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 Erosion is the process that moves material resulting from the breakdown, or weathering, of bedrock. Weathering itself may be distinguished into two broad categories of process: physical and chemical (see weathering). Physical weathering results from the mechanical action of environmental agents on bedrock, including wind, moving water, freezing and thawing of water, crystallization and dissolution of salts, expansion of roots, digging by animals, and fire. Glacial erosion is perhaps the most powerful manifestation of the mechanical breakdown of bedrock. Sometimes, physical erosion acts alone, for example during the erosion of barely consolidated bedrock. Chemical weathering entails the chemical breakdown of bedrock minerals by water; it can also operate alone, as when carbonate or evaporite deposits dissolve. For much of the Earth's surface, however, physical and chemical weathering act in concert. When liquid water interacts with siliceous bedrock, chemical weathering breaks down some of the silicate minerals in the bedrock, converting these to solutes plus clays and aluminium and iron sesquioxides.

Measuring rates

Most weathered materials are removed and transported by water and wind. The discharge of material transported by rivers is normally measured at fixed gauging sites. The material discharge divided by the area of the gauged catchment gives the yield. The yield does not equate directly to erosion or weathering rates because of mass storage during transport downslope and through river channels. Instead, yields are said to equate to rates of denudation, a measure of the removal of material, thus generally leading to a reduction of elevation and relief. Possible sites of storage include (1) colluvium or sediment deposited at the base of hillslopes before even entering a channel, (2) alluvium or sediment deposited by fluvial activity within a channel or on adjacent floodplains, and (3) lacustrine sediments deposited in lakes. Various techniques are used to measure local rates of physical erosion, such as erosion pins, erosional troughs, and isotopic techniques. Particles moved by wind do not conveniently pass by a single gauging point, and intermediate storage in dunes and sediment sheets is almost always important. Thus aeolian denudation rates must be synthesized from many local measurements.

In rivers, the concentrations of individual solutes normally decrease with increasing discharge, whereas suspended sediment concentrations and bedload movement normally increase. Consequently, solute transport is dominated by typical flow conditions, and solid transport by discharge events. Good estimates of sediment yield from a watershed, or catchment area, thus require years of measurement. Many rigorous studies have been undertaken to determine sedi-ment yields from large watersheds, including those on sedimentary substrates. The mass of sediment in motion is large, 30–100 × 10¹⁵ g year⁻¹, with approximately 20 × 10¹⁵ g year⁻¹ being transferred to the ocean (Fig.1). Still larger quantities of sediment are probably deposited on land. Storage of solutes on land appears minimal, and about 9 × 10¹⁵ g year⁻¹ are transferred to the ocean.

Natural erosion in tropical to temperate settings

Under natural conditions, the supply of terrigenous sediments from a watershed is largely controlled by its topographic relief, its bedrock geology, and its Pleistocene climatic history. The fundamental division is between steepland and flatland terrains. Runoff follows in importance, whereas temperature appears less significant. The erosion of Pleistocene glacial and periglacial deposits continues to affect the solute and solid loads of many rivers. Finally, given the same general rock type, topography, runoff, and temperature, rates of chemical weathering of younger igneous rocks, such as those in island arcs, are roughly twice rates in old cratonic settings.

Feedback among the chemical and mechanical processes that erode developing soils strongly influences both the rate of erosion and the composition of solid and dissolved erosion products. This interaction can be examined in a simple framework, the division of erosional regimes into two types: weathering-limited and transport-limited. Typically, soils develop from the weathered and partially weathered mineral grains, while solutes are transported away by surface and ground waters. Loose soil material can then be eroded by physical processes. For weathering-limited erosion, the supply of loose weathering products is controlled by weathering rate. Soils tend to be thin. Most rock types are more vulnerable than young crystalline igneous bedrock to chemical and physical erosion, and weakly cemented sediments, such as some shales, and unconsolidated glacial deposits, especially loess, contribute exceptional quantities of sediments to rivers. For transport-limited erosion, weathering supplies loose solid materials at rates that exceed the capacity of transport processes. As soil thickens, it develops an internal structure which in turn reduces the rate of interaction between water and fresh minerals, thereby reducing weathering rates. Rates may drop exponentially with soil thickness. Weathering rates can be low in the absence of soil, because water fails to make prolonged contact with fresh minerals. Millions of years may be required to reach equilibrium.

Vegetation has two primary roles in weathering and erosion. First, it reduces the power of physical erosion processes. Plants protect and anchor loose material with their roots, while litter and low ground cover protect soil from raindrop impact and surface wash. This allows thicker soils to develop and stabilizes vulnerable bedrock. Physical erosion usually accelerates after loss of vegetation by fire, cutting, cropping, or grazing. Second, plants produce chemicals that promote the breakdown of bedrock and soil minerals. Some are released through roots, perhaps with the help of symbiotic root fungi (mycorrhyzae). Chemicals include an array of complexing agents, organic acids, and carbonic acid. Other chemicals, such as carbonic and nitric acids, are generated during the decay of organic matter. Experiments indicate substantially greater rates of chemical weathering under well-developed plant cover. Soils with plant cover and active soil organisms appear to have greater infiltration rates of water, perhaps allowing more interaction with bedrock minerals. On slopes where soils tend to be thin owing to weathering-limited erosion, plants probably greatly increase erosion rates because of their chemical effects. In transport-limited settings, with thick soils, the influence of plants may be less; carbon dioxide, however, is mobile throughout the soil profile.

The interaction of vegetation with erosional processes promotes styles of erosion where mass movement is episodic. Two types of cycles seem to be especially important. One is a landslide (soil–avalanche) cycle that may be a dominant form of mass wasting in many humid regions. The other is a fire–flood erosional cycle. In the landslide cycle, soil is anchored by vegetation. The soil profile eventually thickens to the point where heavy rainfall or an earthquake destabilizes the slope and causes soil avalanches. This normally involves the entire profile above a hard saprolite (partially weathered rock). Vegetation re-establishes, and the soil profile begins to thicken, starting the cycle once again. In eastern Puerto Rico, the recurrence interval is roughly 10 000 years. In a fire–flood sequence, vegetation anchors loose soil. After sufficient burnable material accumulates in and upon the soil, a hot fire can occur. Such burns commonly transform soils chemically into a hydrophobic form. Instead of infiltrating, heavy rains run off, thereby eroding the soil and scouring channels. Debris torrents and landslides develop. Vegetation re-establishes and the cycle starts once again. The fire–flood sequence requires a threshold accumulation of soil. Thus, the recurrence intervals of the fire–flood sequence are much shorter, probably by several hundred years. Because these processes involve loose material and act minimally on hard saprolite, they cause a characteristic partitioning of major cations in stream waters. Sodium and calcium are preferentially leached from the hard saprolite, and the sodium-to-potassium ratio in water (after correcting for sea salt) exceeds the bedrock ratio, as does the calcium to magnesium ratio.

For landscapes in which chemical weathering represents the rate-limiting step for physical erosion, the rates of physical erosion (solid yields) can be predicted from chemical erosion (Fig.2). A hypothetical starting point for analysing physical erosion in a watershed is to assume that the watershed is in geomorphic equilibrium. This is a state in which the statistical measures of a landscape (hypsography, river-channel density, soil types and thicknesses, vegetation types, disturbance types and frequencies) do not change through time. Most importantly, equilibrium requires that there is no net thickening of soils or accumulation of sediments within the landscape. In this context, we can define trends for equilibrium erosion for geologically young, crystalline, igneous bedrock and carbonate-cemented bedrock. When sediment yields for watersheds are compared with run-off, the predicted equilibrium–erosion trends separate high-sediment-yield rivers that drain steeplands from low-sediment-yield rivers that drain flatlands. Thus, these trends approximate to the transition between weathering-limited erosion in steeplands and transport-limited erosion in flatlands.

Sediment yields of flatland watersheds do not seem to depend on watershed area, whereas yields in mountainous watersheds generally decrease with increasing watershed area. The yields of the world's largest rivers are only a few-fold greater than those of flatland watersheds. Several factors are involved in the apparent decrease in yields with increasing catchment area. In uplands, sediments accumulate as colluvium at the base of hillslopes or in other areas of reduced steepness, or they deposit as alluvium in lower-order streams, without having ever entered a large river channel. Alluvial accumulation is especially significant in flatter regions. Large watersheds tend to have extensive flatland areas. These settings are transport-limited and either fail to contribute sediment through physical erosion or act as depositional areas, where sediments accumulate as alluvium in river channels and adjacent floodways, and as a mix of clastic and organic deposits in lakes and wetlands.

Glacial and cold climate weathering and erosion

Glacial erosion is the most powerful form of physical erosion. Perhaps hundreds of metres of rock were removed during the Pleistocene by continental ice sheets from the Laurentide region and smaller ice sheets in Asia and Europe. Glaciers have also deeply sculpted mountainous areas worldwide. Ice caps still cover much of Antarctica and Greenland. The intensity of glacial erosion is much greater when liquid water is present at the ice base, a condition referred to as a warm-based glacier. Glaciers lacking liquid water near the base, cold-based glaciers, sometimes erode rather little. For some regions with extensive Pleistocene glacial sediments, sediment yields of watersheds increase with increasing area, an indication that Pleistocene sediments have been eroded out of the smaller basins but are still moving downstream in larger watersheds.

Chemical weathering in cold, wet environments contrasts in notable ways from that of warmer settings. Glacierization and freeze-thaw processes fracture bedrock, exposing fresh mineral surfaces. In effect, the bedrock-to-water ratio in these settings is much greater than for warmer settings. Because of this, the weathering of less abundant but easily weathered minerals in siliceous bedrock, especially carbonates and sulphides, contributes disproportionally to solutes in surface waters. Subglacial waters are especially rich in potassium relative to sodium, perhaps because the physical breakdown of micas releases large quantities of potassium.

Human effects

Much of the Earth's land surface is now occupied by people and has been cleared for timber, agriculture, and urbanization. Although land clearing increases rates of physical erosion, the resulting changes in rates of physical erosion have, in general, not been well documented. Agriculture, civil engineering, and mining mobilize vast quantities of soils, unconsolidated sediment, and bedrock. Engineering activities, such as road building and building construction, may, in concert, move more solids over the Earth's surface than all ordinary natural processes combined. The hydraulic architecture of the land has been transformed to serve agriculture, water supplies, land reclamation, navigation, and power generation. Few rivers lack dams and reservoirs; large rivers are confined to their channels by levees; normal floodplains are converted to paddylands and aquiculture; water is mined; irrigation raises soil moisture and creates artificial distributary systems; and so forth. These changes confound the quantification of sediment dispersal from the uplands to the ocean.

Human activities have accelerated physical erosion over equilibrium levels. In flat coastal plains and piedmonts, yields appear to increase five- to ten-fold with agriculture, whereas in loess terrains 100-fold increases are typical. The greatest yields are in the smallest watersheds. In tiny watersheds in the upper Mississippi River valley, yields approaching 10 000 g m⁻² year⁻¹ have been reported.

Research directions

Much of the process-level research relating biogeochemical processes to erosion is routinely undertaken in small research watersheds. Most of these are on igneous and metamorphic rock to minimize groundwater losses. In contrast, most of the world's chemical and physical erosion is from sedimentary rocks, and these are mostly shales. Moreover, sampling for most small watersheds has been from periodic (interval-based) sampling, thereby ignoring a major characteristic of such watersheds, that sediment moves during discharge events. Finally, most research watersheds are not large enough to examine the effects of sediment storage during transport. Thus, many opportunities remain for well-designed process-level studies of weathering and erosion.

Perhaps one of the most fascinating research directions in the study of local erosion is the analysis of cosmogenic radionuclides produced in situ. These are created when cosmic-ray neutrons and muons strike atoms within mineral grains in a soil profile. Some of the important isotopes are ¹⁰Be, ²⁶Al, and ¹⁴C. The concentration of these isotopes depends on erosion rate and irradiation intensity. The latter depends on altitude and latitude and depth below the ground surface. Two situations are easily modelled. One is the erosion event, where it is assumed that a surface was formed at some time in the past, and that there was no subsequent erosion. Isotope concentrations are used to date this event. Settings locally dominated by transport-limited erosion would be suited to this approach. The second situation is to assume that erosion is steady, as might be expected in an erosion-limited setting. The ground-surface isotope concentration equates, for a broad range of erosion rates, to the erosion rate. These methods average erosion over thousands of years.

R. F. Stallard

Bibliography

Milliman, J. D. and and Syvitski, J. P. M. (1992) Geomorphic tectonic control of sediment discharge to the ocean—the importance of small mountainous rivers. Journal of Geology, 100, 525–44.
Stallard, R. F. (1995a) Relating chemical and physical erosion. Reviews in Mineralogy, 31, 543–64.
Stallard, R. F. (1995b) Tectonic, environmental, and human aspects of weathering and erosion—a global review using a steady-state perspective. Annual Review of Earth and Planetary Sciences, 23, 11–39.

Erosion

Erosion is the general term for the processes that wear down Earth's surfaces, exposing the rocks below. The natural forces responsible for this endless sculpting include running water, near-shore waves, ice, wind, and gravity. The material produced by erosion is called sediment or sedimentary particles. Covering most of Earth's surface is a thin layer of sediment known as regolith, which is produced by the erosion of bedrock, or the solid rock surface underlying Earth's surface.

Natural sources of erosion

Running water. Everywhere on the planet, running water continuously reshapes the land by carrying soil and debris steadily downslope. As the sediment and other eroded materials are carried along the bottoms of streams and rivers, they scour away the bedrock underneath, eventually carving deep gorges or openings. A classic example of the erosive power of running water over a great period of time is the Grand Canyon of the Colorado River.

Rain falling on dry land also can result in erosion. When raindrops strike bare ground that is not protected by vegetation, they loosen particles of soil, spattering them in all directions. During heavy rains on sloped surfaces, the dislodged soil is carried off in a flow of water.

Near-shore waves. Along seacoasts, the constant movement of tides and the pounding of waves alter the shoreline. The strong force of waves, especially during storms, erodes beaches and cliffs. Breaking waves often contain small pebbles and stones that scrape away at seacoast rocks, rubbing and grinding them into pieces. Waves can also trap air in small cracks and crevices in the rocks against which they crash. Small explosions in the rock result when air pressure builds up, sending loose chunks of rock toppling down.

Ice. Ice, in the form of huge glaciers, can plow through rock, soil, and vegetation. As the ice moves along, it scoops up great chunks of bedrock from the slopes, creating deep valleys. In turn, the rocks and soil already carried along the bottom of the glacier wear away the bedrock that is not loosened. Along many seacoasts, especially in Norway, glaciers gouged out fjords—long, narrow inlets whose bottoms can reach depths thousands of feet below sea level.

Wind. Wind erosion is referred to as eolian erosion, after Aeolus, the Greek god of wind. Erosion due to wind is more pronounced in dry regions and over land that lacks vegetation. The wind easily picks up particles of soil, sand, and dust and carries them away. Wind cannot carry as large of particles as flowing water, and it cannot carry fine particles more than a few feet or a meter above ground level. However, windblown grains of sand, when carried along at high speeds, effectively act as cutting tools. In desert regions, the bases of rocks and cliffs are often dramatically sandblasted away, resulting in mushroom-shaped rocks with large caps and slender stems.

Gravity. 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 sediment with them. When bare soil on steep slopes becomes waterlogged and fluid, the downward pull of gravity results in a landslide. Sometimes landslides are simple lobes of soil slumped down a hillside; other times they can be an avalanche of rocks and debris hurtling downslope.

Human contributions to erosion

Soil loss results naturally from erosion. A balance exists on Earth between the erosion of land and its rejuvenation by natural forces. However, human activities have overwhelmed this balance in many parts of the world. The removal of vegetation, poor farming practices, strip mining, logging, construction, landscaping, and other activities all increase erosion. In general, any land use or activity that disturbs the natural vegetation or that changes the slope or surface materials of an area will increase the chances of erosion.

The Dust Bowl that took place in the prairie states of America in the 1930s is an example of an ecological disaster resulting from erosion. In the years leading up to the Dust Bowl, farmers planted wheat on lands that were formerly used for livestock grazing. After several growing seasons, the livestock were returned and allowed to graze. Their hooves pulverized the unprotected soil, which strong winds then carried aloft in huge dust clouds. Crops and land were destroyed by the dust storms, and many families were forced to abandon their farms.

[See also Soil ]

Dunes

Dunes are well-sorted deposits of materials by wind or water that take on a characteristic shape and that retain that general shape as material is further transported by wind or water. Desert dunes classifications are based upon shape include barchan dunes, relic dunes, transverse dunes, lineal dunes, and blount (parabolic) dunes. Dunes formed by wind are common in desert areas and dunes formed by water are common in coastal areas. Dunes can also form on the bottom of flowing water (e.g., stream and river beds).

When water is the depositing and shaping agent, dunes are a bedform that are created by saltation and deposition of particles unable to be carried in suspension. Similar in shape to ripples—but much larger in size—dunes erode on the upstream side and extend via deposition the downstream or downslope side.

Regardless of whether deposited by wind or water, dunes themselves move or migrate much more slowly than any individual deposition particle.

In desert regions, dune shape is dependent upon a number of factors including the type of sand , the moisture content of the sand, and the direction and strength of the prevailing wind pattern. Barchan dunes are crescent-shaped small dunes with the terminal points of the crescent pointed downwind (on the lee side of the prevailing wind). Transverse dunes are long narrow dunes (a dune line) formed at right angles to the prevailing wind pattern. Transverse dunes may form from the fusion of individual barchan dunes.

Blount or parabolic dunes may form in regions of higher moisture content where there is sufficient vegetation to retard the migration of sand. Blount dunes take the mirror image shape of barchan dunes—they are crescent-shaped, but the terminal points of the crescent point windward (into the direction of the prevailing winds). Lineal dunes form parallel to prevailing wind patterns. Lineal dunes may be become the dominant relief feature and dunes may measure several hundred yards or meters high and extend for more than 50 miles (80 km).

Desert dunes migrate downwind from prevailing winds. Relic dunes form as migration slows and vegetation forms on a dune.

Ergs are "dune seas" ("erg" derives from Arabic) or large complexes of dunes. Very large (generally over 100 meters high and at least a kilometer long) complexes of dunes form a drass. Globally, dune fields and seas are common between 20° to 40° N, and 20° to 40° S latitudes.

In contrast to well-sorted dunes, a loess is another form of sedimentary, wind-driven deposit usually associated with glacier movements. Loess formations, however, represent layers of settling dust and are not well-sorted.

The formation and movements of dune fields are also of great interest to extraterrestrial or planetary geologists. Analysis of satellite images of Mars, for example, allows calculation of the strength and direction of the Martian winds and provides insight into Martian atmospheric dynamics. Dunes fields are a significant Martian landform and many have high rates of migration.

See also Beach and shoreline dynamics; Bed or traction load; Bedforms (ripples and dunes); Desert and desertification; Eolian processes; Glacial landforms; Landscape evolution

erosion , general term for the processes by which the surface of the earth is constantly being worn away. The principal agents are gravity, running water, near-shore waves, ice (mostly glaciers), and wind. All running water gathers and transports particles of soil or fragments of rock (formed by weathering ), and every stream carries, in suspension or rolling along its bottom, material received from its tributaries or detached from its own banks. These transported particles strike against the bedrock of the stream channel, literally grinding it away and eventually settle out along the channel or find their way to the sea. The Mississippi River is being reduced by erosion at the rate of 1 ft (30 cm) in about 9,000 years. Seacoasts are eroded by ocean waves, which detach loose or nonresistant material. Waves wear the rock by both the force of their own impact and the abrasive action of the detritus they carry. Ice can erode rocks by a freezing-thawing cycle; and ice in the form of glaciers erodes by plucking off loose rocks, by its abrasive action on the surface over which it passes, and by glacial meltwater rivers and streams. In deserts and along beaches, wind transports sand, eroding one area and depositing in another. The wind can also drive sand and other particles against rocks, abrading them. Before human modification, landmasses were probably eroding at rates close to 1 inch (2 to 3 centimeters) per 1,000 years; now rates have doubled. In the United States 30% is natural erosion, while 70% is because of human intervention. Suspended sediment from erosion is one of the world's greatest pollutants. Sediment can fill reservoirs and navigable waterways, impair wildlife habitats, increase flooding and water treatment costs, and deplete valuable topsoil. It can also concentrate harmful chemicals and bacteria. The continuous washing away of the fine rich topsoil of farmland due to poor agricultural practices is a problem in many parts of the world. Accelerated erosion from removal of acres of trees and vegetation, which diminishes the natural erosion protection, is becoming increasingly common in populated areas. Strip mining also removes vegetation and can be a localized cause of erosion. Among the methods of preventing soil erosion are reforestation, maintenance of fallow strips, terracing, underdraining, ditching, deep plowing, and plowing across slopes rather than up and down. See conservation of natural resources .

Erosion

Erosion is the reduction or breakdown of landforms exposed to the forces of weathering (disintegration and decomposition). Weathering and subsequent erosion may be caused by both chemical or mechanical forces. Mechanical weathering agents include wind , water , and ice . Chemical weathering leading to erosion results from bio-organic breakdown, hydration, hydrolysis, and oxidation processes. The process of transportation describes the movements of eroded materials.

Erosion requires a transport mechanism (e.g., gravity , wind, water, or ice). Wind, water, and ice are also agents of erosion that cause the physical breakdown of rock and landforms.

A special form of erosion, mass wasting , describes the transport of material downslope under the influence of gravity. Landslides are a common example of mass wasting.

Erosion processes can also cause indirect landform alteration by breaking down overburden of rock and precipitating a pressure release that can crack and shift rock layers. The cracking process results in peels, exfoliation , or spalling. For example, the erosion of overburden can expose batholiths and these exposed formations can form exfoliation domes.

Organic materials can frequently contribute to erosion by pressure that results in structural cracking or in the formation of acidic compounds that weather rock.

Rapid temperature changes or large diurnal temperature changes (the difference between the highest daytime temperature and the coolest nighttime temperature) can accelerate erosional exfoliation, jointing, and ice wedging.

See also Acid rain; Catastrophic mass movements; Depositional environments; Dunes; Eolian processes; Faults and fractures; Freezing and melting; Glacial landforms; Glaciation; Hydrothermal processes; Ice heaving and ice wedging; Impact crater; Landforms; Landscape evolution; Leaching; Oxidation-reduction reaction; Precipitation; Rapids and waterfalls; Rate factors in geologic processes; Rock; Rockfall; Salt wedging; Seawalls and beach erosion; Soil and soil horizons; Talus pile or talus slope.

wind erosion Removal of soil or sediment by wind can occur either through the action of direct fluid lift and drag forces, or by the impaction of wind-entrained sediment (see saltation) on the sediment surface. Wind is also capable of eroding consolidated sediment through the impact of entrained sediment on a surface, to produce ventifacts and yardangs. Eroded sediment can be transported by creep, saltation, or suspension, and where much fine soil or sediment is present, dust clouds can result (see dust). Sandy soils are particularly prone to wind erosion, but an increasing percentage of clay in a soil, a greater moisture content, or an enhanced vegetation cover will inhibit the erosivity. Hazards resulting from wind erosion can induce damage to crops through abrasion and removal of seedlings and damage to soils through the removal of valuable soil constituents. It can also result in a reduction of visibility and other hazards as a result of dust storms. Most wind erosion sites are situated in arid and semi-arid regions of the world. However, wind erosion and dust emission have also been reported in more temperate countries, such as New Zealand, Sweden, and the United Kingdom. An instance of particular note was the ‘dust bowl’ period in the United States during the 1930s.

T. Linsey

erosion In geology, alteration of landforms by the wearing away of rock and soil, and the removal of any debris (as opposed to weathering). Erosion is carried out by the actions of wind, water, glaciers, and living organisms. In chemical erosion, minerals in the rock react to other substances, such as weak acids found in rainwater, and are broken down. In physical erosion, powerful forces such as rivers and glaciers wear rock down and transport it. Erosion can have disastrous economic results, such as the removal of topsoil, the gradual destruction of buildings, and the alteration of water systems. Inland, erosion occurs most drastically by the action of rivers, and in coastal regions, by waves. See also geomorphology

erosion (i-roh-zhŏn) n. an eating away of surface tissue by physical or chemical processes, including those associated with inflammation. cervical e. an abnormal area of epithelium that may develop at the cervix of the uterus. See ectropion. dental e. loss of surface tooth substance, usually caused by repeated application of acid. It may result from excessive intake of fruit or carbonated drinks or acidic fruits or from regurgitation of acid from the stomach.

erosion
1. The part of the overall process of denudation which includes the physical breaking down, chemical solution, and transportation of material.

2. The movement of soil and rock material by agents such as running water, wind, moving ice, and gravitational creep (or mass movement). See bubnoff unit.

erosion
1. The part of the overall process of denudation that includes the physical breaking down, chemical solution and transportation of material.

2. Movement of soil and rock material by agents such as running water, wind, moving ice, and gravitational creep (or mass movement). See BUBNOFF UNIT.

erosion, wind see wind erosion

erosion •abrasion, Australasian, equation, Eurasian, evasion, invasion, occasion, persuasion, pervasion, suasion, Vespasian •adhesion, cohesion, Friesian, lesion •circumcision, collision, concision, decision, derision, division, elision, envision, excision, imprecision, incision, misprision, precisian, precision, provision, scission, vision •subdivision • television • Eurovision •LaserVision •corrosion, eclosion, erosion, explosion, implosion •allusion, collusion, conclusion, confusion, contusion, delusion, diffusion, effusion, exclusion, extrusion, fusion, illusion, inclusion, interfusion, intrusion, obtrusion, occlusion, preclusion, profusion, prolusion, protrusion, reclusion, seclusion, suffusion, transfusion •Monaghan • Belgian •Bajan, Cajun, contagion, Trajan •Glaswegian, legion, Norwegian, region •irreligion, religion •Injun • Harijan • oxygen • antigen •sojourn • donjon • Georgian •theologian, Trojan •Rügen •bludgeon, curmudgeon, dudgeon, gudgeon, trudgen •dungeon • glycogen • halogen •collagen • Imogen • carcinogen •hallucinogen • androgen •oestrogen (US estrogen) •hydrogen • nitrogen •burgeon, sturgeon, surgeon