Gravity erosion
"Mass wasting" is the down-slope movement of rock and sediments, caused mainly by the force of gravity. Mass wasting is an important part of the erosion process, as it moves material from higher elevations to lower elevations, where transporting agents like streams and glaciers can pick up the material and move it further down. Mass-wasting processes occur continuously on all slopes. Some of these processes take place 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." There are, however, more detailed ways of classifying landslides, reflecting the mechanisms responsible for the movement and the velocity at which the movement occurs. One visible topographical manifestation of a very slow type of landslide is a scree slope.
"Slumping" is a form of mass-wasting in which consolidated materials or rock layers move together a short distance down a slope. Slumping happens on steep hillsides, and often with materials like clay that, once released, may move rapidly downhill. The area left behind by the moving material often shows a spoon-shaped depression. In some cases, the slump is caused by water beneath the slope, weakening layers of rock above 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. It is usually not perceptible, except through extended observation. This term, however, is also used to describe the rolling of dislodged soil particles (0.5 to 1.0 mm in diameter) by wind along the soil surface.
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| A heavily eroded roadside near Ciudad Colon, Costa Rica |
Water erosion
"Splash erosion" is the detachment and airborne movement of small soil particles, caused by the impact of raindrops falling on the soil. "Sheet erosion" is the result of heavy rain on bare soil, where water flows as a sheet down a gradient, carrying soil particles. In any area where precipitation rates exceed the absorption and infiltration rates into the soil, runoff occurs. Surface runoff turbulence can often cause more erosion than the initial raindrop impact. "Gully erosion" occurs in places where water flows along a linear depression, eroding a trench or gully (a furrow in the soil that is more than one meter deep). |
| A nearly perfect sphere of granite, in Trégastel, Brittany. |
"Valley" or "stream erosion" takes place with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside. 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 (twists and turns) across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when greater amounts and faster-moving water is available to carry a larger sediment load. In such processes, erosion is caused not only by water but also by suspended abrasive particles, such as pebbles and boulders, as they traverse the surface.
At extremely high rates of flow, kolks (underwater vortices) are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features. Examples can be seen in the flood regions that resulted from glacial activity at Glacial Lake Missoula, which created the channeled scablands in the Columbia basin region of eastern Washington state.
Shoreline erosion
Shoreline erosion, on both exposed and sheltered coasts, occurs primarily through the action of currents and waves, but sea level (tidal) changes can also play a role. When the sheer energy of a wave hitting a cliff or rock breaks pieces off, this is known as wave pounding. "Abrasion" or "corrasion" is caused by waves launching seaload at the cliff. It is the most effective and rapid form of shoreline erosion. (Corrasion should not to be confused with corrosion, or the dissolving of rock by carbonic acid in seawater. Limestone cliffs are particularly vulnerable to this type of erosion). Finally, "attrition" is where particles/seaload carried by the waves are worn down, as they strike one another and rock surfaces like cliffs, making the material easier to wash away.
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| Coastal erosion at Happisburgh, Norfolk, England |
Sediment is transported along the coast in the direction of the prevailing current (known as "longshore drift"). Erosion occurs when the uptake of new sediment by the upcurrent is less than the amount being carried away. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form. 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 (or "spit"). Underwater sandbanks offshore may also protect parts of a coastline from erosion. Over the years, as the sandbanks gradually shift, the erosion may be redirected to impact different parts of the shore.
Ice erosion
Ice erosion is caused by the movement of ice, typically in the form of glaciers. As glaciers scrape down slopes, they break up rock and transport it, leaving moraines (accumulation of rock debris), drumlins (elongated hills or ridges), and glacial erratics (pieces of glacial residue) in their wake, typically at the glacial terminus or during glacial retreat.
"Ice wedging" is the weathering process in which water trapped in tiny cracks of rock freezes and expands, breaking the rock, allowing for gravity erosion on steep slopes. The scree that forms at the bottom of a steep mountainside is mostly formed from pieces of rock broken away by this means. It is a common engineering problem, wherever rock cliffs are alongside roads, because morning thaws can drop hazardous rock pieces onto the road.
In some places that are cold enough , water seeps into rocks during the daytime, then freezes at night. Ice expands, thus, creating a wedge in the rock. Over time, the repetition in the forming and melting of the ice causes fissures, which eventually breaks the rock down.
Wind erosion
Wind erosion, also known as "eolian erosion," is the movement of rock and sediment by wind. The wind causes dust particles to be lifted up, where they can be moved to other regions. Wind erosion generally occurs in areas with little or no vegetation, often in places where there is insufficient rainfall to support vegetation. An example is the formation of sand dunes on a beach or in a desert. Farmers often plant windbreaks to reduce wind erosion. This includes the planting of trees, shrubs, or other vegetation, usually perpendicular (or nearly so) to the principal wind direction.
Bioerosion
Bioerosion involves the erosion of hard ocean substrates by living organisms, utilizing a number of mechanisms. It can be caused by mollusks, polychaete worms, sponges, crustaceans, echinoids, and fish. It can occur on coastlines and coral reefs, and on the hulls of ships. Mechanisms of bioerosion include biotic boring, drilling, rasping, and scraping.
Bioerosion of coral reefs generates the fine and white coral sand characteristic of tropical islands. The coral is converted to sand by internal bioeroders such as algae, fungi, bacteria, sponges (Clionidae), bivalves (Lithophaga), sipunculans (Aspidosiphon), and polychaetes (Eunicidae), generating extremely fine sediment (10 to 100 micrometers in diameter). External bioeroders include urchins (Diadema) and chitons (Acanthopleura). The combination of these internal and external bioeroders results in a great deal of erosion. Sea urchin erosion of calcium carbonate (CaCO3) at some reefs has been reported to occur at annual rates exceeding 20 kg/m².
Fish also erode coral while eating algae. Parrotfish cause a great deal of bioerosion, as a result of their well developed jaw muscles and tooth armature, and a pharyngeal mill that grinds up ingested material into sand-sized particles
Tectonic effects of erosion
When erosion involves the removal of large amounts of rock from a particular region and its deposition elsewhere, it can lighten the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region. Research undertaken since the early 1990s suggests that the spatial distribution of erosion at the surface of an orogen (mountain-forming belt of rock along a tectonic plate) can exert a key influence on its development and its final internal structure. (NWE)
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