Rainfall and surface runoff and water being
splashed by the impact of a single
raindrop Rainfall, and the
surface runoff which may result from rainfall, produces four main types of
soil erosion:
splash erosion,
sheet erosion,
rill erosion, and
gully erosion. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). In
splash erosion, the
impact of a falling raindrop creates a small crater in the
soil, ejecting soil particles.
Sheet erosion is the transport of loosened soil particles by overland flow.
Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth. A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations. Extreme gully erosion can progress to formation of
badlands. These form under conditions of high relief on
easily eroded bedrock in climates favorable to erosion. Conditions or disturbances that limit the growth of protective vegetation (
rhexistasy) are a key element of badland formation.
Rivers and streams , Scotland, showing two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the stream's banks are
glacial till that was left behind as ice age glaciers flowed over the terrain. exposed by a river eroding through them
Valley or
stream erosion occurs with continued water flow along a
linear feature. The erosion is both
downward, deepening the
valley, and
headward, extending the valley into the hillside, creating
head cuts and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V-shaped cross-section and the stream gradient is relatively steep. When some
base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream
meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles,
pebbles, and
boulders can also act erosively as they traverse a surface, in a process known as
traction.
Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as
scour. Erosion and
changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.
Thermal erosion is the result of melting and weakening
permafrost due to moving water. It can occur both along rivers and at the coast. Rapid
river channel migration observed in the
Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials. Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the
Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a segment of the
Beaufort Sea shoreline averaged per year from 1955 to 2002. Most river erosion happens nearer to the mouth of a river. On a river bend, the longest least sharp side has slower moving water. Here deposits build up. On the narrowest sharpest side of the bend, there is faster moving water so this side tends to erode away mostly. Rapid erosion by a large river can remove enough sediments to produce a
river anticline, as
isostatic rebound raises rock beds unburdened by erosion of overlying beds.
Coastal erosion caused by erosion of cliffs by the sea, at
Southerndown in South Wales Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and
waves but sea level (tidal) change can also play a role. beach,
Wales Hydraulic action takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it.
Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off.
Abrasion or
corrasion is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with
corrosion).
Corrosion is the dissolving of rock by
carbonic acid in sea water.
Limestone cliffs are particularly vulnerable to this kind of erosion.
Attrition is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up 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 (
longshore drift). When the upcurrent
supply of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of
deposition. These banks may slowly migrate along the coast in the direction of the longshore drift, 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 (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.
Chemical erosion Chemical erosion is the loss of matter in a landscape in the form of
solutes. Chemical erosion is usually calculated from the solutes found in streams.
Anders Rapp pioneered the study of chemical erosion in his work about
Kärkevagge published in 1960. Formation of
sinkholes and other features of karst topography is an example of extreme chemical erosion.
Glaciers (
Pirunpesä), the deepest ground erosion in
Europe, located in
Jalasjärvi,
Kurikka,
Finland ,
Bernese Alps, Switzerland
Glaciers erode predominantly by three different processes: abrasion/scouring,
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, such as erosion also control cross-valley variations. In a homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created. Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in
glaciated valleys. Scientists also 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 (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, 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 being the result tectonic forces, such as rock uplift, 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 m. 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 (especially in the
accumulation zone above the glacial equilibrium line altitude), which causes increased rates of erosion of the mountain, decreasing mass faster than
isostatic rebound can add to the mountain. This provides a good example of a
negative feedback loop. Ongoing research is showing that while glaciers tend to decrease mountain size, in some areas, glaciers can actually reduce the rate of erosion, acting as a
glacial armor. These processes, combined with erosion and transport by the water network beneath the glacier, leave behind
glacial landforms such as
moraines,
drumlins, ground moraine (till),
glaciokarst, kames, kame deltas, moulins, and
glacial erratics in their wake, typically at the terminus or during
glacier retreat. The best-developed glacial valley morphology appears to be restricted to landscapes with low rock uplift rates (less than or equal to 2mm per year) and high relief, leading to long-turnover times. Where rock uplift rates exceed 2mm per year, glacial valley morphology has generally been significantly modified in postglacial time. Interplay of glacial erosion and tectonic forcing governs the morphologic impact of glaciations on active orogens, by both influencing their height, and by altering the patterns of erosion during subsequent glacial periods via a link between rock uplift and valley cross-sectional shape.
Floods in
Cornwall after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode; leaving behind a tall sand bank in its place At extremely high flows,
kolks, or
vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called
rock-cut basins. Examples can be seen in the flood regions result from glacial
Lake Missoula, which created the
channeled scablands in the
Columbia Basin region of eastern
Washington. 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. Deflation is divided into three categories: (1)
surface creep, where larger, heavier particles slide or roll along the ground; (2)
saltation, where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3)
suspension, where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%). Wind erosion is much more severe in arid areas and during times of drought. For example, 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.
Mass wasting in
Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks
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 mass-wasting processes act very slowly; 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. 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. However, the term can also describe the rolling of dislodged soil particles in diameter by wind along the soil surface.
Submarine sediment gravity flows of submarine canyons in the
continental slope off the coast of New York and New Jersey On the
continental slope, erosion of the ocean floor to create channels and
submarine canyons can result from the rapid downslope flow of
sediment gravity flows, bodies of sediment-laden water that move rapidly downslope as
turbidity currents. Where erosion by turbidity currents creates oversteepened slopes it can also trigger underwater landslides and
debris flows. Turbidity currents can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock. Almost all continental slopes and deep ocean basins display such channels and canyons resulting from sediment gravity flows and submarine canyons act as conduits for the transfer of sediment from the continents and shallow marine environments to the deep sea.
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. ==Factors affecting erosion rates==