Erosion

Erosion is a dynamic process by which running water, glaciers, winds, waves and currents, gravity and groundwater wear away the earth. These agents of erosion sculpt the earth's surface as they wear it down. Forces within the earth driven by internal heat raise the surface, and the agents of erosion driven ultimately by the heat of the sun wear it down.

As these agents of erosion do their work another process, termed weathering, takes place, the physical disintegration and chemical decomposition of rocks by gases and water in the atmosphere. Minerals in rocks, for example, react with water and oxygen, resulting in the formation of new substances that are susceptible to erosion. In climates where water freezes, the increase in volume in the conversion to ice produces powerful forces that can split rocks. Fractures in rocks are enlarged by tree roots growing in them, eventually splitting the rocks into smaller pieces. On steep slopes loose material becomes susceptible to being pulled down by gravity if the slope is undercut or rain increases the weight of the mass. The process of mass wasting, the downward movement of loose material (regolith) on the earth's surface, includes various types of slides, falls, and flows. Very slow movements called creep are a common process easily seen, for example, in cemeteries, where stone markers have moved downward even on gentle slopes. Much of the debris will end up in streams, the tools of the master sculptor of the earth's surface. Streams flow downhill confined within a channel and carry not only rock debris but also dissolved substances. The amount of material carried is dependent on a number of factors, such as the velocity and amount of water, amount and type of vegetation, nature of the bedrock (solid or loose), and steepness of the stream. Streams transport sediment by rolling and pushing, while hopping takes place with increased velocity and by suspension and in solution. This material becomes the grist for erosion. By a combination of down-slope movements, water running over the surface in sheets and within valleys, the surface is eroded. Rivers are the tools that cut down through the underlying rock and undercut their valley walls, producing spectacular features like the Grand Canyon. Drainage systems develop and evolve through stages controlled by climate and tectonics. Over the long term they constantly adjust to changing conditions, such as increasing or decreasing rainfall, rifting, uplift, subsidence, and volcanism.

Streams tend to be in equilibrium with their environment, and changes imposed by construction projects along valleys alter these dynamics, often increasing the amount of erosion. Deforestation removes a protective cover over the surface, making the surface more susceptible to the forces of erosion. Road-building in some areas can set up conditions that enhance the possibility of landslides. Essentially, road building undercuts steep slopes and makes them unstable, requiring extensive remedial action that may include netting, rock bolts, or simply terracing the cliff back, reducing the threat.

In cold climates or areas that have been subjected to colder climatic regimes in the past, glaciers have changed the landscape. At the present time about 10 percent of the earth's surface is covered with glacial ice, but during the recent glacial maximum, 18,000 years ago, one-third of the earth was covered with ice. As ice moves it picks up loose rock debris or plucks it directly from the bedrock, incorporating the material into the ice. The moving ice behaves like sandpaper, grooving, smoothing, and abrading the bedrock. Alpine glaciers begin their existence in areas protected from lots of sunlight, where snow accumu lates in valleys high on the slopes. As ice forms the areas of accumulation enlarge, eventually forming large, bowl-shaped depressions called cirques. Ice moves out of the cirque and down the valley, deepening, widening, and converting the once V-shaped into the typically U-shaped configuration commonly seen in some mountainous areas of the United States such as the Rocky Mountains. Additional major erosional landforms formed by alpine glaciers are called horns (like the Matterhorn of Switzerland) and hanging valleys (such as Bridal Veil Falls in Yosemite National Park). Horns form where several cirques are formed around a mountain; as they enlarge and intersect, all that remains is a sharp central peak. Hanging valleys result from the fact that tributary glaciers do not erode as deeply as the larger main glaciers. As a result, after the glaciers have melted away the tributary valley terminates high above the main valley, marked by a waterfall.

Ice sheets move over parts of continents, altering the surface and profoundly modifying previous drainage systems. They gouge out valleys deep enough so that when they melt away, deep basins form that fill with water, creating lakes. Glaciers transport material, and when they melt they leave piles of rocky debris producing distinct land forms. Terminal moraines, for example, are ridges of debris deposited at the margins of glaciers; they can produce substantial features. Long Island in New York state owes its existence to two morainal deposits that extend the length of the island. Otherwise, the island would be just a string of small, muddy lumps off the coast. Glacial erosion produces distinct features, letting observers determine where glaciers once existed, implying a climate change for that area.

When the velocity of wind is sufficient, it can pick up small particles that then become an agent of erosion, blasting surfaces and alter

A dust storm whips across the landscape in Namibia. (UN photo/Milton Grant)

ing their appearance. This sand blasting process is most obvious in desert environments. In areas where loose sand is at the surface, wind removes the material, forming shallow depressions called blowouts. Often, in desert areas, the wind removes finer material, leaving behind a lag deposit of coarse material—a striking desert pavement feature. Eventually, when the wind dies down, some of the sand will be piled up, building a variety of sand dunes.

Groundwater has a great capacity to create landforms, especially when the bedrock is composed of limestone. Water falling through the atmosphere picks up carbon dioxide, forming carbonic acid; with organic acids picked up from soil it dissolves limestone, creating most of the world's caves. Both the surface and subsurface features formed by solution of limestone are called karst topography, named for a region in Yugoslavia where caves are well developed. A large number of features such as sinkholes, caves, and disappearing streams are products of this process. Sinkholes form where the cave has enlarged to such a degree that the overlying roof rock is unsupported and caves in.

Along shorelines, waves and currents armed with sediment attack cliffs, undermining them, wearing them away, and producing such compelling shoreline features as stacks and sea arches. Along sandy low areas, the moving sand and gravel grind away at each other, reducing their size. As a result beaches along the coast have finer and finer sand as they get farther away from their source. Coastal erosion, destruction of cliffs, and the disappearance of beaches during storms are of great concern to

Rock erosion off the coast of California (USQS/Arnold R.)

people who live at and visit the shore. People and governments try to battle these erosive processes by building expensive structures to preserve what there is. Often the structures are inappropriate and actually enhance erosion.

—Sidney Horenstein

See also: Climatology; Deposition; Freshwater; Rivers and Streams; Topsoil, Loss of

Bibliography

Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw-Hill Professional Publishing; Pinter, Nicholas, and Mark T. Brandon. 1997. "How Erosion Builds Mountains." Scientific American. 276: 74-79; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill.

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