Encyclopedia
Erosion is the displacement of solids by the agents of wind, water or ice, by downward or down-slope movement in response to
gravity or by living organisms . Erosion is distinguished from
weathering, which is the decomposition of rock and particles through processes where no movement is involved, although the two processes may be concurrent.
Erosion is an intrinsic natural process but in many places it is increased by human land use. Poor land use practices include
deforestation, overgrazing, unmanaged construction activity and road or trail building. However, improved land use practices can limit erosion, using techniques like terrace-building and tree planting.
A certain amount of erosion is natural and, in fact, healthy for the ecosystem. For example,
gravels continually move downstream in watercourses. Excessive erosion, however, can cause problems, such as receiving water
sedimentation, ecosystem damage and outright loss of soil.
Causes
What causes erosion to be severe in some areas and minor elsewhere is a combination of many factors, including the amount and intensity of
precipitation, the
texture of the soil, the gradient of the slope, ground cover and land use. The first factor, rain, is the agent for erosion, but the degree of erosion is governed by other factors.
The first three factors can remain fairly constant over time. In general, given the same kind of vegetative cover, you expect areas with high-intensity precipitation,
sandy or
silty soils and steep slopes to be the most erosive. Soils with a greater proportion of
clay that receive less intense precipitation and are on gentle slopes tend to erode less.
The factor that is most subject to change is the amount and type of ground cover. When fires burn an area or when vegetation is removed as part of timber operations or building a house or a road, 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. A road that has a lot of rock and one that is "hydrologically invisible" has the best chance of not causing increased erosion.
Understandably, many human activities remove vegetation from an area, making the soil easily eroded.
Logging and heavy grazing can reduce vegetation enough to increase erosion. Changes in the kind of vegetation in an area can also effect erosion rates. Different kinds of vegetation effect 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 treatment of
tropical forests. 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 high central
plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of
vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the
slash and burn method in some regions of the world.
When land is overused by animal activities , 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 stampeding such as the
Blue Wildebeest on the Serengeti 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 of one species.
In the case of human use, the effects are also generally linked to
overpopulation. For when large numbers 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
China, on the middle reaches of the
Yellow River and the upper reaches of the
Yangtze River. From the
Yellow River, over
1.6 billion tons of sediment flow each year into the ocean. The
sediment originates primarily from water erosion in the
Loess Plateau region of northwest China.
Erosion processes
Gravity erosion
Mass wasting is the down-slope movement of rock and sediments, mainly due to the force of
gravity. Mass wasting is an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and
glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are occurring continuously on all slopes; some mass-wasting 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 depression, within 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 erosion
Splash erosion is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soil. Sheet erosion is the result of heavy rain on bare soil where water flows as a sheet down any gradient, carrying soil particles. Where precipitation rates exceed soil infiltration rates, runoff occurs. Surface runoff turbulence can often cause more erosion than the initial raindrop impact.
Gully erosion results where water flows along a linear depression eroding a trench or gully.
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.
Shoreline erosion
Shoreline erosion, on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level 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 it. 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 lasuching 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 vulnereable to this kind of erosion. Finally, Attrition is where particles/seaload carried by the waves are worn down as they hit each other and the cliffs. This then makes it easier to wash away. Sediment is transported along the coast in the direction of the prevailing current . 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 .
Ice erosion
Ice erosion is caused by movement of ice, typically as glaciers.
Glaciers can scrape down a slope and break up rock and then transport it, leaving
moraines,
drumlins and
glacial erratics in their wake, typically at the terminus or during
glacier retreat.
Ice wedging is the weathering process in which water trapped in tiny rock cracks freezes and expands, breaking the rock. 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 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.
Wind erosion
Wind erosion, also known as
eolian erosion, is the movement of rock and sediment by the wind. Windbreaks are often planted by farmers to reduce wind erosion. This includes the planting of trees, shrubs, or other vegetation, usually perpendicular or nearly so to the principal wind direction. The wind causes dust particles to be lifted and therefore moved to another region. 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.
Tectonic effects of erosion
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.
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.
Figurative use
The concept of erosion is commonly employed by
analogy to various forms of perceived or real homogenization , "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 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
External links
Reference
- World Bank 2001: China: Air, Land, and Water.
