Erosion

Introduction

Erosion is the process of transportation of weathered rock and soil material by the processes of water, wind, or ice movement and/or by gravity or the actions of some organisms. For erosion to begin, rock or soil must first be subjected to the processes of weathering, including disintegration (breaking apart) and decomposition (chemical breakdown). Erosion ends with deposition, which is the process of eroded material (rock fragments, sediments, or dissolved chemical elements) coming to rest in a place that is apart from where erosion began. Erosion may be natural, human-induced, or the result of a combination of these two factors.

Historical Background and Scientific Foundations

Erosion has played a huge part in the history of planet Earth. Mountain building through geological time has raised vast mountain ranges, which in turn have been reduced by weathering and erosion to low plains. Tec-tonic forces within Earth, which have routinely affected

the crust over time, move rock to higher elevations. Yet, these materials are moved to lower elevations by erosion. In fact, if mountains were not continually built at a faster rate than erosion, the continental surface of Earth would have been reduced to a nearly featureless landscape long ago.

Eroded material can be picked up and moved by water, wind, and ice. Each of these transportation media (water, wind, and ice) requires a minimum velocity of movement to pick up (or entrain) eroded particles. For water and wind, these velocities are relatively high compared to ice, which can move materials of any size at almost any velocity. As the velocity of water and wind increases, progressively larger particles can be moved. As velocity decreases, particles are deposited or laid to rest.

During transportation, water and wind move particles by rolling, bouncing, and in suspension (floating). Particles interact during transportation and thus they are abraded (worn) or comminuted (broken into smaller pieces) during transportation. Transportation of particles moves them from the point of origin (for example, an eroded hillside) to the point of deposition (for example, a river delta or sand dune). The point of deposition may be a temporary location from which the particles are moved again at another time. An example of a temporary location is a gravel or sand bar in a river, which is a temporary location for river sediment. During a flood on the river, such bars may be eroded and the sediment moved downstream to another bar or ultimately downstream to the river's delta.

Gravity pulls on all natural and human-made materials. The effect of this is the natural tendency for materials that can slide or flow to move downslope at varying rates. The movement of water, wind, and ice noted earlier is in response to gravity. However, gravity acts directly upon rocks, sediment, and soil to move these materials downslope as well. If the affected rock, sediment, or soil moves as a single large mass, it is called a slump. If the rock, sediment, or soil moves as a layer where the upper part is moving faster than the lower parts, it is called a flow. An example of a common type of flow is soil creep, which is the slow, progressive flow of soil down the side of a hill or other sloping surface. Gravity effects like slump and flow work in combination with water, wind, and ice to move large quantities of materials all the time.

The actions of some organisms may affect erosion in some situations. The trampling effect of animals walking on sloping surfaces may weaken the rock, sediment, or soil in that area and thus contribute to erosion. Trampling may remove vegetation as well, which weakens natural materials or allows water, wind, or ice to more effectively transport such materials. Some organisms in the seas actively eat rock and sediment and thus disinte-grate (by chewing) and decompose (by digesting) rock fragments and sediment. Some agricultural, construction, timbering, and related practices affect erosion in some situations. Like the actions of organisms mentioned earlier, these practices can assist gravity and the flow of water and wind in the process of erosion of landscapes.

Impacts and Issues

WORDS TO KNOW

DECOMPOSITION: The breakdown of matter by bacteria and fungi. It changes the chemical makeup and physical appearance of materials.

DISINTEGRATION: Spontaneous nuclear transformation characterized by the emission of energy and/or mass from the nucleus.

SEDIMENTS: Disintegration and decomposition products of natural materials

TECTONIC FORCES: Forces that shape Earth's crust over geological time. The largest tectonic force is supplied by the slow convection of Earth's interior, driven by radiation of heat to space through the crust, much like the roiling of liquid in a boiling pot

Erosion may be human-induced, but much of natural erosion may be induced by climate change and its attendant effects. For example, increased erosion due to increased rainfall in an area may be climatically controlled. Climate control necessarily implies that the change is long-term and part of a long-term trend, as opposed to short-term effects from weather systems that come and go over shorter time spans.

See Also Dust Storms; Floods; Glaciation.

BIBLIOGRAPHY

Books

Montgomery, David R. Dirt: The Erosion of Civilization. Berkeley: University of California Press, 2007.

Periodicals

U.S. Department of Agriculture. “Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE).” Agriculture Handbook 703 (1996).

Web Sites

Pidwirny, Michael. “Introduction to the Lithosphere: Erosion and Deposition.” PhysicalGeography.net, 2007. < http://www.physicalgeography.net/fundamentals/10w.html> (accessed December 3, 2007).

David T. King Jr .