Agents of erosion

Erosion by Water

  • Liquid water is the major agent of erosion on Earth. Rain, rivers, floods, lakes, and the ocean carry away bits of soil and sand and slowly wash away the sediment.
  • Rainfall produces four types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion. 
      • Splash erosiondescribes the impact of a falling raindrop, which can scatter tiny soil particles as far as 6 meters (2 feet).
      • Sheet erosion describes erosion caused by runoff.
      • Rill erosion describes erosion that takes place as runoff develops into discrete streams (rills).
      • Gully erosion is the stage in which soil particles are transported through large channels. Gullies carry water for brief periods of time during rainfall or snowmelt but appear as small valleys or crevasses during dry seasons.
  • Valley erosion is the process in which rushing streams and rivers wear away their banks, creating larger and larger valleys. The Fish River Canyon, in southern Namibia, is the largest canyon in Africa and a product of valley erosion. 
  • The ocean is a huge force of erosion.
      • Coastal erosion—the wearing away of rocks, earth, or sand on the beach—can change the shape of entire coastlines. During the process of coastal erosion, waves pound rocks into pebbles and pebbles into sand. Waves and currents sometimes transport sand away from beaches, moving the coastline farther inland.
      • Coastal erosion can have a huge impact on human settlement as well as coastal ecosystems.
      • The battering force of ocean waves also erodes seaside cliffs.
      • The action of erosion can create an array of coastal landscape features.
      • For example, erosion can bore holes that form caves.
      • When water breaks through the back of the cave, it can create an arch. The continual pounding of waves can cause the top of the arch to fall, leaving nothing but rock columns called sea stacks.
      • The seven remaining sea stacks of Twelve Apostles Marine National Park, in Victoria, Australia, are among the most dramatic and well-known of these features of coastal erosion.

Erosion by Wind

  • Wind is a powerful agent of erosion. Aeolian (wind-driven) processes constantly transport dust, sand, and ash from one place to another.
  • Wind can sometimes blow sand into towering dunes. Some sand dunes in the Badain Jaran section of the Gobi Desert in China, for example, reach more than 400 meters (1,300 feet) high. 
  • In dry areas, windblown sand can blast against a rock with tremendous force, slowly wearing away the soft rock. It polishes rocks and cliffs until they are smooth—giving the stone a so-called “desert varnish.”
  • Wind is responsible for the eroded features that give Arches National Park, in the U.S. state of Utah, its name.
  • Wind can also erode material until little remains at all.
  • Ventifacts are rocks that have been sculpted by wind erosion.
  • The enormous chalk formations in the White Desert of Egypt are ventifacts carved by thousands of years of wind roaring through the flat landscape.
  • Some of the most destructive examples of wind erosion are the dust storms that characterized the “Dust Bowl” of the 1930s in North America. Made brittle by years of drought and agricultural mismanagement, millions of tons of valuable topsoil were eroded away by strong winds in what came to be known as “black blizzards.” These dust storms devastated local economies, forcing thousands of people who depended on agriculture for their livelihoods to migrate.

Erosion by Ice

  • Ice, usually in the form of glaciers, can erode the earth and create dramatic landforms.
  • In frigid areas and on some mountaintops, glaciers move slowly downhill and across the land. As they move, they transport everything in their path, from tiny grains of sand to huge boulders.
  • Rocks carried by glaciers scrape against the ground below, eroding both the ground and the rocks.
  • In this way, glaciers grind up rocks and scrape away the soil. Moving glaciers gouge out basins and form steep-sided mountain valleys.
  • Eroded sediment called moraine is often visible on and around glaciers.
  • Ice Age glaciers scoured the ground to form what are now the Finger Lakes in the U.S. state of New York, for example. They carved fjords, deep inlets along the coast of Scandinavia.
  • Today, in places such as Greenland and Antarctica, glaciers continue to erode the earth. Ice sheets there can be more than a mile thick, making it difficult for scientists to measure the speed and patterns of erosion. However, ice sheets do erode remarkably quickly—as much as half a centimeter (.2 inch) every year.