Erosion

I

Introduction

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.

II

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.

III

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.

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IV

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.

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