EROSION

CONCEPT

Erosion is a broadly defined group of processes involving the movement of soil and rock. This movement is often the result of flowing agents, whether wind, water, or ice, which sometimes behaves like a fluid in the large mass of a glacier. Gravitational pull may also influence erosion. Thus, erosion, as a concept in the earth sciences, overlaps with mass wasting or mass movement, the transfer of earth material down slopes as a result of gravitational force. Even more closely related to erosion is weathering, the breakdown of rocks and minerals at or near the surface of Earth owing to physical, chemical, or biological processes. Some definitions of erosion even include weathering as an erosive process. Though most widely known as a by-product of irresponsible land use by humans and for its negative effect on landforms, erosion is neither unnatural nor without benefit. Far more erosion occurs naturally than as a result of land development, and a combination of weathering and erosion is responsible for producing the soil from which Earth's plants grow.

HOW IT WORKS

Weathering

The first step in the process of erosion is weathering. Weathering, in a general sense, occurs everywhere: paint peels; metal oxidizes, resulting in its tarnishing or rusting; and any number of products, from shoes to houses, begin to show the effects of physical wear and tear. The scuffing of a shoe, cracks in a sidewalk, or the chipping of glass in a gravel-spattered windshield are all examples of physical weathering. On the other hand, the peeling of paint is usually the result of chemical changes, which have reduced the adhesive quality of the paint. Certainly oxidation is a chemical change, meaning that it has not simply altered the external properties of the item but also has brought about a change in the way that the atoms are bonded.

Weathering, as the term is used in the geologic sciences, refers to these and other types of physical and chemical changes in rocks and minerals at or near the surface of Earth. A mineral is a substance that occurs naturally and is usually inorganic, meaning that it contains carbon in a form other than that of an oxide or a carbonate, neither of which is considered organic. It typically has a crystalline structure, or one in which the constituent parts have a simple and definite geometric arrangement repeated in all directions. Rocks are simply aggregates or combinations of minerals or organic material or both.

TWO AND ONE-HALF KINDS OF WEATHERING.

There are three kinds of weathering (or perhaps two and one-half, since the third incorporates aspects of the first two): physical or mechanical, chemical, and biological. Physical or mechanical weathering takes place as a result of such factors as gravity, friction, temperature, and moisture. Gravity may cause a rock to drop from a height, such that it falls to the ground and breaks into pieces, while the friction of wind-borne sand may wear down a rock surface. Changes in temperature and moisture cause expansion and contraction of materials, as when water seeps into a crack in a rock and then freezes, expanding and splitting the rock.

Minerals are chemical compounds; thus, whereas physical weathering attacks the rock as a whole, chemical weathering effects the breakdown of the minerals that make up the rock. This breakdown may lead to the dissolution of the minerals, which then are washed away by water or wind or both, or it may be merely a matter of breaking the minerals down into simpler compounds. Reactions that play a part in this breakdown may include oxidation, mentioned earlier, as well as carbonation, hydrolysis (a reaction with water that results in the separation of a compound to form a new substance or substances), and acid reactions. For instance, if coal has been burned in an area, sulfur impurities in the air react with water vapor (an example of hydrolysis) to produce acid rain, which can eat away at rocks. Rainwater itself is a weak acid, and over the years it slowly dissolves the marble of headstones in old cemeteries.

As noted earlier, there are either three or two and one-half kinds of weathering, depending on whether one considers biological weathering a third variety or merely a subset of physical and chemical weathering. The weathering exerted by organisms (usually plants rather than animals) on rocks and minerals is indeed chemical and physical, but because of the special circumstances, it is useful to consider it individually. There is likely to be a long-term interaction between the organism and the geologic item, an obvious example being a piece of moss that grows on a rock. Over time, the moss will influence both physical and chemical weathering through its attendant moisture as well as its specific chemical properties, which induce decomposition of the rock's minerals.

Unconsolidated Material

The product of weathering in rocks or minerals is unconsolidated, meaning that it is in pieces, like gravel, though much less uniform in size. This is called regolith, a general term that describes a layer of weathered material that rests atop bedrock. Sand and soil, including soil mixed with loose rocks, are examples of regolith. Regolith is, in turn, a type of sediment, material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

Every variety of unconsolidated material has its own angle of repose, or the maximum angle at which it can remain standing. Piles of rocks may have an angle of repose as high as 45°, whereas dry sand has an angle of only 34°. The addition of water can increase the angle of repose, as anyone who has ever strengthened a sand castle by adding water to it knows. Suppose one builds a sand castle in the morning, sloping the sand at angles that would be impossible if it were dry. By afternoon, as wind and sunlight dry out the sand, the sand castle begins to fall apart, because its angle of repose is too high for the dry sand.

Water gives sand surface tension, the same property that causes water that has been spilled on a table to bead up rather than lie flat. If too much water is added to the sand, however, the sand becomes saturated and will flow, a process called lateral spreading. On the other hand, with too little moisture, the material is susceptible to erosion. Unconsolidated material in nature generally has a slope less than its angle of repose, owing to the influence of wind and other erosive forces.

Introduction to Mass Wasting

There are three general processes whereby a piece of earth material can be moved from a high out-cropping to the sea: weathering, mass wasting, and erosion. In the present context, we are concerned primarily with the last of these processes, of course, and secondarily with weathering, inasmuch as it contributes to erosion. A few words should be said about mass wasting, however, which, in its slower forms (most notably, creep), is related closely to erosion.

Mechanical or chemical processes, or a combination of the two, acting on a rock to dislodge it from a larger sample (e.g., separating a rock from a boulder) is an example of weathering, as we have seen. If the pieces of rock are swept away by a river in a valley below the outcropping, or if small pieces of rock are worn away by high winds, the process is erosion. Between the out-cropping and the river below, if a rock has been broken apart by weathering, it may be moved farther along by mass-wasting processes, such as creep or fall.

REAL-LIFE APPLICATIONS

Mass Wasting in Action

One of the principal sources of erosion is gravity, which is also the force behind creep, the slow downward movement of regolith along a hill slope. The regolith begins in a condition of unstable equilibrium, like a soda can lying on its side rather than perpendicular to a table's surface: in both cases, the object remains in place, yet a relatively small disturbance would be enough to dislodge it.

Changes in temperature or moisture are among the leading factors that result in creep. A variation in either can cause material to expand or contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. Water also can provide lubrication, or additional weight, that assists the material in moving. Though it is slow, over time creep can produce some of the most dramatic results of any mass-wasting process. It can curve tree trunks at the base, break or dislodge retaining walls, and overturn objects ranging from fence posts to utility poles to tombstones.

OTHER VARIETIES OF FLOW.

Creep is related to another slow mass-wasting process, known as solifluction, that occurs in the active layer of permafrost—that is, the layer that thaws in the summertime. The principal difference between creep and solifluction is not the speed at which they take place (neither moves any faster than about 0.5 in. [1 cm] per year) but the materials involved. Both are examples of flow, a chaotic form of mass wasting in which masses of material that are not uniform move downslope. With the exception of creep and solifluction, most forms of flow are comparatively rapid, and some are extremely so.

Because it involves mostly dry material, creep is an example of granular flow, which is composed of 0% to 20% water; on the other hand, solifluction, because of the ice component, is an instance of slurry flow, consisting of 20% to 40% water. If the water content is more than 40%, a slurry flow is considered a stream. Types of granular flow that move faster than creep range from earth flow to debris avalanche. Both earth flow and debris flow, its equivalent in slurry form, move at a broad range of speeds, anywhere from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Grain flow can be as fast as 60 mi. (100 km) per hour, and mud flow is even faster. Fastest of all is debris avalanche, which may achieve speeds of 250 mi. (400 km) per hour.

OTHER TYPES OF MASS WASTING.

Other varieties of mass wasting include slump, slide, and fall. Slump occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl.) The result is the formation of a small, crescent-shaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Slump often is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat or planar surface. These movements are sometimes called rock slides, debris slides, or, in common parlance, landslides.

In contrast to most other forms of mass wasting, in which there is movement along slopes that are considerably less than 90°, fall occurs at angles almost perpendicular to the ground. The "Watch for Falling Rock" signs on mountain roads may be frightening, and rock or debris fall is certainly one of the more dramatic forms of mass wasting. Yet the variety of mass wasting that has the most widespread effects on the morphology or shape of landforms is the slowest one—creep. (For more about the varieties of mass wasting, see Mass Wasting.)

What Causes Erosion?

As noted earlier, the influences behind erosion are typically either gravity or flowing media: water, wind, and even ice in glaciers. Liquid water is the substance perhaps most readily associated with erosion. Given enough time, water can wear away just about anything, as proved by the carving of the Grand Canyon by the Colorado River.

Dubbed the universal solvent for its ability to dissolve other materials, water almost never appears in its pure form, because it is so likely to contain other substances. Even "pure" mountain water contains minerals and pieces of the rocks over which it has flowed, a testament to the power of water in etching out landforms bit by bit. Nor does it take a rushing mountain stream or crashing waves to bring about erosion; even a steady drip of water is enough to wear away granite over time.

MOVING WATER.

Along coasts, pounding waves continually alter the shoreline. The sheer force of those walls of water, a result of the Moon's gravitational pull (and, to a lesser extent, the Sun's), is enough to wear away cliffs, let alone beaches. In addition, waves carry pieces of pebble, stone, and sand that cause weathering in rocks. Waves even can bring about small explosions in pockmarked rock surfaces by trapping air in small cracks; eventually the pressure becomes great enough that the air escapes, loosening pieces of the rock.

In addition to the erosive power of saltwater waves on the shore, there is the force exerted by running water in creeks, streams, and rivers. As the river moves, pushing along sediment and other materials eroded from the streambed or riverbed, it carves out deep chasms in the bedrock beneath. These moving bodies of water continually reshape the land, carrying soil and debris downslope, or from the source of the river to its mouth or delta. A delta is a region of sediment formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition (depositing) of sediment. It is so named because its triangular shape resembles that of the Greek letter delta, Δ .

Water at the bottom of a large body, such as a pond or lake, also exerts erosive power. Then there is the influence of falling rain. Assuming ground is not protected by vegetation, raindrops can loosen particles of soil, sending them scattering in all directions. A rain that is heavy enough may dislodge whole layers of topsoil and send them rushing away in a swiftly moving current. The land left behind may be rutted and scarred, much of its best soil lost for good.

Just as erosion gives to the soil, it also can take away. Whereas erosion on the Nile delta acted to move rich, black soil into the region (hence, the ancient Egyptians' nickname for their country, the "black land"), erosion also can remove soil layers. As is often the case, it is much easier to destroy than to create: 1 in. (2.5 cm) of soil may take as long as 500 years to form, yet a single powerful rainstorm or windstorm can sweep it away.

Glaciers

Ice, of course, is simply another form of water, but since it is solid, its physical (not its chemical) properties are quite different. Generally, physical sciences, such as physics or chemistry, treat as fluid all forms of matter that flow, whether they are liquid or gas. Normally, no solids are grouped under the heading of "fluid," but in the earth sciences there is at least one type of solid object that behaves as though it were fluid: a glacier.

A glacier is a large, typically moving mass of ice either on or adjacent to a land surface. It does not flow in the same way that water does; rather, it is moved by gravity, as a consequence of its extraordinary weight. Under certain conditions, a glacier may have a layer of melted water surrounding it, which greatly enhances it mobility. Regardless of whether it has this lubricant, however, a glacier steadily moves forward, carrying pieces of rock, soil, and vegetation with it.

These great rivers of ice gouge out pieces of bedrock from mountain slopes, fashioning deep valleys. Ice along the bottom of the glacier pulls away rocks and soil, which assist it in wearing away bedrock. The fjords of Norway, where high cliffs surround narrow inlets whose depths extend many thousands of feet below sea level, are a testament to the power of glaciers in shaping the Earth. The fact that the fjords came into existence only in the past two million years, a product of glacial activity associated with the last ice age, is evidence of something else remarkable about glaciers: their speed.

"Speed," of course, is a relative term when speaking about processes involved in the shaping of the planet. A "fast" glacier, one whose movement is assisted by a wet and warm (again, relatively warm!) maritime climate, moves at the rate of about 980 ft. (300 m) per year. Examples include not only the glaciers that shaped the fjords, but also the active Franz Josef glacier in southern New Zealand. By contrast, in the dry, exceptionally cold, inland climate of Antarctica, the Meserve glacier moves at the rate of just 9.8 ft. (3 m) per year.

Wind

The erosion produced by wind often is referred to as an eolian process, the name being a reference to Aeolus, the Greek god of the winds encountered in Homer's Odyssey and elsewhere. Eolian processes include the erosion, transport, and deposition of earth material owing to the action of wind. It is most pronounced in areas that lack effective ground cover in the form of solidly rooted, prevalent vegetation.

Eolian erosion in some ways is less forceful than the erosive influence of water. Water, after all, can lift heavier and larger particles than can the winds. Wind, however, has a much greater frictional component in certain situations. This is particularly true when the wind carries sand, every grain of which is like a cutting tool. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a mushroom-shaped formation. The wind could not lift the fine grains of sand very high, but in places where it has been able to do its work, it has left an indelible mark.

The Dust Bowl and Human Contribution to Erosion

Though human actions are not a direct cause of erosion, human negligence or mismanagement often has prepared the way for erosive action by wind, water, or other agents. Interesting, soil itself, formed primarily by chemical weathering and enhanced by biological activity in the sediment, is a product of nature's erosive powers. Erosion transports materials from one place to another, robbing the soil in one place and greatly enhancing it in another.

This is particularly the case where river deltas are concerned. By transporting sediment and depositing it in the delta, the river creates an area of extremely fertile soil that, in some cases, has become literally the basis for civilizations. The earliest civilizations of the Western world, in Egypt and Sumer, arose in the deltas of the Nile and the Tigris-Euphrates river systems, respectively.

EROSION ON THE GREAT PLAINS.

An extreme example of the negative effects on the soil that can come from erosion (and, ultimately, from human mismanagement) took place in Texas, Oklahoma, Colorado, and Kansas during the 1930s. In the preceding years, farmers unwittingly had prepared the way for vast erosion by overcultivating the land and not taking proper steps to preserve its moisture against drought. In some places farmers alternated between wheat cultivation and livestock grazing on particular plots of land.

The soil, already weakened by raising wheat, was damaged further by the hooves of livestock, and thus when a period of high winds began at the height of the Great Depression (1929-41), the land was particularly vulnerable. The winds carried dust to places as far away as the eastern seaboard, in some cases removing topsoil to a depth of 3-4 in. (7-10 cm). Dunes of dust as tall as 15-20 ft. (4.6-6.1 m) formed, and the economic blight of the Depression was compounded for the farmers of the plains states, many of whom lost everything.

Out of the Dust Bowl era came some of the greatest American works of art: the 1939 film Wizard of Oz, John Steinbeck's book The Grapes of Wrath and the acclaimed motion picture (1939 and 1940, respectively), as well as Dorothea Lange's haunting photographs of Dust Bowl victims. The Dust Bowl years also taught farmers and agricultural officials a lesson about land use, and in later years farming practices changed. Instead of alternating one year of wheat growing with one year in which a field lay fallow, or unused, farmers discovered that a wheat-sorghum-fallow cycle worked better. They also enacted other measures, such as the planting of trees to serve as windbreaks around croplands.

The Striking Landscape of Erosion

Among the by-products of erosion are some of the most dramatic landscapes in the world, many of which are to be found in the United States. A particularly striking example appears in Colorado, where the Arkansas River carved out the Royal Gorge. Though it is not nearly as deep as the Grand Canyon, this one has something the more famous gorge does not: a bridge. Motorists with the stomach for it can cross a span 1,053 ft. (0.32 km) above the river, one of the most harrowing drives in America.

Another, perhaps equally taxing, drive is that down California 1, a gorgeous scenic highway whose most dramatic stretches lie between Carmel and San Simeon. Drivers headed south find themselves pressed up against the edge of the cliffs, such that the slightest deviation from the narrow road would send an automobile and its passengers plummeting to the rocks many hundreds of feet below. These magnificent, terrifying landforms are yet another product of erosion, in this case, the result of the pounding Pacific waves.

Also striking is the topography produced by the erosion of material left over from a volcanic eruption. As discussed in the Mountains essay, Devils Tower National Monument in Wyoming is the remains of an extinct volcano whose outer surface long ago eroded, leaving just the hard lava of the volcanic "neck." Erosion of lava also can produce mesas. Lava that has settled in a river valley may be harder than the rocks of the valley walls, such that the river eventually erodes the rocks, leaving only the lava platform. What was once the floor of the valley thus becomes the top of a mesa.

Controlling Erosion

The force that shapes valleys and coastlines is certainly enough to destroy hill slopes, often with disastrous consequences for nearby residents. Such has been the case in California, where, during the 1990s, areas were dealt a powerful onetwo punch of drought followed by rain. The drought killed off much of the vegetation that might have held the hillsides, and when rains came, they brought about mass wasting in the form of mudflows and landslides.

Over the surface of the planet, the average rate of erosion is about 1 in. (2.2 cm) in a thousand years. This is the average, however, meaning that in some places the rate is much, much higher, and in others it is greatly lower. The rate of erosion depends on several factors, including climate, the nature of the materials, the slope and angle of repose, and the role of plant and animal life in the local environment.

Whereas many types of plants help prevent erosion, the wrong types of planting can be detrimental. The dangers of improper land usage for crops and livestock are illustrated by the Dust Bowl experience, which highlights the fact that the organism most responsible for erosion is humanity itself. On the other hand, people also can protect against erosion by planting vegetation that holds the soil, by carefully managing and controlling land usage, and by lessening slope angle in places where gravity tends to erode the soil.

WHERE TO LEARN MORE

Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991.

"Coastal and Nearshore Erosion." United States Geological Survey (USGS) (Web site). <http://walrus.wr.usgs.gov/hazards/erosion.html>.

Dean, Cornelia. Against the Tide: The Battle for America's Beaches. New York: Columbia University Press, 1999.

Hecht, Jeff. Shifting Shores: Rising Seas, Retreating Coastlines. New York: Scribners, 1990.

Middleton, Nick. Atlas of Environmental Issues. Illus. Steve Weston and John Downes. New York: Facts on File, 1989.

Protecting Your Property from Erosion (Web site). <http://www.abag.ca.gov/bayarea/enviro/erosion/erosion.html>.

Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Land. Hillside, NJ: Enslow Publishers, 1993.

"Soil Erosion on Farmland." New Zealand Ministry of Agriculture and Forestry (Web site). <http://www.maf.govt.nz/MAFnet/publications/erosion-risks/httoc.htm>.

Weathering and Erosion (Web site). <http://vishnu.glg.nau.edu/people/jhw/GLG101/Weathering.html>.

Wind Erosion Research Unit. United States Department of Agriculture/Kansas State University (Web site). <http://www.weru.ksu.edu/>.

KEY TERMS

CREEP:

A form of mass wasting involving the slow downward movement of regolith as a result of gravitational force.

DELTA:

A region of sediment formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition of sediment.

DEPOSITION:

The process wherebysediment is laid down on the Earth's surface.

EROSION:

The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved.

FLOW:

A form of mass wasting in which a body of material that is not uniform moves rapidly downslope.

GEOMORPHOLOGY:

An area of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth.

GLACIER:

A large, typically moving mass of ice either on or adjacent to a land surface.

LANDFORM:

A notable topographicalfeature, such as a mountain, plateau, or valley.

MASS WASTING:

The transfer of earth material, by processes that includecreep, slump, slide, flow, and fall, downslopes. Also known as mass movement.

MORPHOLOGY:

Structure or form or the study thereof.

REGOLITH:

A general term describing a layer of weathered material that rests atopbedrock.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

SLIDE:

A variety of mass wasting in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat or planar surface.

SLUMP:

A form of mass wasting that occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl).

TOPOGRAPHY:

The configuration of Earth's surface, including its relief as well as the position of physical features.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical, chemical, or biological processes.

Erosion

Sources of erosional energy

Erosional settings

Weathering

Agents and mechanisms of transport

Products and impacts of erosion

Controls on erosion

Vegetation

Climate

Surface material

Slope angle

Land use

Erosion and rejuvenation

Erosion research

Resources

Erosion is a group of processes that act to slowly decompose, disintegrate, remove, and transport materials on the surface of Earth. Erosion can include processes that remove and transport materials such as weathering (decomposition and disintegration).

Erosion operates at the surface of Earth. The material produced by erosion is called sediment (sedimentary particles, or grains). A thin layer of sediment, known as regolith, covers most of Earth’s surface. Erosion of the underlying solid rock surface, known as bedrock, produces this layer of regolith. Erosion constantly wears down Earth’s surface, exposing the rocks below.

Sources of erosional energy

The energy for erosion comes from five sources: gravity, the sun, Earth’s rotation, chemical reactions, and organic activity. These forces work together to break down and carry away the surface materials of Earth.

Gravity exerts a force on all matter. Gravity, acting alone, moves sediment down slopes. Gravity also causes water and ice to flow down slopes, transporting earth materials with them. Gravity and solar energy work together to create waves and some types of ocean currents. Earth’s rotation, together with gravity, also creates tidal currents. All types of water movement in the ocean (waves and currents) erode and transport sediment.

Solar energy, along with gravity, produces weather in the form of rain, snow, wind, temperature changes, etc. These weather elements act on surface materials, working to decompose and disintegrate them. In addition, chemical reactions act to decompose earth materials. They break down and dissolve any compounds that are not stable at surface temperature and pressure. Organic activity, by both plants and animals, can also displace or disintegrate sediment. An example of this is the growth of a tree root moving or fracturing a rock.

Erosional settings

Erosion can occur almost anywhere on land or in the ocean. However, erosion does occur more rapidly in certain settings. Erosion happens faster in areas with steep slopes, such as mountains, and especially in areas where steep slopes combine with flowing water (mountain streams) or flowing ice (alpine glaciers). Erosion is also rapid where there is an absence of vegetation, which stabilizes material. Some settings where the absence of vegetation helps accelerate erosion are deserts, mountaintops, or agricultural lands. Whatever the setting, water is a more effective agent of erosion than wind or ice—even in the desert.

Weathering

The first step in erosion is weathering. Weathering of solid rock, produces loose sediment, and makes the sediment available for transport. Weathering consists of a number of related processes that are of two basic types: mechanical or chemical.

Mechanical weathering

Mechanical weathering processes serve to physically break large rocks or sedimentary particles into smaller ones. That is, mechanical weathering disintegrates earth materials. An example of mechanical weathering is when water, which has seeped down into cracks in a rock, freezes. The pressure created by freezing and expanding of the water breaks the rock apart. By breaking up rock and producing sediment, mechanical weathering increases the surface area of the rock and so speeds up its rate of chemical weathering.

Chemical weathering

Chemical weathering processes attack the minerals in rocks. Chemical weathering either decomposes minerals to produce other, more stable compounds or simply dissolves them away. Chemical weathering usually requires the presence of water. You may have noticed during a visit to a cemetery that the inscription on old marble headstones is rather blurred. This is because rainwater, which is a weak acid, is slowly

dissolving away the marble. This dissolution of rock by rainwater is an example of chemical weathering.

Chemical weathering results in the formation of dilute chemical solutions (minerals dissolved in water) as well as weathered rock fragments. Chemical weathering, along with biological activity, contributes to the formation of soils. Besides surface area, both the temperature and the amount of moisture present in an environment control the rate of chemical weathering. Chemical weathering usually happens fastest in warm, moist places like a tropical jungle, and slowest in dry, cold places like the Arctic.

Agents and mechanisms of transport

Transport of sediment occurs by one or more of four agents: gravity, wind, flowing water, or flowing ice. A simple principle controls transport; movement of sediment occurs only as long as the force exerted on the sediment grain by the agent exceeds the force that holds the grain in place (friction due to gravity). For example, the wind can only move a grain of sand if the force generated by the wind exceeds the frictional force on the bottom of the grain. If the wind’s force is only slightly greater than the frictional force, the grain will scoot along on the ground. If the wind’s force is much greater than the frictional force, the grain will roll or perhaps bounce along on the ground. The force produced by flowing air (wind), water, or ice is a product of its velocity.

When gravity alone moves rocks or sediment, this is a special type of transport known as mass wasting (or mass movement). This refers to the fact that most mass wasting involves a large amount of sediment

moving all at once rather than as individual grains. Along a highway, if a large massof soil and rock from a hillside suddenly gives way and rapidly moves downhill, this would be a type of mass wasting known as a landslide. Mudflows and rock falls are two othercommon types of mass wasting.

Products and impacts of erosion

Sediment grains and chemical solutions are common products of erosion. Wind, water, ice, or gravity transport these products from their site of origin and lay them down elsewhere in a process known as deposition. Deposition, which occurs in large depressions known as basins, is considered to be a separate process from erosion.

Soil is also a product of erosion. Soil is formed primarily by chemical weathering of loose sediments or bedrock along with varying degrees of biological activity, and the addition of biological material. Some soil materials may also have undergone a certain amount of transport before they were incorporated into the soil.

Another important product of erosion is the landscape that is left behind. Erosional landscapes are present throughout the world and provide some of our most majestic scenery. Some examples are mountain ranges such as the Rocky Mountains, river valleys like the Grand Canyon, and the rocky sea cliffs of Northern California. Anywhere that you can see exposed bedrock, or where there is only a thin layer of regolith or soil covering bedrock, erosion has been at work creating a landscape. In some places one erosional agent may be responsible for most of the work; in other locations a combination of agents may have produced the landscape. The Grand Canyon is a good example of what the combination of flowing river water and mass wasting can do.

In addition to producing sediment, chemical solutions, soil, and landscapes, erosion also has some rather negative impacts. Two of the most important of these concern the effect of erosion on soil productivity and slope stability.

Soils are vital to both plants and animals. Without soils plants cannot grow. Without plants, animals cannot survive. Unfortunately, erosion can have a very negative impact on soil productivity because it decreases soil fertility. Just as erosion can lead to the deposition of thick layers of nutrient rich material, thereby increasing soil fertility, erosion can also remove existing soil layers. Soil forms very slowly—a 1-in (2.5-cm) thick soil layer typically takes 50-500 years to form. Yet an inch of soil or more can be eroded by a single rainstorm or windstorm, if conditions are right. Farming and grazing, which expose the soil to increased rates of erosion, have a significant impact on soil fertility worldwide, especially in areas where soil conservation measures are not applied. High rates of soil erosion can lead to crop loss or failure and in some areas of the world, mass starvation. On United States farmland, even with widespread use of soil conservation measures, the average rate of soil erosion is three to five times the rate of soil formation. Over time, such rates cut crop yields and can result in unproductive farmland.

Erosion is also a very important control on slope stability. Slopes, whether they are small hillsides or large mountain slopes, tend to fail (mass waste) due to a combination of factors. However, erosion is a significant contributor to nearly all slope failures. For example, in California after a drought has killed much of the vegetation on a hillside, the rains that signal the end of the drought lead to increased erosion. Eventually, due to the increased erosion, the slope may fail by some type of mass wasting, such as a mudflow or landslide.

Controls on erosion

The average rate of erosion at the surface of Earth is about 1 inch (2.5 centimeter) per thousand years. However, the rate of erosion varies tremendously from place to place. Soil erosion in some areas exceeds one inch per year—one hundred times its rate of formation. This range in rates is dependent on several different controlling factors. These factors include the type and amount of plant cover and animal activity, the climate, the nature of surface materials, the slope angle, and human land use. However, many of these factors routinely help increase erosion in some ways, while decreasing it in others. In addition, a complex interplay between the different factors may exist. For example, a particular combination of surface materials and plant cover may accelerate erosion in one climate, while decreasing it in another. The individual controls can be difficult to recognize and their effects difficult to discern as well.

Vegetation

Generally, plants tend to secure and stabilize sediment, but they may also be instrumental in helping to weather bedrock (for example, by prying open cracks during root growth). Animals may increase erosion by loosening soil, but they can also help stabilize it. An earthworm’s sticky slime, for example, increases soil particle cohesion and helps the particles resist erosion.

Climate

As was mentioned above, warm, moist climates increase the rate of weathering and so speed up erosion as well. However, the plant cover in this setting usually helps decreasesoil loss. Deserts tend to be very susceptible to erosion due to the limited amounts of vegetation. Fortunately, the low rainfall characteristic of deserts helps to limit erosional effects.

Surface material

Bedrock is more resistant to erosion than are sediments and soil. However, bedrock does display a range of susceptibility to erosion due to the different types of rock that may be present. Here again, the type of climate can have a major impact on erosion rates. In the desert, nearly all types of bedrock are very resistant to erosion, whereas in the tropics, nearly all types of rock weather rapidly.

Slope angle

The angle of a slope is one of the few consistent controls on erosion. The steeper the slope, when all other factors being equal, the more susceptible the slope will be to erosion.

Land use

Agriculture increases the likelihood of erosion by exposing soil to wind and rainfall. However, agriculture is not the only human land use that increases the likelihood of erosion. Logging, construction, landscaping, as well as many other activities make land more susceptible to erosion. Generally, any land use or activity that disturbs the natural vegetation or involves a change in slope, surface materials, etc., will increase the likelihood of erosion. There are some obvious exceptions, though. For example, pavement can temporarily halt erosion in almost all cases. However, nothing resists the erosive power of nature forever—all human-made structures will eventually weather and then fail.

Erosion and rejuvenation

Studies of erosion and the landscapes it leaves behind have been going on for over a century. This area of geologic inquiry, known as geomorphology, has long recognized that a balance exists between the erosion of land and its rejuvenation. If this were not the case, after a few tens to hundreds of millions of years, Earth’s mountains would wear down to flat, relatively featureless plains and the basins would fill up with the sediment shed by the mountains. Instead, after billions of years of erosion, we still have mountains such as Mt. Everest in the Himalayas, which stands over 5.5 miles (8.8 kilometers) above sea level, and ocean trenches such as the Marianas Trench, which reaches depths of more than 6.5 miles (10.4 kilmeters) below sea level.

The continued existence of rugged landscapes on the face of Earth is a result of a process of rejuvenation known as plate tectonics. Forces within the interior of Earth periodically re-elevate, or uplift, Earth’s surface in various regions, while causing the lowering, or subsidence, of other regions. Plate tectonics therefore serves to maintain existing landscapes or build new ones. Currently, the Himalayas are an area of activeuplift, but someday uplift will cease and erosion will

KEY TERMS

Bedrock— The unweathered or partially weatheredsolid rock layer, which is exposed at Earth’s surface or covered by a thin mantle of soil or sediment.

Chemical weathering— The decomposition and decay of Earth materials caused by chemical attack. Deposition—The accumulation of sediments after transport by wind, water, ice, or gravity.

Geomorphology— The study of Earth’s landforms and the processes that produce them.

Mechanical weathering— The break up or disintegration of earth materials caused by the creation and widening of fractures. Also known as physical weathering.

Regolith— A thin layer of sediment that covers most of Earth’s surface. Erosion of the underlying solid rock surface produces this layer.

Slope stability— The ability of the materials on a slope to resist mass wasting.

Soil productivity— The ability of a soil to promote plant growth.

Surficial material— Any type of loose Earth material, for example sediment or soil, found at the surface of Earth.

slowly, but completely, wear them down. Someday the Marianas Trench may be filled in with sediment deposits. At the same time, new dramatic landscapes will be forming elsewhere on Earth.

Erosion research

Research continues to focus on the factors that control erosion rates and ways to lessen the impact of land use on soil productivity. New methods of soil conservation are continually being developed and tested to decrease the impact of soil erosion on crop production.

Conventional tillage techniques leave fields bare and exposed to the weather for extended periods of time, which leaves them vulnerable to erosion. Conservation tillage techniques are planting systems that employ reduced or minimum tillage and leave 30% or more of the field surface protected by crop residue after planting is done. Leaving crop residue protects the soil from the erosive effects of wind and rain. Direct drilling leaves the entire field undisturbed. Specialized machines poke holes through the crop residue, and seeds or plant starts are dropped directly into the holes. No-till planting causes more disturbances to the crop residue on the field. Using this technique, the farmer prepares a seedbed 2 in (5 cm) wide or less, leaving most of the surface of the field undisturbed and still covered with crop residues. Strip rotary tillage creates a wider seedbed, 4-8 inches (10-20 centimeters) wide, but still leaves crop residue between the seedbeds. Conservation tillage techniques are particularly effective at reducing erosion from farm lands; in some cases reducing erosion by as much as 90%.

Other erosion research is focused on the factors that control mass wasting, especially where it is hazardous to humans. Stabilization of slopes in high risk areas is an increasingly important topic of study, as more people populate these areas every day.

Resources

BOOKS

Douglass, Scott L. Saving America’s Beaches: The Causes of and Solutions to Beach Erosion. Singapore: World Scientific Publishing Company, 2002.

Morgan, R.P.C. Soil Erosion and Conservation. Boston: Blackwell Publishing Professional, 2005.

Redlin, Janice. Land Abuse & Soil Erosion. New York: Weigl Publishers, 2006.

Clay Harris

Erosion

Erosion is a group of processes that, acting together, slowly decompose, disintegrate, remove, and transport materials on the surface of Earth . Among geologists, there is no general agreement on what processes to include as a part of erosion. Some limit usage to only those processes that remove and transport materials. Other geologists also include weathering (decomposition and disintegration). This broad definition is used here.

Erosion is a sedimentary process. That is, it operates at the surface of the earth to produce, among other things, surficial materials. The material produced by erosion is called sediment (sedimentary particles, or grains). A thin layer of sediment, known as regolith, covers most of the earth's surface. Erosion of the underlying solid rock surface, known as bedrock , produces this layer of regolith. Erosion constantly wears down the earth's surface, exposing the rocks below.

Sources of erosional energy

The energy for erosion comes from five sources: gravity, the Sun , Earth's rotation , chemical reactions , and organic activity. These forces work together to break down and carry away the surficial materials of Earth.

Gravity exerts a force on all matter , Earth materials included. Gravity, acting alone, moves sediment down slopes. Gravity also causes water and ice to flow down slopes, transporting earth materials with them. Gravity and solar energy work together to create waves and some types of ocean currents. Earth's rotation , together with gravity, also creates tidal currents. All types of water movement in the ocean (waves and currents) erode and transport sediment.

Solar energy, along with gravity, produces weather in the form of rain, snow, wind , temperature changes, etc. These weather elements act on surface materials, working to decompose and disintegrate them. In addition, chemical reactions act to decompose earth materials. They break down and dissolve any compounds that are not stable at surface temperature and pressure . Organic activity, by both plants and animals, can also displace or disintegrate sediment. An example of this is the growth of a tree root moving or fracturing a rock.

Erosional settings

Erosion can occur almost anywhere on land or in the ocean. However, erosion does occur more rapidly in certain settings. Erosion happens faster in areas with steep slopes, such as mountains , and especially in areas where steep slopes combine with flowing water (mountain streams) or flowing ice (alpine glaciers ). Erosion is also rapid where there is an absence of vegetation, which would stabilize the surficial materials. Some settings where the absence of vegetation helps accelerate erosion are deserts, mountain tops, or agricultural lands. Whatever the setting, water is a more effective agent of erosion than wind or ice—even in the desert .

Weathering

The first step in erosion is weathering. Weathering "attacks" solid rock, produces loose sediment, and makes the sediment available for transport. Weathering consists of a number of related processes that are of two basic types: mechanical or chemical.

Mechanical weathering

Mechanical weathering processes serve to physically break large rocks or sedimentary particles into smaller ones. That is, mechanical weathering disintegrates earth materials. An example of mechanical weathering is when water, which has seeped down into cracks in a rock, freezes. The pressure created by freezing and expanding of the water breaks the rock apart. By breaking up rock and producing sediment, mechanical weathering increases the surface area of the rock and so speeds up its rate of chemical weathering.

Chemical weathering

Chemical weathering processes attack the minerals in rocks. Chemical weathering either decomposes minerals to produce other, more stable compounds or simply dissolves them away. Chemical weathering usually requires the presence of water. You may have noticed during a visit to a cemetery that the inscription on old marble headstones is rather blurred. This is because rainwater, which is a weak acid, is slowly dissolving away the marble. This dissolution of rock by rainwater is an example of chemical weathering.

Chemical weathering results in the formation of dilute chemical solutions (minerals dissolved in water) as well as weathered rock fragments. Chemical weathering, along with biological activity, contributes to the formation of soils. Besides surface area, both the temperature and the amount of moisture present in an environment control the rate of chemical weathering. Chemical weathering usually happens fastest in warm, moist places like a tropical jungle, and slowest in dry, cold places like the Arctic.

Agents and mechanisms of transport

Transport of sediment occurs by one or more of four agents: gravity, wind, flowing water, or flowing ice. A simple principle controls transport; movement of sediment occurs only as long as the force exerted on the sediment grain by the agent exceeds the force that holds the grain in place (friction due to gravity). For example, the wind can only move a grain of sand if the force generated by the wind exceeds the frictional force on the bottom of the grain. If the wind's force is only slightly greater than the frictional force, the grain will scoot along on the ground. If the wind's force is much greater than the frictional force, the grain will roll or perhaps bounce along on the ground. The force produced by flowing air (wind), water, or ice is a product of its velocity .

When gravity alone moves rocks or sediment, this is a special type of transport known as mass wasting (or mass movement). This name refers to the fact that most mass wasting involves a large amount of sediment moving all at once rather than as individual grains. Along a highway, if a large mass of soil and rock from a hillside suddenly gives way and rapidly moves downhill, this would be a type of mass wasting known as a landslide. Mudflows and rockfalls are two other common types of mass wasting.

Products and impacts of erosion

As already mentioned, sediment grains and chemical solutions are common products of erosion. Wind, water, ice, or gravity transport these products from their site of origin and lay them down elsewhere in a process known as deposition. Deposition, which occurs in large depressions known as basins, is considered to be a separate process from erosion.

Soil is also an erosional product. Soil is formed primarily by chemical weathering of loose sediments or bedrock along with varying degrees of biological activity, and the addition of biological material. Some soil materials may also have undergone a certain amount of transport before they were incorporated into the soil.

Another important product of erosion is the landscape that is left behind. Erosional landscapes are present throughout the world and provide some of our most majestic scenery. Some examples are mountain ranges such as the Rocky Mountains, river valleys like the Grand Canyon, and the rocky sea cliffs of Northern California. Anywhere that you can see exposed bedrock, or where there is only a thin layer of regolith or soil covering bedrock, erosion has been at work creating a landscape. In some places one erosional agent may be responsible for most of the work; in other locations a combination of agents may have produced the landscape. The Grand Canyon is a good example of what the combination of flowing river water and mass wasting can do.

In addition to producing sediment, chemical solutions, soil, and landscapes, erosion also has some rather negative impacts. Two of the most important of these concern the effect of erosion on soil productivity and slope stability.

Soils are vital to both plants and animals. Without soils plants cannot grow. Without plants, animals cannot survive. Unfortunately, erosion can have a very negative impact on soil productivity because it decreases soil fertility. Just as erosion can lead to the deposition of thick layers of nutrient rich material, thereby increasing soil fertility, erosion can also remove existing soil layers. Soil forms very slowly—a 1-in (2.5-cm) thick soil layer typically takes 50-500 years to form. Yet an inch of soil or more can be eroded by a single rainstorm or windstorm, if conditions are right. Farming and grazing, which expose the soil to increased rates of erosion, have a significant impact on soil fertility worldwide, especially in areas where soil conservation measures are not applied. High rates of soil erosion can lead to crop loss or failure and in some areas of the world, mass starvation. On United States farmland, even with widespread use of soil conservation measures, the average rate of soil erosion is three to five times the rate of soil formation. Over time , such rates cut crop yields and can result in unproductive farmland.

Erosion is also a very important control on slope stability. Slopes, whether they are small hillsides or large mountain slopes, tend to fail (mass waste) due to a combination of factors. However, erosion is a significant contributor to nearly all slope failures. For example, in California after a drought has killed much of the vegetation on a hillside, the rains that signal the end of the drought lead to increased erosion. Eventually, due to the increased erosion, the slope may fail by some type of mass wasting, such as a mudflow or landslide.

Controls on erosion

The average rate of erosion at the surface of Earth is about 1 in (2.5 cm) per thousand years. However, the rate of erosion varies tremendously from place to place. Soil erosion in some areas exceeds one inch per year—one hundred times its rate of formation. This range in rates is dependent on several different controlling factors. These factors include the type and amount of plant cover and animal activity, the climate, the nature of surface materials, the slope angle , and human land use . However, many of these factors routinely help increase erosion in some ways, while decreasing it in others. In addition, a complex interplay between the different factors may exist. For example, a particular combination of surficial materials and plant cover may accelerate erosion in one climate, while decreasing it in another. The individual controls can be difficult to recognize and their effects difficult to discern as well.

Vegetation

Generally, plants tend to secure and stabilize sediment, but they may also be instrumental in helping to weather bedrock (for example, by prying open cracks during root growth). Animals may increase erosion by loosening soil, but they can also help stabilize it. An earthworm's sticky slime, for example, increases soil particle cohesion and helps the particles resist erosion.

Climate

As was mentioned above, warm, moist climates increase the rate of weathering and so speed up erosion as well. However, the plant cover in this setting usually helps decrease soil loss. Deserts tend to be very susceptible to erosion due to the limited amounts of vegetation. Fortunately, the low rainfall characteristic of deserts helps to limit erosional effects.

Surface material

Bedrock is more resistant to erosion than are sediments and soil. However, bedrock does display a range of susceptibility to erosion due to the different types of rock that may be present. Here again, the type of climate can have a major impact on erosion rates. In the desert, nearly all types of bedrock are very resistant to erosion, whereas in the tropics, nearly all types of rock weather rapidly.

Slope angle

The angle of a slope is one of the few consistent controls on erosion. The steeper the slope, when all other factors being equal, the more susceptible the slope will be to erosion.

Land use

Agriculture increases the likelihood of erosion by exposing soil to wind and rainfall. However, agriculture is not the only human land use that increases the likelihood of erosion. Logging, construction, landscaping, as well as many other activities make land more susceptible to erosion. Generally, any land use or activity that disturbs the natural vegetation or involves a change in slope, surface materials, etc., will increase the likelihood of erosion. There are some obvious exceptions, though. For example, pavement can temporarily halt erosion in almost all cases. However, nothing resists the erosive power of nature forever—all man-made structures will eventually weather and then fail.

Erosion and rejuvenation

Studies of erosion and the landscapes it leaves behind have been going on for over a century. This area of geologic inquiry, known as geomorphology, has long recognized that a balance exists between the erosion of land and its rejuvenation. If this were not the case, after a few tens to hundreds of millions of years, the earth's mountains would wear down to flat, relatively featureless plains and the basins would fill up with the sediment shed by the mountains. Instead, after billions of years of erosion, we still have mountains such as Mt. Everest in the Himalayas, which stands over 5.5 mi (8.8 km) above sea level , and ocean trenches such as the Marianas Trench, which reaches depths of more than 6.5 mi (10.4 km) below sea level.

The continued existence of rugged landscapes on the face of the earth is a result of a process of rejuvenation known as plate tectonics . Forces within the interior of the earth periodically re-elevate, or uplift , the earth's surface in various regions, while causing the lowering, or subsidence , of other regions. Plate tectonics therefore serves to maintain existing landscapes or build new ones. Currently, the Himalayas are an area of active uplift, but someday uplift will cease and erosion will slowly, but completely, wear them down. Someday the Marianas Trench may be filled in with sediment deposits. At the same time, new dramatic landscapes will be forming elsewhere on the Earth.

Erosion research

Research continues to focus on the factors that control erosion rates and ways to lessen the impact of land use on soil productivity. New methods of soil conservation are continually being developed and tested to decrease the impact of soil erosion on crop production.

Conventional tillage techniques leave fields bare and exposed to the weather for extended periods of time, which leaves them vulnerable to erosion. Conservation tillage techniques are planting systems that employ reduced or minimum tillage and leave 30% or more of the field surface protected by crop residue after planting is done. Leaving crop residue protects the soil from the erosive effects of wind and rain. Direct drilling leaves the entire field undisturbed. Specialized machines poke holes through the crop residue, and seeds or plant starts are dropped directly into the holes. No-till planting causes more disturbance to the crop residue on the field. Using this technique, the farmer prepares a seedbed 2 in (5 cm) wide or less, leaving most of the surface of the field undisturbed and still covered with crop residues. Strip rotary tillage creates a wider seed bed, 4-8 in (10-20 cm) wide, but still leaves crop residue between the seed beds. Conservation tillage techniques are particularly effective at reducing erosion from farm lands; in some cases reducing erosion by as much as 90%.

Other erosion research is focused on the factors that control mass wasting, especially where it is hazardous to humans. Stabilization of slopes in high risk areas is an increasingly important topic of study, as more people populate these areas every day.

See also Sediment and sedimentation.

Resources

books

Dixon, D. The Practical Geologist. New York: Simon & Schuster, 1992.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Leopold, L.B. A View of the River. Cambridge: Harvard University Press, 1994.

Woodhead, James A. Geology. Boston: Salem Press, 1999.

periodicals

Reganold, J. P., R. K. Papendick, and J.F. Parr. "Sustainable Agriculture." Scientific American (June 1990): 112-120.

Clay Harris

KEY TERMS

Bedrock

—The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.

Chemical weathering

—The decomposition and decay of Earth materials caused by chemical attack.

Deposition

—The accumulation of sediments after transport by wind, water, ice, or gravity.

Geomorphology

—The study of Earth's land forms and the processes that produce them.

Mechanical weathering

—The break up or disintegration of earth materials caused by the creation and widening of fractures. Also known as physical weathering.

Regolith

—A thin layer of sediment that covers most of the earth's surface. Erosion of the underlying solid rock surface produces this layer.

Slope stability

—The ability of the materials on a slope to resist mass wasting.

Soil productivity

—The ability of a soil to promote plant growth.

Surficial material

—Any type of loose Earth material, for example sediment or soil, found at the surface of Earth.

Erosion

Erosion is the wearing away of the land surface by running water, wind, ice, or other geologic agents, including such processes as gravitational creep.

The term geologic erosion refers to the normal, natural erosion caused by geological processes acting over long periods of time, undisturbed by humans. Accelerated erosion is a more rapid erosion process influenced by human, or sometimes animal, activities. Accelerated erosion in North America has only been recorded for the past few centuries, and in research studies, postsettlement erosion rates were found to be eight to 350 times higher than presettlement erosion rates.

Soil erosion has been both accelerated and controlled by humans since recorded history. In Asia, the Pacific, Africa, and South America, complex terracing and other erosion control systems on arable land go back thousands of years. Soil erosion and the resultant decreased food supply have been linked to the decline of historic, particularly Mediterranean, civilizations, though the exact relationship with the decline of governments such as the Roman Empire is not clear.

A number of terms have been used to describe different types of erosion, including gully erosion, rill erosion, interrill erosion, sheet erosion, splash erosion, saltation, surface creep, suspension, and siltation .In gully erosion, water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 1.5 ft (0.5 m) to as much as 82–98 ft (25–30 m).

Rill erosion refers to a process in which numerous small channels of only a few inches in depth are formed, usually occurring on recently cultivated soils. Interrill erosion is the removal of a fairly uniform layer of soil on a multitude of relatively small areas by rainfall splash and film flow.

Usually interpreted to include rill and interril erosion, sheet erosion is the removal of soil from the land surface by rainfall and surface runoff . Splash erosion, the detachment and airborne movement of small soil particles, is caused by the impact of raindrops on the soil.

Saltation is the bouncing or jumping action of soil and mineral particles caused by wind, water, or gravity. Saltation occurs when soil particles 0.1–0.5 mm in diameter are blown to a height of less than 6 in (15 cm) above the soil surface for relatively short distances. The process includes gravel or stones effected by the energy of flowing water, as well as any soil or mineral particle movement downslope due to gravity.

Surface creep, which usually requires extended observation to be perceptible, is the rolling of dislodged particles 0.5–1.0 mm in diameter by wind along the soil surface. Suspension occurs when soil particles less than 0.1 mm diameter are blown through the air for relatively long distances, usually at a height of less than 6 in (15 cm) above the soil surface. In siltation, decreased water speed causes deposits water-borne sediments, or silt , to build up in stream channels, lakes, reservoirs, or flood plains.

In the water erosion process, the eroded sediment is often higher (enriched) in organic matter, nitrogen , phosphorus , and potassium than in the bulk soil from which it came. The amount of enrichment may be related to the soil, amount of erosion, the time of sampling within a storm, and other factors. Likewise, during a wind erosion event, the eroded particles are often higher in clay, organic matter, and plant nutrients. Frequently, in the Great Plains, the surface soil becomes increasingly more sandy over time as wind erosion continues.

Erosion estimates using the Universal Soil Loss Equation (USLE) and the Wind Erosion Equation (WEE) estimate erosion on a point basis expressed in mass per unit area. If aggregated for a large area (e.g., state or nation), very large numbers are generated and have been used to give misleading conclusions. The estimates of USLE and WEE indicate only the soil moved from a point. They do not indicate how far the sediment moved or where it was deposited. In cultivated fields, the sediment may be deposited in other parts of the field with different crop cover or in areas where the land slope is less. It may also be deposited in riparian land along stream channels or in flood plains.

Only a small fraction of the water-eroded sediment leaves the immediate area. For example, in a study of five river watersheds in Minnesota, it was estimated that from less than 1–27% of the eroded material entered stream channels, depending on the soil and topographic conditions. The deposition of wind-eroded sediment is not well quantified, but much of the sediment is probably deposited in nearby areas more protected from the wind by vegetative cover, stream valleys, road ditches, woodlands, or farmsteads.

While a number of national and regional erosion estimates for the United States have been made since the 1920s, the methodologies of estimation and interpretations have been different, making accurate time comparisons impossible. The most extensive surveys have been made since the Soil, Water and Related Resources Act was passed in 1977. In these surveys a large number of points were randomly selected, data assembled for the points, and the Universal Soil Loss Equation (USLE) or the Wind Erosion Equation (WEE) used to estimate erosion amounts. While these equations were the best available at the time, their results are only estimations, and subject to interpretation. Considerable research on improved methods of estimation is underway by the U.S. Department of Agriculture .

In the cornbelt of the United States, water erosion may cause a 1.7–7.8% drop in soil productivity over the next one hundred years, as compared to current levels, depending on the topography and soils of the area. The U.S.D.A. results, based on estimated erosion amounts for 1977, only included sheet erosion, not losses of plant nutrients. Though the figures may be low for this reason, other surveys have produced similar estimates.

In addition to depleting farmlands, eroded sediment causes off-site damages that, according to one study, may exceed on-site loss. The sediment may end up in a domestic water supply, clog stream channels, even degrade wetlands , wildlife habitats, and entire ecosystems.

See also Environmental degradation; Gillied land; Soil eluviation; Soil organic matter; Soil texture

[William E. Larson ]

RESOURCES

BOOKS

Paddock, J. N., and C. Bly. Soil and Survival: Land Stewardship and the Future of American Agriculture. San Francisco: Sierra Club Books, 1987.

Resource Conservation Glossary. 3rd ed. Ankeny, IA: Soil Conservation Society of America, 1982.

PERIODICALS

Steinhart, P. "The Edge Gets Thinner." Audubon 85 (November 1983): 94–106+.

OTHER

Brown, L. R., and E. Wolf. "Soil Erosion: Quiet Crisis in the World Economy." Worldwatch Paper #60. Washington DC: Worldwatch Institute, 1984.

Erosion

Introduction

Erosion is the process of transportation of weathered rock and soil material by the processes of water, wind, or ice movement and/or by gravity or the actions of some organisms. For erosion to begin, rock or soil must first be subjected to the processes of weathering, including disintegration (breaking apart) and decomposition (chemical breakdown). Erosion ends with deposition, which is the process of eroded material (rock fragments, sediments, or dissolved chemical elements) coming to rest in a place that is apart from where erosion began. Erosion may be natural, human-induced, or the result of a combination of these two factors.

Historical Background and Scientific Foundations

Erosion has played a huge part in the history of planet Earth. Mountain building through geological time has raised vast mountain ranges, which in turn have been reduced by weathering and erosion to low plains. Tec-tonic forces within Earth, which have routinely affected

the crust over time, move rock to higher elevations. Yet, these materials are moved to lower elevations by erosion. In fact, if mountains were not continually built at a faster rate than erosion, the continental surface of Earth would have been reduced to a nearly featureless landscape long ago.

Eroded material can be picked up and moved by water, wind, and ice. Each of these transportation media (water, wind, and ice) requires a minimum velocity of movement to pick up (or entrain) eroded particles. For water and wind, these velocities are relatively high compared to ice, which can move materials of any size at almost any velocity. As the velocity of water and wind increases, progressively larger particles can be moved. As velocity decreases, particles are deposited or laid to rest.

During transportation, water and wind move particles by rolling, bouncing, and in suspension (floating). Particles interact during transportation and thus they are abraded (worn) or comminuted (broken into smaller pieces) during transportation. Transportation of particles moves them from the point of origin (for example, an eroded hillside) to the point of deposition (for example, a river delta or sand dune). The point of deposition may be a temporary location from which the particles are moved again at another time. An example of a temporary location is a gravel or sand bar in a river, which is a temporary location for river sediment. During a flood on the river, such bars may be eroded and the sediment moved downstream to another bar or ultimately downstream to the river's delta.

Gravity pulls on all natural and human-made materials. The effect of this is the natural tendency for materials that can slide or flow to move downslope at varying rates. The movement of water, wind, and ice noted earlier is in response to gravity. However, gravity acts directly upon rocks, sediment, and soil to move these materials downslope as well. If the affected rock, sediment, or soil moves as a single large mass, it is called a slump. If the rock, sediment, or soil moves as a layer where the upper part is moving faster than the lower parts, it is called a flow. An example of a common type of flow is soil creep, which is the slow, progressive flow of soil down the side of a hill or other sloping surface. Gravity effects like slump and flow work in combination with water, wind, and ice to move large quantities of materials all the time.

The actions of some organisms may affect erosion in some situations. The trampling effect of animals walking on sloping surfaces may weaken the rock, sediment, or soil in that area and thus contribute to erosion. Trampling may remove vegetation as well, which weakens natural materials or allows water, wind, or ice to more effectively transport such materials. Some organisms in the seas actively eat rock and sediment and thus disinte-grate (by chewing) and decompose (by digesting) rock fragments and sediment. Some agricultural, construction, timbering, and related practices affect erosion in some situations. Like the actions of organisms mentioned earlier, these practices can assist gravity and the flow of water and wind in the process of erosion of landscapes.

Impacts and Issues

WORDS TO KNOW

DECOMPOSITION: The breakdown of matter by bacteria and fungi. It changes the chemical makeup and physical appearance of materials.

DISINTEGRATION: Spontaneous nuclear transformation characterized by the emission of energy and/or mass from the nucleus.

SEDIMENTS: Disintegration and decomposition products of natural materials

TECTONIC FORCES: Forces that shape Earth's crust over geological time. The largest tectonic force is supplied by the slow convection of Earth's interior, driven by radiation of heat to space through the crust, much like the roiling of liquid in a boiling pot

Erosion may be human-induced, but much of natural erosion may be induced by climate change and its attendant effects. For example, increased erosion due to increased rainfall in an area may be climatically controlled. Climate control necessarily implies that the change is long-term and part of a long-term trend, as opposed to short-term effects from weather systems that come and go over shorter time spans.

See Also Dust Storms; Floods; Glaciation.

BIBLIOGRAPHY

Books

Montgomery, David R. Dirt: The Erosion of Civilization. Berkeley: University of California Press, 2007.

Periodicals

U.S. Department of Agriculture. “Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE).” Agriculture Handbook 703 (1996).

Web Sites

Pidwirny, Michael. “Introduction to the Lithosphere: Erosion and Deposition.” PhysicalGeography.net, 2007. < http://www.physicalgeography.net/fundamentals/10w.html> (accessed December 3, 2007).

David T. King Jr .