Erosion

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

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

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

Mechanical weathering

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

Severe soil erosion caused by overgrazing and clearance of vegetation in Eastern Province, Kenya, Africa. Mark Boulton (1990). Photo Researchers, Inc. Reproduced by permission.

Chemical weathering

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

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

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

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

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

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