most divergent boundaries occur along the axis of the
mechanism for the movement of continents.
oceanic ridge system and are associated with seafloor spreading.
By 1968, continental drift was replaced by a far more encom- New divergent boundaries may form within a continent (for
passing theory known as plate tectonics. According to plate example, the East African Rift Valleys), where they may fragment
tectonics, Earth’s rigid outer layer (lithosphere) overlies a weaker a landmass and develop a new ocean basin.
Convergent plate boundaries occur where plates move together, The theory of plate tectonics is supported by (1) the ages of
resulting in the subduction of oceanic lithosphere into the man- sediments from the floors of the deep-ocean basins; (2) the exis-
tle along a deep-ocean trench. Convergence of an oceanic and tence of island groups that formed over hot spots and that provide
continental block results in subduction of the oceanic slab and a frame of reference for tracing the direction of plate motion; and
the formation of a continental volcanic arc such as the Andes of (3) Paleomagnetism, the direction and intensity of Earth’s
South America. Oceanic–oceanic convergence results in an arc- magnetism in the geologic past.
shaped chain of volcanic islands called a volcanic island arc.
Two basic models for mantle convection are currently being
When two plates carrying continental crust converge, the
evaluated. Mechanisms that contribute to this convective flow are
buoyant continental blocks collide, resulting in the formation
slab pull and ridge push. Slab pull occurs where cold, dense
of a mountain belt as exemplified by the Himalayas.
oceanic lithosphere is subducted and pulls the trailing
Transform fault boundaries occur where plates grind past each lithosphere along. Ridge push results when gravity sets the
other without the production or destruction of lithosphere. Most elevated slabs astride oceanic ridges in motion. Hot, buoyant
transform faults join two segments of a mid-ocean ridge where mantle plumes are considered the upward flowing arms of mantle
they provide the means by which oceanic crust created at a ridge convection. One model suggests that mantle convection occurs
crest can be transported to its site of destruction—a deep-ocean in two layers separated at a depth of 660 kilometers (410 miles).
trench. Still others, like the San Andreas Fault, cut through Another model proposes whole-mantle convection that stirs the
continental crust. entire 2900-kilometer-thick (1800-mile-thick) rocky mantle.
Key Terms
asthenosphere (p. 369) fracture zones (p. 376) normal polarity (p. 382) seafloor spreading (p. 370)
continental drift (p. 363) hot-spot (p. 380) oceanic ridge slab pull (p. 387)
continental rift (p. 371) hot-spot track (p. 380) system (p. 368) spreading centers (p. 369)
continental volcanic island arc (p. 374) paleomagnetism (p. 382) subduction zones (p. 372)
arcs (p. 374) lithosphere (p. 368) Pangaea (p. 363) supercontinent (p. 363)
convergent boundaries (p. 369) lithospheric plates (p. 369) partial melting (p. 374) tectonic plates (p. 369)
Curie point (p. 381) magnetometers (p. 382) plate tectonics (p. 368) transform fault
deep-ocean trenches (p. 372) magnetic reversal (p. 382) reverse polarity (p. 382) boundaries (p. 369)
divergent boundaries (p. 369) magnetic time scale (p. 382) ridge push (p. 387) volcanic island arc (p. 374)
fossil magnetism (p. 382) mantle plume (p. 380) rift valley (p. 370)
GIVE IT SOME THOUGHT

1 After referring to the section in Chapter 1 entitled “The Nature of 2 Referring to the accompanying diagrams that illustrate the three
Scientific Inquiry” answer the following: types of convergent plate boundaries, complete the following:
a. What observation led Alfred Wegener to develop his continental a. Identify each type of convergent boundary.
drift hypothesis? b. Volcanic island arcs develop on what type of crust?
b. What evidence did he gather to support his proposal? c. Why are volcanoes largely absent where two continental blocks collide?
c. Why was the continental drift hypothesis rejected by the majority d. Describe two ways that oceanic–oceanic convergent boundaries
of the scientific community? are different from oceanic–continental boundaries? How are they
d. Do you think Wegener followed the basic principles of scientific similar?
inquiry? Support your answer.
Companion Website 391
3 Some predict that California will sink into the ocean. Is this idea 5 Volcanoes, such as the Hawaiian chain, that form over mantle
consistent with the theory of plate tectonics? Explain. plumes are some of the largest on Earth. However, several
4 Refer to the accompanying hypothetical plate map to answer the volcanoes on Mars are gigantic compared to those on Earth.
following questions: What does this difference tell us about how, or if, the process of
a. Portions of how many plates are shown? plate motion operates on Mars? Explain.
6 Imagine you are studying seafloor spreading along two different
b. Are continents A, B, and C moving toward or away from each
other? How did you determine your answer? oceanic ridges. Along the first ridge the magnetic stripes are uni-
formly narrow. Along the second ridge they are wide near the ridge
c. Explain why active volcanoes are found on both continent A and
crest, but they become narrower as you move away from the crest.
continent B.
What can you say about the history of motion in each example?
d. Why does continent C lack active volcanoes? Provide at least one 7 Australian marsupials (kangaroos, koala bears, etc.) have direct fossil
scenario in which volcanic activity might be triggered on this
links to marsupial opossums found in the Americas. Yet the modern
continent.
marsupials in Australia are markedly different from their American
relatives. How does the break-up of Pangaea help to explain these
differences (see Figure 15.26)?
8 Density is a key component in the behavior of Earth materials and is
especially important in understanding key aspects of plate tectonics.
Describe three different ways that density and /or density differ-
ences play a role in plate tectonics.
9 Refer to the accompanying map to complete the following:
a. List the cities (in pairs) that are moving farther apart as a result of
plate motion.
b. List the cities (in pairs) that are moving closer together as a result
of plate motion.
c. List the cities (in pairs) that are presently not moving relative to
each other.
Companion Website
www.mygeoscienceplace.com
The Essentials of Geology, 11e companion Website contains numerous multimedia resources accompanied by assessments
to aid in your study of the topics in this chapter. The use of this site's learning tools will help improve your understanding
of geology. Utilizing the access code that accompanies this text, visit www.mygeoscienceplace.com in order to:
• Review key chapter concepts.
• Read with links to the Pearson eText and to chapter-specific web resources.
• Visualize and comprehend challenging topics using the learning activities in GEODe: Essentials of Geology and the
Geoscience Animations Library.
• Test yourself with online quizzes.

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C H A P T E R F I F T E E N

P LATE TECTONICS IS THE the important concepts in this

economically significant mineral

This hiker is walking across glacial debris in Pakistan’s

From Continental chains (FIGURE 15.1). Furthermore,

Armed with these more advanced tools, FIGURE 15.2 Reconstructions of

DID YOU KNOW?

GLOSSOPTERIS. Wegener also cited the

Evidence: Ancient Climates swamps existed in several locations in the

FIGURE 15.7 Paleoclimatic evidence for continental drift. A. Glacial

FIGURE 15.8 Illustration of Earth’s major CONCEPT CHECK 15.3

North PLATE TECTONICS

The lithosphere, in turn, overlies a between two locations on different plates,

FIGURE 15.9 A mosaic of rigid

the ridge crest. Rift

The mechanism that

operates along the oceanic

average around 5 centime- ge t Arc

ts. Divergent boundaries can

Nazca overlying lithosphere

plate (FIGURE 15.11A). As a

Rift asthenosphere. In general, oceanic litho-

Spreading in the western Pacific, including the Mari-

center ana and Tonga trenches, tend to be even

Upwarping DID YOU KNOW?

Continental eventually build a chain of volcanic

Volcanic island arc (FIGURE 15.14A). Whereas oceanic

Continental volcanic arc

FIGURE 15.14 The ongoing

B. Position of India in relation 38 million

spectacular mountain range on Earth

The Alps were created by the collision of the

Fracture zone FIGURE 15.15 Transform fault boundaries.

Like the Mendocino Fault, most

ples are the earthquake-prone San

tures are preserved as linear topographic

zones roughly parallels the direction of Juan

Thus, these structures are useful in plate

plate plate zone

vide the means by which the oceanic crust

situation. Notice that the Juan de Fuca San Francisco

plate moves in a southeasterly direction,

eventually being subducted under the west PACIFIC ea

end of this plate is bounded by a transform

Testing the Plate Tectonics Model

Evidence: Ocean Drilling Thickness of sediments increase

dant in lava flows of basaltic

The JOIDES Resolution, one of the drilling ships

This is evidence that North America and 200 Ma 300 Ma

FIGURE 15.22 Time scale of Earth’s magnetic field in the recent

reference will need to be reevaluated. using space-age technology, researchers

5 centimeters Antarctic plate

evidence. Techniques using GPS devices Plate–Mantle Convection

ridge that causes mantle upwelling, not the Ridge push

Lower mantle WHOLE-MANTLE CONVECTION. Other researchers favor

form supercontinents roughly every 500 million years.

A few geologists have even speculated on the nature of

the globe 250 million years into the future. As shown in