Erosion is the process of weathering Weathering is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity and transport of solids (sediment Sediment is naturally-occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself, soil Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics, rock In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids and other particles) in the natural environment or their source and deposits them elsewhere. It usually occurs due to transport Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. An understanding of sediment transport is typically used in natural systems, where the particles are clastic rocks (sand, gravel, boulders, etc.), by wind, water, or ice; by down-slope creep Downhill creep, or commonly just creep, is the slow downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of prolonged pressure and stress. Creep may appear to an observer to be continuous, but it really is the sum of numerous minute, discrete movements of slope material of soil and other material under the force of gravity Gravitation, or gravity, is one of the four fundamental interactions of nature , in which objects with mass attract one another. In everyday life, gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, thus accounting for; or by living organisms, such as burrowing animals, in the case of bioerosion Bioerosion describes the erosion of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling,.

Erosion is a natural process, but it has been increased dramatically by human land use Land use is the human modification of natural environment or wilderness into built environment such as fields, pastures, and settlements. The major effect of land use on land cover since 1750 has been deforestation of temperate regions. More recent significant effects of land use include urban sprawl, soil erosion, soil degradation, salinization,, especially industrial agriculture Industrial agriculture is a form of modern farming that refers to the industrialized production of livestock, poultry, fish, and crops. The methods of industrial agriculture are technoscientific, economic, and political. They include innovation in agricultural machinery and farming methods, genetic technology, techniques for achieving economies of, deforestation Deforestation occurs for many reasons: trees or derived charcoal are used as, or sold, for fuel or as a commodity, while cleared land is used as pasture for livestock, plantations of commodities, and settlements. The removal of trees without sufficient reforestation has resulted in damage to habitat, biodiversity loss and aridity. It has adverse, and urban sprawl Urban sprawl, also known as suburban sprawl, is a multifaceted concept, which includes the spreading outwards of a city and its suburbs to its outskirts to low-density, auto-dependent development on rural land, with associated design features that encourage car dependency. As a result, some critics argue that sprawl has certain disadvantages,.^[1][2] Land that is used for industrial agriculture generally experiences a significant greater rate of erosion than that of land under natural vegetation, or land used for sustainable agricultural Sustainable agriculture uses ecological principles to farm, hence the prefex agro- to farm and ecology- the science of the relationship between organisms and their environment. It has been defined as "an integrated system of plant and animal production practices having a site-specific application that will, over the long term: practices. This is particularly true if tillage is used, which reduces vegetation cover on the surface of the soil and disturbs both soil structure and plant roots that would otherwise hold the soil in place. However, improved land use practices can limit erosion, using techniques such as terrace In agriculture, a terrace is a leveled section of a hill cultivated area, designed as a method of soil conservation to slow or prevent the rapid surface runoff of irrigation water. Often such land is formed into multiple terraces, giving a stepped appearance. The human landscapes of rice cultivation in terraces that follow the natural contours of-building, conservation tillage practices, and tree planting.

A certain amount of erosion is natural and, in fact, healthy for the ecosystem An ecosystem consists of all the organisms living in a particular area, as well as all the nonliving, physical components of the environment with which the organisms interact, such as air, soil, water, and sunlight. It is all the organisms in a given area, along with the nonliving factors with which they interact; a biological community and its. For example, gravels Gravel is rock that is of a specific particle size range. Specifically, it is any loose rock that is larger than 2 mm in its smallest dimension (about 1/12 of an inch) and no more than 64 mm (2.5 in). The next smaller size class in geology is sand, which is >0.0625 to 2 mm (0.0025 to 0.0787 in) in size. The next larger size is cobble, which is & continuously move downstream in watercourses. Excessive erosion, however, causes serious problems, such as receiving water sedimentation Sediment is naturally-occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself, ecosystem damage and outright loss of soil.

Erosion is distinguished from weathering Weathering is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity, which is the process of chemical or physical breakdown of the minerals in the rocks, although the two processes may occur concurrently.

Causes

The rate of erosion depends on many factors. Climatic factors include the amount and intensity of precipitation Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the condensation of atmospheric water vapor into drops of water heavy, the average temperature, as well as the typical temperature range, and seasonality, the wind speed, storm frequency. The geologic factors include the sediment or rock type, its porosity and permeability, the slope (gradient) of the land, and whether the rocks are tilted, faulted, folded, or weathered. The biological factors include ground cover from vegetation Vegetation is the plant life or the plant ground cover of a region, not the particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader than the term flora which refers exclusively to species composition. Perhaps the closest synonym is plant community, but vegetation can, and or lack thereof, the type of organisms inhabiting the area, and the land use.

In general, given similar vegetation and ecosystems, areas with high-intensity precipitation, more frequent rainfall, more wind, or more storms are expected to have more erosion. Sediment with high sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles or silt Silt is granular material of a grain size between sand and clay derived from soil or rock. Silt may occur as a soil or as suspended sediment in a surface water body. It may also exist as soil deposited at the bottom of a water body contents and areas with steep slopes erode more easily, as do areas with highly fractured or weathered rock. Porosity and permeability of the sediment or rock affect the speed with which the water can percolate into the ground. If the water moves underground, less runoff is generated, reducing the amount of surface erosion. Sediments containing more clay Clay is a naturally occurring material composed primarily of fine-grained minerals. Clay deposits are mostly composed of clay minerals, a subtype of phyllosilicate minerals, which impart plasticity and harden when fired or dried; they also may contain variable amounts of water trapped in the mineral structure by polar attraction. Organic materials tend to erode less than those with sand or silt. Here, however, the impact of atmospheric sodium on erodibility of clay should be considered.^[3]

The factor that is most subject to change is the amount and type of ground cover. In an undisturbed forest, the mineral soil is protected by a litter layer and an organic layer. These two layers protect the soil by absorbing the impact of rain drops. These layers and the underlying soil in a forest are porous and highly permeable to rainfall. Typically, only the most severe rainfall and large hailstorm events will lead to overland flow in a forest. If the trees are removed by fire or logging, infiltration rates become high and erosion low to the degree the forest floor remains intact. Severe fires can lead to significantly increased erosion if followed by heavy rainfall. In the case of construction or road building, when the litter layer is removed or compacted, the susceptibility of the soil to erosion is greatly increased.

Roads are especially likely to cause increased rates of erosion because, in addition to removing ground cover, they can significantly change drainage patterns, especially if an embankment has been made to support the road. A road that has a lot of rock and one that is "hydrologically invisible" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion.

Many human activities remove vegetation from an area, making the soil easily eroded. Logging Logging is the process in which certain trees are cut down by a lumberjack or machine, such as the feller buncher, for forest management and timber can cause increased erosion rates due to soil compaction Soil compaction occurs when the weight of livestock or heavy machinery compresses the soil, causing it to lose pore space. Soil compaction may also occur due to a lack of water in the soil. Affected soils become less able to absorb rainfall, thus increasing runoff and erosion. Plants have difficulty in compacted soil because the mineral grains are, exposure of mineral soil, for example roads and landings. However it is the removal of or compromise to the forest floor not the removal of the canopy that can lead to erosion. This is because rain drops striking tree leaves coalesce with other rain drops creating larger drops. When these larger drops fall (called throughfall) they again may reach terminal velocity In fluid dynamics an object is moving at its terminal velocity if its speed is constant due to the restraining force exerted by the air, water or other fluid through which it is moving and strike the ground with more energy then had they fallen in the open. Terminal velocity of rain drops is reached in about 8 meters. Because forest canopies are usually higher than this, leaf drop can regain terminal velocity. However, the intact forest floor, with its layers of leaf litter and organic matter, absorbs the impact of the rainfall.^[4]

Heavy grazing Grazing generally describes a type of predation in which a herbivore feeds on plants , and also on other multicellular autotrophs (such as algae). Grazing differs from true predation because the organism being eaten is not killed, and it differs from parasitism as the two organisms do not live together, nor is the grazer necessarily so limited in can reduce vegetation enough to increase erosion. Changes in the kind of vegetation in an area can also affect erosion rates. Different kinds of vegetation lead to different infiltration rates of rain into the soil. Forested areas have higher infiltration rates, so precipitation will result in less surface runoff, which erodes. Instead much of the water will go in subsurface flows, which are generally less erosive. Leaf litter and low shrubs are an important part of the high infiltration rates of forested systems, the removal of which can increase erosion rates. Leaf litter also shelters the soil from the impact of falling raindrops, which is a significant agent of erosion. Vegetation can also change the speed of surface runoff flows, so grasses and shrubs can also be instrumental in this aspect.

One of the main causes of erosive soil loss in the year 2006 is the result of slash and burn Slash and burn consists of cutting and burning of forests or woodlands to create fields for agriculture or pasture for livestock, or for a variety of other purposes. It is sometimes part of shifting cultivation agriculture, and of transhumance livestock herding treatment of tropical The tropics is a region of the Earth by the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N and the Tropic of Capricorn in the southern hemisphere at 23°26' (23.5°) S. The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone) forest A forest is an area with a high density of trees. There are many definitions of a forest, based on the various criteria. These plant communities cover approximately 9.4% of the Earth's surface (or 30% of total land area), though they once covered much more (about 50% of total land area), in many different regions and function as habitats for. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive. For example, on the Madagascar Madagascar, or Republic of Madagascar , is an island nation in the Indian Ocean off the southeastern coast of Africa. The main island, also called Madagascar, is the fourth-largest island in the world high central plateau In geology and earth science, a plateau, also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau. A volcanic plateau is a plateau produced by volcanic activity, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation Vegetation is the plant life or the plant ground cover of a region, not the particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader than the term flora which refers exclusively to species composition. Perhaps the closest synonym is plant community, but vegetation can, and, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation Shifting cultivation is an agricultural system in which plots of land are cultivated temporarily, then abandoned. This system often involves clearing of a piece of land followed by several years of wood harvesting or farming, until the soil loses fertility. Once the land becomes inadequate for crop production, it is left to be reclaimed by natural is a farming system which sometimes incorporates the slash and burn Slash and burn consists of cutting and burning of forests or woodlands to create fields for agriculture or pasture for livestock, or for a variety of other purposes. It is sometimes part of shifting cultivation agriculture, and of transhumance livestock herding method in some regions of the world. This degrades the soil and causes the soil to become less and less fertile.

Effects

Approximately 40% of the world's agricultural land is seriously degraded.^[5] According to the UN The United Nations Organization or simply United Nations (UN) is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and the achieving of world peace. The UN was founded in 1945 after World War II to replace the League of, an area of fertile soil the size of Ukraine is lost every year because of drought A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short,, deforestation Deforestation occurs for many reasons: trees or derived charcoal are used as, or sold, for fuel or as a commodity, while cleared land is used as pasture for livestock, plantations of commodities, and settlements. The removal of trees without sufficient reforestation has resulted in damage to habitat, biodiversity loss and aridity. It has adverse and climate change Climate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average . Climate change may be limited to a specific region, or may occur across the whole Earth.^[6] In Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population, if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025, according to UNU The United Nations University (UNU) is a United Nations agency established in Tokyo in 1973 to "research into the pressing global problems of human survival, development and welfare that are the concern of the United Nations and its agencies". It is a think tank for the United Nations and the member states. Its creation was set in motion's Ghana-based Institute for Natural Resources in Africa.^[7]

When land is overused by animal activities (including humans), there can be mechanical erosion and also removal of vegetation leading to erosion. In the case of the animal kingdom, this effect would become material primarily with very large animal herds A herd is a large group of animals and is a form of collective animal behavior. The term is usually applied to mammals, particularly ungulates. Other terms are used for similar phenomena in other types of animal. For example, a large group of birds is usually called a flock and a large group of carnivores is usually called a pack. In addition, stampeding such as the Blue Wildebeest The Blue Wildebeest , also called the Common Wildebeest and the Brindled Gnu, is a large antelope and one of two species of wildebeest. It grows to 115-145 cm shoulder height and attains a body mass of 168-274 kg. They range the open plains, bushveld and dry woodlands of Southern and East Africa, realizing a life span in excess of twenty years on the Serengeti The Serengeti ecosystem is a geographical region located in north-western Tanzania and extends to south-western Kenya between latitudes 1 and 3 S and longitudes 34 and 36 E. It spans some 30,000 km2 plain. Even in this case there are broader material benefits to the ecosystem, such as continuing the survival of grasslands, that are indigenous to this region. This effect may be viewed as anomalous or a problem only when there is a significant imbalance or overpopulation Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. In common parlance, the term often refers to the relationship between the human population and its environment, the Earth of one species.

In the case of human use, the effects are also generally linked to overpopulation Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. In common parlance, the term often refers to the relationship between the human population and its environment, the Earth. When large number of hikers use trails or extensive off road vehicle use occurs, erosive effects often follow, arising from vegetation removal and furrowing of foot traffic and off road vehicle tires. These effects can also accumulate from a variety of outdoor human activities, again simply arising from too many people using a finite land resource.

One of the most serious and long-running water erosion problems worldwide is in the People's Republic of China b. ^ Simple characterizations of the political structure since the 1980s are no longer possible, on the middle reaches of the Yellow River The Yellow River is called "the cradle of Chinese civilization", as its basin is the birthplace of the northern Chinese civilizations and was the most prosperous region in early Chinese history. But frequent devastating flooding largely due to the elevated river bed in its lower course, has also earned it the unenviable names "China' and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flows into the ocean each year. The sediment originates primarily from water erosion in the Loess Plateau region of the northwest.

Processes

Gravity

Mass wasting is the down-slope movement of rock and sediments, mainly due to the force of gravity. Mass movement is an important part of the erosional process, as it moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope.

Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression, in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence.

Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm in diameter by wind along the soil surface.

Water

Splash erosion is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soil.

Sheet erosion is the detachment of soil particles by raindrop impact and their removal downslope by water flowing overland as a sheet instead of in definite channels or rills. The impact of the raindrop breaks apart the soil aggregate. Particles of clay, silt and sand fill the soil pores and reduce infiltration. After the surface pores are filled with sand, silt or clay, overland surface flow of water begins due to the lowering of infiltration rates. Once the rate of falling rain is faster than infiltration, runoff takes place. There are two stages of sheet erosion. The first is rain splash, in which soil particles are knocked into the air by raindrop impact. In the second stage, the loose particles are moved downslope by broad sheets of rapidly flowing water filled with sediment known as sheetfloods. This stage of sheet erosion is generally produced by cloudbursts, sheetfloods commonly travel short distances and last only for a short time.

Rill erosion refers to the development of small, ephemeral concentrated flow paths, which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically on the order of a few centimeters or less and slopes may be quite steep. These conditions constitute a very different hydraulic environment than typically found in channels of streams and rivers. Eroding rills evolve morphologically in time and space. The rill bed surface changes as soil erodes, which in turn alters the hydraulics of the flow. The hydraulics is the driving mechanism for the erosion process, and therefore dynamically changing hydraulic patterns cause continually changing erosional patterns in the rill. Thus, the process of rill evolution involves a feedback loop between flow detachment, hydraulics, and bed form. Flow velocity, depth, width, hydraulic roughness, local bed slope, friction slope, and detachment rate are time and space variable functions of the rill evolutionary process. Superimposed on these interactive processes, the sediment load, or amount of sediment in the flow, has a large influence on soil detachment rates in rills. As sediment load increases, the ability of the flowing water to detach more sediment decreases.

Where precipitation rates exceed soil infiltration rates, runoff occurs. Surface runoff turbulence can often cause more erosion than the initial raindrop impact.

Gully erosion, also called ephemeral gully erosion, occurs when water flows in narrow channels during or immediately after heavy rains or melting snow. This is particularly noticeable in the formation of hollow ways, where, prior to being tarmacked, an old rural road has over many years become significantly lower than the surrounding fields.

A gully is sufficiently deep that it would not be routinely destroyed by tillage operations, whereas rill erosion is smoothed by ordinary farm tillage. The narrow channels, or gullies, may be of considerable depth, ranging from 1 to 2 feet to as much as 75 to 100 feet. Gully erosion is not accounted for in the revised universal soil loss equation.

Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles and boulders can also act erosively as they traverse a surface.

At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called Rock-cut basins. Examples can be seen in the flood regions result from glacial Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington.^[8]

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as scour. Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.^[9]

Shoreline

Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.

Hydraulic action takes place when air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion is caused by waves launching seaload at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition is where particles/seaload carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is the boring, scraping and grinding of organisms, a process termed bioerosion.

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent amount of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a build up of eroded material occurs forming a long narrow bank (a spit). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.

Ice

Ice erosion can take one of two forms. It can be caused by the movement of ice, typically as glaciers, in a process called glacial erosion. It can also be due to freeze-thaw processes in which water inside pores and fractures in rock may expand cause further cracking.

Glaciers erode predominantly by three different processes: abrasion/scouring, plucking, and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. These processes, combined with erosion and transport by the water network beneath the glacier, leave moraines, drumlins, ground moraine (till), kames, kame deltas, moulins, and glacial erratics in their wake, typically at the terminus or during glacier retreat.

Cold weather causes water trapped in tiny rock cracks to freeze and expand, breaking the rock into several pieces. This can lead to gravity erosion on steep slopes. The scree which forms at the bottom of a steep mountainside is mostly formed from pieces of rock (soil) broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads, because morning thaws can drop hazardous rock pieces onto the road.

In some places, water seeps into rocks during the daytime, then freezes at night. Ice expands, thus, creating a wedge in the rock. Over time, the repetition in the forming and melting of the ice causes fissures, which eventually breaks the rock down.

Wind

In arid climates, the main source of erosion is wind.^[10] The general wind circulation moves small particulates such as dust across wide oceans thousands of kilometers downwind of their point of origin,^[11] which is known as deflation. Erosion can be the result of material movement by the wind. There are two main effects. First, wind causes small particles to be lifted and therefore moved to another region. This is called deflation. Second, these suspended particles may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation. An example is the formation of sand dunes, on a beach or in a desert.^[12] Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (aeolian) sediment.^[13] It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.^[14] Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.^[15] Loess deposits are geologically unstable by nature, and will erode very readily. Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.^[10]

Thermal

Thermal erosion is the result of melting and weakening permafrost due to moving water.^[16] It can occur both along rivers and at the coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials.^[17] Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a 100-kilometer segment of the Beaufort Sea shoreline averaged 5.6 meters per year from 1955 to 2002.^[18]

Soil erosion and climate change

The warmer atmospheric temperatures observed over the past decades are expected to lead to a more vigorous hydrological cycle, including more extreme rainfall events.^[19] In 1998 Karl and Knight reported that from 1910 to 1996 total precipitation over the contiguous U.S. increased, and that 53% of the increase came from the upper 10% of precipitation events (the most intense precipitation).^[20] The percent of precipitation coming from days of precipitation in excess of 50 mm has also increased significantly.

Studies on soil erosion suggest that increased rainfall amounts and intensities will lead to greater rates of erosion. Thus, if rainfall amounts and intensities increase in many parts of the world as expected, erosion will also increase, unless amelioration measures are taken. Soil erosion rates are expected to change in response to changes in climate for a variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include: a) changes in plant canopy caused by shifts in plant biomass production associated with moisture regime; b) changes in litter cover on the ground caused by changes in both plant residue decomposition rates driven by temperature and moisture dependent soil microbial activity as well as plant biomass production rates; c) changes in soil moisture due to shifting precipitation regimes and evapo-transpiration rates, which changes infiltration and runoff ratios; d) soil erodibility changes due to decrease in soil organic matter concentrations in soils that lead to a soil structure that is more susceptible to erosion and increased runoff due to increased soil surface sealing and crusting; e) a shift of winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures; f) melting of permafrost, which induces an erodible soil state from a previously non-erodible one; and g) shifts in land use made necessary to accommodate new climatic regimes.

Studies by Pruski and Nearing indicated that, other factors such as land use not considered, we can expect approximately a 1.7% change in soil erosion for each 1% change in total precipitation under climate change.^[21]

Tectonic effects

The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region. Research undertaken since the early 1990s suggests that the spatial distribution of erosion at the surface of an orogen can exert a key influence on its growth and its final internal structure (see erosion and tectonics).^[22]

Materials science

In materials science, erosion is the recession of surfaces by repeated localized mechanical trauma as, for example, by suspended abrasive particles within a moving fluid. Erosion can also occur from non-abrasive fluid mixtures. Cavitation is one example.

In hard particle erosion, the hardness of the impacted material is a large factor in the mechanics of the erosion. A soft material will typically erode fastest from glancing impacts.^[23] Harder material will typically erode fastest from perpendicular impacts. Hardness is a correlative factor for erosion resistance, but a higher hardness does not guarantee better resistance. Factors that affect the erosion rate also include impacting particle speed, size, density, hardness, and rotation. Coatings can be applied to retard erosion, but normally can only slow the removal of material. Erosion rate for solid particle impact is typically measured as mass of material removed divided by the mass of impacting material.^[24]

Figurative use

The concept of erosion is commonly employed by analogy to various forms of perceived or real homogenization (i.e. erosion of boundaries), "leveling out", collusion or even the decline of anything from morals to indigenous cultures. It is a common trope of the English language to describe as erosion the gradual, organic mutation of something thought of as distinct, more complex, harder to pronounce or more refined into something indistinct, less complex, easier to pronounce or (disparagingly) less refined.

Origin of term

The first known occurrence of the term "erosion" was in the 1541 translation by Robert Copland of Guy de Chauliac's medical text The Questyonary of Cyrurygens. Copland used erosion to describe how ulcers developed in the mouth. By 1774 'erosion' was used outside medical subjects. Oliver Goldsmith employed the term in the more contemporary geological context, in his book Natural History, with the quote

"Bounds are thus put to the erosion of the earth by water."

See also

• Badland
• Bioerosion
• Biorhexistasy
• Bridge scour
• Cellular confinement
• Certified Professional in Erosion and Sediment Control CPESC
• Deforestation
• Desertification
• Erosion control
• Erosion prediction
• Food security
• Geography
• Groundwater sapping
• Lessivage
• Riparian strips
• Sediment transport
• Soft engineering
• Sphericity scale
• Surface runoff
• TERON (Tillage erosion)
• Vegetation and slope stability
• Vetiver System
• Ocean surface wave

Notes

1. ^ Montgomery, David (October 2, 2008). Dirt: The Erosion of Civilizations (1st ed.). University of California Press. ISBN 0520258061.
2. ^ Kötke, William (November 30, 2007). The Final Empire. AuthorHouse. ISBN 143433130X.
3. ^ Schmittner Karl-Erich and Pierre, 1999. The impact of atmospheric sodium on erodibility of clay in a coastal Mediterranean region. Environmental Geology 37/3: 195-206.
4. ^ http://treesearch.fs.fed.us/pubs/14744 Concepts about forests and water Author: Stuart, Gordon W.; Edwards, Pamela J.
5. ^ Global food crisis looms as climate change and population growth strip fertile land
6. ^ 2008: The year of global food crisis
7. ^ Africa may be able to feed only 25% of its population by 2025
8. ^ Alt, David (2001). Glacial Lake Missoula & its Humongous Floods. Mountain Press Publishing Company. ISBN 0-87842-415-6.
9. ^ Nancy D. Gordon (2004-06-01). "Erosion and Scour". Stream hydrology: an introduction for ecologists. ISBN 9780470843574. http://books.google.com/?id=_PJHw-hSKGgC&pg=PA113
10. ^ ^{a b} Vern Hofman and Dave Franzen (1997). "Emergency Tillage to Control Wind Erosion". North Dakota State University Extension Service. http://www.ag.ndsu.edu/disaster/drought/emergencytillagetocontrolerosion.html. Retrieved 2009-03-21.
11. ^ James K. B. Bishop, Russ E. Davis, and Jeffrey T. Sherman (2002). "Robotic Observations of Dust Storm Enhancement of Carbon Biomass in the North Pacific". Science 298. pp. 817–821. http://www-ocean.lbl.gov/people/bishop/bishoppubs/paparobots.html. Retrieved 2009-06-20.
12. ^ United States Geological Survey (2004). "Dunes – Getting Started". http://geomaps.wr.usgs.gov/parks/coast/dunes/index.html. Retrieved 2009-03-21.
13. ^ F. von Richthofen (1882). On the mode of origin of the loess. pp. 293–305.
14. ^ Glossary of Geology. Springer-Verlag, New York. 2005. p. 779. ISBN 3-540-27951-2.
15. ^ Introduction to Geography, Seventh Edition. McGraw Hill. 2000. p. 99. ISBN 0-697-38506-X.
16. ^ "Thermal Erosion". NSIDC Glossary. National Snow and Ice Data Center. http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion. Retrieved 21 December 2009.
17. ^ Costard, F.; Dupeyrat, L.; Gautier, E.; Carey-Gailhardis, E. (2003). "Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)". Earth Surface Processes and Landforms 28: 1349. doi:10.1002/esp.592.
18. ^ Jones, B.M.; Hinkel, K.M., Arp, C.D. and Eisner, W.R. (2008). "Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska" (). Arctic (The Arctic Institute of North America) 61 (4): 361–372. http://digitization.ucalgary.ca/arctic/index.php/arctic/article/view/44/115.
19. ^ IPCC. 1995. Second Assessment Synthesis of Scientific-Technical Information relevant to interpreting Article 2 of the UN Framework Convention on Climate Change. Intergovernmental Panel on Climate Change. Geneva, Switzerland. 64 pp.
20. ^ Karl, T.R. and R. W. Knight. 1998. Secular trend of precipitation amount, frequency, and intensity in the United States. Bulletin of the American Meteorological Society 79:231-242.
21. ^ Pruski, F. F. and M.A. Nearing. 2002. Runoff and soil loss responses to changes in precipitation: a computer simulation study. Journal of Soil and Water Conservation 57(1), 7-16.
22. ^ Willett, Sean D., et al., Tectonics, Climate and Landscape Evolution, Geological Society of America Special Paper 398, 2006 ISBN 0-8137-2398-1 PDF of Introduction
23. ^ Finnie, I., Erosion of surfaces by solid particles. Wear, 1960. 3(2): p. 87.
24. ^ ASTM G76-07

• World Bank 2001: China: Air, Land, and Water.
• Pan European Soil Erosion Assessment

Further reading

• Boardman, John; Poesen, Jean (2006). Soil erosion in Europe. Chichester: Wiley. ISBN 9780470859100.
• Montgomery, David R. (2007) Soil erosion and agricultural sustainability PNAS 104: 13268-13272.
• Brown, Jason; Drake, Simon (2009). Classic Erosion. Venture West: Wiley.

External links

• The Soil Erosion Site
• International Erosion Control Association
• USDA National Soil Erosion Laboratory
• The Soil and Water Conservation Society
• International Soil Conservation Organization
• Bioerosion website at The College of Wooster
• Pulawy Erosion Research Center
• Southwest Watershed Research Center

Categories: Geomorphology | Geological processes | Soil science | Agronomy | Industrial agriculture | Environmental soil science | Environmental issues

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