Erosion

Erosion, removal of rock and soil material by natural processes, principally running water, glaciers, waves, and wind. Erosion transports rocky material after the process of weathering has broken bedrock down into smaller, moveable pieces.
Through erosion the surface of the earth is constantly being sculptured into new forms. The shapes of continents are continuously changing, as waves and tides cut into old land while silt from rivers builds up new land. As rivulets, streams, and rivers cut their channels deeper, gullies become ravines and ravines become valleys. The Grand Canyon, more than 1500 m (more than 5000 ft) deep, was produced by erosion probably within the past 5 million years. The overall effect of the wearing down of mountains and plateaus is to level the land; the tendency is toward the reduction of all land surfaces to sea level. For example, in each 7000- to 9000-year period the entire area drained by the Mississippi River loses an average of 30 cm (12 in) of altitude. Opposing this tendency are volcanic eruptions and movements of the crust of the earth that raise mountains, plateaus, and new islands. See Geology; Geomorphology.

WEATHERING

Weathering initiates the erosion of rock, causing alterations in the surface layers. In dry climates, the top layer of a rock may expand from the heat of the sun and crack off from the lower layers. If the rock consists of several minerals, the minerals may expand at different rates and break up the rock. In cold climates, frost breaks up rocks because rainwater, which seeps into cracks and pores in the rock, expands when it freezes. Rain in damp climates acts chemically as well as mechanically in the weathering of rocks. As the rain passes through the atmosphere it absorbs carbon dioxide, forming carbonic acid, which dissolves some minerals and decomposes others. Feldspar, a common family of minerals in granite, is changed into clays, and certain minerals in basalt combine with oxygen and water to form iron oxides, such as limonite. Plants play a role in weathering as roots can split rocks and extract soluble nutrients.

WATER EROSION
Water plays an important role in erosion by carrying away material that has been weathered and broken down. When an area receives more water (in the form of rain, melting snow, or ice) than the ground can absorb, the excess water flows to the lowest level, carrying loose material with it. Gentle slopes are subject to sheet and rill erosion, in which the runoff removes a thin layer of topsoil without leaving visible traces on the eroded surface. This erosion may be balanced by the formation of new soil. Often, however, especially in arid areas having little vegetation, the runoff leaves a pattern of gullies formed by rivulets. Water can even erode solid rock, especially along streambeds where the stones that are carried with the current scour and abrade. Every year rivers deposit about 3.5 million tons of eroded material into the oceans.

GLACIAL EROSION
Glaciers are important agents of erosion. Although a glacier moves slowly, it gradually removes all the loose material from the surface over which it travels, leaving bare rock surfaces when the ice melts. Besides removing loose material, glaciers actively erode the solid rock over which they travel. Rock fragments that become embedded in the bottom and sides of the moving ice mass act as an abrasive, grinding and scouring the bedrock which forms the walls and floors of mountain valleys.

COASTAL EROSION
Coastal erosion of rocky cliffs and sandy beaches results from the action of ocean waves and currents. This is especially severe during storms. In many parts of the world the loss of land due to coastal erosion represents a serious problem. The action of waves, however, does not extend to a great depth, and the sea tends to cut a flat platform, characteristic of marine erosion, into coastal rocks.

WIND EROSION
Wind is another active agent of erosion, especially in arid climates with little vegetation. Wind blowing across bare land lifts particles of sand and silt but leaves behind larger pebbles and cobbles. Eventually, a surface layer of closely packed stones, called a desert pavement, is formed as the sand and silt is removed. The removal of large quantities of loose material is called deflation. Deflation lowers the landscape slowly, usually less than a meter (3 ft) in a thousand years. However, deflation can occur more rapidly, as it did during the 1930s in the southern parts of the Great Plains of the United States (see Dust Bowl). Winds may sometimes deposit sand in large piles, known as sand dunes.

HUMAN IMPACTS ON EROSION
Without human activities, losses of soil through erosion would in most areas probably be balanced by the formation of new soil. On virgin land a mantle of vegetation protects the soil. When rain falls on a surface of grass or on the leaves of trees, some of the moisture evaporates before it can reach the ground. Trees and grass serve as windbreaks, and a network of roots helps to hold the soil in place against the action of both rain and wind. Agriculture and lumbering, as well as housing, industrial development, and highway construction, however, partially or wholly destroy the protective canopy of vegetation and greatly speed up erosion of certain kinds of soils. Erosion is less severe with crops such as wheat, which cover the ground evenly, than with crops such as corn and tobacco, grown in rows.
Overgrazing, which in time can change grassland to desert, and careless cultivation have had disastrous effects in certain parts of the United States (see Drought; Dust Bowl). Some historians believe that soil erosion has been a determinant in the complex of causes underlying various population shifts and the fall of certain civilizations. Ruins of towns and cities have been found in arid regions, such as the deserts of Mesopotamia, indicating that agriculture was once widespread in the surrounding territory.