55061 15 Ch15 p410 437 pp2 - Cengage · Under what climatic conditions are chemical weathering processes ... Earth’s surface leading to its disintegration and decomposition

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15Weathering and Mass Wasting

Rocks exposed at Earth’s surface are subject to disintegra-tion and decomposition by physical and chemical weathering processes.

What is there about the environment at Earth’s surface that makes rocks vulnerable to this breakdown?Under what climatic conditions are chemical weathering processes generally more effective than physical weathering processes?

Not all rocks weather at the same rate in the same environmen-tal setting.

Why are some rocks more vulnerable to weathering than others in a given environment?What factors might cause different outcrops of the same rock type to vary in their vulnerability to weathering in the same region?

Weathering and mass wasting are exogenic processes, which means they originate at the surface of Earth and contribute to an overall decrease in relief.

Why are weathering and mass wasting the initial exogenic processes experienced by rock matter at Earth’s surface?How are weathering and mass wasting related to erosion, transportation, and deposition?

Various categories of mass wasting are distinguished on the basis of the materials moved and the nature of their motion.

Why is climate important in determining the type of mass wasting that can occur?Why is rock type important in determining the type of mass wasting that can occur?

Weathering and mass wasting are the most commonly occurring exogenic processes.

How do they affect the appearance of the landscape?What makes weathering and mass wasting important to people and society?

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CHAPTER PREVIEW

T he previous two chapters concerned the materials,

processes, and structures associated with the

construction of topographic relief at the surface of Earth.

However, the relief-building endogenic geomorphic

processes of tectonism and volcanism, which arise from

within Earth, are opposed by the relief-reducing exogenic

geomorphic processes, which originate at the surface.

Exogenic processes break down rocks and erode rock

materials from higher energy sites and transport them to

locations of lower energy. The relocation of rock fragments

can be accomplished by the force of gravity alone or with

the help of one of the geomorphic agents—flowing water,

wind, moving ice, or waves. This chapter focuses on the

exogenic processes that cause rocks to decay and on the

ways that erosion, transportation, and deposition of surficial

Earth materials are accomplished when gravity, rather than a

geomorphic agent, is the dominant factor in transporting the

Earth materials.

Gravity constantly pulls downward on all Earth surface

materials. Weakened rock and broken rock fragments

are especially susceptible to downslope movement by

gravity, which may be slow and barely noticeable, or rapid

and catastrophic. Slope instability causes costly damage

to buildings, roadways, pipelines, and other types of

construction, and is also responsible for injury and loss of life.

Opposite: Rockfall deposits in Glacier National Park, Montana, have accumulated into a talus cone, the slope of angular clasts.USGS/P. Carrara

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C H A P T E R 1 5 • W E AT H E R I N G A N D M A S S W A S T I N G412

Some gravity-induced slope movements are entirely natural in origin, but human actions contribute to the occurrence of others. Understanding the processes involved and circumstances that lead to slope movements can help people avoid these costly and often hazardous events.

Nature of Exogenic ProcessesEarth’s surface provides a harsh environment for rocks. Most rocks originate under much higher temperatures and pressures and in very different chemical settings than those occurring at Earth’s surface. Thus, surface and near-surface conditions of compara-tively low temperature, low pressure, and extensive contact with water cause rocks to undergo varying amounts of disintegration and decomposition ( ● Fig. 15.1). This breakdown of rock mate-rial at and near Earth’s surface is known as weathering. Rocks weakened and broken by weathering become susceptible to the other exogenic processes—erosion, transportation, and deposi-tion. A rock fragment broken (weathered) from a larger mass will be removed from that mass (eroded), moved (transported), and set down (deposited) in a new location. Together, weathering, ero-sion, transportation, and deposition actually represent a chain or continuum of processes that begins with the breakdown of rock.

Erosion, transportation, and deposition of weathered rock mate-rial often occurs with the assistance of a geomorphic agent, such as stream flow, wind, moving ice, and waves. Sometimes, however, the only factor involved is gravity. Gravity-induced downslope move-ment of rock material that occurs without the assistance of a geo-morphic agent, as in the case of a rock falling from a cliff, is known as mass wasting. Although gravity also plays a role in the redistribu-tion of rock material by geomorphic agents, the term mass wasting

is reserved for movement caused by gravity alone. Whether it is mass wasting or a geomorphic agent doing the work, fragments and ions of weathered rock are removed from high-energy locations and transported to positions of low energy, where they are deposited.

The variations in elevation at Earth’s surface as well as the appearance of various landforms reflect the opposing tendencies of endogenic and exogenic processes. The relief created by tec-tonism and volcanism will decrease over time if endogenic pro-cesses cease or operate at a slow rate compared to the exogenic processes ( ● Fig. 15.2). Rates of the exogenic processes depend on such factors as rock resistance to weathering and erosion, the amount of relief, and climate.

Different weathering processes often impart visually distinc-tive features to a landform or landscape. Typically, weathering, mass wasting, or one of the various geomorphic agents do not work alone in shaping and developing a landform. More often they work together to modify the landscape, and evidence of multiple processes can be discerned in the appearance of the resulting land-form ( ● Fig. 15.3). For example, both the northern Rockies and the Sierra Nevada were produced by tectonic uplift, but much of the spectacular terrain seen there today is the result of weathering, mass wasting, running water, and glacial activity that have sculpted the mountains and valleys, often in distinctive ways ( ● Fig. 15.4).

WeatheringEnvironmental conditions at and near Earth’s surface subject rocks to temperatures, pressures, and substances, especially water, that contribute to physical and chemical breakdown of exposed rock. Broken fragments of rock, called clasts, that detach from the original rock mass can be large or small. These detached

pieces continue to weather into smaller particles. Fragments may accumulate close to their source or be widely dispersed by mass wasting and the geo-morphic agents. Many weathered rock fragments be-come sediments deposited to form landforms such as floodplains, beaches, or sand dunes, while others blanket hillslopes and comprise regolith, the inorganic portion of soils. Weathering is the principal source of inorganic soil constituents, without which most vegetation could not grow. Likewise, ions chemically removed from rocks during weathering are trans-ported in surface or subsurface water to close or dis-tant locations. They are a major source of nutrients in terrestrial as well as aquatic ecosystems, including rivers, ponds, lakes, and the oceans.

The kinds of rock weathering fall into two ba-sic categories. Physical weathering, also known as mechanical weathering, disintegrates rocks, break-ing smaller fragments from a larger block or outcrop of rock. Chemical weathering decomposes rock through chemical reactions that change the original rock-forming minerals. Many different physical and chemical processes lead to rock weathering, and wa-ter plays an important role in almost all of them.

● FIGURE 15.1This boulder, which was once hard and solid, has been subjected to conditions at Earth’s surface leading to its disintegration and decomposition.Why are some exposed parts of the boulder darker than others?

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Physical WeatheringThe mechanical disintegration of rocks by physical weathering is especially important to landscape modification in two ways. First, smaller clasts are more easily eroded and transported than larger ones. Second, the breakup of a large rock into smaller ones en-courages additional weathering because it increases the surface area exposed to weathering processes. There are several ways by which rocks can be physically weathered. For example, a person breaking a rock with a hammer is carrying out physical weather-ing. Although people, animals, and plants are responsible for break-ing some rocks ( ● Fig. 15.5), most physical weathering occurs in other ways. Five principal types of physically weathering are discussed here.

Unloading Most rocks form under much higher pressures (weight per unit area) than the 1013.2 millibars (29.92 in. Hg), or 10 Newtons per square centimeter (15 lbs/in2), of average atmospheric pressure that exists at Earth’s surface. Intrusive igneous rocks solidify slowly deep beneath the surface under great pressure from the weight of the overlying rocks. Sedimen-tary rocks solidify partly or wholly due to compaction from the weight of overlying sediments. Most metamorphic rocks

originate when high pressure and tempera-ture substantially change preexisting rocks. Commonly, rocks that or iginated under conditions of high pressure from deep burial are uplifted through mountain-building tectonic processes to eventually be exposed at the surface. For example, a large pluton of the intrusive igneous rock granite can be uplifted in a fault block during tectonism.High elevation helps drive erosional strip-ping of the overlying rocks, and ultimately through this removal of overlying weight, the unloading process, the rock is exposed at the surface, where it is subjected to the low pressure of the atmosphere. As a result of the pressure differential between high pres-sure at depth and low atmospheric pressure, the outer few centimeters to meters of the rock mass expand outward toward the atmo-sphere ( ● Fig. 15.6). This expansion causes the rock mass to crack in a roughly concen-tric form. These expansion cracks are joints, not faults. Concentric sheets of granite or other massive rocks form, broken and sepa-rated by these concentric joints. The granite expands and breaks, weathering mechani-cally like this because of the erosional re-moval of overlying rock masses. This physical weathering process of unloading is especially common with granite, but can affect other types of rocks as well.

As the outer sheet of an unloaded rock breaks further, segments of it may slide off and slough away or weather away, reducing

the load on underlying rock and allowing additional concentric joints to form. The successive removal of these outer rock sheets is known as exfoliation. Weathering by unloading leads to ex-foliation, and each concentric broken layer of rock is called an exfoliation sheet. Unloaded granite rock masses often have a domelike surface form, a topographic feature called an exfoliation dome. Well-known examples of granite exfoliation domes are Stone Mountain in Georgia, Half Dome in Yosemite National Park, Sugar Loaf Mountain, which overlooks Rio de Janeiro, Brazil, and Enchanted Rock in central Texas ( ● Fig. 15.7).

Thermal Expansion and Contraction Many early Earth scientists believed that the extreme diurnal tempera-ture changes common in deserts caused physical weathering of rocks by expansion and contraction as they warmed and cooled. They cited widespread existence of split rocks in arid regions as evidence of the effectiveness of this thermal expansion and contraction weathering ( ● Fig. 15.8). Laboratory studies in the early 20th century seemed to refute this idea, but the notion re-mained somewhat popular, although controversial. Recent field studies in the American Southwest deserts lend support to the notion that alternating heating and cooling can indeed lead to

● FIGURE 15.2(a) Tectonic (endogenic) uplift is opposed by (b) the exogenic processes of weathering, mass wasting, erosion, transportation, and deposition that (c) eventually decrease relief at Earth’s surface significantly if no additional uplift occurs.

(c) Reduced relief

Uplift

(a) Tectonic uplift

DepositionDepositionErosion

(b) Exogenic processes dominate

Erosionalremnants

Depositionalplain

Depositionalplain

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the mechanical splitting of rocks. Less controversial has been the notion that differential thermal expansion and contraction of in-dividual mineral grains in coarse crystalline rocks contributes to granular disintegration, the breaking free of individual mineral

grains from a rock ( ● Fig.15.9). The broken rocks found after a forest or brush fire provide additional evidence for the effective-ness of thermal expansion and contraction weathering, but under much greater extremes of temperature.

Originalsurfacecreatedby uplift

Valley deepeningby stream erosion

Valley widening bymass wasting and

water erosion

Slope rougheningby freeze/thaw

Mass wasting Debrisaccumulation

Postglacialstreamincision

Valley enlargingby glacial

erosion (ice)

Freeze/thaw weathering

● FIGURE 15.3The landscape of most high mountain regions has been produced by at least three different phases of land-forming processes: stream cutting and valley formation during tectonic uplift; glacial enlargement of former stream valleys, with intense freeze—thaw weathering above the level of the ice; and postglacial weathering and mass wasting with recent stream cutting.How does the cross-sectional profile of the valley change at each phase?

● FIGURE 15.4In the foreground of this aerial view of mountain summits in the northern Rockies are the White Cloud Peaks in the Sawtooth Range of Idaho.Can you identify evidence of the three phases shown in Figure 15.3?

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Freeze–Thaw Weathering In areas subject to numerous diurnal cycles of freeze/thaw, water repeatedly freezing in fractures and small cracks in rocks contributes significantly to rock break-age by freeze–thaw weathering, sometimes referred to as frost weathering, or ice wedging. When water freezes, it expands in volume up to 9%, and this can cause large pressures to be exerted on the walls and bottom of the crack, widening it and eventually leading to a piece of rock breaking off ( ● Fig. 15.10). The damaging ef-fects of freezing water expanding is why vehicles driven in regions that experience freezing temperatures must have antifreeze instead

● FIGURE 15.5A growing tree has broken up a sidewalk and retaining wall made of concrete, just as trees and other plants can break up natural rock into smaller fragments, contributing to physical weathering.How might an animal cause physical weathering?

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● FIGURE 15.6Outward expansion due to weathering by unloading has caused this granite in the Sierra Nevada, California, to break, forming joints and sheets of rock that parallel the surface.

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● FIGURE 15.7Enchanted Rock in central Texas is a huge granite exfoliation dome.Why is granite so susceptible to unloading and exfoliation?

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● FIGURE 15.8This split rock may be the result of physical weathering by thermal expansion and contraction.

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of water in their radiator systems. Likewise, water pipes to and within buildings will burst if they are not sufficiently insulated to protect the water inside them from freezing. Freeze–thaw weath-ering is particularly effective in the upper-middle and lower-high latitudes, and results of freeze–thaw weathering are especially no-ticeable in mountainous regions near tree lines where angular blocks of rock attributed to freeze–thaw weathering are common ( ● Fig. 15.11). Freeze–thaw weathering is not significant at lower latitudes except in areas of high elevation.

Salt Crystal Growth The development of salt crystals in cracks, fractures, and other void spaces in rocks causes physical dis-integration in a way that is similar to freeze–thaw weathering. With salt crystal growth, water with dissolved salts accumulates in these

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spaces and then evaporates, and the growing salt crystals wedge pieces of rock apart. This physical weathering pro-cess is most common in arid regions and in rocky coastal locations where salts are abundant, but people also con-tribute to salt weathering when they use salt to melt ice from roads and sidewalks in winter. Salt crystal growth can lead to granular disintegration in coarse crystalline (intrusive igneous) rocks or to the removal of clastic particles from sedimentary rocks, especially sandstones. Continued salt weathering, accompanied by removal of the weathered fragments, contributes to the creation of numerous hollows, small caves, and large overhangs in exposed sandstone cliffs ( ● Fig. 15.12). Large caves and overhangs (called alcoves) provided important living spaces for prehistoric Native American communities in the American Southwest, for example, at Mesa Verde, Colorado, and Canyon de Chelly, Arizona.

Hydration In weathering by hydration, water molecules attach to the crystalline structure of a mineral without causing a permanent change in that mineral’s composition. The water molecules are able to join and leave the “host” mineral during hydration and dehydra-tion, respectively. A mineral will expand when hydrated and shrink when dehydrated. As in freeze–thaw and salt crystal growth weathering, when hydrating materials expand in cracks or voids, pieces of the rock can wedge apart. Clasts, mineral grains, and thin flakes can be bro-ken from a rock mass because of hydration weathering. Salts and clay minerals, which are clay-sized materials formed during chemical weathering, commonly oc-cupy cracks and voids in rocks and are subject to hydra-tion and dehydration.

Chemical WeatheringChemical reactions between substances at Earth’s surface and rock-forming minerals also work to break down rocks. In chemi-cal weathering, ions from a rock are either released into water or recombine with other substances to form new materials, such as clay minerals. New materials made by chemical weathering are more stable at Earth’s surface than the original rocks. The most important catalysts and reactive agents performing chemical weathering are water, oxygen, and carbon dioxide, all of which are common in soil, precipitation, surface water, groundwater, and air.

Oxidation Water that has regular contact with the atmo-sphere contains plenty of oxygen. When oxygen in water comes in contact with certain elements in rock-forming minerals, a chemi-cal reaction can occur. In this reaction, the element releases its bond with the mineral, leaving behind a substance with an altered chemical formula, and establishes a new bond with the oxygen. This chemical union of oxygen atoms with another substance to create a new product is called oxidation. Metals, particularly iron and aluminum, are commonly oxidized in rock weathering and

● FIGURE 15.10Water expands when it freezes into ice. The freezing of water within rock fractures creates a force of expansion great enough to cause some rock weathering.How important is freeze–thaw weathering where you live?

● FIGURE 15.9Weathering of this intrusive igneous boulder has taken the form of granular disinte-gration, as evidenced by the numerous, individual mineral grains that have collected around the base of the boulder.What other evidence exists on the boulder to suggest that it has been subjected to considerable weathering?

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form iron and aluminum oxides as the new products. Compared to the origi-nal rock, these oxides, including Fe

2O

3

and Al2O

3, are chemically more stable,

not as hard, larger in volume, and have a distinctive color. Iron oxides pro-duced in this way often have a red, or-ange, or yellow color, while oxidized aluminum from rock weathering fre-quently appears yellow ( ● Fig. 15.13). Oxidation of iron is very common, and when it affects steel and iron ob-jects we call it rust.

Carbonation and Solution Water at and near the surface also typi-cally contains a considerable amount of carbon dioxide obtained from the air and from decaying organic matter in the soil. Weathering by carbonation occurs when carbon dioxide in water reacts with rock material to produce bi-carbonate ions (HCO

3–) and other ions

that vary with the composition of the decomposing rock. Carbonation weath-ering is most effective on carbonate rocks (those containing CO

3), particu-

larly limestone, which is an abundant chemical precipitate sedimentary rock

● FIGURE 15.11Angular blocks of rock attributed to freeze–thaw weathering, like these near the tree line in Great Basin National Park, Nevada, are common in mountainous areas that experience numerous freeze–thaw cycles every year.Why are these rocks angular in shape rather than rounded?

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● FIGURE 15.12Physical weathering by salt crystal growth helps break apart rocks, especially along bedding planes exposed in cliffs, and on rocky seacoasts. Weathered fragments are removed by gravity, wind, and water, leaving behind hollows.Once a small hollow is formed, how might it impact further weathering at that site?

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composed of calcium carbonate (CaCO3). When water with suffi-

cient carbon dioxide comes into contact with limestone, the chemical reaction creates calcium ions as well as bicarbonate ions. During carbonation, the calcium and carbonate in limestone detach from each other, thereby decomposing the limestone ( ● Fig. 15.14). The weathering event occurs as follows: H

2O + CO

2 + CaCO

3 = Ca2+ +

2HCO3–. Water carries away the ions of calcium and bicarbonate

produced in the reaction. Similar reactions take place when other carbonate rocks undergo carbonation weathering. Because of the role of water in carbonation, limestone in particular, but carbonate rocks overall, tend to weather extensively in humid regions, but they are re-sistant and often form cliffs in arid climates. Because water can obtain carbon dioxide by moving through the soil, carbonation operates on rocks impacted by soil water or ground water in the subsurface as well as on rocks exposed at the surface.

Carbonation is a specific chemical reaction that leads to solu-tion, the dissolving of rock matter in water. Rock salt, which con-tains the mineral halite (NaCl), is especially susceptible to solution in water more directly, without involving the carbonation reaction. Most minerals that are insoluble or only slightly soluble in pure water will dissolve more readily if the water is acidic. Carbon dioxide in water creates acidic conditions because the two substances react to make car-bonic acid (H

2CO

3). Other acids present in water, derived primarily

from decaying organic matter in the soil, can also facilitate the solu-tion of minerals. In some cases this operates on exposed rock, as when lichens and mosses that grow on rock surfaces secrete acidic substances and that mix with water from precipitation ( ● Fig. 15.15).

Hydrolysis In the weathering process of hydrolysis, water molecules alone, rather than oxygen or carbon dioxide in wa-ter, react with chemical components of rock-forming miner-als to create new compounds, of which the H+ and OH– ions

of water are a part. Many common minerals are susceptible to hydrolysis, particularly the silicate minerals that comprise igneous rocks. Hydrolysis of silicate minerals often produces clay minerals.

● FIGURE 15.13The reddish orange coloration on the surface of this boulder reveals that its constituents include minerals containing iron, and that it has been subjected to chemical weathering by oxidation.What is a likely chemical formula for the reddish orange substance?

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● FIGURE 15.14This limestone shows the effects of chemical weathering by carbonation. Weathered limestone often takes on a fluted, pitted, or even honey-combed appearance.Why does the rock near the bottom seem to be more weathered than that at the top of this rock ledge?

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● FIGURE 15.15The green patches of lichens growing on this black and white granitic rock contribute to solution weathering with the acids that they secrete.How does the fact that lichens retain moisture also contribute to weathering?

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xpansion and contraction of rocks, sediments, or of materials in frac-tures and other void spaces in rocks and sediments is a common theme

in weathering and mass wasting. Physi-cal weathering by thermal expansion and contraction, freeze/thaw of water, and hydration/dehydration of salts and clay minerals all involve cycles of expansion and contraction, as does downslope movement of soil by creep.

Soils consist of inorganic and organic materials, soil water, and soil air. The inor-ganic fraction of soils—rock fragments and minerals—derives from rock weathering. These fragments are of sand, silt, and clay size. As weathering continues, clasts be-come smaller and smaller. In addition, the chemical weathering process of hydrolysis produces very small substances called clay minerals. Clay minerals are colloidal in size, meaning they are smaller than 0.0001 mil-limeter. In addition to rock fragments and clay minerals, some decaying organic mat-ter (humus) in the soil is colloidal in size.

Having colloidal-sized materials in the soil benefits soil fertility because colloids increase the cation exchange capacity of the soil, which is essentially the ability of the soil to hold plant nutrients. Colloids

are so small that their negative electrical charge becomes an important characteristic. Plant nutrients, such as potassium, calcium, magnesium, phosphorous, copper, zinc, and several others, are positively charged ions, or cations. The negatively charged colloids, therefore, attract nutrient cations. When a plant takes up soil water, nutrient cations are drawn up as well. A high cation exchange capacity means that the soil has plenty of colloids and therefore the poten-tial to maintain a large number of nutrient cations in the soil for plants to use. The spe-cific capacity of a colloid to hold cations is a function of its composition and structure. The cation exchange capacity of a clay min-eral depends on the type of clay mineral, which, in turn, depends on the kind of par-ent material and the extent of weathering. Illite and vermiculite are two clay minerals associated with slightly weathered soils; montmorillonite is an important component of moderately weathered soils; and kaolin-ite is characteristic of highly weathered soils.

Nutrient cations are not the only sub-stances that join and leave some colloids. Water molecules come into and leave the crystal structure of colloidal-sized clay minerals during hydration and dehydration. Clay minerals have layered, “sandwichlike”

structures. Water molecules enter and exit their layered crystalline structure and cause the clay mineral to expand when wet and contract when dry. Of the four basic clay min-erals, montmorillonite expands and contracts the most upon hydration and dehydration. The expansion and contraction, or swelling and shrinking, of clay minerals applies physi-cal pressure on surrounding soil particles and rock surfaces and contributes to physical weathering. According to some sources, the expansion in clay mineral volume can vary from a very small percentage to more than 100%. In most cases, however, the expan-sion in volume is less than 50%.

A high content of clay minerals advanta-geous for soil fertility sometimes interferes with the human-built environment. Soils with a significant amount of expansive clays swell and shrink considerably as they become wet and dry. The expansion and contraction in soil volume can shift and crack roads, sidewalks, and building foundations. In regions with typically fer-tile, expansive clay-rich soils, this causes an estimated $7 billion damage a year to structures in the United States alone. Spe-cial construction techniques can be used to mitigate the negative effects of shrinking and swelling on the built environment.

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

Expanding and Contracting Soils

The concrete driveway slab here has been heaved upward by expansive soils, also causing damage to the adjacent railing and retaining wall.

Foundation cracks due to expansive soils can cause serious damage to buildings that is expensive to repair.

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The specific type of clay mineral produced by hydrolysis depends on the composition of the preexisting mineral and other sub-stances that may be present and active in the water. Because water is the weathering agent, hydrolysis is not limited to rocks that are exposed at the surface, but can occur in the subsurface through the action of soil and groundwater; in tropical humid climates, it can decay rock to a depth of 30 meters (100 ft) or more.

The chemical weathering process of hydrolysis differs from the physical weathering process of hydration, in which water molecules join and leave a substance, causing it to swell and shrink without changing its inherent chemical formula. Both weathering processes, however, may involve clay minerals because those clay-sized sub-stances are often the product of hydrolysis, and many clay minerals swell and shrink substantially during hydration and dehydration.

Variability in WeatheringHow and why types and rates of weathering vary, at both the regional and local spatial scales, are of particular interest to physi-cal geographers. In addition to climate, the type of rock (lithology) and the nature and amount of fractures or other weaknesses in it are major influences on the effectiveness of the various rock weathering processes. How specific rocks weather in different natural settings is impor-tant for explaining the amount and formation of regolith, soil, and relief, but it also impacts the cultural environment because weathering affects building stone as well as rock in its nat-ural setting.

ClimateIn almost all environments, physical and chemical weathering processes operate together, but usu-ally one of these categories dominates. Although water plays a role in all but two of the physical weathering processes, it is essential for all types of chemical weathering. Also, chemical weath-ering increases as more water, comes in contact with rocks. Chemical weathering, then, is par-ticularly effective and rapid in humid climates ( ● Fig. 15.16). Most arid regions have enough moisture to allow some chemical weathering, but it is much more restricted than in humid climates. Arid regions typically receive sufficient moisture for physical weathering by salt crys-tal growth and the hydration of salts. Abundant salts, high humidity, and contact with seawater make salt weathering processes very effective in marine coastal locations.

The other principal climatic variable, temperature, also influences dominant types and rates of weather ing. Most chemical reactions proceed faster at higher temperatures.

Low-latitude regions with humid climates consequently experi-ence the most intense chemical weathering. In the tropical rainfor-est, savanna, and monsoon climates, chemical weathering is more significant than physical weathering, soils are deep, and landforms appear rounded. Although chemical weathering is somewhat less extreme in the mid-latitude humid climates, its influence is appar-ent in the moderate soil depth and rounded forms of most land-scapes in those regions. In contrast, the landforms and rocks of both arid and cold regions, where physical weathering dominates, tend to be sharper, angular, and jagged, but this depends to some extent on rock type, and rounded features may remain in an arid land-scape as relicts from prehistoric times when the climate was wetter ( ● Fig. 15.17). Comparatively low rates of chemical weathering are reflected in the thin (or absent) soils found in arid, subarc-tic, and polar climatic regimes. We have already noted the role of daily temperature ranges in weathering processes, including ther-mal expansion and contraction weathering in arid climates and freeze–thaw weathering in areas with cold winters.

Air pollution that contributes to the acidity of atmospheric moisture accelerates weathering rates. Extensive damage has

● FIGURE 15.16This diagram of weathering regions summarizes the relationships between climate and weath-ering processes. Physical weathering is most active where temperature and rainfall are both low. Chemical weathering is most active in regions of high temperature and rainfall. Most world regions experience a combination of both physical and chemical weathering.In which weathering region would we find a site that has an annual mean temperature of 5°C (41°F) and an annual rainfall of 100 centimeters (40 in.)?

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THEORETICAL WEATHERING REGIONSAnnual rainfall (in.)

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Strong physical

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Moderate chemicaland physical frost action

Moderate chemicalStrong chemical

Almostno chemical

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Annual rainfall (cm)


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already occurred in some regions to historic cultural artifacts made of limestone and marble (metamorphosed limestone), both containing calcium carbonate ( ● Fig. 15.18). Because many of the world’s great monuments and sculptures are made of limestone or marble, there is a growing concern about weathering damage to these treasures. The Parthenon in Greece, the Taj Mahal in India, and the Great Sphinx in Egypt are examples of structures made of rock where pollution-induced chemical solution, and related growth and hydration of salts, are damaging and rotting away monument surfaces.

Rock TypeWherever many different rock types occupy a landscape, some will be more resistant and others will be less resistant to the weather-ing processes operating there. Because erosion removes weathered rock fragments more easily than large, intact rock masses, areas of

diverse rock types undergo differen-tial weathering and erosion; easily eroded rocks exhibit more extensive ef-fects of weathering and erosion than the resistant rocks.

A rock that is strong under certain environmental conditions may be eas-ily weathered and eroded in a differ-ent environmental setting. Rocks that are resistant in a climate dominated by chemical weathering may be weak where physical weathering processes dominate, and vice versa. Quartzite is a good example. It is chemically nearly inert and harder than steel, but it is brittle and can be fractured by physi-cal weathering. Shale is chemically in-ert but mechanically weak. In humid regions, limestone is highly susceptible to carbonation and solution, but under arid conditions, limestone is much more resistant. Granitic outcrops in an arid or semiarid region resist weathering. How-ever, the minerals in granite are suscepti-ble to alteration by oxidation, hydration, and hydrolysis, particularly in regions with warm, humid conditions. Accord-ingly, granitic areas are often covered by a deeply weathered regolith when they have been exposed to a tropical humid environment.

Structural WeaknessesIn addition to rock type, the relative resistance of a rock to weathering de-pends on other characteristics, such as the presence of joints, faults, folds, and bedding planes that make rocks suscep-

tible to enhanced weathering. Sandstone is only as strong as its cement, which varies from soluble calcium carbonate to inert and resistant silica. In general, the more massive the rock, that is, the fewer the joints and bedding planes it has, the more resistant it is to weathering.

The processes of volcanism, tectonism, and rock formation produce fractures in rocks that can be exploited by exogenic pro-cesses, including weathering. Joints can be found in any solid rock that has been subjected to crustal stresses, and some rocks are in-tensely jointed ( ● Fig. 15.19). Joints and other fractures that com-monly develop in igneous, sedimentary, and metamorphic rocks represent zones of weakness that expose more surface area of rock, provide space for flow or accumulation of water, collect salts and clay minerals, and offer a foothold for plants ( ● Fig.15.20). Rock surfaces along fractures tend to experience pronounced weathering. Chemi-cal and physical weathering both proceed faster along any kind of gap, crack, or fracture than in places without such voids.

● FIGURE 15.17(a) Due to the dominance of slow physical weathering and the sparseness of vegetation, slopes in arid and semiarid environments tend to be bare and angular. Slope angles reflect differences in component rock resistance to weathering and erosion. (b) Because humid region hillsides are affected by the more rapid chemical weathering processes and regolith is held longer on the slope by vegetation, slopes in wetter climates are much more rounded with a deeper weathered mantle than arid region slopes.

(a)

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V A R I A B I L I T Y I N W E AT H E R I N G



Because joints are sites of concentrated weathering, the spa-tial pattern of joints strongly influences the landforms and the appearance of the landscapes that develop. Multiple joints that parallel each other form a joint set, and two sets, each composed

of multiple parallel joints, will cross each other at an angle ( ● Fig. 15.21). Joints divide rocks into many different configura-tions, most commonly resembling blocks or columns, which are often visible in the topography. Over time, preferential weather-ing and erosion in crossing joint sets leave rock in the central area between the fractures only slightly weathered while the rock near the fractures acquires a more rounded appearance. This distinc-tive, rounded weathered form, known as spheroidal weather-ing, develops especially well on jointed crystalline rocks, such as granite ( ● Fig. 15.22). Spheroidal weathering is a result that can occur from the interaction of many weathering processes; it does not refer to a specific weathering process. Once a rock becomes rounded, weathering rates of spheroidal outcrops and boulders de-crease because there are no more sharp, narrow corners or edges for weathering to attack, and a sphere exposes the least amount of surface area for a given volume of rock.

Topography Related to Differential Weathering and ErosionIn the previous chapter, we learned that structural upfolds (anti-clines) do not always form topographic ridges and that structural downfolds (synclines) do not always form topographic valleys. The reason for this stems from the resistance of the folded rock units to the weathering and erosion processes. If resistant rocks are found in the center of a syncline, they will eventually create a topographic high regardless of the structural downfold. Weak rocks, even when forming an anticline, are too easily attacked by weathering and erosion to exist in the landscape as a topographic high for very long. Variation in rock resistance to weathering

● FIGURE 15.18These limestone tombstones have undergone extensive chemical weath-ering, accelerated by air pollution.What kind of chemical weathering has impacted the iron fence?

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● FIGURE 15.19(a) Massive jointing influences the location and shape of canyons in and around Zion National Park, Utah. To get an idea of the scale of this satellite image, note the smoke from the wildfire at upper left. (b) Farther north, in Bryce Canyon National Park, Utah, numerous closely spaced vertical joints in sedimentary rocks are sites of preferential weathering and erosion, leaving narrow rock spires between joints.

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exerts a strong and often highly visible influence on the appearance of landforms and landscapes. Given sufficient time, rocks that are resistant to weathering and erosion tend to stand higher than less resistant rocks. Resistant rocks stand out in the topography as cliffs, ridges, or mountains, while weaker rocks undergo greater weath-ering and erosion to create gentler slopes, valleys, and subdued hills.

An outstanding example of how differential weath-ering and erosion can expose rock structure and enhance its expression in the landscape is the scenery at Arizona’s Grand Canyon ( ● Fig. 15.23). In the arid climate of that region, limestone is resistant, as are sandstones and conglomerates, but shale is relatively weak. Strong and resistant rocks are necessary to maintain steep or vertical cliffs. Thus, the stair-stepped walls of the Grand Canyon have cliffs composed of limestone, sandstone, or con-glomerate, separated by gentler slopes of shale. At the

canyon base, ancient resistant metamorphic rocks have produced a steep-walled inner gorge. The topographic effects of differential weathering and erosion tend to be more prominent and obvious in landscapes of arid and semiarid climates. In these dry envi-ronments, chemical weathering is minimal, so slopes and varying rock units are not generally covered under a significant mantle of soil or weathered rocks. In addition, vegetation typically does not mask the topography in arid regions as it does in humid regions.

Another example of differential weathering and erosion can be seen in the Appalachian Ridge and Valley region of the eastern United States ( ● Fig. 15.24). The rock structure here

● FIGURE 15.20Vertical joints in rocks are sites of concentrated weathering and erosion as water preferentially accumulates in and moves along them. Higher up at a narrow spot, this joint has accumulated enough soil for the cactus to grow, while enhanced weathering and erosion lower down has notice-ably widened the joint.

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● FIGURE 15.21Multiple cross-cutting joint sets are visible in this aerial view of part of the Colorado Plateau.With north at the top of this photo, what directions do the two most apparent joint sets trend?

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● FIGURE 15.22Cross-jointed granite east of the Sierra Nevada in California forms this hill of spheroi-dally weathering blocks of rock.

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● FIGURE 15.24A satellite image of Pennsylvania’s Ridge and Valley section of the Appalachians clearly shows the effects of weathering and erosion on folded rock layers of different resistance. Resistant rocks form ridges, and weaker rocks form valleys.Can you see how the topography of the Ridge and Valley section influences human settlement patterns?

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consists of sandstone, conglomerate, shale, and limestone folded into anticlines and synclines. These folds have been eroded so that the edges of steeply dipping rock layers are exposed as prominent ridges. In this humid climate region, forested ridges

composed of resistant sandstones and conglomerates stand up to 700 meters (2000 ft) above agricultural lowlands that have been excavated by weathering and erosion out of weaker shales and soluble limestones.

● FIGURE 15.23The Grand Canyon of the Colorado River in Arizona is a classic example of differential weathering and erosion in an arid climate. Weathering, mass wasting, and erosion work together to make differences in rock structure and strength visible in the landscape. Rock layers of varying thickness and resistance result in a distinctive array of cliffs formed by strong rocks and slopes formed by less resistant rocks.

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Mass WastingMass wasting, also called mass movement, is a collective term for the downslope transport of surface materials in direct response to gravity. Everywhere on the planet’s surface, gravity pulls ob-jects toward Earth’s center. This gravitational force is represented by the weight of each object. Heavier objects have a greater downward pull from gravity than lighter objects. The force of gravity encourages rock, sediment, and soil to move downhill on sloping surfaces.

Mass wasting operates in a wide variety of ways and at many scales. A single rock rolling and tumbling downhill is a form of this gravitational transfer of materials ( ● Fig. 15.25), as is an entire hillside sliding hundreds or thousands of meters downslope, bury-ing homes, cars, and trees. Some mass movements act so slowly that they are imperceptible by direct observation and their effects appear gradually over long periods of time. In these cases, tilted telephone poles, gravestones, fence posts, retaining walls, trees, or cracks in buildings can reveal how mass wasting processes are af-fecting the ground beneath those objects. Other types of mass wasting are disastrous in scale and produce instantaneous violence.

The cumulative impact of all forms of mass wasting rivals the work of running water as a modifier of physical landscapes be-cause gravitational force is always present. Wherever there is loose rock, regolith, or soil on a slope, gravity will cause some move-ment downslope. Friction and rock strength are factors that resist this downslope movement of materials. Friction increases with the roughness and angularity of a rock fragment and the rough-ness of the surface on which it rests. Rock strength depends on physical and chemical properties of the rock and is especially de-creased by any kind of break or gap in the rock. Fractures, joints, faults, bedding planes, and spaces between mineral grains or clasts

all weaken the rock. Furthermore, because all of these gaps invite the accumulation of water, their bonds to the outcrop continue to weaken further over time through weathering.

Slope angle also helps determine whether or not mass wast-ing will occur. Gravitational forces act to pull objects straight downward, toward the center of Earth. The closer a slope is to being parallel to that downward direction, in other words, the steeper the slope angle, the easier it is for the gravitational forces to overcome the resistance of friction and rock strength. Gravity is more effective at pulling rock materials downslope on steep hillsides and cliffs than on gently sloping or level surfaces. The steeper the slope, the stronger the friction or rock strength must be to resist downslope motion ( ● Fig. 15.26). Any surface materi-als on a slope that do not have the strength or stability to resist the force of gravity will respond by creeping, falling, sliding, or flowing downslope until stopping at the bottom of the slope or wherever there is enough friction to resist further movement. As a result, soil and regolith are thinner on steep slopes and thicker on gentle slopes, and intense mass wasting is one reason why bedrock tends to be exposed in areas of steep terrain.

Gravity is the principal force responsible for mass wasting, but water is often a contributing factor. Water (1) contributes to weathering, which prepares rock material for mass movement, (2) adds weight to porous materials on a slope, (3) decreases the strength of unconsolidated slope material, and (4) can increase the slope angle. We have seen that water is involved in many weather-ing processes that break and weaken rocks, making them more susceptible to mass movement. Unconsolidated soil and regolith have a considerable volume of voids, or pore spaces, between par-ticles. Usually some of these voids contain air and some contain water, but storms, wet seasons, broken pipelines, irrigation, and other situations can cause the voids to fill with water. Saturated conditions encourage mass wasting because water adds weight

● FIGURE 15.25A massive boulder, which was loosened by heavy rains and pulled downhill by the force of gravity, blocks a road in Southern California. The boulder weighed an estimated 270,000 kilograms (300 tons) and had to be dynamited to clear the highway.What other kinds of problems on roads are related to mass wasting?

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● FIGURE 15.26The occurrence of mass wasting is strongly related to the slope angle and the strength of materials that make up a slope. Other factors, such as amount and type of vegetation cover and amount of soil moisture or groundwater, also influence downslope move-ment of materials by mass wasting. G = total force of gravity (weight of the block); F = component of the block’s weight resisting motion; f = frictional forces resisting motion; D = downslope com-ponent of gravity.How might vegetative cover or moisture content affect the potential for downslope movement of soil?

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to the sediments, and an object’s weight represents the amount of gravitational force pulling on it. As water replaces air in the voids, unconsolidated sediments and soil also experience decreas-ing strength as the rock fragments come into greater contact with the liquid, which tends to flow downslope. Finally, streams flowing along the base of a slope can undercut it by erosion. This increases the slope angle, thereby facilitating mass movement.

Undercutting and steepening of slopes also occurs by waves in coastal locations, and through the actions of people, especially when removing rock material from the base of a slope for con-struction projects. The strength or stability of slope material is of-ten reduced by a triggering event, such as an earthquake. But, the vibrations produced by explosions and even the movements of heavy trucks or trains can be enough to shake material loose from a slope. Shaking reduces the support and friction between particles, so it can trigger mass movement.

Understanding the conditions and processes that affect mass wasting is important because gravity-induced movements of Earth materials are common and frequently impact people and the built environments in which they live. Although this natural hazard cannot be eradicated, people can avoid actions that aggra-vate the hazard potential and pay close attention to evidence of impending failure in susceptible terrain.

Classification of Mass WastingPhysical geographers classify mass wasting events according to the kinds of Earth materials involved and the ways in which they move. Mass wasting events are categorized by using a descrip-tive name, for example rockfall, that summarizes the type of mate-rial and the type of motion. The various kinds of gravity-induced motions are separated into two general groups according to the speed with which they occur. After an overview of material cat-egories and speed, types of motion are described.

Types of Earth Material Anything on Earth’s surface that exists in or on an unstable, or potentially unstable, land sur-face is susceptible to gravity-induced movement and can there-fore be transported downslope as a result of mass wasting. Mass wasting involves almost all kinds of surface materials. Rock, snow and ice, soil, earth, debris, and mud commonly experience downslope movements. In a mass wasting sense, soil refers to a relatively thin unit of predominantly fine-grained, unconsoli-dated surface material. A thicker unit of the same type of mate-rial is referred to as earth. Debris specifies sediment with a wide range of grain sizes, including at least 20% gravel. Mud indicates saturated sediment composed mainly of clay and silt, which are the smallest particle sizes.

Speed of Motion Surface materials move in response to gravity in many different ways, depending on the material, its water content, and characteristics of the setting. Some types of mass movement happen so slowly that no one can watch the mo-tion occurring. With these slow mass wasting types we can only measure the movement and observe its effects over long periods of time. The motion of fast mass wasting can be witnessed

by people. The speed of downslope movement of material varies greatly according to details of the slope, the material, and if a trig-gering factor is involved. In addition, slow and fast mass move-ments often work in combination. Mass movement that is initially slow may be a precursor to more destructive rapid motion, and most materials that have undergone rapid mass wasting continue to shift with slow movements. Specific types of motion, and com-mon Earth materials associated with them, are discussed in the following sections on slow and fast mass wasting. These are sum-marized in Table 15.1.

Slow Mass WastingSlow mass wasting has a significant, cumulative effect on Earth’s surface. In general, slow mass wasting produces rounded hillcrests and a landscape free of sharp angular features.

Creep Most hillslopes covered with weathered rock material or soils undergo creep, the slow migration of particles to suc-cessively lower elevations. This gradual downslope motion, often occurs as soil creep, primarily affecting a relatively thin layer of weathered rock material. Creep is so gradual that it is visually im-perceptible; the rate of movement is usually less than a few cen-timeters per year. Yet creep is the most widespread and persistent form of mass wasting because it affects nearly all slopes where there are weathered materials at the surface.

Creep typically results from some kind of heaving process, which causes individual soil particles or rock fragments to be first pushed upward perpendicular to the slope, and then eventually

Motion CommonMaterial

TypicalSpeed Effect

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Flow Debrisor mud

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slide)Earth Fast

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TABLE 15.1Different Kinds of Mass Wasting Processes


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fall straight downward due to gravity. The freezing and thawing of soil water, as well as the wetting and drying of soils or clays, can lead to soil heaving. For example, when soil water freezes, it expands, pushing overlying soil particles upward relative to the surface of the slope. When that ice thaws, the soil particles move back, but not to their original position because the force of gravity pulls downward on them, as shown in ● Figure 15.27. Soil creep results from repeated cycles of expansion and con-traction related to freezing and thawing, or wetting and drying, which cause lifting followed by the downslope movement of soil and rock particles.

Organisms can also contribute to soil creep and other kinds of mass wasting. Small burrowing animals, including ground squirrels and chipmunks are effective soil movers. When they dig their tun-nels on a sloping surface, the excavated material tends to fall downslope. Plant roots can also move soils outward and in a downward direction on a slope. Even the traversing of slopes by people and animals tends to push surface material downhill. Every step up, down, or across a steep slope shifts some surface material downhill to a slightly lower position.

Rates of soil creep are greater near the surface of a slope and diminish into the subsurface, because the factors instigating it are more frequent near the surface, and because frictional resistance to move-ment increases with depth. As a result, telephone poles, fence posts, other human structures, and even trees ( ● Fig. 15.28)—all of which are anchored at a level below the surface—exhibit a downslope tilt when affected by the downward movement of creep.

For the most part, creep does not produce discrete distinc-tive landforms, but it contributes to rounded, rather than angular, topography in hilly terrain. Once deposited at the base of the slope by gravity, these surface sediments can subsequently be carried away by one of the geomorphic agents, usually running water.

● FIGURE 15.27Repeated cycles of expansion and contraction cause soil particles to be lifted at right angles to the surface slope but to fall straight downward by the force of gravity, resulting in soil creep.Are there places near where you live that show evidence of soil creep?

Ground surface liftedby expansion of surface

Position of particlewhen lifted

Position of particleafter contraction of surface

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wallsBedrock

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● FIGURE 15.28(a) Visible landscape effects of soil creep are common on natural and cultural features. (b) Trees attempt to grow vertically, but their trunks become bent if surface creep is occurring.What other constructed features might be damaged by creep?

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Solifluction The word solifluction, which literally means “soil flow,” refers to the relatively slow downslope movement of water-saturated soil and/or regolith. Solifluction is most com-mon in high-latitude or high-elevation tundra regions that have permafrost, a subsurface layer of permanently frozen ground. Above the permafrost layer lies the active layer, which freezes during winter and thaws during summer. During the summer thaw of the centimeters- to meters-thick active layer, the permanently frozen ground beneath it prevents downward percolation of melted soil water. As a result, the active layer becomes a heavy, water-saturated soil mass that, even on a gentle incline, sags slowly downslope by the force of gravity until the next surface freeze arrives. Move-ment rates are typically only several centimeters per year.

Evidence of solifluction exists in many tundra landscapes. It consists of irregular lobes of soil that produce hummocky ter-rain or mounds ( ● Fig. 15.29). Slopes affected by solifluction typically exhibit cracks and tongue-shaped lobes formed during downslope movement.

Fast Mass WastingFour major kinds of mass wasting usually occur so quickly—from seconds to days—that people can watch the material move. The speed of movement varies with the situation and depends on the quantity and composition of the material, the steepness of slope,

the amount of water involved, the vegetative cover, and the trig-gering factor. The effects of fast mass wasting events on the land surface tend to be more dramatic than those of slow mass wasting. Rapid mass movements usually leave a visible upslope scar on the landscape, revealing where material has been removed, and a defi-nite deposit where transported Earth material has come to rest at a lower elevation.

Falls Mass wasting events that consist of Earth materials plummeting downward freely through the air are called falls. Rockfalls are probably the most common type of fall. Rocks fall from steep bedrock cliffs, either (1) one by one as weather-ing weakens the bonds between individual clasts and the rest of the cliff, or (2) as large rock masses that fall from a cliff face or an overhanging ledge ( ● Fig. 15.30). Large slabs that fall typically

● FIGURE 15.29Solifluction has formed these tongue-shaped masses of soil on a slope near Suslositna, Alaska.How does solifluction differ from soil creep?

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● FIGURE 15.30Eventually this overhanging sandstone ledge will fail in a rockfall. Rockfall beneath the ledge has already occurred, creating a zone with enhanced weathering because it stays wet longer in the shade. As more rocks fall from beneath, increasing the size of the overhang, the force of gravity will increase until it exceeds the strength of the bonds holding the ledge in place.What weathering processes might be acting on the sandstone beneath the overhang when it becomes wet?

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break into angular individual clasts when they hit the ground at the base of the cliff.

In steep mountainous areas, rockfalls are particularly com-mon during the spring when snowmelt, rains, and alternating freezing and thawing can disturb precariously balanced rock masses, loosening them from their previously secure positions. Ground shaking caused by earthquakes is another common trig-ger for rockfalls.

Over time, a sloping accumulation of angular, broken clasts piles up at the base of a cliff that is subject to rockfall. This slope is known as a talus, talus slope, or, where cone-shaped, a talus cone ( ● Fig. 15.31). The presence of talus is good evi-dence that the cliff above is undergoing rockfall. The steepest slope angle that a pile of loose sediment can stand at without having particles tumble or slide downslope is called the angle of repose. Like other accumulations of unconsolidated sedi-ment, such as the slopes of cinder cones or piles of gravel in a gravel pit, talus slopes typically lie at or near their angle of re-pose ( ● Fig. 15.32). The angle of repose for these different fea-tures varies somewhat depending on the size and angularity of the particles, but commonly ranges between about 30° and 34°. Large angular clasts have a steeper angle of repose than small, more rounded rock fragments.

Falling rocks create hazardous conditions in mountain-ous regions and wherever steep roadcuts expose bedrock cliffs. Rockfall hazard mitigation is a high priority along mountain-ous stretches of Interstate 70 and other highways in Colorado. In Yosemite Valley, California, massive rockfalls have originated from the area’s towering and steep granite cliffs. In July 1996, one hiker was killed and numerous others injured by such an

event. A huge 180,000-kilogram (200-ton) mass of rock broke away from a cliff, slid 200 meters (650 ft) down a steep slope, and then fell airborne for another 550 meters (1800 ft) before hitting the ground with great force. The rockfall was estimated to have moved downslope at more than 250 kilometers per hour (160 mph). The huge mass of moving rock also generated a de-structive blast of compressed air that destroyed trees hundreds of meters from the cliff as the rock crashed to the valley floor ( ● Fig. 15.33).

Avalanches An avalanche is a type of mass movement in which much of the involved material is pulverized, that is, bro-ken into small, powdery fragments, which then flow rapidly as a density current along Earth’s surface. Although the word avalanche may bring to mind billowing torrents of snow and ice roaring down a steep mountainside, snow avalanches are not the only kind. Avalanches of pulverized bedrock, called rock avalanches, and those of a very poorly sorted mixture of gravel, sands, silts, and clays, called debris avalanches, are also common and have caused con-siderable loss of life and destruction to mountain communities. Many avalanches are triggered by falls of snow and ice, rock, or debris that pulverize when they impact a lower surface. Snow av-alanches are the best-known type of avalanche to the public, and they present serious hazards to skiers, mountaineers, and people who live in steep mountain communities that experience snowy winters. Regardless of the specific type of Earth material involved, avalanches can be very dangerous and powerful, traveling up to 100 kilometers per hour (60 mph). They easily knock down trees and demolish buildings ( ● Fig. 15.34), and have even destroyed entire towns.

● FIGURE 15.31A steep slope of large, angular clasts has accumulated by rockfall in the talus built at the base of a limestone cliff in southern Idaho.

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Slides In slides, a cohesive or semicohesive unit of Earth material slips downslope in continuous contact with the land surface. Water plays a somewhat greater role in most slides than it does in falls. Slides of all kinds threaten the lives and property of people who live in regions with considerable slope as well as such characteristics as tilted layers of alternating strong and weak rocks.

Slides of large units of bedrock, called rockslides, are frequent in mountainous terrain where originally horizontal sedimen-tary rock layers have been tilted by tectonism. The importance of water in reducing the resisting forces of rock strength and friction is seen in the fact that rockslides are most common in

wet years, or after a rainstorm or snowmelt. As weathering and erosion by water weaken contacts between successive rock layers, the force of gravity can exceed the strength of the bonds be-tween two rock layers. When this happens, a unit of rock, often of massive size, detaches and slides along the tilted planar surface of the contact ( ● Fig. 15.35). Rockslides sometimes end as rock-falls if the topography is such that free fall is needed to transport the rock to a stable position on more level ground. Deposits from rockslides tend to consist of larger blocks of rock than comprise rockfall deposits.

Some rockslides are enormous, with volumes measured in cubic kilometers. Anything in their path is obliterated. In addi-tion, rockslides may form dams across river valleys, which soon become filled with lakes. When the lakes become deep enough, they may wash out the rockslide dams, producing sudden and disastrous downstream floods. Thus, immediately after this kind of a major rockslide, workers do what is necessary to stabilize the resulting dam and control the overflow outlet of the water trapped in the newly formed lake. This was done successfully

● FIGURE 15.33The part of this cliff that is lighter in tone marks the point of origin for the 1996 Happy Isles rockfall in Yosemite Valley, California. The rockfall killed one hiker and injured many others. The fall was so large and rapid, traveling at an estimated 250 kilometers per hour (160 mph), that it created an air blast which set off a giant dust cloud from the valley floor.

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● FIGURE 15.32(a) The angle of repose is the steepest natural slope angle that loose material can maintain. Particles are held at this angle by friction between clasts. (b) The same size and shape of particles and the same-sized dump truck loads formed these nearly identical piles, all with the same height and slope angle.Would angular particles form steeper or gentler slopes in comparison to rounded particles if dumped in this manner?

Angle of repose30-34 for sand

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after the Hebgen Lake slide in southwestern Montana in 1959 ( ● Fig. 15.36). Triggered by an earthquake, this rockslide, one of the largest in North American history, killed 28 people camped along the Madison River.

Huge rockslides have also resulted from instability related to rock structure and to the undercutting of slopes by streams, glaciers, or waves. Today, there are many locations in mountain

● FIGURE 15.34Snow avalanches like this one in Alaska can block roads, knock down trees, carry rocks and tree trunks downslope, and damage structures. Note the heavy-packed nature of the snow, partly a result of pressure at impact. Many people erroneously believe that snow avalanche deposits are light and powdery.

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● FIGURE 15.35Rock units that dip in the same direction as the topographic slope of the land are especially susceptible to rock-slide, as recognized along this stretch of highway in Wyoming.

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● FIGURE 15.36The 1959 earthquake-induced rockslide on the Madison River, Montana, completely blocked the river valley and created a new body of water, Earthquake Lake, seen in the background. The massive slide killed 28 people in a valley campground.Why can earthquakes trigger landslides?

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regions where enormous slabs of rock supported by weak materials are poised on the brink of detachment, waiting only for an unusually wet year or a jarring earthquake to set them in motion.

Rock is not the only Earth material prone to mass wasting by sliding. Debris slides, which contain a poorly sorted mixture of gravel and fines, and mudslides, which are dominated by wet silts and clays, are also common.

Slumps are rotational slides where a thick block of soil, called earth, moves along a concave, curved surface. Because of this curved surface of failure, slump blocks undergo a back-ward rotation as they slide ( ● Fig. 15.37), causing what used to be the ground surface at the top of the slump to tilt back-ward. Slumps are most common in wet years and during wet seasons in many regions with substantial relief, including the

Appalachians, New England, and mountainous parts of thewestern United States. During exceptionally wet winters in Mediterranean climate regions, like California, slumps frequently damage hillside homes. Like rockslides, slumps can be triggered by earthquakes, which greatly reduce friction and material strength. People can also contribute to slumping by purposefully or accidentally adding water to hillslope sediments and by increasing slope angles by excavating for construction purposes.

Landslide has become a general term popularly used to refer to any form of rapid mass movement. How-ever, sometimes large, rapid mass wasting events are dif-ficult to classify because they contain elements of more than one category of motion or because multiple types of materials—rock, debris, earth, soil, and mud—are involved in a single massive slide. In some cases, Earth scientists call these large failures landslides. Such large slides are relatively rare but are often newsworthy because of their destructive qualities ( ● Fig. 15.38).

Flows Mass wasting flows are masses of water- saturated unconsolidated sediments that move downslope

by the force of gravity. Flows carry wa-ter in moving sediments whereas rivers carry sediments in moving water. Com-pared to slides, which tend to move as cohesive units, flows involve considerable churning and mixing of the materials as they move.

When a relatively thick unit of pre-dominantly fine-grained, unconsolidated hillside sediment or shale becomes satu-rated and mixes and tumbles as it moves, rather than moving as a cohesive unit along a curved surface (a slump), the mass move-ment is an earthflow. Earthflows occur as independent gravity-induced events or in association with slumps in a compound feature called a slump-earthflow (see again Fig. 15.38). A slump-earthflow moves as a cohesive unit along a concave surface in the middle and upper reaches of the failure. In the downslope reach, however, the sedi-ments flow beyond the failure plane, and the mass flows in a less cohesive manner as an earthflow.

Debris flows and mudflows differ from each other primarily grain size and sediment attr ibutes. Both flow faster

● FIGURE 15.37Slump is the common name for a rotational earthslide. Many slumps transition into the more fluid motion of an earthflow in their lower reaches.How does the earthflow component differ from the slump component?

Slump

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● FIGURE 15.38Very large slides that move a variety of materials are referred to as landslides. Landslides modify the landscape but can also cause much destruction when buildings are constructed in areas susceptible to these large mass movements.

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than earthflows, often move down gullies or canyon stream channels for at least part of their travel, create raised chan-nel rims called flow levees, and leave lobate (tongue-shaped) deposits where they spill out of the channel. They result from torrential rainfall or rapid snowmelt on steep, poorly vege-tated slopes, and are the most fluid of all mass movements. Debris flows transport more coarse-grained sediment than mudflows do.

Debris flows often originate on steep slopes, especially in arid or seasonally dry regions. They also occur on steep slopes in humid regions that have been deforested by human activity or wildfire. The rain or meltwater flush weathered rock mate-rial into canyons where it acquires additional water from sur-face runoff. The result is a chaotic, saturated mixture of fine and coarse sediment, ranging in size from tiny clays to large boul-ders. As it flows down stream channels, some of the debris is


The massive 1903 rockfall-avalanche known as the Frank Slide left a huge scar on Turtle Mountain, Alberta, that remains very obvious in the landscape more than a century later.

any mass wasting events are compound, having elements of more than one type of motion.

Avalanches and flows often begin as falls or slides. Slumps commonly grade into earthflows in their lower reaches. The mas-sive and deadly slope failure that occurred over a century ago at Turtle Mountain in the Canadian Rockies appears to have comprised two of the most catastrophic types of mass movement, rockfall and rock avalanche. This 1903 Turtle Mountain failure is known as the Frank Slide after the town of Frank, Alberta, a portion of which was obliterated by the very rapidly moving 30 million cubic meters (82 million tons) of rock, resulting in the loss of an esti-mated 70 lives.

Frank was situated at the base of Turtle Mountain along the Canadian Pacific Rail-road line. Many of the town’s 600 citizens worked as underground coal miners within the steep mountain. Most of the towns-folk didn’t know what hit them as they lay sleeping at 4:10 a.m. when the mountain gave way. Others were at work in the mine inside the mountain when the force of gravity overcame the strength of the 1 kilo-meter (3280 ft) wide, 425 meter (1395 ft) high, and 150 meter (490 ft) thick mass of limestone. The resistance of the rock mass was weakened by underground mining activities, including blasting, and weather-ing and erosion along fractures near the mountain summit. Severe weather condi-tions may have also played a role. In less

than 2 minutes, the rockfall that became an avalanche on impact destroyed homes, buildings, roads, and the railroad line in its path, and left a huge expanse of broken rock that extends across to the far side of the valley. Amazingly, 17 miners survived and dug their way out of the mountain through the rubble.

Despite having occurred over a century ago, tremendous evidence of the huge rock failure still exists in the landscape today. The scar on the flank of Turtle Mountain and the rock rubble strewn over more than 3 square kilometers (1.2 sq mi) across the valley floor serve as a reminder of the incredible and deadly power that can be unleashed by the force of gravity.

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

The Frank Slide

Rubble from the rapidly moving rockfall-avalanche was strewn across the valley floor, well beyond the partly buried town of Frank, in Alberta, Canada.

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piled along the sides as levees. Where a flow spills out of the channel, the unconfined mass spreads out and velocity decreases, resulting in deposition of a lobe of sediment ( ● Fig. 15.39). Debris flows are powerful mass wasting events that can destroy bridges, buildings, and roads ( ● Fig. 15.40). With dry summers and wet winters, Mediterranean climate regions are susceptible to mudflows and debris flows, particularly in rainy seasons that follow dry seasons during which devastating wildfires destroyed hillside vegetation.

Serious mudflow hazards exist in many active volcanic regions. Here, steep slopes may be covered with hundreds of me-ters of volcanic ash. During eruptions, emitted steam, cooling and falling as rain, saturates the ash, sending down dangerous and fast-moving volcanic mudflows, known as lahars. Of particular

concern are high volcanic peaks capped with glaciers and snow-fields. Should an eruption melt the ice and snow, rapid and cata-strophic lahars rush down the mountains with little warning and bury entire valleys and towns. In the United States, there is concern that some of the high Cascade volcanoes in the Pacific Northwest may pose a risk of eruptions and resulting lahars. Lahars accompanied the 1980 eruption of Mount St. Helens, and Mounts Rainier, Baker, Hood, and Shasta all have the conditions in place, including nearby populated areas, for potentially disas-trous mudflows to occur ( ● Fig. 15.41).

Weathering, Mass Wasting, and the Landscape

In this chapter we have concentrated on the exogenic processes of weathering and mass movement. Although neither weather-ing nor the slower forms of mass movement usually attract much attention from the general public, they are critical to soil forma-tion and, like faster forms of gravity-induced motion, they are significant processes in shaping the landscape. While weathering

● FIGURE 15.39Although small, this recent debris flow in western Utah left well-developed levees on either side of the fresh channel and deposited a tongue-shaped mass (lobe) of sediment where the flow spread out at the base of the slope.What evidence is there to indicate this is a site of repeated debris flows?

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● FIGURE 15.40A 1995 debris flow in La Conchita, California, destroyed several homes. Steep slopes consisting of weak unstable sediments failed during a period of heavy rainfall. A similar precipitation event triggered massive movement there again 10 years later, damaging 36 homes and killing ten people.Why might a specific site experience repeated slope failures over time?

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by the properties of the rocks and the lo-cal climatic factors. Slow weathering of resistant rocks leaves steep hillslopes, while rapid weathering of weak rocks produces gentle hillslopes that are typically blan-keted by a thick mantle of soil or regolith. Differential weathering and erosion in areas of multiple rock types or variations in structural weakness produce complex landscapes of variable slopes.

Chemical weather ing is most in-tense in warm humid climates where moisture and warmth help to acceler-ate the chemical reactions that cause minerals and rocks to decompose. In contrast, rocks in ar id, semiar id, and cold climates tend to weather slowly, mainly by physical processes. Physi-cal weathering, particularly those pro-cesses related to the freezing of water, is especially intense in climates that experience many cycles of freezing and thawing in a year, such as high-latitude and high-elevation locations. Because ar id and semiar id regions often lack a weathered mantle and have a sparse

vegetative cover, these regions typically exhibit barren, angu-lar slopes of exposed bedrock that reflect a tendency toward fast mass wasting. Loose, weathered rock fragments in ar id lands are also easily mobilized during intense precipitation events. In the following chapters, it will be important to re-member that rock weathering and mass wasting are key parts of the geomorphic system of processes that interact to shape the landforms and topography of Earth.

and mass wasting processes shape the landscape and create new landforms, they also have impacts, at times catastrophic and deadly, on people and the built environment. The impact is re-ciprocal, however; our construction and recreational activities can influence the breakdown of rocks and induce the occurrence of mass movements.

Every slope reflects the local weathering and mass wasting processes that have occurred. These, in turn, are largely determined

weatheringmass wastingphysical (mechanical) weatheringchemical weatheringunloadingexfoliationexfoliation sheetexfoliation domethermal expansion and contractiongranular disintegration

freeze–thaw weatheringsalt crystal growthhydrationoxidationcarbonationsolutionhydrolysisdifferential weathering and erosionjoint setspheroidal weathering

soil (as a mass wasting material)earth (as a mass wasting material)debrismudslow mass wastingfast mass wastingcreepheavingsolifluctionactive layer

Chapter 15 ActivitiesDefine & Recall

● FIGURE 15.41The violent 1980 eruption of Mount St. Helens in Washington generated lahars—mudflows consisting of volcanic ash. This house was half buried in lahar deposits associated with that volcanic eruption.

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1. Based on Figure 15.16, in what weathering regions are the following sites located?a. Your local areab. Brazil’s Amazon Basinc. The North Slope of Alaskad. The summit of Pike’s Peak, Coloradoe. The Mojave Desert of Southern Californiaf. The Appalachian Mountains of Pennsylvania

2. What would you recommend as a solution to prevent the loss of valuable historical monuments to weathering processes?

3. If you were an urban planner in a city with numerous steep slopes, what major hazards would you have to plan for? What recommendations would you make to lessen these dangers to the community?

Consider & Respond

1. Over a period of 2 hours, a thick, wet mass of unconsoli-dated sediment traveled down a mountain canyon stream channel before reaching and spreading out onto a desert plain in the arid western United States. Analyses determined that the sediment contained 18% clay, 29% silt, 27% sand, and 26% gravel, consisting of some very large blocks of rock. Based on this information, what specific type of event was it? What landform features would you look for to support your answer?

2. Find the climate data for the place where you live, or for the nearest climate data station with a similar climate to that of where you live. Using that data, determine which theoretical weather-ing region you live in, from the graph in Figure 15.16. Based on your observational evidence of weathering in the local environ-ment (perhaps of natural rocks, tombstones, or building materials like stone, asphalt, and so on), write a short explanation and cite examples of why or why not the rock weathering in your area fits the theoretical weathering region from Figure 15.16.

Apply & Learn

fallrockfalltalus (talus slope, talus cone)angle of repose

avalancheslideslumplandslide

flowdebris flowmudflowlahar

1. In what ways is mass wasting similar to, yet different from, the action of the geomorphic agents?

2. How does physical weathering encourage chemical weather-ing in rock?

3. How are joints, fractures, and other voids in a rock related to the rate at which weathering takes place?

4. What are several ways in which expansion and contraction can affect the weathering of rock?

5. Why is chemical weathering more rapid in humid climates than in more arid climates?

6. Distinguish between hydration and hydrolysis. 7. What are the impacts of differential weathering and erosion

on shaping landforms? 8. Distinguish among the principal types of Earth materials

moved in mass wasting. 9. What factors facilitate creep? 10. Describe the principal differences between a rockslide and

a mudflow. 11. What role does climate play in mass wasting?

Discuss & Review


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Note: Please read the About Locate & Explore Activities section of the Preface before beginning these exercises. 1. Using Google Earth, identify the landforms at the following

locations (latitude, longitude) and provide a brief discussion of how the landform developed. Include a brief discussion of why the landform is found in that general area of North America. Tip: Use the zoom, tilt, rotate, and elevation exaggeration tools to help view and interpret the landform and the area in which it is found.a. 33.805°N, 84.145°Wb. 51.56°N, 116.36°Wc. 37.75°N, 119.53°W

d. 22.19°N, 159.61°We. 49.305°N, 121.241°Wf. 49.59°N, 114.40°W

2. Using Google Earth and the Landslide Hazard Layer, provide an explanation for why the landslide hazard is high in some areas of the United States and low in others. Make sure that you consider geology, topography, precipitation, soils, and vegetation cover. Tip: Use the zoom, tilt, rotate and elevation exaggeration tools to help view and interpret the landscape where there is a high landslide hazard.

Locate & Explore

C H A P T E R 1 5 A C T I V I T I E S


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55061 15 Ch15 p410 437 pp2 - Cengage · Under what climatic conditions are chemical weathering processes ... Earth’s surface leading to its disintegration and decomposition