Text 18. Erosion

Erosion is the 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 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. Opposing this tendency are volcanic eruptions and movements of the crust of the earth that raise mountains, plateaus, and new islands.

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 in a thousand years. Also winds may sometimes deposits sand in large piles, known as sand dunes.

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NOTES:

• bedrock - подстилающая порода, коренная порода;

• rill - источник, родник, ручей;

• rivulet - ручей, маленькая речка;

• scour - промывание, промоина, размыв;

• silt – ил, тина, шлам;

• deflation – выкачивание воздуха, газа.