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R E S O U R C E L I B R A R Y E N C Y C L O P E D I C E N T RY

Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.

G R A D E S 6 - 12+

S U B J E C T S Earth Science, Geology, Geography, Physical Geography

C O N T E N T S 9 Images

For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/erosion/

Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water. A similar process, weathering, breaks down or dissolves rock, but does not involve movement.

Erosion is the opposite of , 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 are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called .

Physical Erosion

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 . Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin.

Landslides and other forms of are associated with physical weathering. These processes cause rocks to dislodge from hillsides and crumble as they tumble down a slope.

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. 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. , , , lakes, and the ocean carry away bits of soil and and slowly wash away the sediment.

Rainfall produces four types of : splash erosion, sheet erosion, erosion, and erosion. • Splash erosion describes 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 (). • Finally, gully erosion is the stage in which soil particles are transported through large channels. 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 , in southern Namibia, is the largest canyon in Africa and a product of erosion. Over millions of years, the Fish River wore away at the hard gneiss , carving a canyon about 160 kilometers (99 miles) in length, 27 kilometers (17 miles) wide, and 550 meters (1,084 feet) deep.

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 as coastal ecosystems. The Cape Hatteras Lighthouse, for example, was nearly destroyed by coastal erosion. The Cape Hatteras Lighthouse was built on the Outer Banks, a series of barrier islands off the of the U.S. state of North Carolina, in 1870. At the time, the lighthouse was nearly 457 meters (1,500 feet) from the ocean. Over time, the ocean eroded most of the beach near the lighthouse. By 1970, the pounding surf was just 37 meters (120 feet) away and endangered the structure. Many people thought the lighthouse would collapse during a strong storm. Instead, thanks to a significant engineering feat completed in 1999, it was moved 880 meters (2,900 feet) inland.

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 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 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.

Several times in Earth’s history, vast glaciers covered parts of the Northern Hemisphere. These glacial periods are known as ice ages. Ice Age glaciers carved much of the modern northern North American and European landscape.

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. The snout of a glacier eroded Cape Cod Bay, Massachusetts, and formed the recognizable fishhook shape of Cape Cod itself.

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.

Other Forces of Erosion

Thermal erosion describes the erosion of along a river or coastline. Warm temperatures can cause ice-rich permafrost to break off coastlines in huge chunks, often carrying valuable topsoil and vegetation with them. These eroded “floating islands” can disintegrate into the ocean, or even crash into another piece of land—helping spread new life to different landscapes.

Mass wasting describes the downward movement of rocks, soil, and vegetation. Mass wasting incidents include , rockslides, and avalanches. Mass wasting can erode and transport millions of tons of earth, reshaping hills and mountains and, often, devastating communities in its path.

Factors Impacting Erosion

Some of the natural factors impacting erosion in a landscape include climate, topography, vegetation, and tectonic activity.

Climate is 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.

Topography, the shape of surface features of an area, can contribute to how erosion impacts that area. The earthen of river valleys are much more prone to erosion than rocky 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.

Finally, tectonic activity shapes 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 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.

Erosion and People

Deposition, , and Sediments Eroded sediments have profoundly influenced the development of civilizations around the world.

Agricultural development is 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 San Joaquin and Sacramento rivers in northern California, 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 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 Control Erosion is a natural process, but human activity can make it happen more quickly.

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.

Global warming, the 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.

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. 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 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 . 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. Vocabulary

Part of Term Definition Speech Part of Term Definition Speech accumulation noun a buildup of something. adhere verb to stick to or support. aeolian noun geographic feature created by wind. landform modern farming methods that include mechanical, chemical, agricultural noun engineering and technological methods. Also called industrial development agriculture. the art and science of cultivating land for growing crops (farming) or agriculture noun raising livestock (ranching). array noun large group. atmosphere noun layers of gases surrounding a planet or other celestial body. large mass of snow and other material suddenly and quickly tumbling avalanche noun down a mountain. a slope of land adjoining a , or a large elevated area of noun the sea floor. long, narrow strip of sandy land built up by waves and tides that barrier island noun protects the mainland shore from erosion. basin noun a dip or depression in the surface of the land or ocean floor. batter verb to beat and cause damage. beach noun narrow strip of land that lies along a body of water. bedrock noun solid rock beneath the Earth's soil and sand. the process in which a living organism wears away at rock or another bioerosion noun hard substance. canyon noun deep, narrow valley with steep sides. carbonation noun absorption of, or reaction with, carbon dioxide. carbonic acid noun chemical produced as carbon dioxide dissolves in water. underground chamber that opens to the surface. Cave entrances can cave noun be on land or in water. chalk noun a soft mineral. Also called limestone and calcium carbonate. noun waterway between two relatively close land masses. chemical attraction between atoms, ions or molecules that enables the noun bond formation of chemical compounds. chemical noun process of rocks changing their chemical composition as they erode. erosion Part of Term Definition Speech chemical process that involves a change in atoms, ions, or molecules of the noun reaction substances (reagents) involved. complex way of life that developed as humans began to develop civilization noun urban settlements. clastic rock composed of fragments of older rocks that have been noun sediment transported from their place of origin. cliff noun steep wall of rock, earth, or ice. climate noun all weather conditions for a given location over a period of time. climate gradual changes in all the interconnected weather elements on our noun change planet. coastal wearing away of earth or sand on the beach by natural or man-made noun erosion methods. coastline noun outer boundary of a shore. crevasse noun deep crack, especially in a glacier. crevice noun crack in a rock. crop noun agricultural produce. cumulative adjectivegrowing in quantity or strength. current noun steady, predictable flow of fluid within a larger body of that fluid. the flat, low-lying plain that sometimes forms at the mouth of a noun from deposits of sediments. deposition noun process of and sediment building up in an area. area of land that receives no more than 25 centimeters (10 inches) of desert noun precipitation a year. dark, hard layer of yellowish or blackish oxides formed on exposed desert varnish noun rock surfaces in windblown, arid regions. discrete adjectiveindividual or distinct. disintegrate verb to fall apart and disappear. dissolve verb to break up or disintegrate. dissolve verb to break up or disintegrate. drought noun period of greatly reduced precipitation. dry season noun time of year with little precipitation. dune noun a mound or ridge of loose sand that has been deposited by wind. dust noun tiny, dry particles of material solid enough for wind to carry. (1930-1940) term for the Great Plains of the U.S. and Canada when Dust Bowl noun severe dust storms forced thousands of people off their farms. Part of Term Definition Speech dust storm noun weather pattern of wind blowing dust over large regions of land. earth noun soil or dirt. economic adjectivehaving to do with money. ecosystem noun community and interactions of living and nonliving things in an area. endanger verb to put at risk. person who plans the building of things, such as structures engineer noun (construction engineer) or substances (chemical engineer). erode verb to wear away. erosion noun act in which earth is worn away, often by water, wind, or ice. erosion noun process of preventing or reducing erosion by wind and water. control farmer noun person who cultivates land and raises crops. fertile adjectiveable to produce crops or sustain agriculture. industry or occupation of harvesting fish, either in the wild or through fishery noun aquaculture. fjord noun long, narrow ocean inlet between steep slopes. floating island noun a mass of soil and plants torn from a coast. flood noun overflow of a body of water onto land. flood plain noun flat area alongside a stream or river that is subject to flooding. fluid noun material that is able to flow and change shape. forest noun ecosystem filled with trees and underbrush. frequent adjectiveoften. frigid adjectivevery cold. noun wire frame filled with rock. geological noun method by which the Earth changes. process time of long-term lowering of temperatures on Earth. Also known as glacial period noun an ice age. glacier noun mass of ice that moves slowly over land. global noun increase in the average temperature of the Earth's air and oceans. warming gouge noun hand tool with a partly curved blade, used for carving. groundwater noun water found in an . gully erosion noun removal of soil along lines by runoff. Part of Term Definition Speech environment where an organism lives throughout the year or for habitat noun shorter periods of time. hedgerow noun line of bushes and trees forming a boundary. tropical storm with wind speeds of at least 119 kilometers (74 miles) per hurricane noun hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region. hydration noun process of a substance or solution chemically combining with water. process in which a compound is split into other compounds by hydrolysis noun reacting with water. ice noun water in its solid form. long period of cold climate where glaciers cover large parts of the ice age noun Earth. The last ice age peaked about 20,000 years ago. Also called glacial age. ice sheet noun thick layer of glacial ice that covers a large area of land. indicate verb to display or show. inlet noun small indentation in a shoreline. island noun body of land surrounded by water. landform noun specific natural feature on the Earth's surface. landscape noun the geographic features of a region. the fall of rocks, soil, and other materials from a mountain, hill, or noun slope. structure displaying large, bright lights to warn and help ships lighthouse noun navigate coastal waters. living method of creating coastal land by using stones and marine grasses to noun shoreline trap soil, sand, and mud. loess noun windblown soil or silt. mass wasting noun downward movement of rock, soil, and other material. inorganic material that has a characteristic chemical composition and mineral noun specific crystal structure. moraine noun material, such as earth, sand, and , transported by a glacier. natural event in the physical environment that is destructive to human noun hazard activity. Northern noun half of the Earth between the North Pole and the Equator. Hemisphere nutrient noun substance an organism needs for energy, growth, and life. Part of Term Definition Speech chemical process of a substance combining with oxygen to change the oxidation noun substance's physical and molecular structure. permafrost noun permanently frozen layer of the Earth's surface. physical process of rocks and earthen materials breaking apart and being noun erosion transported without changing their chemical composition. noun, tool used for cutting, lifting, and turning the soil in preparation for plow verb planting. polish verb to make smooth and shiny by rubbing. precipitation noun all forms in which water falls to Earth from the atmosphere. prone adjectivevulnerable or tending to act in a certain way. rill noun very small stream. process of soil removal by water running through little streamlets, or rill erosion noun headcuts. riparian buffernoun area of grass, trees, or shrubs near a river bank. rock noun natural substance composed of solid mineral matter. runoff noun overflow of fluid from a farm or industrial factory. red or orange coating that forms on the surface of iron when it is rust noun exposed to oxygen and moisture. Also called iron oxide or ferric oxide. sand noun small, loose grains of disintegrated rocks. sand dune noun mound of sand created by the wind. region and name for some countries in Northern Europe: Iceland, Scandinavia noun Norway, Sweden, Finland, and Denmark. scour verb to rub harshly, often to polish. increase in the average reach of the ocean. The current sea level rise is sea level rise noun 1.8 millimeters (.07 inch) per year. column-shaped rock formation created by waves eroding parts of sea stack noun coastal cliffs. sediment noun solid material transported and deposited by water, ice, and wind. sedimentationnoun process of accumulating small solid deposits. sheet erosion noun removal of soil in thin layers by the forces of rain and stream flow. shelterbelt noun line of bushes and trees forming a boundary. shrub noun type of plant, smaller than a tree but having woody branches. snout noun end of a glacier. soil noun top layer of the Earth's surface where plants can grow. Part of Term Definition Speech substance in which a gas, liquid, or solid is evenly distributed in solution noun another medium. splash erosion noun soil displacement by the impact of a falling raindrop. severe weather indicating a disturbed state of the atmosphere storm noun resulting from uplifted air. abnormal rise in sea level accompanying a hurricane or other intense storm surge noun storm. Also called a storm tide. stream noun body of flowing fluid. surf noun waves as they break on the shore or reef. suspend verb to temporarily stop an activity. tectonic movement of tectonic plates resulting in geologic activity such as noun activity volcanic eruptions and . movement of plates beneath the Earth's surface that causes one part tectonic uplift noun of the landscape to rise higher than the surrounding area. degree of hotness or coldness measured by a thermometer with a temperature noun numerical scale. thermal erosion of permafrost by the combined thermal and mechanical action noun erosion of moving water. topography noun study of the shape of the surface features of an area. topsoil noun the most valuable, upper layer of soil, where most nutrients are found. tropical storm with wind speeds of at least 74 miles (119 kilometers) per typhoon noun hour. Typhoons are the same thing as hurricanes, but usually located in the Pacific or Indian Ocean region. valley noun depression in the Earth between hills. process in which rushing streams and rivers wear away their banks, valley erosion noun creating larger and larger valleys. vegetation noun all the plant life of a specific place. measurement of the rate and direction of change in the position of an velocity noun object. ventifact noun rock that has been shaped by wind-driven sand, dust, or ice particles. water noun chemical compound that is necessary for all forms of life. wave noun moving swell on the surface of water. the breaking down or dissolving of the Earth's surface rocks and weathering noun minerals. wetland noun area of land covered by shallow water or saturated by water. Part of Term Definition Speech movement of air (from a high pressure zone to a low pressure zone) wind noun caused by the uneven heating of the Earth by the sun. windbreak noun structure that serves to interrupt an air current or flow of wind. Articles & Profiles SF Gate: How to Control Erosion Around a House SoftSchools.com: Physical Weathering Examples National Parks Service: What’s the Difference Between Weathering and Erosion? Agriculture Victoria: Gully Erosion National Park Service: Grand Canyon—Geologic Formations LiveScience: Glaciers Erode Land Faster Than Thought

Websites National Geographic: Erosion and Weathering

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