Erosion

As erosion (from Latin erodere, dt nibble ) is called in geoscientific sense the destruction or creation of surface forms by the removal of more or less heavily weathered rocks including the soils ( see → soil erosion ). A distinction is essentially between their linear (linear ) and areal erosion.

Under their linear erosion is defined as the erosion of the earth's surface by runoff water in small, ephemeral rivulets ( grooves erosion) or in rivers ( then as fluvial or river erosion refers ). Another and presently on earth less widespread form their linear erosion is done by mountain glaciers ( Exaration or glacial erosion ). The resulting valley forms are V-shaped ( V-shaped valley ) in fluvial and U-shaped ( U-shaped valley ) in glacial erosion.

In contrast, extensive erosion occurs aeolian ( wind), marine ( by ocean surf and flow ), glacial ice through and occasionally directly by precipitation. The large-scale removal and leveling of entire land surface is called denudation.

Linear and extensive erosion are not completely distinguishable from each other and scale-dependent. Thus erosion of small-scale grooves can ultimately act also areally (especially eg soil erosion ). Also goes beyond the linear erosion of mountain glaciers in the extensive erosion of Inlandeisgletschern eg ice ages or in particularly high latitudes.

Basic

Natural erosion occurs with the participation of a flowing medium, it is in most cases either to liquid water, ice, or air in the form of wind.

The material of the substrate, which was previously often transformed by weathering physically and chemically to loose material ( rock debris, sand, soil) is discharged when the shear stress generated by the flowing medium more responsive to be eroded particles than the static friction between the particles. The flow rate required for this is the higher, the lower the density of the medium, that is, Erosion by wind erosion requires the same effect higher flow rates than the erosion by water. An exception is the erosion by ice. Although the density of ice is slightly lower than that of water, is carried out erosion by ice even at very low speeds. Here is more of the size and thus the mass of an ice body, usually a glacier, crucial for the erosion effect. In addition, no one speaks for the movement of ice " flows ", but of " flow" or "creep "

Eroded material that is already being carried by the flowing or flowing medium can increase its erosion effect significantly.

The extent of erosion, which is caused by the mechanisms mentioned above, also depends largely on the geology, say, the nature of the substrate from. The erosion effect is stronger the lower the erosion resistance of the substrate. Specifically, line-like erosion preference is to where the ground locally has the lowest erosion resistance.

The erosion resistance of the surface depends on the rock type, from which it is made. Unconsolidated material is more erodible than solid rock, sand that is more erodible than sandstone. Fine-grained material is more erodible than coarse-grained material, argillite that is more erodible than sandstone. Igneous rocks, such as basalt or granite are usually more difficult to erode than sedimentary rocks. Tectonic weakening of faults and fractures generally reduces the erosion resistance of the substrate.

Forms of erosion

Depending on erodierendem medium and its appearance differentiates:

River erosion ( fluvial erosion)

River erosion ( fluvial erosion), a linear erosion form, is the creation of incisions in the land surface by the action of flowing waters ( streams, rivers ); the extent of their effect is specifically dependent on:

• Amount of water of the watercourse
• Water turbulence and entrained material
• Existing terrain morphology (slope )

Starting point for any fluvial erosion is a source exit, from where the water, the force of gravity, flows into deeper terrain. The larger the gradient is, the greater the flow rate of the water and thus the stronger the erosion effect. A watercourse formed so over the course of millennia, a V- shaped valley ( V-shaped valley ).

Due to the material removal (also known as removal ) of the river cuts incessantlyit in source direction in the Underground. This process is called headward erosion and can result in special cases to Flussanzapfung.

The associated with the erosive action of a stream deepening of the riverbed is as deep erosion ( or bed erosion, as the sole, the deepest area of ​​the flow channel, is preferred eroded ), describes his widening after page as lateral erosion. For fast- flowing mountain rivers with steep gradients outweighs the deep erosion. In slow -moving rivers that descend outweighs the lateral erosion, which then usually leads to meandering.

Because the slope of a river decreases towards the mouth, and their ability to transport material is always lower. The strongest gradient difference experienced long rivers that originate in the high mountains, then go through a deep level, and finally flow into the sea. Can such flows initially carry pebbles and sand, there are in the mouth region usually only suspended. All material for which the transport power is no longer sufficient, is deposited on the path from the source to the mouth of the river bed ( sedimentation). The absolute base level of a river is the height level below which its erosion effect is zero, usually this is the sea level. In the course of the river lying plains and lakes can, however, represent local erosion bases with increased sedimentation rate.

About a given geological period it may be in an area come to several changes between erosion and Sedimentationsphasen, either caused by tectonic uplift and subsidence processes of the earth's crust and its effect on the flow gradient or the raising or lowering of the absolute base level of erosion due to fluctuations in sea level. Is more than sedimented eroded, this periodical change in the sedimentary sequence of the corresponding area are recorded by the appearance of Erosionsdiskordanzen. If more than eroded sediments, there is no sedimentary sequence for the considered period.

In a relatively short time periods may be subject to the erosive action of a river strong fluctuations. This is primarily for mountain rivers, which provide the bulk of their erosion work when they carry out intense or prolonged rainfall many times their usual amount of water.

Anthropogenic influence on the natural river erosion

The straightening and damming of streams and rivers, the drainage and landfill alluvial areas, the additional injection of water in rivers (eg sewage treatment plants ) or, in some cases, the artificial modification of riparian vegetation result in numerous rivers, at least locally in a change of flow regime, which manifests itself primarily in an increase in the flow rate. This causes an increase in the depth and bed erosion rate and thus an accelerated erosion of the flow channel of the affected waters out (in this context, the term bed erosion also specifically caused anthropogenic, accelerated erosion depth defined). This causes damage to the associated with the respective waters ecosystems, including by the dehydration of the increasingly above the water level, remaining natural flood plains, as well as greater risk of flooding of the regions in the lower reaches through the fast flowing flood the headwaters (see also → Anthropocene ).

Grooves erosion

Grooves erosion is a small scale with linear, areal large scale erosion form in which precipitation water surface in small, ephemeral rills downslope flows while expanding the material of the ground with it. In places where the water collects, it washes most of gutters. Often the discharged material to flow along with the water through the gutters into the nearest body of water. Groove erosion plays a role in broadening of valleys and mountains of the planarization

Grooves erosion is also an important mechanism in soil erosion.

A special gully, which occurs only in dry areas, the wadi ( Arroyo ).

Glacial erosion ( glacial erosion)

In areas with cold climates accordingly ( high mountains, polar regions) to form glaciers. These also move down the valley as the water of the rivers, but with only a few meters a year, but to other, equally significant erosion. In contrast to the generally V-shaped river valleys ( V-shaped valleys ) the glaciers produce U-shaped Talquerschnitte ( U-shaped valleys ), whose typical shape even after melting can still be close to their glacial origin.

Glacial erosion is also primarily linear. During ice ages, or in today's polar regions ( Antarctica, Greenland) glacier cover huge areas leading to their erosion effect is there area.

Abrasion ( marine erosion)

This erosion form attacks the mainland across the board and can be particularly well observed on cliffs. There the work of the surf on the cliff leads to the emergence of surf fillets and other similar cavities in the rock, the collapse with time. Thus, the shoreline is forced back inland and at sea level created an ever -widening area Abrasionsplatte (also: surf platform or Felsschorre ).

Abrasion is linear up area.

Wind erosion ( wind erosion )

Wind affects especially erosive if it carries with it a lot of material (dust, sand) ( aeolian transport), then the similar gnaws a sandblaster on the surrounding rock of the substrate ( see, eg, mushroom rocks ). This occurs primarily in arid areas ( desert ) on at low vegetation and strong physical weathering. Wind erosion is the entire surface.

When wind erosion distinguished by deflation (denoted blowing off of fine particles that are accumulated during the weathering ) and corrasion (designated the active erosion, so the abrasive effect on rocks and stones by sand particles).

Under terms, small forms of erosion

• Rinsing, and denudation, is extensive erosion by rainwater.
• Piping ( tube formation) is a form of internal erosion, resulting from the tubular channels.
• Efforation is erosion under high water pressure.
• Space erosion: surface destroyed by meteorite impacts, solar wind and cosmic rays - plays on Earth because of the dampening effect of the atmosphere and the magnetic field is less important, and is in the geology of astronomical significance

Submarine erosion

Besides all the above mentioned erosion processes occurring above the sea level, erosion also takes place below sea level. Best known is the line-like erosion of the continental slope by turbidity currents.

Erosion on other planets

Wherever similar physical conditions how on earth prevail, appropriate erosion processes can also take place on other planets. This is especially well known from the neighboring planet Mars, where gullies were observed which clearly go back to running water. It is not there but genuine fluvial erosion, because the water currents that formed these channels, are very short-lived due to the thin atmosphere of Mars.

But the most single both on Earth and Mars active erosion form is wind erosion, the wind speeds that can generate sufficient shear stress for the material handling, due to the low density of the atmosphere on Mars is many times need to be higher than on Earth.