Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.
Erosion is the opposite of deposition, the geological process in which earthen materials are deposited, or built up, on a landform.
Most erosion is performed by liquid water, wind, or ice (usually in the form of a glacier). If the wind is dusty, or water or glacial ice is muddy, erosion is taking place. The brown color indicates that bits of rock and soil are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called sediment.
Physical erosion describes the process of rocks changing their physical properties without changing their basic chemical composition. Physical erosion often causes rocks to get smaller or smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin.
Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up earthen materials as they take root, and can create cracks and crevices in rocks they encounter.
Ice and liquid water can also contribute to physical erosion as their movement forces rocks to crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River rocks are often much smoother than rocks found elsewhere, for instance, because they have been eroded by constant contact with other river rocks.
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.
Some of the natural factors impacting erosion in a landscape include climate, topography, vegetation, and tectonic activity.
- Climateis perhaps the most influential force impacting the effect of erosion on a landscape.
- Climate includes precipitation and wind.
- Climate also includes seasonal variability, which influences the likelihood of weathered sediments being transported during a weather event such as a snowmelt, breeze, or hurricane.
- Global warming, the current period of climate change, is speeding erosion. The change in climate has been linked to more frequent and severe storms.
- Storm surges following hurricanes and typhoons can erode kilometers of coastline and coastal habitat. These coastal areas are home to residences, businesses, and economically important industries, such as fisheries.
- The rise in temperature is also quickly melting glaciers. The slower, more massive form of glacial erosion is being supplanted by the cumulative impact of rill, gully, and valley erosion. In areas downstream from glacial snouts, rapidly melting glaciers are contributing to sea level rise. The rising sea erodes beaches more quickly.
- Topography, the shape of surface features of an area, can contribute to how erosion impacts that area.
- The earthen floodplains of river valleys are much more prone to erosion than rocky flood channels, which may take centuries to erode.
- Soft rock like chalk will erode more quickly than hard rocks like granite.
- Vegetation can slow the impact of erosion.
- Plant roots adhere to soil and rock particles, preventing their transport during rainfall or wind events.
- Trees, shrubs, and other plants can even limit the impact of mass wasting events such as landslides and other natural hazards such as hurricanes.
- Deserts, which generally lack thick vegetation, are often the most eroded landscapes on the planet.
- Human activity altering the vegetation of an area is perhaps the biggest human factor contributing to erosion. Trees and plants hold soil in place.
- When people cut down forests or plow up grasses for agriculture and development, the soil is more vulnerable to washing or blowing away. Landslides become more common. Water rushes over exposed soil rather than soaking into it, causing flooding.
- Tectonic activityshapes the landscape itself, and thus influences the way erosion impacts an area.
- Tectonic uplift, for example, causes one part of the landscape to rise higher than others.
- In a span of about 5 million years, tectonic uplift caused the Colorado River to cut deeper and deeper into the Colorado Plateau, land in what is now the U.S. state of Arizona.
- It eventually formed the Grand Canyon, which is more than 1,600 meters (1 mile) deep and as much as 29 kilometers (18 miles) wide in some places.
Eroded sediments have profoundly influenced the development of civilizations around the world.
- Agricultural developmentis often reliant on the nutrient-rich soils created by the accumulation of eroded earth.
- When the velocity of wind or water slows, eroded sediment is deposited in a new location.
- The sediment builds up in a process called sedimentation and creates fertile land.
- River deltas are made almost entirely of sediment that has eroded from the banks and bed of a river.
- The rich delta soils of the Gangetic rivers in India , for example, have created one of the most agriculturally productive areas in the world.
- Loess is an agriculturally rich sediment made almost entirely of wind-blown, eroded sediment.
- The Yellow River in central China gets its name from the yellow loess blown into and suspended in its water.
- The fertile lands around the Yellow River have been among China’s most productive for thousands of years.
Erosion is a natural process, but human activity can make it happen more quickly.
Erosion control is the process of reducing erosion by wind and water. Farmers and engineers must regularly practice erosion control.
- Sometimes, engineers simply install structures to physically prevent soil from being transported.
- Gabions are huge wireframes that hold boulders in place, for instance. Gabions are often placed near cliffs. These cliffs, often near the coast, have homes, businesses, and highways near them. When erosion by water or wind threatens to tumble the boulders toward buildings and cars, gabions protect landowners and drivers by holding the rocks in place.
- Erosion control also includes physically changing the landscape.
- Communities often invest in windbreaks and riparian buffers to protect valuable agricultural land.
- Windbreaks, also called hedgerows or shelterbelts, are lines of trees and shrubs planted to protect cropland from wind erosion.
- Riparian buffers describe plants such as trees, shrubs, grasses, and sedges that line the banks of a river. Riparian buffers help contain the river in times of increased stream flow and flooding.
- Living shorelines are another form of erosion control in wetland areas.
- Living shorelines are constructed by placing native plants, stone, sand, and even living organisms such as oysters along wetland coasts.
- These plants help anchor the soil to the area, preventing erosion.
- By securing the land, living shorelines establish a natural habitat.
- They protect coastlines from powerful storm surges as well as erosion.
- Living shorelines are another form of erosion control in wetland areas.