Erosion is a geological process that shapes the landscape of the Earth, both on the surface and under water. It involves the breakdown of rock through either the individual or combined effects of wind, precipitation, glacial movement and gravitational forces.
Types of Erosion
There are four main types forces that cause the erosion of solid rock and formation of sediments:
- Water - as running water or wave action
- Wind - occurs in two forms: deflation and abrasion
- Glaciers - by scraping rock and transporting all sizes of sediment thousands of miles
- Gravity - as a constant force on unstable hills and cliffs
Water
Powerful currents of moving water can be very destructive forces of nature. There are two ways in which water can act as a transport of sediment: by running water, such as rivers and streams, or by waves. Included with erosion by running water is erosion due to heavy rainfall. Heavy rainfall can cause sediment to be dislodged due to the impact of continuous high-velocity raindrops or it can cause flooding, which causes erosion in the same manner as rivers and streams, also known as sheet erosion. Vegetation is very important for slowing down erosion due to heavy rainfall and flooding. The leaves and branches of trees slow down falling raindrops before they impact the surface. Moreover, a well-developed network of tree, bush and grass roots holds soil in place.
Rivers and streams erode and transport sediment by picking up particles along the channel or at the bottom of the channel. The ability for streams to transport sediment depends on the turbulence and velocity of the water. The higher the velocity of the water, the more turbulence, and therefore the more effective the water is able to carry particles downstream. The velocity of the water is generally greater as the depth increases. Therefore, larger particles are transported by water at depth whereas water at the surface transports fine particles. This is evident in stream and river bed deposits, where coarser-grained material is found at the bottom of the stream and finer-grained material is found along the edges.
Wind
Like water, erosion due to wind processes is dependent on the rate of speed the wind is blowing or for how long a rock formation has been subject to wind. Also like water, wind erodes in two ways: by a process called deflation and by another process called abrasion. Deflation comes from the Latin word meaning “to blow away”. As the name implies, deflation occurs when the flow of air is strong enough to pick up particles and blow them elsewhere. Areas of significant deflation are deserts and beaches, where little to no vegetation is present to protect sediments from the forces of wind. Abrasion occurs when wind-driven sediments strike and chip away at rock. The rock being abraded begins to take on distinctive forms and can even be slightly polished.
Glaciers
Glaciers are some of the most powerful transporters of sediment. They do not discriminate between the sizes of particles being transported and can transport any size particle thousands of kilometers. There are multiple ways in which glaciers erode rock surfaces and formations. In one way, glaciers act like huge bulldozers, picking up loose sediment – from silt and sand to huge boulders – and pushing it along to be deposited elsewhere. Another way is like a file, literally scratching and scraping away at rock formations, leaving marks called striations in its path. This occurs when the sediment at the base of the glacier is scraped against bedrock. In the case of huge boulders, the scraping can be much more dramatic and large chunks of rock can be ripped away.
Gravity
Landslides are a dramatic consequence of erosion due to gravitational forces. Heavy rainfall can contribute to the problem, but it is the weight of the rock and the angle of its slope that is the main reason for failures in stability. The constant pull of gravity makes all hill slopes and mountain faces susceptible to landslides, but areas where the material is unconsolidated are the most hazardous.
Even animal activity can cause erosion – called bioerosion – as soil and sand is displaced from this activity. The effects of erosion depend on the kind of erosion occurring, the rate at which it occurs and the period of time over which it has occurred.
Related Articles:
The Geology of the Grand Canyon
Prehistoric Ice Sheets of Africa
Source:
Skinner, Brian J., Porter, Stephen C, and Park, Jeffrey. Dynamic Earth - An Introduction to Physical Geology. NJ: John Wiley and Sons Inc., 2004.
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