Erosion, the wearing away of the earth's surface by natural forces. The major causes of erosion include running water, wind, glaciers, and waves. Erosion begins with weathering, the weakening and breaking down of rocks and earth on the earth's surface. Some weathering results from air and water combining chemically with rocks; some results from water freezing in rock crevices—as the water freezes it expands, cracking the rock. Living things, including lichens, bacteria, and burrowing animals, also help bring about weathering.

Erosion takes place when particles of earth and broken-down rock are carried away by wind, runoff from rain, or in some-other way. The particles themselves contribute to erosion by grinding against exposed rock and earth. In this way, rivers can cut deep channels and even canyons. As glaciers advance they cause extreme erosion, scraping away large amounts of rock and earth. The pounding of ocean waves results in the erosion of seacoasts.

Erosion is constantly sculpturing the earth's surface into new forms. The general tendency of erosion is to bring all land surfaces down to sea level. However, this tendency is opposed by the formation of volcanoes and by movements in the earth's crust that gradually thrust upward large sections of the earth's surface.

By disturbing the surface of the earth through farming, mining, and construction, humans are a major cause of erosion. Of particular concern is the erosion of soil. In general, forests and natural vegetation help prevent erosion because plant roots form a network that grips the soil and they absorb some of the water that would otherwise wash away soil.