Floor creep
Ground creep is a slow downward movement of a hill or mountain slope without the formation of demolition forms. The decisive factor for this is the continuous flow of movement. Soil creep arises in loose material by expansion and contraction of the material by freeze-thaw cycles ( Regelationsfließen ) and the swelling and shrinking of clayey material, or overlapping in soft, deformable rocks by the load rock layers (Continuous creep, engl. Continuous creep ). In terms of importance, because of the very low rates of movement, ground creep can also be caused by displacement as a result of the impact of raindrops ( splash creep ) or the formation of kammeis .
overview
The processes of soil creep can be traced back to various causes, in contrast to solifluction, however, not to flow due to water saturation. The displacement denudation is caused by the displacement of material on inclined surfaces, such as mountain slopes, during the expansion and shrinkage of loose material or lifting due to ice formation. Continuous creep is caused by the deformation of soft rock under load, and splash creep is caused by the impact of raindrops.
In contrast to denudation by sliding or sliding, floor creeping does not take place on a specific sliding surface. Thus, the downhill material displacement is not the same at every point of the sliding mass; the offset amounts are usually highest at the surface and lowest near the initial surface. It is a seamless, plastic deformation.
Ground creep is first recognizable by the sickle growth of trees and the hooking of rock layers, sometimes also by subsidence on the lower slope, on plaiks and increased erosion during heavy rain . The creeping speed is on the order of one to two centimeters per year, but can be significantly lower.
Offset denudation
The displacement denudation results in a downhill movement in that the soil particles are displaced downhill (hence the term). In the case of displacement denudation through expansion and contraction, the soil material expands perpendicularly to the slope and contracts again later. However, the movement is no longer reversed perpendicular to the slope, but following the force of gravity vertically downwards, i.e. diagonally downwards.
This directed movement depends on the following factors:
- the amount of volume increase,
- the slope - the greater it is, the greater the amounts of movement,
- and the frequency of the expansion and contraction cycles or the precipitation events.
The amount of descent is greatest at the surface and decreases to the base of the freezing and thawing soil. This is due to the fact that the expansion of the soil adds up from bottom to top, so that the top soil particles are lifted the furthest from their original position and are therefore also moved the furthest down the slope when they sink.
Ground creep due to alternation of frost
Ground creep when there is a change in frost ( regulation flow , derived from regulation ) is caused by the freezing of ground water and the resulting increase in volume ( ice has a larger volume than water ). It occurs in freezing climates and when the soil is sufficiently moist. The expansion of the loose material is only possible upwards and corresponds to the change in volume of the water present in the soil pores. This process is the type of floor creep with the greatest amounts of movement in temperate latitudes .
Floor creep due to swelling and shrinking
The creep of the soil due to swelling and shrinking is just as much a result of alternating expansion and shrinkage as with regular flow. In this case, the increase in volume results from the absorption and release of water from hydrophilic , swellable clay minerals (especially montmorillonites ). Swellable clay minerals are sheet silicates that have the ability to store water in their crystal lattice and to release it again. This process is associated with an increase and decrease in volume. With alternating dehydration and moisture penetration, similar to regular flow, the result is an offset down the slope. Since the sinking of the soil particles when drying out due to the cohesion (cohesion) of the clay-rich soil material is not only directed vertically downwards following the force of gravity, but always contains an upward component perpendicular to the slope, the offset amount is lower than in the case of regular flow.
Ground creep due to precipitation
Offset denudation by precipitation is of a completely different nature. During heavy rain, soil particles are thrown away in all directions by the impact of raindrops. If this happens on inclined surfaces, the trajectory of the soil particles is greatest down the slope, so that the uppermost soil particles move down the slope overall. This process occurs especially with fine soil material with a small grain size ( sand , silt , clay ). If large raindrops hit the edge, even coarse soil particles, including gravel , can start moving. In their case, the strength of the impact is not sufficient to throw the gravel through the air, but there is a slight shift away from the location of the impact. If this happens on an inclined surface, a downhill movement occurs in this case too.
Ground creep due to hoof load
The alternately recurring hoof load from grazing animals can also trigger ground creep.
Floor crawling through Kammeis
When kammeis are formed as a result of the sublimation of water in the uppermost soil layers, the uppermost soil particles are raised by the ice needles growing like a comb. When the ice thaws, the soil particles sink vertically downwards or the needles fall downhill, creating a downhill movement. Since this process is limited to the uppermost soil layers, the amounts of this movement are only small. However, the soil crawling through Kammeis loosens this soil layer so that other erosion processes such as rinsing and blowing out can attack more easily.
Continuous creep
If soft, deformable rock layers are heavily stressed by overlying rock or soil layers, the slope can start moving due to the deformation of the material. This movement is very slow and depends on whether the material has a high clay content and the degree of moisture penetration. Furthermore, the steepness of the slope and the size of the load play an important role in whether the process gets going. In many cases, the continuation leads to creep of the continuous loss of strength of the overlying layers and thus to the formation of rockfalls , landslides and Slumps .
literature
- Frank Ahnert: Introduction to Geomorphology . 1st edition. Verlag Eugen Ulmer, Stuttgart 1996, ISBN 3-8252-8103-5 , p. 134 ff .
- Lexicon of Geosciences. 1. Bd A to Edi . Spectrum / Academic Publishing House, Heidelberg 2000, ISBN 3-8274-0299-9 .
