Download Tectonic Forces, Rock Structure, and Landforms

391
Copyright and photograph by Dr. Parvinder S. Sethi
Copyright and photograph by Dr. Parvinder S. Sethi
T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
● FIGURE
● FIGURE
14.20
14.22
The igneous rock of this exposed dike in New Mexico was intruded into
a near-vertical fracture in weaker sandstone. Later much of the sandstone was eroded away, leaving the resistant dike exposed.
How do laccoliths deform the rocks they are intruded into?
How does a dike differ from a sill? How are they alike?
Anthony G. Taranto Jr., Palisades Interstate Park – NJ Section
Copyright and photograph by Dr. Parvinder S. Sethi
The La Sal Mountains in southern Utah, near Moab, are composed of
a laccolith that was exposed at the surface by uplift and subsequent
erosion of the overlying sedimentary rocks.
● FIGURE
14.23
Shiprock, New Mexico, is a volcanic neck exposed by erosion of surrounding rock. Volcanic necks are resistant remnants of the intrusive pipe of a
volcano.
● FIGURE
14.21
Sills develop where magma intrudes between parallel layers of surrounding rocks. The Palisades of the Hudson River, the impressive cliffs
found along the river’s western bank in the vicinity of New York City, are
made from a thick sill of igneous rock that was intruded between layers
of sedimentary rocks.
Why do you think this feature is called Shiprock?
volcano situated above it about 30 million years ago. Erosion has removed the volcanic cone, exposing the resistant dikes and neck that
were once internal features of the volcano at Shiprock.
Why does the sill at the Palisades form a cliff?
surrounding rocks. As it solidifies, the magma forms a wall-like structure of igneous rock known as a dike. When exposed by erosion,
dikes often appear as vertical or near-vertical walls of resistant rock
rising above the surrounding topography ( ● Fig. 14.22). At Shiprock,
in New Mexico, resistant dikes many kilometers long rise vertically to more than 90 meters (300 ft) above the surrounding plateau
( ● Fig. 14.23). Shiprock is a volcanic neck, a tall rock spire made
of the exposed (formerly subsurface) pipe that fed a long-extinct
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Tectonic Forces, Rock Structure,
and Landforms
Tectonic forces, which at the largest scale move the lithospheric
plates, also cause bending, warping, folding, and fracturing of
Earth’s crust at continental, regional, and even local scales. Such
deformation is documented by rock structure, the nature,
orientation, inclination, and arrangement of affected rock
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C H A P T E R 14 • V O LC A N I C A N D T E C TO N I C P R O C E S S E S A N D L A N D F O R M S
G EO G R A P H Y ’ S S PAT I A L SC I E N C E P E R S P EC T I V E
Spatial Relationships between Plate Boundaries,
Volcanoes, and Earthquakes
T
he geographic distributions of
volcanism and earthquake activity
are quite similar. Both tend to be
concentrated in linear patterns along the
boundaries of lithospheric plates. Although
the locations of volcanic and earthquake
activity correlate fairly well, there are exceptions, and their nature and severity differ
from place to place. In general, the frequency and severity of volcanic eruptions
or earthquakes vary according to their proximity to a specific type of lithospheric plate
boundary or specific site in the central part
of a plate.
Regardless of whether it breaks a
continent or the seafloor, plate divergence creates fractures that provide
avenues for molten rock to reach the
surface. The divergent midoceanic ridges
experience rather mild volcanic eruptions and small to moderate earthquakes
that originate at a shallow depth. People
are impacted when these volcanic and
tectonic activities occur on islands associated with midocean ridges, such as the
Azores and Iceland.
Volcanism also arises where continental crust is breaking and diverging. In these
regions, earthquakes tend to be small to
moderate, but continental crust mixed
with mafic magma produces a wider variety of volcanic eruptions, some of which
are potentially quite violent. Examples of
resulting volcanoes in the East African rift
valleys include Mount Kilimanjaro and
Mount Kenya.
The potential severity of earthquakes
and volcanic eruptions is much greater
where plates are converging rather than
diverging. Along the oceanic trenches
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where crustal rock material is subducted,
volcanoes typically develop along the
edge of the overriding plate. The largest
region where this occurs is the “Pacific
Ring of Fire,” the volcanically active and
earthquake-prone margin around the
Pacific Ocean. Where oceanic crust subducts beneath continental crust along
an oceanic trench, some of it melts
into magma that moves upward under
the continental crust. Subduction along
the Pacific Ocean is associated with
extensive volcanoes in the Andes, the
Cascades, and the Aleutians; the Kuril
Islands and the Kamchatka Peninsula
in Russia; and Japan, the Philippines,
New Guinea, Tonga, and New Zealand.
Many of these volcanoes erupt rock and
lava of andesitic composition and can
be dangerously explosive. Earthquakes
are also common events along the
Pacific Rim. Although most are small to
moderate, the largest earthquakes ever
recorded have been related to subduction in this region. Points where earthquakes originate along an oceanic trench
become deeper toward the overriding
plate, indicating the subducting plate’s
progress downward toward where it is
recycled into the mantle.
Another volcanic and seismic belt
occupies the collision zone between
northward-moving Southern Hemisphere
lithospheric plates and the Eurasian
plate. The volcanoes of the Mediterranean region, Turkey, Iran, and Indonesia
are located along this collision zone.
Seismic activity is common in that zone
and has included some major, deadly
earthquakes.
Transform plate boundaries, where
lateral sliding occurs, also experience
many earthquakes. The potential for
major earthquakes mainly exists in
places such as along the San Andreas
Fault zone in California where thick
continental crust is resistant to sliding
easily. Volcanic activity along transform
plate boundaries ranges from moderate
on the seafloor to slight in continental
locations.
Areas far from active plate boundaries
are not necessarily immune from earthquakes and volcanism. The Hawaiian
Islands, the Galapagos Islands, and the
Yellowstone National Park area are examples of intraplate “hot spots” located
away from plate margins and associated
with a plume of magma rising from the
mantle. Oceanic crustal areas that lie
over hot spots, like the Hawaiian Islands,
have strong volcanic activity and moderate earthquake activity. In midcontinental
areas large earthquakes occur in suture
zones where continents are colliding,
such as in the Himalayas, or where broken edges of ancient landmasses shift
even though they are today situated in
midcontinent and are deeply buried by
more recent rocks.
Volcanic and earthquake activities
that are located away from active plate
margins are intriguing and show that we
still have much to learn about Earth’s
internal processes and their impact on
the surface. Still, plate tectonics has contributed greatly to our understanding of
the variations in volcanism, earthquake
activity, and the landforms associated
with these processes.
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Smithsonian Institution Hologlobe Project with NASA/GSFC/SVS/GCRP/NOAA/USGS/NSF/DARPA/DMA/
New York Film and Animation Company/SGI/Hughes STX Corporation
T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
Smithsonian Institution Hologlobe Project with NASA/GSFC/SVS/GCRP/NOAA/USGS/NSF/DARPA/DMA/
New York Film and Animation Company/SGI/Hughes STX Corporation
The spatial correspondence among plate margins, active volcanoes, earthquake activity, and hot spots is not coincidental but is
strongly related to lithospheric plate boundaries. This map shows plate boundaries and the global distribution of active volcanoes
(1960–1994).
This map shows plate boundaries and the global distribution of earthquake activity (magnitude 4.5+, 1990–1995).
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C H A P T E R 14 • V O LC A N I C A N D T E C TO N I C P R O C E S S E S A N D L A N D F O R M S
layers. For example, rock layers that have undergone significant tectonic forces may be
tilted, folded, or fractured, or, relative to adjacent rocks masses, offset, uplifted, or downdropped. Sedimentary rocks are particularly
useful for identifying tectonic deformation
because they are usually horizontal when
they are formed, and older rock layers are
originally overlain by successively younger
rock layers. If strata are bent, fractured, offset, or otherwise out of sequence, some kind
of structural deformation has occurred.
Earth scientists describe the orientations
of inclined rock layers by measuring their
strike and dip. Strike is the compass direction of the line that forms at the intersection
of a tilted rock layer and a horizontal plane.
A rock layer, for example, might strike northeast, which could also be expressed correctly
as striking southwest ( ● Fig. 14.24). The inclination of the rock layer, the dip, is always
measured at right angles to the strike and in
degrees of angle from the horizontal (0° dip =
horizontal). The direction toward which the
rock dips down is expressed with the general
compass direction. For example, a rock layer
that strikes northeast and dips 11° from the
horizontal down to the southeast would
have a dip of 11° to the southeast (see again
Fig. 14.24).
Earth’s crust has been subjected to tectonic
forces throughout its history, although the forces
have been greater during some geologic periods
than others and have varied widely over Earth’s
surface. Most of the resulting changes in the
crust have occurred over hundreds of thousands
or millions of years, but others have been rapid
and cataclysmic.The response of crustal rocks to
tectonic forces can yield a variety of configurations in rock structure, depending on the nature of the rocks and the nature of the applied
forces.
Tectonic forces are divided into three
principal types that differ in the direction of the
applied forces ( ● Fig. 14.25). Compressional
tectonic forces push crustal rocks together.
Tensional tectonic forces pull parts of the
crust away from each other. Shearing tectonic forces slide parts of Earth’s crust past
each other.
NE
IKE
Co
ng
● FIGURE
Sa
nd
lom
Granite
era
te
Sh
ale
sto
ne
Sa
Horizontal
nd
sto
ne
DIP
40° SE
14.24
Geoscientists use the properties of strike and dip to describe the orientation of sedimentary rock
layers. Strike is the compass direction of the line created by the intersection of a rock layer with
a horizontal plane. Dip is the angle from the horizontal and compass direction toward which the
rock layer angles down. Dip direction lies at a 90° angle to the strike.
What are the strike and dip of the upper layer of sandstone in this diagram?
Compression
(a)
Tension
(b)
Shear
(c)
● FIGURE
14.25
Three types of tectonic force cause deformation of rock layers. (a) Compressional forces push
rocks together. Compressional forces can bend (fold) rocks, or they can cause the rocks to break
and slide along the breakage zone, which is called a fault. (b) Tensional forces pull rocks apart
and may also lead to the breaking and shifting of rock masses along faults. (c) Shearing forces
work to slide rocks past each other horizontally, rather than into or away from each other. If the
shearing forces are greater than the resistance of the rocks to them, the rocks will break and
slide in opposite directions past each other along the breakage zone (fault).
Compressional Tectonic Forces
Tectonic forces that push two areas of crustal rocks together
tend to shorten and thicken the crust. How the affected rocks
respond to compressional forces depends on how brittle (breakable) the rocks are and the speed with which the forces are applied.
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R
ST
SW
Folding, which is a bending or wrinkling of rock layers, occurs
when compressional forces are applied to rocks that are ductile
(bendable), as opposed to brittle. Rocks that lie deep within the
crust and that are therefore under high pressure are generally
ductile and particularly susceptible to behaving plastically, that is,
deforming without breaking. As a result rocks deep within the
crust typically fold rather than break in response to compressional
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T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
J. Petersen
forces ( ● Fig. 14.26). Folding is also more likely than fracturing when the compressional forces are applied slowly. Eventually,
however, if the force per unit area, the stress, is great enough, the
rocks may still break with one section pushed over another.
As elements of rock str ucture, upfolds are called
anticlines, and downfolds are called synclines ( ● Fig. 14.27).
The rock layers that form the flanks of anticlinal crests and
synclinal troughs are the fold limbs. Folds in some rock layers are very small, covering a few centimeters, while
others are enormous with vertical distances between
the upfolds and downfolds measured in kilometers.
● FIGURE 14.26
Folds can be tight or broad, symmetrical or asymCompressional forces have made complex folds in these layers of sedimentary rock.
How can solid rock be folded without breaking?
metrical. Folds are symmetrical—that is, each limb
has about the same dip angle—if they formed by
compressional forces that were relatively equal from
both sides. If compressional forces were stronger
from one side, a fold may be asymmetrical, with the
dip of one limb being much steeper than that of
the other. Eventually, asymmetrically folded rocks
may become overturned and perhaps so compressed
that the fold lies horizontally; these are known as
recumbent folds (see again Fig. 14.27).
Much of the Appalachian Mountain system is an
example of folding on a large scale. Spectacular folds exist in the Rocky Mountains of Colorado,Wyoming, and
Montana and in the Canadian Rockies. Highly complex
folding created the Alps, where folds are overturned,
sheared off, and piled on top of one another. Almost all
mountain systems exhibit some degree of folding.
Rock layers that are near Earth’s surface, and not
under high confining pressures, are too rigid to bend
into folds when experiencing compressional forces.
If the tectonic force is large enough, these rocks will
break rather than bend and the rock masses will move
● FIGURE
14.27
Folded rock structures become increasingly complex as the applied compressional forces become more
unequal from the two directions.
Anticline
b
Li
Syn ine
cl
Syn ine
cl
b
m
m
Li
Simple fold
Symmetrical
(simple)
fold
Asymmetrical
fold
Overturn
Recumbent
Overthrust
Pressure increasingly one-sided
Increasingly distorted folds
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C H A P T E R 14 • V O LC A N I C A N D T E C TO N I C P R O C E S S E S A N D L A N D F O R M S
relative to each other along the fracture. Faulting is the slippage
or displacement of rocks along a fracture surface, and the fracture
along which movement has occurred is a fault. When compressional forces cause faulting either one mass of rock is pushed up
along a steep-angled fault relative to the other or one mass of rock
slides along a shallow, low-angle fault over the other. The steep,
high-angle fault resulting from compressional forces is termed a
reverse fault ( ● Fig. 14.28a). Where compression pushes rocks
along a low-angle fault so that they override rocks on the other
side of the fault, the fracture surface is called a thrust fault, and
the shallow displacement is an overthrust (Fig. 14.28b). In both
reverse and thrust faults, one block of crustal rocks is wedged up
relative to the other. Direction of motion along all faults is always
given in relative terms because even though it may seem obvious
that one block was pushed up along the fault, the other block
may have slid down some distance as well, and it is not always
possible to determine with certainty if one or both blocks moved.
Reverse or thrust faulting can also result from compressional
forces that are applied rapidly and in some cases to rocks that
have already responded to the force by folding. In the latter case,
the upper part of a fold breaks, sliding over the lower rock layers
along a thrust fault forming an overthrust. Major overthrusts occur along the northern Rocky Mountains and in the southern
● FIGURE
Appalachians. Together, recumbent folds and overthrusts are important rock structures that have formed in complex mountain
ranges such as the Andes, Alps, and Himalayas.
Tensional Tectonic Forces
Tensional tectonic forces pull in opposite directions in a way
that stretches and thins the impacted part of the crust. Rocks,
however, typically respond by faulting, rather than bending
or stretching plastically, when subjected to tensional forces.
Tensional forces commonly cause the crust to be broken
into discrete blocks, called fault blocks, that are separated
from each other by normal faults (Fig. 14.28c). In order
to accommodate the extension of the crust, one crustal fault
block slides downward along the normal fault relative to the
adjacent fault block. Notice that the direction of motion along
a normal fault is opposite to that along a reverse or thrust fault
(see again Fig. 14.28a).
In map view, regional scale tensional forces frequently cause
a roughly parallel succession of normal faults to occur, creating
a series of alternating downdropped and upthrown fault blocks.
Each block that slid downward between two normal faults, or
that remained in place while blocks on either side slid upward
14.28
The major types of faults are illustrated here along with the direction of tectonic forces that cause them
(indicated by large arrows). Compressional forces may create reverse (a) or thrust (b) faults. Tensional
tectonic forces break rocks along normal faults (c). Shearing forces move rocks horizontally past each other
along strike-slip faults (d).
How does motion along a normal fault differ from that along a reverse fault?
(a) Reverse fault
(a)
Reverse fault
(b) Thrust fault or overthrust
(b)
Thrust fault or overthrust
(c) Normal fault
(d) Strike-slip fault
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T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
Courtesy Sheila Brazier
along the faults, is called a graben ( ● Fig. 14.29). A fault block
that moved relatively upward between two normal faults—that is,
it actually moved up or remained in place while adjacent blocks
slid downward—is a horst. The great Ruwenzori Range of East
Africa is a horst, as is the Sinai Peninsula between the fault troughs
in the Gulfs of Suez and Aqaba (see again Fig. 13.31). Horsts and
grabens are rock structural features that can be identified by the
nature of the offset of rock units along normal faults. Topographically, horsts form mountain ranges and grabens form basins. The
Basin and Range region of the western United States that extends
eastward from California to Utah and southward from Oregon to
New Mexico is an area undergoing tensional tectonic forces that
are pulling the region apart to the west and
east. A transect from west to east across that
● FIGURE 14.29
region, for example from Reno, Nevada, to
Horsts and grabens are blocks of Earth material that are bounded by normal faults. A block that
Salt Lake City, Utah, encounters an extenhas moved upward along a normal fault relative to adjacent blocks is a horst. A block that has slid
sive series of alternating downdropped and
down along a normal fault relative to adjacent blocks is a graben.
upthrown fault blocks comprising the basins
What kind of tectonic force causes these kinds of fault blocks?
and ranges for which the region is named.
Some of the ranges and basins are simple
Graben
Horst
Graben
horsts and grabens, but others are tilted
fault blocks that result from the uplift of
one side of a fault block while the other end
of the same block rotates downward ( ● Fig.
14.30). Death Valley, California, is a classic
example of the down-tilted side of a tilted
fault block ( ● Fig. 14.31).
Large-scale tensional tectonic forces
can create rift valleys, which are composed of relatively narrow but
● FIGURE 14.30
long reg ions of crust downThis diagram of a tilted fault block indicates its strike and dip. The east-facing cliff is an erosion-modified fault
dropped along nor mal faults.
scarp. This configuration is a simplified version of the kind of faulting that produced Death Valley, which
Examples of rift valleys include
occupies the downtilted part of a tilted fault block.
the Rio Grande r ift of New
Mexico and Colorado, the Great
)
Rift Valley of East Africa, and the
d
N
e
ifi
od
IKE
m
Dead Sea rift valley where that
(
R
rp
ST
sca
body of water lies at an elevation
t
l
Fau
S
some 390 meters (1280 ft) below
DIP
30° W
the Mediterranean Sea, which is
Tilted fault
only 64 kilometers (40 mi) away.
block
Rift valleys also run along the
centers of oceanic ridges.
A n escar pment, o f t e n
● FIGURE 14.31
shortened to scarp, is a steep
Death Valley, California, is a classic example of a topographic basin created by tilted fault blocks. The valley
cliff, which may be tall or short.
floor is 86 meters (282 ft) below sea level, which is the lowest elevation in North America.
Scarps can form on Earth surface terrain for many reasons
and in many different settings. A
cliff that results from movement
along a fault is specifically a fault
scar p. Fault scarps are commonly visible in the landscape
along normal fault zones, where
they may consist of rock faces on
fault blocks that have undergone
extensive amounts of uplift over
long periods of time. Piedmont
fault scarps offset unconsolidated
sediments that have been eroded
from uplifted fault blocks and deposited along the base of the fault
block ( ● Fig. 14.32).
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side was faulted upward and the west side tilted
down (see again Fig. 14.30). The equally dramatic Grand Tetons of Wyoming also rise along
a fault scarp facing eastward. In Big Bend
National Park, Texas, the fault block that forms
the walls of Santa Elena Canyon is an excellent
example of a fault scarp. Other than the 500meter-deep canyon that the Rio Grande has
cut, the fault block is modified so little by erosion that it preserves much of its blocklike shape
( ● Fig. 14.34). In the southwestern United
States, the Colorado Plateau steps down to the
Great Basin by a series of fault scarps that face
westward in southern Utah and northern Arizona.
Major uplift of faulted mountain ranges
can have a strong impact on other physical
systems, and an excellent example is the Sierra Nevada. As the mountains rose, stream
erosion accelerated because of the increase
in slope. Precipitation on the windward side
of the Sierra increased because of orographic
lifting. The steep lee side of the tilted fault
● FIGURE 14.32
block became more arid than before because
This piedmont fault scarp in Nevada is the topographic expression of a normal fault. Moveit was situated in the rain shadow of the Sierra.
ment along the fault that created this scarp occurred about 30 years before the photograph
Increased precipitation and lower temperatures
was taken.
at higher elevations changed the climate of the
On which side of the fault does the horst lie?
uplifted range significantly, and climate change
influenced the vegetation, soils, and animal life.
Fault scarps can account for spectacular mountain walls, esSoils have also been affected by increased runoff and erosion.
pecially in regions like much of the western United States with
The uplift of the Sierra has extended over several million years
a history of recent tectonic activity. The east face of the 645in an episodic sequence of faulting. The Sierra Nevada Range
kilometer-long (405 mi) Sierra Nevada Range in California is
is continuing to rise rapidly, in a geologic sense—on average
a classic example of a fault scarp that rises steeply 3350 meters
about a centimeter per year. Weathering and erosion have at(11,000 ft) above the desert ( ● Fig. 14.33). In contrast, the west
tacked the rocks as uplift progressed. The Sierra Nevada, like
side of the Sierra (the “back slope”) descends very gently over a
most high mountain ranges, have been altered and etched by
distance of 100 kilometers (60 mi) through rolling foothills. The
glaciation, stream erosion, and downslope gravitational moveSierra Nevada Range is a great tilted fault block where the east
ment of rock material. These processes have carved and shaped
● FIGURE
14.33
©Terry Husebye/Getty Images
The east front of the Sierra Nevada in California is essentially the steep scarp side of a tilted fault block.
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T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
J. Petersen
and weakened by faulting marks the trace of
the San Andreas Fault zone ( ● Fig. 14.35).
The amount that Earth’s surface can be
offset during instantaneous movement along
a fault varies from fractions of a centimeter
to several meters. Faulting can move rocks
laterally, vertically, or both. The maximum
horizontal displacement along the San Andreas Fault in California during the 1906 San
Francisco earthquake was more than 6 meters (21 ft). A vertical displacement of more
than 10 meters (33 ft) occurred during the
Alaskan earthquake of 1964. Over millions
of years, the cumulative displacement along a
major fault may be tens of kilometers vertically or hundreds of kilometers horizontally,
although the majority of faults have offsets
that are much smaller.
● FIGURE
14.34
The steep fault scarp at Santa Elena Canyon, along the Texas–Mexico border, has undergone
limited modification by weathering and erosion. The Rio Grande has cut a canyon into the
uplifted and tilted fault block. In this photo, the wall to the left of the canyon is in Mexico and
that to the right is in the United States.
Shearing Tectonic Forces
Vertical displacement along a fault occurs when the rocks on
one side move up or drop down in relation to rocks on the
other side. Faults with this kind of movement, up or down along
the dip of the fault plane extending into Earth, are known as
dip-slip faults. Normal and reverse faults, for example, have
dip-slip motion. There also exists, however, a completely different category of fault along which displacement of rock units
is horizontal rather than vertical. In this case, the direction of
slippage is parallel to the surface trace, or strike, of the fault; thus
it is called a strike-slip fault or, because of the horizontal motion, a lateral fault (see again Fig. 14.28d). Offset along strikeslip faults is most easily seen in map view (from above), rather
than in cross-sectional view. Active strike-slip faults can cause
horizontal displacement of roads, railroad tracks, fences, streambeds, and other features that extend across the fault. The motion
along a strike-slip fault is described as left lateral or right lateral,
depending on the direction of movement of the blocks. To determine whether motion is left or right lateral, imagine yourself
standing on one block and looking across the strike-slip fault to
the other block. The relative direction of motion of the block
across the fault determines whether it is a left lateral or right
lateral fault. The San Andreas Fault, which runs through much of
California, has right lateral strike-slip movement. A long and narrow, rather linear valley composed of rocks that have been crushed
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Tectonic activity can result in a variety of
structural features that range from microscopic
fractures to major folds and fault blocks. At
the surface, structural features comprise various topographic features (landforms) and are subject to modification by weathering,
erosion, transportation, and deposition. It is important to distinguish
between structural elements and topographic features because rock
structure reflects endogenic factors while landforms reflect the balance between endogenic and exogenic factors. As a result, a specific
● FIGURE
14.35
The San Andreas Fault along the Carrizo Plain in California runs
from left to right across the center of this photo. The area west
(background) of the fault is moving northwestward, in relation
to the area on the east (foreground) side. Valleys of creeks that
cross the fault have been offset about 130 meters (427 ft) by
numerous episodes of earthquake displacement.
What type of fault is the San Andreas?
USGS/R.E. Wallace
valleys in the Sierran fault block, leaving the spectacular canyons
and mountain peaks.
Relationships between
Rock Structure
and Topography
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G EO G R A P H Y ’ S E N V I R O N M E N TA L SC I E N C E P E R S P EC T I V E
Mapping the Distribution of Earthquake Intensity
W
hen an earthquake strikes a
populated area, one of the first
pieces of scientific information
released is the magnitude of the tremor.
Magnitude is a numerical expression of an
earthquake’s size at its focus in terms of
energy released. In this sense, earthquakes
can be compared to explosions. For example, a magnitude 4.0 earthquake releases
energy equivalent to exploding 1000 tons
of TNT. Because the scale is logarithmic,
a 6.9 magnitude is the equivalent of 22.7
million tons of TNT.
Because of their greater energy, earthquakes of greater magnitude have the
potential to cause much more damage
and human suffering than those of smaller
magnitude, but the reality is much more
complex than that. A moderate earthquake in a densely populated area may
cause great injury and damage, while a
very large earthquake in an isolated region
may not affect humans at all. Many factors
relating to physical geography can influence an earthquake’s impact on people
and their built environment. In general, the
farther a location is from the earthquake
epicenter, the less the effect of shaking,
but this generalization does not apply in
every case. An earthquake in 1985 caused
great damage in Mexico City, including
the complete collapse of buildings, even
though the epicenter was 385 kilometers
(240 mi) away.
Montg
y
omer
ss
Van Ne
St.
ia St.
Californ
The Mercalli Scale of earthquake intensity (I–XII) was devised to measure
the impact of a tremor on people, their
homes, buildings, bridges, and other elements of human habitation. Although
every earthquake has only one magnitude, intensity can vary greatly from
place to place, so a range of intensities
will typically be encountered for a single
tremor. The impact of an earthquake on
a region varies spatially, and the patterns
of Mercalli intensity can be mapped.
Earthquake intensity maps use lines of
equal shaking and earthquake damage,
called isoseismals, expressed in Mercalli
intensity levels. Patterns of isoseismals are
useful in assessing what local conditions
San
Francisco
Bay
N
.
et
k
ar
Pacific
Ocean
Dolores St.
M
St
16th St.
Bay mud (in places covered
by artificial fill as of 1906)
Army St.
Alluvium (>30 m thick)
Alluvium (<30 m thick)
Bedrock
0
1
2
3
km
(a)
The location of different Earth materials during the 1906 San Francisco earthquake.
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T E C TO N I C F O R C E S , R O C K S T R U C T U R E , A N D L A N D F O R M S
contributed to spatial variations in shaking
and impact. Earthquake intensity factors
vary geographically according to the nature of the substrate, population density,
construction type and quality, and topography. Areas with unconsolidated Earth
materials, poor construction, or high population densities generally suffer more from
shaking and experience greater damage.
The 1906 San Francisco earthquake
and ensuing fire caused the destruction of a great many buildings, numerous injuries, and an estimated 3000
deaths. The fire resulted from earthquakedamaged electrical and gas lines. Neither the magnitude nor the intensity
scales existed in 1906. Subsequent
studies, however, suggest that the earthquake magnitude was about 8.3, and
cartographers have prepared maps of
the spatial distributions of Mercalli intensity. These show the great variations
in ground shaking and damage that the
earthquake caused. The geographic
patterns of the isoseismals have been
analyzed to explain why certain areas
suffered more than others did. Areas
of bedrock were shown to have experienced lower intensities (less damage)
in comparison to areas of unconsolidated Earth materials. Much of the
worst damage occurred on artificially
filled lands along the bayfront and on
areas that had been stream valleys but
were covered over with loose Earth
materials in order to construct buildings
on the land. In some cases, buildings
on one side of a street were destroyed,
Montg
ss St.
omery
Van Ne
ia St.
Californ
401
while those on the other side of the
street suffered little damage.
Analyzing the nature of sites where
intensities in an earthquake were
higher or lower than expected helps
us understand the reasons for spatial
variations in earthquake hazards. The
geographic patterns of Mercalli intensity
that are generated even by small tremors help in planning for larger earthquakes in the same area. The overall
patterns of ground shaking and isoseismals should be similar for a larger
earthquake having the same epicenter
as a smaller one, but the amount of
ground shaking, the level of intensity,
and the size of the area affected
would be greater for the larger
magnitude tremor.
San
Francisco
Bay
N
.
t
ke
ar
Pacific
Ocean
Dolores St.
M
St
16th St.
X+
VIII+
Army St.
VII-VIII
VI-VII
VI
0
1
2
3
km
(b)
Geographic patterns of Mercalli intensity caused by the 1906 earthquake. The areas of intensity VIII+ in the northeast
quarter of the city are on artificial landfill.
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402
C H A P T E R 14 • V O LC A N I C A N D T E C TO N I C P R O C E S S E S A N D L A N D F O R M S
located anywhere from near the surface
to a depth of 700 kilometers (435 mi).
c.
The earthquake epicenter is the point
on Earth’s surface that lies directly above
a.
the focus and is where the strongest
shock is normally felt ( ● Fig. 14.37).
The vast majority of earthquakes are
so slight that we cannot feel them, and
they produce no injuries or damage. Most
earthquakes occur at a focus that is deep
● FIGURE 14.36
enough so that no displacement is visThis example cross section from a region of folded rocks illustrates the distinction between rock
ible at the surface. Others may cause mild
structure and surface topography (landforms). Structure is the rock response to applied tectonic
shaking that rattles a few dishes, while a
forces. Rock structure may or may not be represented directly in the surface topography, which
depends on the nature and rate of exogenic as well as endogenic geomorphic processes. Strucfew are strong enough to topple buildings
tural upfolds do not always comprise topographic mountains, nor do all downfolds form valleys.
and break power lines, gas mains, and wa(a) The structure is an anticline, but the surface landform is a plain of low relief. (b) Here, the
ter pipes. Surface offset or ground shakerosionally resistant center of a downfold (a syncline) supports a mountain peak. (c) A valley has
ing during an earthquake can also trigger
been eroded into the crest of an anticline.
rockfalls, landslides, avalanches, and tsuWhy is it that not all anticlines form mountains?
namis. Aftershocks may follow the main
earthquake as crustal adjustments continue
type of structural element can assume a variety of topographic
to occur. Geophysicists are currently investigating the possibility that
expressions ( ● Fig. 14.36). For instance, an upfolded structural
foreshocks may alert us to major earthquakes, although evidence is
feature is an anticline even though geomorphically it may comat present inconclusive.
prise a ridge, a valley, or a plain, depending on erosion of broken
or weak rocks. Nashville, Tennessee, occupies a topographic valMeasuring Earthquake Size
ley, yet it is sited in the remains of a structural dome (a circular
domal anticline). Likewise, even though synclines are structural
An earthquake’s severity can be expressed in two ways: (1) the
downfolds, topographically a syncline may contribute to the forsize of the event as a physical Earth process, and (2) the degree
mation of a valley or a ridge. Some mountain tops in the Alps
of its impact on humans. These two methods may be related in
are the erosional remnants of synclines. Words like mountain, ridge,
that, all other factors being equal, powerful earthquakes should
valley, basin, and fault scarp are geomorphic terms that describe
the surface topography, while anticline, syncline, horst, graben, and
● FIGURE 14.37
normal fault are structural terms that describe the arrangement of
The point of energy release for an earthquake, that is, the location
rock layers. Elements of rock structure may or may not be directly
where movement along the fault began, is the earthquake focus, which
reflected in the surface topography. It is important to remember
is typically at some depth beneath the surface. The earthquake epicenthat the topographic variation on Earth’s surface results from the
ter is the location on Earth’s surface directly above the focus.
interaction of three major factors: endogenic processes that create
Why is the epicenter in this example not located where the fault
relief, exogenic processes that shape landforms and reduce relief,
crosses Earth’s surface?
and the relative strength or resistance of different rock types to
Surface fault trace
weathering and erosion.
b.
Earthquakes
Earthquakes, evidence of present-day tectonic activity, are
ground motions of Earth caused when accumulating tectonic
stress is suddenly relieved by displacement of rocks along a fault.
The sudden, lurching movement of crustal blocks past one another represents a release of energy that generates these internal earthquake motions, the seismic waves that were discussed in
Chapter 13 as helpful in understanding Earth’s interior. Seismic
waves, however, can also have a great impact on Earth’s surface. It
is primarily when these waves pass along the crustal exterior or
emerge at Earth’s surface from below that they cause the damage
and subsequent loss of life that we associate with major tremors.
The subsurface location where the rock displacement and resulting earthquake originated is the earthquake focus, which may be
55061_14_Ch14_p378_409 pp2.indd 402
Fault
Epicenter
Focus
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