# Erosion

In Géomorphologie, the erosion is the process of degradation and transformation of the Relief, and thus of the rocks, which is caused by any external agent (thus other that the Tectonique).

A relief whose Modelé is explained mainly by erosion is known as “Relief of erosion. ” Factors of erosion sont :

• climate;
• the slope;
• physics (hardness) and the chemistry (solubility e.g.) of the rock;
• the absence or not of vegetable cover and the nature of the plants;
• tectonic history (fracturing for example);
• the action of the man (husbandries, urbanization).

Erosion acts at various rates/rhythms and can, on several tens of million years, to level the mountains, to dig valleys, to make move back cliffs

Natural phenomena violent one such as a Avalanche, a Lahar or a Orage can modify the landscape considerably in an almost instantaneous way.

## Mechanisms of erosion

In the processes of erosion, one generally distinguishes three phases distinctes :
• destruction of the rock material;
• transport;
• accumulation of the remains.

Erosion implies a surface disintegration of the Roche or ground called Météorisation. It occurs on the spot, and produced remains. The degree of erosion depends on the characters of the roche :

• of the Hardness for example: the most resistant rocks and minerals are the Diamant and the Corindon; the least resistant are the Talc, the Gypse and the Calcite. the presence of quartz returns the Granite more résistant : erosion will be thus more lente  ;
• of the capacity of Dilation of the rock;
• of its Chemistry.

#### Mechanical erosion

Mechanical disintegration occurs under the action of a physical force which tears off pieces of rock more or less volumineux :
• bursting due to the Freezing or heat;
• wear by frottement : Glacier, flow of water (Cavitation) or wind; they are the remains carted by these factors (rocks, gravels or sand) which are effective in the process of erosion. Mechanical erosion is particularly active in the cold mediums (gel and thaws) and/or arid.

##### Erosion by water
It is mechanical and chemical, with like principal altérations  ; Hydroclastie, Effect splash (impact of the water drops which falls on the ground), the Reptation, the Solifluxion. Erosion by water is reinforced by the slope (torrent S). On the littoral, it is necessary to take account of the waves and the current . In the river or channels, it is the Batillage which accelerates erosion.

If a fluid as the Eau runs it can take care of suspended particles. The sedimentation test is minimal speed that a flood must have to transport, rather than to deposit, sediments and is given by the law of Stoke :

$w= \ frac \left\{\left(\ rho_p- \ rho_f\right) gr^2\right\} \left\{4.5 \ driven\right\}$

where W is the sedimentation test, ρ is the Density (the indices p and F indicate particle and fluid respectively), G is acceleration due to gravity, R is the ray of the particle and μ is the dynamic viscosity of the fluid. If the rate of the flow is larger than that of deposit, the aggregate continues towards the downstream. As there are always different diameters in the flood, largest settle (Décantation) while being able to continue to go down by mechanisms like the saltation (collisions particle-wall), rolling and slipping whose traces are often preserved in the solid rocks, and, can be used to estimate the current velocity.

• the Ruissellement is the most frequent type of erosion on Earth. It can be concentrated (torrents, Oued S) or diffuse (water film resulting from the snow melt, erosion Littoral E).

• fluviatile erosion is produced by River.
• Hydroclastie : alternation moistening-desiccation
• Effect splash: impact of the water drops on the ground.
• the Erosion fluvioglaciaire: the ice exerts a strong pressure on itself which thus makes it fluid and erosive with stones.

##### Erosion by the wind

Wind erosion attacks the rocks by removing particles (Déflation, Abrasion) or by polishing surface (Corrosion). It is all the more effective as the obstacles are non-existent and that the wind is powerful, regular and charged with dust or spray.

##### Erosion related to the differences in temperature
In the areas of strong thermal amplitude (climate continental, polar, deserts, high mountain, etc), the thermal shocks repeated by the repetition of the cycles day/night , splits then made burst certain rocks, on various scales microphone and/or macroscopique  ; it is the Thermoclastie.

Erosion related to the temperature also utilizes water like agent of erosion in the presence of porous rocks and/or cracks which burst in the event of freezing. The Cryoclastie is an example of erosion by thermoclastie : the rock bursts because of alternation freezing-thawing of the water which infiltrates, when water freezes, it occupies more volume and exerts a force able to explode a rock. The pieces released by freezing are called Gélifract S. the cycle freezing/thaw is seasonal worker (in Siberia for example) or daily newspaper in high mountain.

They are the processes of the Gélifraction or Gélivation. In mountain, the gélifraction produces crumblings which accumulate in bottom of slope to form falls or stone drains.

#### Chemical erosion

The chemical decomposition of the rocks gives rise to “modelled disintegration” which one meets in the old solid masses the Appalachian Mountains, Massif Central.
• the principal process is dissolution, in particular limestones by the more or less acid rain, one speaks then about Karst.

The Dissolution is a form of Météorisation which affects primarily the solid masses limestones. It gives place to landscapes of Karst. Water, charged in organic acids and Carbon dioxide, infiltrates by the cracks and model the carbonated rocks; it constitutes a “complex of deterioration”. It releases the chemical elements of the rock in the form of Ion S dissolved in water. Indeed, contrary to the sandstone silicieux, limestones are particularly vulnerable to dissolution. Also, other rocks and minerals are soluble:
• less than 0,05 grams per liter of quartz
• 1 gram per liter for the Carbonate S
• 2,4 grams per liter of Gypsum
• 2,5 grams for the Sulfate S
• 300 grams per liter of Rock salt
• the chemical weathering modifies the chemical compounds of the roches : Hydration, Oxidation, Oxydoreduction, Hydrolysis.

• In the intertropical zone, the deterioration of the ic rocks Feldspath by scrubbing allows the formation of Latérite S, Roche S reds or brown consisted of Hydroxyde S of Aluminum and Fer and which forms a true armor on the surface of the plates of the hot and wet areas.
• the hydrolysis is the process of rupture of the chemical bonds of minerals. It gives rise to Oxyde S such as the Limonite, or of clays and ends up forming a ground.

#### The erosion caused by the living beings

• Micro-organism S
• Mollusc S punchers, Pholade S for example
• Végétaux can contribute to erosion by their roots for example
• the man: Deforestation, Ploughing S, various urbanizations

#### Exceptional and brutal phenomena

• Avalanche
• Landslide
• Seism
• volcanic Phenomena
• Lahars

## Transport

The transport of materials resulting from the disintegration of the rock is carried out either in dissolved form in the circulation of continental water, or in solid form. In this last case, it can be a question of gravitating processes acting at short distance by gravitating processes or of transport with more long distance when the materials are dealt with by agent of transport : glacier, water, wind. The transported materials can possibly be stored, creating sedimentary accumulations, before being again put moving. In the long run, they lead in the seas and the oceans.
• the material mass transported in dissolved form by continental water is important. It is the essential process of the ic areas Karst.
• Of multiple gravitating processes (crumbling, Avalanche, Snaking, Streaming, Solifluction) nourishes a coat of deterioration in the vicinity immediate of the source area. On the slopes or at their base, one finds alluvial cones, scree cones or screes.
• the glaciers transport materials of any size (erratic blocks, moraines, sands).

On the long run, the sedimentation of the remains gives rise to detrital rocks. The wind constitutes a formidable freight agent, in particular in the desert areas. The wind can also transport gravels and Sable (by Saltation) and of the silt S (by suspension) starting from zones of (deflation). They carry and deposit sometimes the Lœss to thousands of kilometers of their place of origin.

## Modelled erosion

Erosion uses the rock material and works very diverse forms.

#### The shapes in hollow

Erosion can dig the rock and give rise to modelled dissection  :

The Ravin ement affects the named landscapes " enfers" in France or Bad Lands in North America. Precipitations, while running on the slopes made up of common excavations (clay, sediments), dig drains and furrows.

#### Other forms

Erosion can give rise to modelled accumulation

#### The littoral

The retreat and the transformation of the littorals depend on very many facteurs :

• configuration of the coast;
• the nature of the rock;
• the force and orientation of the running S, the waves, the littoral drift and the Swell;
• the presence of Roller S;
• the Anthropisation.

One can thus have several cases of figure :

• littoral with cliff different according to the rocks;
• the Calanque S belong to the karstic relief;
• rias, river mouths and fjords;
• marshes, deltas, estuaries;
• dunes.

#### The erosion of the agricultural grounds

The erosion of the agricultural grounds produces crusts (gypseous or limestones), ferruginous armours and lateritic This erosion is due mainly to the action of the homme :

• the Clearing S;
• the agricultural methods intensive, the Monoculture, culture in row spaced, mechanization, ploughing, naked ground in wintry time, grubbing, furrows in the direction of the slope, etc  ;
• road and urban installations increase surfaces of streaming;
• it on Pasture: in the countries of the Sahel, the Désertification is the consequence of the overgrazing;
• the regrouping of the Years 1960, in France, has leads to the increase in the size of the pieces and correlatively to the removal of the hedges, the slopes and the ditches. Surfaces in cultures of spring, encouraged by the subsidies, increase (sunflower, corn, beet) and leave the ground to naked in winter. The sloping grounds are gradually colonized by the vine. Lastly, the destruction of the adventitious plants by the weedkillers leaves the ground to naked between the cultivated seedlings.
• the zones destroyed by the Incendie S are particularly exposed with erosion.

## Appendices

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