tectonic forces, rock structure, and landforms

391T 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

surrounding rocks. As it solidifies, the magma forms a wall-like struc-ture 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 verti-cally 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

volcano situated above it about 30 million years ago. Erosion has re-moved the volcanic cone, exposing the resistant dikes and neck that were once internal features of the volcano at Shiprock.

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, or ientation, inclination, and arrangement of affected rock

● FIGURE 14.20The 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.How do laccoliths deform the rocks they are intruded into?

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● FIGURE 14.21Sills develop where magma intrudes between parallel layers of sur-rounding 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 does the sill at the Palisades form a cliff?

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● FIGURE 14.22The igneous rock of this exposed dike in New Mexico was intruded into a near-vertical fracture in weaker sandstone. Later much of the sand-stone was eroded away, leaving the resistant dike exposed.How does a dike differ from a sill? How are they alike?

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● FIGURE 14.23Shiprock, New Mexico, is a volcanic neck exposed by erosion of surround-ing rock. Volcanic necks are resistant remnants of the intrusive pipe of a volcano.Why do you think this feature is called Shiprock?

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55061_14_Ch14_p378_409 pp2.indd 39155061_14_Ch14_p378_409 pp2.indd 391 6/6/08 12:59:48 PM6/6/08 12:59:48 PM

C H A P T E R 1 4 • V O L C 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 S392

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 excep-tions, and their nature and severity differ from place to place. In general, the fre-quency and severity of volcanic eruptions or earthquakes vary according to their prox-imity 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 diver-gence creates fractures that provide avenues for molten rock to reach the surface. The divergent midoceanic ridges experience rather mild volcanic erup-tions and small to moderate earthquakes that originate at a shallow depth. People are impacted when these volcanic and tectonic activities occur on islands associ-ated with midocean ridges, such as the Azores and Iceland.

Volcanism also arises where continen-tal 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 vari-ety 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

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 sub-ducts 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 subduc-tion in this region. Points where earth-quakes 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 Mediterra-nean 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 earth-quakes and volcanism. The Hawaiian Islands, the Galapagos Islands, and the Yellowstone National Park area are ex-amples 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 moder-ate earthquake activity. In midcontinental areas large earthquakes occur in suture zones where continents are colliding, such as in the Himalayas, or where bro-ken 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 con-tributed greatly to our understanding of the variations in volcanism, earthquake activity, and the landforms associated with these processes.

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G E O G R A P H Y ’ S S P A T I A L S C I E N C E P E R S P E C T I V E

Spatial Relationships between Plate Boundaries, Volcanoes, and Earthquakes



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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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layers. For example, rock layers that have un-dergone significant tectonic forces may be tilted, folded, or fractured, or, relative to ad-jacent rocks masses, offset, uplifted, or down-dropped. 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, off-set, 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 direc-tion of the line that forms at the intersection of a tilted rock layer and a horizontal plane. A rock layer, for example, might strike north-east, which could also be expressed correctly as striking southwest ( ● Fig. 14.24). The in-clination 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 configu-rations in rock structure, depending on the na-ture 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 tec-tonic forces slide parts of Earth’s crust past each other.

Compressional Tectonic ForcesTectonic 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 (break-able) the rocks are and the speed with which the forces are applied.

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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● FIGURE 14.24Geoscientists 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?

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● FIGURE 14.25Three 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).

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forces ( ● Fig. 14.26). Folding is also more likely than fractur-ing 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 structure, upfolds are cal led 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 lay-

ers are very small, covering a few centimeters, while others are enormous with vertical distances between the upfolds and downfolds measured in kilometers. Folds can be tight or broad, symmetrical or asym-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 ex-ist 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

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 14.26Compressional forces have made complex folds in these layers of sedimentary rock.How can solid rock be folded without breaking?

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● FIGURE 14.27Folded rock structures become increasingly complex as the applied compressional forces become more unequal from the two directions.



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 compres-sional 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 oc-cur along the northern Rocky Mountains and in the southern

Appalachians. Together, recumbent folds and overthrusts are im-portant rock structures that have formed in complex mountain ranges such as the Andes, Alps, and Himalayas.

Tensional Tectonic ForcesTensional 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

● FIGURE 14.28The 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?

Reverse fault Thrust fault or overthrust(a) (b)(a) Reverse fault (b) Thrust fault or overthrust

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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. Topographi-cally, 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 region, for example from Reno, Nevada, to Salt Lake City, Utah, encounters an exten-sive series of alternating downdropped and upthrown fault blocks comprising the basins and ranges for which the region is named. Some of the ranges and basins are simple 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 com-

posed of relatively narrow but long regions of crust down-dropped along normal faults. Examples of rift valleys include the Rio Grande r ift of New Mexico and Colorado, the Great Rift Valley of East Africa, and the Dead Sea rift valley where that body of water lies at an elevation some 390 meters (1280 ft) below the Mediterranean Sea, which is only 64 kilometers (40 mi) away. Rift valleys also run along the centers of oceanic ridges.

An escarpment, o f ten shortened to scarp, is a steep cliff, which may be tall or short. Scarps can form on Earth sur-face terrain for many reasons and in many different settings. A cliff that results from movement along a fault is specifically a fault scarp. Fault scarps are com-monly 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 de-posited along the base of the fault block ( ● Fig. 14.32).


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● FIGURE 14.29Horsts and grabens are blocks of Earth material that are bounded by normal faults. A block that has moved upward along a normal fault relative to adjacent blocks is a horst. A block that has slid down along a normal fault relative to adjacent blocks is a graben.What kind of tectonic force causes these kinds of fault blocks?

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● FIGURE 14.30This diagram of a tilted fault block indicates its strike and dip. The east-facing cliff is an erosion-modified fault scarp. This configuration is a simplified version of the kind of faulting that produced Death Valley, which occupies the downtilted part of a tilted fault block.

● FIGURE 14.31Death Valley, California, is a classic example of a topographic basin created by tilted fault blocks. The valley floor is 86 meters (282 ft) below sea level, which is the lowest elevation in North America.

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Fault scarps can account for spectacular mountain walls, es-pecially in regions like much of the western United States with a history of recent tectonic activity. The east face of the 645-kilometer-long (405 mi) Sierra Nevada Range in California is a classic example of a fault scarp that rises steeply 3350 meters (11,000 ft) above the desert ( ● Fig. 14.33). In contrast, the west side of the Sierra (the “back slope”) descends very gently over a distance of 100 kilometers (60 mi) through rolling foothills. The Sierra Nevada Range is a great tilted fault block where the east

side was faulted upward and the west side tilted down (see again Fig. 14.30). The equally dra-matic 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 500-meter-deep canyon that the Rio Grande has cut, the fault block is modified so little by ero-sion 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 Si-erra 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 block became more arid than before because it was situated in the rain shadow of the Sierra. Increased precipitation and lower temperatures at higher elevations changed the climate of the uplifted range significantly, and climate change influenced the vegetation, soils, and animal life.

Soils have also been affected by increased runoff and erosion. The uplift of the Sierra has extended over several million years in an episodic sequence of faulting. The Sierra Nevada Range is continuing to rise rapidly, in a geologic sense—on average about a centimeter per year. Weathering and erosion have at-tacked the rocks as uplift progressed. The Sierra Nevada, like most high mountain ranges, have been altered and etched by glaciation, stream erosion, and downslope gravitational move-ment of rock material. These processes have carved and shaped

● FIGURE 14.32This piedmont fault scarp in Nevada is the topographic expression of a normal fault. Move-ment along the fault that created this scarp occurred about 30 years before the photograph was taken.On which side of the fault does the horst lie?

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● FIGURE 14.33The east front of the Sierra Nevada in California is essentially the steep scarp side of a tilted fault block.

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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 An-dreas Fault in California during the 1906 San Francisco earthquake was more than 6 me-ters (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 verti-cally or hundreds of kilometers horizontally, although the majority of faults have offsets that are much smaller.

Relationships between Rock Structure and TopographyTectonic 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 fea-tures (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 bal-ance between endogenic and exogenic factors. As a result, a specific

valleys in the Sierran fault block, leaving the spectacular canyons and mountain peaks.

Shearing Tectonic ForcesVertical 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 differ-ent 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 mo-tion, a lateral fault (see again Fig. 14.28d). Offset along strike-slip 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, stream-beds, 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 de-termine 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 nar-row, rather linear valley composed of rocks that have been crushed


● FIGURE 14.35The 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?

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● FIGURE 14.34The 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.



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 exam-ple, 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, earth-quakes 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 earth-quake 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 influ-ence 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.

The Mercalli Scale of earthquake in-tensity (I–XII) was devised to measure the impact of a tremor on people, their homes, buildings, bridges, and other ele-ments of human habitation. Although every earthquake has only one mag-nitude, 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

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The location of different Earth materials during the 1906 San Francisco earthquake.

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Mapping the Distribution of Earthquake Intensity

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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.

contributed to spatial variations in shaking and impact. Earthquake intensity factors vary geographically according to the na-ture of the substrate, population density, construction type and quality, and topog-raphy. Areas with unconsolidated Earth materials, poor construction, or high popu-lation densities generally suffer more from shaking and experience greater damage.

The 1906 San Francisco earthquake and ensuing fire caused the destruc-tion of a great many buildings, numer-ous injuries, and an estimated 3000 deaths. The fire resulted from earthquake- damaged electrical and gas lines. Nei-ther the magnitude nor the intensity scales existed in 1906. Subsequent studies, however, suggest that the earth-quake magnitude was about 8.3, and

cartographers have prepared maps of the spatial distributions of Mercalli in-tensity. 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 experi-enced lower intensities (less damage) in comparison to areas of uncon-solidated 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,

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 trem-ors help in planning for larger earth-quakes in the same area. The overall patterns of ground shaking and iso-seismals 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.

(b)



type of structural element can assume a variety of topographic expressions ( ● Fig. 14.36). For instance, an upfolded structural feature is an anticline even though geomorphically it may com-prise a ridge, a valley, or a plain, depending on erosion of broken or weak rocks. Nashville, Tennessee, occupies a topographic val-ley, yet it is sited in the remains of a structural dome (a circular domal anticline). Likewise, even though synclines are structural downfolds, topographically a syncline may contribute to the for-mation of a valley or a ridge. Some mountain tops in the Alps are the erosional remnants of synclines. Words like mountain, ridge, valley, basin, and fault scarp are geomorphic terms that describe the surface topography, while anticline, syncline, horst, graben, and normal fault are structural terms that describe the arrangement of rock layers. Elements of rock structure may or may not be directly reflected in the surface topography. It is important to remember that the topographic variation on Earth’s surface results from the interaction of three major factors: endogenic processes that create relief, exogenic processes that shape landforms and reduce relief, and the relative strength or resistance of different rock types to weathering and erosion.

EarthquakesEarthquakes, 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 an-other represents a release of energy that generates these inter-nal 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 result-ing earthquake originated is the earthquake focus, which may be

located anywhere from near the surface to a depth of 700 kilometers (435 mi). The earthquake epicenter is the point on Earth’s surface that lies directly above 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 enough so that no displacement is vis-ible at the surface. Others may cause mild shaking that rattles a few dishes, while a few are strong enough to topple buildings and break power lines, gas mains, and wa-ter pipes. Surface offset or ground shak-ing during an earthquake can also trigger rockfalls, landslides, avalanches, and tsu-namis. Aftershocks may follow the main earthquake as crustal adjustments continue

to occur. Geophysicists are currently investigating the possibility that foreshocks may alert us to major earthquakes, although evidence is at present inconclusive.

Measuring Earthquake SizeAn earthquake’s severity can be expressed in two ways: (1) the size of the event as a physical Earth process, and (2) the degree of its impact on humans. These two methods may be related in that, all other factors being equal, powerful earthquakes should

Surface fault trace

Epicenter

Fault

Focus

● FIGURE 14.37The point of energy release for an earthquake, that is, the location where movement along the fault began, is the earthquake focus, which is typically at some depth beneath the surface. The earthquake epicen-ter is the location on Earth’s surface directly above the focus.Why is the epicenter in this example not located where the fault crosses Earth’s surface?

a.

b.

c.

● FIGURE 14.36This example cross section from a region of folded rocks illustrates the distinction between rock structure and surface topography (landforms). Structure is the rock response to applied tectonic 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. Struc-tural upfolds do not always comprise topographic mountains, nor do all downfolds form valleys. (a) The structure is an anticline, but the surface landform is a plain of low relief. (b) Here, the erosionally resistant center of a downfold (a syncline) supports a mountain peak. (c) A valley has been eroded into the crest of an anticline.Why is it that not all anticlines form mountains?


tectonic forces, rock structure, and landforms

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