Erosion

Erosion
By Susan Soberhad
Fall 97

What is erosion?

Erosion occurs when rock and soil are broken loose from the earth's surface in one place, and carried to another place. Erosion is usually a slow and gradual process that takes thousands or even millions of years. But man's activities such a farming, mining, and urban development can speed up the erosion process.

How does erosion occur?

Erosion begins with the weathering, or breaking down, of rocks and soil. Chemical processes like carbonation or acid rain, and physical processes like freezing and the pull of gravity can cause weathering to occur. Once rocks and soil are broken down, wind and water can erode the landscape and carry the loose rocks and soil away.

Water runoff is the major cause of erosion. Runoff is water that flows over the earth's surface. It picks up and carries weathered rocks and soils to other places. It also creates many features on the earth's surface.

Do rivers cause erosion?

Rivers cause erosion by transporting weathered rock and soil downstream. Fast-moving rivers cause more erosion than slow-moving rivers, and are responsible for some of Earth's most spectacular features.

The Grand Canyon is one spectacular feature that was created by river erosion. Over millions of years, the Colorado River carried weathered sediments away until the mile-deep canyon was created as we know it today.

How do floods affect erosion?

Floods can cause brief periods of heavy erosion. Raging waters can carry huge load of rocks, soil and debris. These same waters can deposit soils in wide, flat areas on both sides of the river in the aftermath of a flood. While these flood plains are often very fertile, the flooding potential can make flood plain areas a dangerous place to live and a risky place to grow crops.

What are some features of erosion?

Runoff first forms tiny grooves called rills to occur. Over time, the rills widen and deepen to form gullies, and the gullies act as channels for runoff.

Chemical weathering can actually dissolve rocks, and erosion can carry the dissolved particles away. Caves and depressions in rock faces may be formed in this way.

Ripple marks look like small waves in the rocks, and are formed by water moving over loose sediments. In time, the sediments harden into rock and the ripple marks are preserved.

How is erosion helpful?

When erosion occurs, broken down rocks form soils and are deposited in very fertile layers on valley floors and river deltas. This fertile soil provides excellent locations for growth of plants and agricultural products.

How is erosion harmful?

Erosion can be harmful when this same kind of fertile topsoil is washed away. Plowed fields don't have any protection against running water, and often the valuable topsoil is carried away during heavy rains or floods. Strip mining also removes trees and other vegetation from large areas, leaving the topsoil open to erosion. Urban development results in replacement of vegetation with concrete and highways, increasing runoff and erosion in surrounding areas.

What can prevent erosion?

Natural deterrents of erosion are the root systems of plants and trees. The roots help hold topsoil in place. Contour plowing and terracing are also ways that farmers can help prevent erosion. Urban developers can help reduce erosion by including green areas and by replanting vegetation that may have been lost during the building process.

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