that is associated with broad upwarping of the overlying lithosphere (figure 5.1 iA). As a result, the lithosphere is stretched, causing the brittle crustal rocks to break into large slabs. As the tectonic forces continue to pull the crust apart, these crustal fragments sink, generating an elongated depression called a continental rift (figure 5.1 ib). A modern example of an active continental rift is the East African Rift (figure s. i 2). Whether this rift will eventually result in the breakup of Africa is a topic of continued research. Nevertheless, the East African Rift is an excellent model of the initial stage in the breakup of a continent. Here, tensional forces have stretched and thinned the crust, allowing molten rock to ascend from the mantle. Evidence for recent volcanic activity includes several large volcanic mountains including Mount Kilimanjaro and Mount Kenya, the tallest peaks in Africa. Research suggests that if rifting continues, the rift valley will lengthen and deepen, eventually extending out to the margin of the landmass (r;<;ur.E 5.1 ic). At this point, the rift will become a narrow sea with an outlet to the ocean. The Red Sea, which formed when the Arabian Peninsula split from Africa, is a modern example of such a feature. Consequently, the Red Sea provides us with a view of how the Atlantic Ocean may have looked in its infancy (figure 5.1 id). QEOD^ Forces Within sSWHBe Plate Tectonics New lithosphere is constantly being produced at the oceanic ridges; however, our planet is not growing larger—its total surface area remains constant. A balance is maintained because older, denser portions of oceanic lithosphere descend into the mantle at a rate equal to seafloor production. This activity occurs along convergent (con = together, vergere = to move) boundaries, where two plates move toward each other and the leading edge of one is bent downward, as it slides beneath the other. Convergent boundaries are also called subduction zones, because they are sites where lithosphere is descending (being subducted) into the mantle. Subduction occurs because the density of the descending tectonic plate is greater than the density of the underlying asthenosphere. In general, oceanic lithosphere is more dense than the asthenosphere, whereas continental lithosphere is ( Upwarping figure 5.11 Continental rifting and the formation of a new ocean basin. A. The initial stage of con tinental rifting tends to include upwelling in the mantle that is associated with broad doming of the lith-osphere.Tensional forces and buoyant uplifting of the heated lithosphere cause the crust to be broken into large slabs. b. A s the crust is pulled apart, large slabs of rock sink, generating a rift valley. C. Further spreading generates a narrow sea, similar to the present-day Red Sea. D. Eventually, an expansive ocean basin and ridge system are created. )less dense and resists subduction. As a consequence, only oceanic lithosphere will subd ...

1. that is associated with broad upwarping of the overlying
lithosphere (figure 5.1 iA). As a result, the lithosphere is
stretched, causing the brittle crustal rocks to break into large
slabs. As the tectonic forces continue to pull the crust apart,
these crustal fragments sink, generating an elongated depression
called a continental rift (figure 5.1 ib).
A modern example of an active continental rift is the East
African Rift (figure s. i 2). Whether this rift will eventually
result in the breakup of Africa is a topic of continued research.
Nevertheless, the East African Rift is an excellent model of the
initial stage in the breakup of a continent. Here, tensional forces
have stretched and thinned the crust, allowing molten rock to
ascend from the mantle. Evidence for recent volcanic activity
includes several large volcanic mountains including Mount
Kilimanjaro and Mount Kenya, the tallest peaks in Africa.
Research suggests that if rifting continues, the rift valley will
lengthen and deepen, eventually extending out to the margin of
the landmass (r;<;ur.E 5.1 ic). At this point, the rift will become
a narrow sea with an outlet to the ocean. The Red Sea, which
formed when the Arabian Peninsula split from Africa, is a
modern example of such a feature. Consequently, the Red Sea
provides us with a view of how the Atlantic Ocean may have
looked in its infancy (figure 5.1 id).
QEOD^
Forces Within sSWHBe Plate Tectonics
New lithosphere is constantly being produced at the oceanic
ridges; however, our planet is not growing larger—its total
surface area remains constant. A balance is maintained because
older, denser portions of oceanic lithosphere descend into the
mantle at a rate equal to seafloor production. This activity
occurs along convergent (con = together, vergere = to move)
boundaries, where two plates move toward each other and the

2. leading edge of one is bent downward, as it slides beneath the
other.
Convergent boundaries are also called subduction zones,
because they are sites where lithosphere is descending (being
subducted) into the mantle. Subduction occurs because the
density of the descending tectonic plate is greater than the
density of the underlying asthenosphere. In general, oceanic
lithosphere is more dense than the asthenosphere, whereas
continental lithosphere is
(
Upwarping
figure 5.11
Continental rifting and the formation of a new ocean basin.
A.
The initial stage of con tinental rifting tends to include
upwelling in the mantle that is associated with broad doming of
the lith-osphere.Tensional forces and buoyant uplifting of the
heated lithosphere cause the crust to be broken into large slabs.
b.
A
s the crust is pulled apart, large slabs of rock sink, generating a
rift valley.
C.
Further spreading generates a narrow sea, similar to the present-
day Red Sea.
D.
Eventually, an expansive ocean basin and ridge system are
created.
)less dense and resists subduction. As a consequence, only
oceanic lithosphere will subduct to great depths.
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,

3. 180
Pg. 152
Convergent Boundaries I 53
(
figure
S.
1
2 East African
rift valleys and associated features.
)
Deep-ocean trenches are the surface manifestations produced as
oceanic lithosphere descends into the mantle (figure 5.13).
These linear depressions are remarkably long and deep. The
Peru-Chile trench along the west coast of South America is
more than 4500 kilometers (3000 miles) long and its base is as
much as 8 kilometers (5 miles) below sea level. The trenches in
the western Pacific, including the Mariana and Tonga trenches,
tend to be even deeper than those of the eastern Pacific.
Slabs of oceanic lithosphere descend into the mantle at angles
that vary from a few degrees to nearly vertical (90 degrees).
The angle at which oceanic lithosphere descends depends
largely on its density. For example, when a spreading center is
located near a subduction zone, as is the case along the Peru-
Chile trench, the subducting lithosphere is young and, therefore,
warm and buoyant. Because of this, the angle of descent is
small, which results in considerable interaction between the
descending slab and the overriding plate. Consequently, the

4. region around the Peru-Chile trench experiences great
earthquakes, including the 1960 Chilean earthquake—the largest
on record.
As oceanic lithosphere ages (gets farther from the spreading
center), it gradually cools, which causes it to thicken and
increase in density. In parts of the western Pacific, some
oceanic lithosphere is 180 million years old. This is the thickest
and densest in today's oceans. The very dense slabs in this
region typically plunge into the mantle at angles approaching 90
degrees. This largely explains the fact that most trenches in the
western Pacific are deeper than trenches in the eastern Pacific.
Although all convergent zones have the same basic
characteristics, they are highly variable features. Each is
controlled by the type of crustal material involved and the
tectonic setting. Convergent boundaries can form between two
oceanic plates, one oceanic and one continental plate, or two
continental plates.
Oceanic-Conti nental Convergence
Whenever the leading edge of a plate capped with continental
crust converges with a slab of oceanic lithosphere, the buoyant
continental block remains "floating," and the denser oceanic
slab sinks into the mantle (figure 5.i4a). When a descending
oceanic slab reaches a depth of about 100 kilometers (60 miles),
melting is triggered within the wedge of hot asthenosphere that
lies above it. But how does the subduction of a cool slab of
oceanic lithosphere cause mantle rock to melt? The answer lies
in the fact that water contained in the descending plate 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.
Sediments and oceanic crust contain a large amount of water
that is carried to great depths by a subducting plate. As the
plate plunges downward, heat and pressure drive water from the
voids in the rock. At a depth of roughly 100 kilometers, the
wedge of mantle rock is sufficiently hot that the introduction of

5. water from the slab below leads to some melting. This process,
called partial melting, is thought to generate about 10 percent
molten material, which is intermixed with unmelted mantle
rock. Being less dense than the surrounding mantle, this hot
mobile material gradually rises toward the surface. Depending
on the environment, these mantle-derived masses of molten rock
may ascend through the crust and give rise to a volcanic
eruption. However, much of this material never reaches the
surface; rather, it solidifies at depth—a process that thickens
the crust.
The volcanoes of the towering Andes are the product of molten
rock generated by the subduction of the Nazca plate beneath the
South American continent (figure 5.i4b). Mountain systems
such as the Andes, which are produced in part by volcanic
activity associated with the subduction of
Measuring the Size of Earthquakes 179
I ij r- rr/r
(
Distance in mil
es 500 1000 1500 2000 2500 3000
Distance in kilometers
figure
6.
ii
A travel-time graph is used to determine the distance to the
epicenter.The difference in arrival times of the first P and S
waves in the example is 5 minutes.Thus.
the epicenter is roughly 3400 kilometers (2100 miles) away.
) (
figure 6.1
2 Determining an earthquake epicenter using the distances ob
tained from three or more siesmic stations—a method called

6. trongutat/on.
)
Historically, seismologists have employed a variety of methods
to determine two fundamentally different measures that describe
the size of an earthquake—intensity and magnitude. The first of
these to be used was intensity—a measure of the degree of
earthquake shaking at a given locale based on observed damage.
Later, with the development of seismographs, it became
possible to measure ground motion using instruments. This
quantitative measurement, called magnitude, relies on data
gleaned from seismic records to estimate the amount of energy
released at an earthquake's source.
Intensity and magnitude provide useful, though different,
information about earthquake strength. Consequently, both
measures are used to describe earthquake severity.
Intensity Scales
Until a little more than a century ago, historical records
provided the only accounts of the severity of earthquake
shaking and destruction. Perhaps the first attempt to
"scientifically" describe the aftermath of an earthquake came
following the great Italian earthquake of 1857. By
systematically mapping effects of the earthquake, a measure of
the intensity of ground shaking was established. The map
generated by this study employed lines to connect places of
equal damage and hence equal ground shaking. Using this
technique, zones of intensity were identified, with the zone of
highest intensity located near the center of maximum ground
shaking and often (but not always) the earthquake epicenter
(FIGURE 6.13).
VI
_ i Oakland
San Francisco

7. San
V|| Jose
VIII VI
+ Epicenter
Santa Cruz
DID YOU KNOW?
During the 1811-1812 New Madrid earthquake, the ground
subsided as much as 4.5 meters (15 feet) and created Lake St.
Francis *est of the Mississippi and enlarged Reelfoot Lake to
the east. Other regions rose, creating temporary waterfalls in the
bed of the Mississippi River.
Montery
figure 6.13 Zones of destruction associated with the Loma
Prieta earthquake that devastated portions of California's San
Francisco Bay Area, January 1994. Intensity levels based on the
Modified Mercalli Intensity Scale. Note the areas of heavy
destruction in San Francisco and Oakland, located almost 100
kilometers (60 miles) from the epicenter.
I 80 CHAPTER 6 Restless Earth: Earthquakes, Geologic
Structures, and Mountain Building
(
24 sec
Seismograph record
0 10 20
) (
Distance, S-R km sec.
figure
6.
14

8. Illustration showing how the Richter magnitude of an earth
quake can be determined graphically using a seismograph record
from aWood-Anderson instrument. First, measure the height
(amplitude) of the largest wave on the seismogram (23 mm) and
then th
e distance to the focus using the time inter
val between S and P waves (24 seconds). Next, draw a line
between the distance scale (left) and the wave amplitude scale
(right). By doing this, you should obtain the Richter magnitude
(M
L
) of S. (Data from Cali
fornia Institute of Technology)
)In 1902, Giuseppe Mercalli developed a more reliable intensity
scale, which in a modified form is still used today. The
Modified Mercalli Intensity Scale, shown in Table 6.1, was
developed using California buildings as its standard. For
example, if some well-built wood structures and most masonry
buildings were destroyed by an earthquake, the affected area
would be assigned an intensity of X (ten) on the Mercalli scale
(Table 6.1).
Despite their usefulness in providing seismologists with a tool
to compare earthquake severity, intensity scales have significant
drawbacks. Such intensity scales are based on effects (largely
destruction) that depend not only on the severity of ground
shaking but also on factors such as building design and the
nature of surface materials. For example, the modest 6.9-
magnitude 1988 Armenian earthquake mentioned earlier was
extremely destructive, mainly because of inferior building
practices. A quake that struck Mexico City in 1985 was deadly
because of the soft sediment upon which part of the city rests.
Thus, the destruction wrought by an earthquake is frequently
not a good measure of the amount of energy that was unleashed.
Magnitude Scales
To more accurately compare earthquakes across the globe, a

9. measure was needed that does not rely on parameters that vary
considerably from one part of the world to another. As a
consequence, a number of magnitude scales were developed.
Richter Magnitude
In 1935 Charles Richter of the California Institute of
Technology developed the first magnitude scale using seismic
records. As shown in hgure 6.14 (top), the Richter scale is
based on the amplitude of the largest seismic wave (P, S, or
surface wave) recorded on a seismogram. Because seismic
waves weaken as the distance between the focus and the
seismograph increases, Richter developed a method that
accounts for the decrease in wave amplitude with increasing
distance. Theoretically, as long as equivalent instruments are
used, monitoring stations at various locations will obtain the
same Richter magnitude for each recorded earthquake. In
practice, however, different recording stations often obtain
slightly different Richter magnitudes for the same earthquake—
a consequence of the variations in rock types through which the
waves travel.
Table 6.1 Modified Mercalli Intensity Scale
I Not felt except by a very few under especially favorable
circumstances.
II Felt only by a few persons at rest, especially on upper floors
of buildings.
III Felt quite noticeably indoors, especially on upper floors of
buildings, but many people do not recognize it as an earthquake.
IV During the day felt indoors by many, outdoors by few.
Sensation like heavy truck striking building.
V Felt by nearly everyone, many awakened. Disturbances of
trees, poles, and other tall objects sometimes noticed.
VI Felt by all; many frightened and run outdoors. Some heavy
furniture moved; few instances of fallen plaster or damaged
chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in buildings of

10. good design and construction; slight-to-moderate in well-built
ordinary structures; considerable in poorly built or badly
designed structures.
VIII Damage slight in specially designed structures;
considerable in ordinary substantial buildings with partial
collapse; great in poorly built structures. (Fall of chimneys,
factory stacks, columns, monuments, walls.)
IX Damage considerable in specially designed structures.
Buildings shifted off foundations. Ground cracked
conspicuously.
X Some well-built wooden structures destroyed. Most masonry
and frame structures destroyed. Ground badly cracked.
XI Few, if any, (masonry) structures remain standing. Bridges
destroyed. Broad fissures in ground.
XII Damage total. Waves seen on ground surfaces. Objects
thrown upward into air.