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Erosion

The surface of the earth is continually being worn away by natural geologic processes. The displacement of this weathered material is called erosion. Agents such as water, wind, ice, and gravity work to level the land surfaces of the earth across vast expanses of geologic time. The shapes of continents constantly change as waves and tides cut into old land, while rivers deposit silt and build new land. Erosion eventually wears away mountains, but movements of the crust and volcanic activity raise new ones. Although erosion is generally a slow and gradual process that occurs over thousands or millions of years, human activity greatly increases the rate of erosion on fertile land. Farming, mining, and industrial development are all ultimately destructive processes.

Types of Erosion

There are two major categories of erosion: geologic erosion and soil erosion. Geologic erosion is initiated by the weathering of rock. Weathering can be caused by natural physical or chemical means. Heat from the sun may expand the uppermost layer of rock, causing cracking on the surface. Wind and rain then carry the particles away, completing the erosion process. In damp climates, minerals in the rock can react chemically with rainwater, causing the rock to gradually dissolve or decompose. Cold climates will cause water that has seeped into crevasses to freeze and expand, splitting rock at the surface. Roots of nearby plants may also grow into cracks and cause further breaking. Water is a very powerful erosional agent. Running water in streams and rivers not only wears away or dissolves rock but also can carry stones that cause further abrasion along the banks. Glaciers pick up and carry away all loose surface material. When the ice melts, only bare rock remains. Ocean currents and waves erode coastlines, sometimes carving into cliffs and other times depositing sandy beaches.

Soil erosion on uninhabited, untouched land is balanced by soil formation. Soil is protected by natural vegetation. Trees, shrubs, and grasses serve as windbreaks and protect the soil from the force of the rain. Roots hold soil in place. Unfortunately, many human activities involve removing the vegetation from the land. Agriculture, logging, construction, mining, and heavy animal grazing all greatly increase the rates of erosion. Some historians believe that soil erosion is partially responsible for population shifts and the collapse of some civilizations throughout history. Ruins of cities with agricultural artifacts have been found in arid deserts, demonstrating that farming was widespread in these areas in the distant past.

Rates of Erosion

Over geologic time, approximately the past 500 million years, there has been about 60 feet of erosion each million years. Currently, in areas of the United States being eroded by human agricultural activity, erosion is occurring at a calculated 1,500 feet per million years. The rate is even higher in other parts of the world. Soil formation varies, but it takes about 10,000 years for 1 foot of top-soil to form. Today, approximately 40% of the world's agricultural land is seriously degraded, The resulting in lower productivity and poorer crop quality. Virtually all arable land is in use.

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