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Erosion
Erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. This natural force has been quietly reshaping the planet for billions of years, yet its effects are often invisible to the naked eye until a landscape has been fundamentally altered. While weathering breaks down rocks, erosion is the movement of those broken pieces, a distinction that defines the dynamic nature of our planet's surface. From the smallest raindrop splashing soil particles a few millimetres away to massive rivers carrying sediment thousands of kilometres to the ocean, the scale of this process is as varied as the environments it affects. The agents of erosion include rainfall, bedrock wear in rivers, coastal waves, glacial plucking, wind abrasion, and even the slow creep of gravity on steep slopes. These forces do not act in isolation; they interact with climate, vegetation, and topography to determine how fast a surface is worn away. In some cases, the feedback between erosion rates and the amount of material already carried by a river or glacier can accelerate the process, creating a self-reinforcing cycle that reshapes the land faster than expected.
Water's Relentless Work
Rainfall and surface runoff produce four main types of soil erosion, each more severe than the last, progressing from splash erosion to gully erosion. Splash erosion, the first stage, occurs when the impact of a falling raindrop creates a small crater in the soil, ejecting particles as much as 0.5 metres vertically and 1.5 metres horizontally on level ground. If the soil becomes saturated or rainfall exceeds the infiltration rate, surface runoff begins, transporting loosened soil particles down the slope in a process known as sheet erosion. This is followed by rill erosion, where small, ephemeral concentrated flow paths develop, acting as both sediment source and delivery systems on hillslopes. These rills, typically only a few centimetres deep, exhibit hydraulic physics distinct from the wider channels of streams and rivers. The most severe stage, gully erosion, occurs when runoff water accumulates and rapidly flows in narrow channels, removing soil to a considerable depth. A gully is distinguished from a rill by a critical cross-sectional area of at least one square foot, a size that can no longer be erased via normal tillage operations. Extreme gully erosion can progress to the formation of badlands, which form under conditions of high relief on easily eroded bedrock in climates favorable to erosion. In rivers, valley erosion deepens the valley and extends it headward, creating head cuts and steep banks. During floods, the most erosion occurs as faster-moving water carries a larger sediment load, with suspended abrasive particles, pebbles, and boulders acting erosively in a process known as traction. Bank erosion, the wearing away of stream banks, is measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times. Thermal erosion, the result of melting and weakening permafrost due to moving water, causes rapid river channel migration in places like the Lena River of Siberia, where weakened banks fail in large slumps.
Common questions
What is erosion and how does it differ from weathering?
Erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. While weathering breaks down rocks, erosion is the movement of those broken pieces, a distinction that defines the dynamic nature of our planet's surface.
What are the four main types of soil erosion caused by rainfall?
Rainfall and surface runoff produce four main types of soil erosion progressing from splash erosion to gully erosion. Splash erosion occurs when the impact of a falling raindrop creates a small crater in the soil, followed by sheet erosion, rill erosion, and finally gully erosion which removes soil to a considerable depth.
How do glaciers erode the landscape and what landforms do they create?
Glaciers erode predominantly by three different processes: abrasion, plucking, and ice thrusting. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield and creates U-shaped parabolic steady-state shapes in glaciated valleys.
How has human activity increased the rate of soil erosion globally?
Human activities have increased by 10 to 40 times the rate at which soil erosion is occurring globally. At agriculture sites in the Appalachian Mountains, intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region.
How long does it take to erode a mountain mass similar to the Himalaya into an almost-flat peneplain?
Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode a mountain mass similar to the Himalaya into an almost-flat peneplain if there are no significant sea-level changes. Erosion of mountain massifs can create a pattern of equally high summits called summit accordance.
Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves, with sea level change also playing a role. Hydraulic action takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint, cracking it. Wave pounding involves the sheer energy of the wave hitting the cliff or rock, breaking pieces off. Abrasion or corrasion is caused by waves launching sea load at the cliff, making it the most effective and rapid form of shoreline erosion. Corrosion is the dissolving of rock by carbonic acid in sea water, a process to which limestone cliffs are particularly vulnerable. Attrition is where particles or sea load carried by the waves are worn down as they hit each other and the cliffs, making the material easier to wash away as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping, and grinding of organisms, a process termed bioerosion. Sediment is transported along the coast in the direction of the prevailing current, known as longshore drift. When the upcurrent supply of sediment is less than the amount being carried away, erosion occurs, but when the upcurrent amount is greater, sand or gravel banks tend to form as a result of deposition. These banks may slowly migrate along the coast, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a buildup of eroded material occurs, forming a long narrow bank known as a spit. Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore. Erosion of a coastal surface, followed by a fall in sea level, can produce a distinctive landform called a raised beach.
Ice and Wind's Grip
Glaciers erode predominantly by three different processes: abrasion, plucking, and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes control cross-valley variations. In a homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created, eventually reaching a U-shaped parabolic steady-state shape as we now see in glaciated valleys. Scientists provide a numerical estimate of the time required for the ultimate formation of a steady-shaped U-shaped valley, approximately 100,000 years. In a weak bedrock pattern, the amount of over deepening is limited because ice velocities and erosion rates are reduced. Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. Differences in the height of mountain ranges are not only the result of tectonic forces but also local climate variations. Scientists use global analysis of topography to show that glacial erosion controls the maximum height of mountains, as the relief between mountain peaks and the snow line are generally confined to altitudes less than 1500 metres. The erosion caused by glaciers worldwide erodes mountains so effectively that the term glacial buzzsaw has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges. As mountains grow higher, they generally allow for more glacial activity, which causes increased rates of erosion of the mountain, decreasing mass faster than isostatic rebound can add to the mountain. Wind erosion is a major geomorphological force, especially in arid and semi-arid regions, and is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage. Wind erosion is of two primary varieties: deflation, where the wind picks up and carries away loose particles; and abrasion, where surfaces are worn down as they are struck by airborne particles carried by wind. Saltation is responsible for the majority of wind erosion, followed by suspension, and then surface creep.
Gravity's Heavy Hand
Mass wasting or mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity. Mass wasting is an important part of the erosional process and is often the first stage in the breakdown and transport of weathered materials in mountainous areas. It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are always occurring continuously on all slopes; some act very slowly, while others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of rapid rockfall activity is a scree slope, which consists of accumulated loose rock debris at the base of cliffs or steep slopes. Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression, in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it, while in many cases it is simply the result of poor engineering along highways where it is a regular occurrence. Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. Submarine sediment gravity flows, bodies of sediment-laden water that move rapidly downslope as turbidity currents, can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock. Turbidites, which are the sedimentary deposits resulting from turbidity currents, comprise some of the thickest and largest sedimentary sequences on Earth, indicating that the associated erosional processes must also have played a prominent role in Earth's history.
The Human Acceleration
While erosion is a natural process, human activities have increased by 10, 40 times the rate at which soil erosion is occurring globally. At agriculture sites in the Appalachian Mountains, intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region. Excessive erosion causes both on-site and off-site problems. On-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, this leads to desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide. Intensive agriculture, deforestation, roads, anthropogenic climate change, and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. In the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years. In Taiwan, where typhoon frequency increased significantly in the 21st century, a strong link has been drawn between the increase in storm frequency with an increase in sediment load in rivers and reservoirs, highlighting the impacts climate change can have on erosion. Often in the United States, farmers cultivating highly erodible land must comply with a conservation plan to be eligible for agricultural assistance. The intentional removal of soil and rock by humans is a form of erosion that has been named lisasion, a term that underscores the scale of human intervention in natural processes.
Mountains and Time
Mountain ranges take millions of years to erode to the degree they effectively cease to exist. Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode a mountain mass similar to the Himalaya into an almost-flat peneplain if there are no significant sea-level changes. Erosion of mountain massifs can create a pattern of equally high summits called summit accordance. It has been argued that extension during post-orogenic collapse is a more effective mechanism of lowering the height of orogenic mountains than erosion. Examples of heavily eroded mountain ranges include the Timanides of Northern Russia. Erosion of this orogen has produced sediments that are now found in the East European Platform, including the Cambrian Sablya Formation near Lake Ladoga. Studies of these sediments indicate that it is likely that the erosion of the orogen began in the Cambrian and then intensified in the Ordovician. If the erosion rate exceeds soil formation, erosion destroys the soil. Lower rates of erosion can prevent the formation of soil features that take time to develop. Inceptisols develop on eroded landscapes that, if stable, would have supported the formation of more developed Alfisols. Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to the local slope, this will change the rates of erosion in the uplifted area. However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause tectonic or isostatic uplift in the region. In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates, for example, beneath the extremely steep terrain of Nanga Parbat in the western Himalayas. Such a place has been called a tectonic aneurysm.