Erosion
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Erosion is the displacement of solids (soil, mud, rock, and so forth) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). Although the processes may be simultaneous, erosion is to be distinguished from weathering, which is the decomposition of rock. Erosion is an important natural process, but in many places it is increased by human activities. Some of those activities include deforestation, overgrazing and road or trail building. Likewise, humans have sought to limit erosion by 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. Too much erosion, however, can cause problems, clogging streams with gravel, filling reservoirs with sediment, reducing soil fertility and water quality.
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Causes
What causes erosion to be severe in some areas and minor elsewhere? It is a combination of many factors, including the amount and intensity of precipitation, the texture of the soil, the steepness of the slope, and ground cover (from vegetation, rocks, etc.).
The first three factors do not change much. 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 lot 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, 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" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion.
One of the most serious and long-running water erosion problems on the planet 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.
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.
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.
Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay, that, once released, may move quite rapidly downhill. They often will 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 soils. Sheet erosion is the result of heavy rain on bare soil where water flows as a sheet down any gradient carrying soil particles. 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 dominately 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 flooplain. 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.
Ice erosion
Ice erosion is caused by movement of ice, typically as glaciers. Glaciers can scrape and break up rock and then transport it, leaving moraines, drumlins, and glacial erratics in its wake typically at the terminus or during glacial retreat. Ice wedging is the weathering process where water trapped in tiny rock cracks freezes and expands, causing the breakup of the rock. This can lead to gravity erosion on steep slopes. The scree which form 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 and 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/or 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.