Patterned ground

Frost pattern floor (also patterned ground ) is a collective term for surface forms with regular structures, which are formed by the action of soil frost. For the emergence of various geomorphological processes are responsible, but above all thermal contraction and frost heave. This can cause a sorting of soil particles. Frost pattern soils occur mainly in the periglacial zone, but can also occur outside of these areas with intense frost.

For the very different forms of frost -patterned ground, there are various classifications, almost all go on the classification proposed by Albert Lincoln Washburn from. It is first differentiated between sorted and unsorted forms - the sorted are contrary to the unsorted sorted on the surface according to grain size and usually surrounded by stones. In German the term structure of soil used as a synonym for ground frost pattern is related only to the sorted patterned ground frost for a definition of Carl Troll, the unsorted forms are referred to as texture soil. Within this dichotomy, a distinction according to the form - here there are mainly stone rings, stone polygons and stone nets in the predominantly flat terrain, as well as stone stripes and stone garlands on inclined surfaces. The most common form in permafrost areas are Eiskeilpolygone, a special form of unsorted polygons.

Even on Mars show high-resolution images patterns corresponding to the frost -patterned ground on Earth, especially in polygonal form. In the areas where these patterns can be seen, ground ice has been shown elsewhere, some of these structures very similar to those on Earth.

Classification

The proposed in 1956 by Washburn classification is based on the appearance of the frost -patterned ground and unaware of the development process leading to this pattern. The division is not in the pedological sense. For this reason, it is variously detailed or slightly modified.

Assorted frost -patterned ground

Assorted frost -patterned ground, for which the term structure of soils used have a sorted particle size spectrum. The forms are thereby structured by large stones, inside the cells is finer material. The dimension of these structures, ranging from less than 20 centimeters, to large shapes having a plurality of meters in diameter.

The main forms are as follows:

Unsorted frost -patterned ground

Unsorted frost -patterned ground, also referred to as texture soils, have not sorted according to grain size. You have a homogeneous, mostly fine grained substrate. The pattern-forming role of stones in the sorted frost -patterned ground often plays the vegetation in the unsorted.

In the unsorted frost -patterned ground mainly following are distinguished:

Formation

There are some different formation processes that lead to the different forms of frost -patterned ground. In the unsorted forms, this seems to apply particularly, but for the unsorted stone rings assumed to various causes. Not all forms of today there is a generally accepted explanation.

Frost pattern floors are very common in areas with permafrost, there is the main cause in thermal contraction and the formation of ice wedges. However, repeated freezing and thawing processes play a role. Processes such as differentiating frost heave, Kryoturbation, kryostatischer pressure and diapirartige relocation of water saturated material are involved, on slopes and solifluction.

The trigger for the formation of the frequent in permafrost areas Eiskeilpolygone is thermal contraction. Through deep winter temperatures of the ground ruptures, and the columns are filled with snow, frost, re- freezing water or other material. At the higher summer temperatures, these cracks close again. In the following winter they tear at the same place again, as the predominantly composed of ice filling the former columns less tension endures as the frozen ground, so increasing the process. This is not the volume increase during freezing of water significantly, which is also because it can be seen that there are polygons in dry regions, the columns of which are filled with only sand. By means of numerical models help to understand that the contraction cracks form polygonal patterns whose shape and size depends mainly on the soil and the temperature differences. Decisive than the mean temperatures, however, are irregularly occurring rapidly drops in temperature. This makes it difficult to draw conclusions from the shape and size of the pattern to draw conclusions about the past climate.

In contrast to the Eiskeilpolygonen play in the sorted frost -patterned ground repeated freezing and thawing a crucial role. In a work published in 2003, it is demonstrated that the various forms of sorted patterned ground frost can be traced by means of a numerical model, if the principles of self-organization are applied. Here, the simulation model only two parameters was influenced, on the one hand the slope, on the other hand, the degree of " harassment " by neighboring cells. The assumed in this simulation model development process can be divided into several phases: First, the formation of ice lenses and frost heave, this resulted transported the stones against the direction of the effects of frost, so an upward trend, which is also known as freezing-. An isolated block in the bottom surface now represents an unstable and prone to interference state because the bottom limit of the stone - frost heave acting inconsistently - due to the different moisture under and next to the stone. This leads to the sorting and formation of separated stone and soil areas. Through the " pressure " that the expanding floor area continues to exert on the stone area, it now comes to a longitudinal extension of the stone area. Depending on the degree of competition by neighboring cells now form the different shapes, with strong competition at polygonal shapes, circular at low. With greater slope to strip yield.

Fossil frost -patterned ground

Frost pattern soils that have arisen in the past and which allow the climatic conditions of the present no creation or further development of such more formations are called fossil - in contrast to the active or recent frost -patterned ground. Examples are inactive Eiskeilpolygone, which are found in the former permafrost regions. If these thawing the ice was replaced by other filler material, it is called ice wedge casts. In some cases, the polygonal pattern of former ice wedges reflect in vegetation, for example in the form of crop marks in cornfields. In the middle latitudes, some of these structures are found, in particular, for example in meltwater valleys and areas with debris deposits from the Saale glaciation. From the air, only relatively young structures are usually but to discover, as older are covered by sediments. Other forms of frost -patterned ground are often still recognizable as areas with former Kryoturbation, but it is often not clear what form they originally met. Assorted frost -patterned ground found only relatively rare in the fossil record, possibly because they are not recognized as such.

Polygon -like structures on Mars

Already on pictures of the Viking missions of the 1970s polygonal structures have been found in the great northern plain. However, the lying between two and ten kilometers in diameter of the polygons excludes almost that there is an analogy to the terrestrial periglacial phenomena - tectonic faults were assumed to be the cause of these forms. With the high-resolution images of Mars Global Surveyor and Mars Reconnaissance Orbiter structures were visible that seem far more frost -patterned ground on Earth at the same. If you categorize these in terms of shape and cell size, resulting in many of these categories, that the respective forms are found at corresponding latitudes in the northern and southern hemisphere - which points to a climatic context. In these areas, it was also found with the neutron spectrometer ground ice. It is believed that many of these polygons in accordance with the result on the earth Eiskeilpolygonen frozen through thermal contraction of the soil. Whether and to what extent, in some cases, repeated freezing and thawing processes are involved is unclear. In particular, in the past, this would be possible, since the inclination of the axis of Mars - is subject to much greater fluctuations than that of the Earth's axis - due to the lack of stabilization by a large moon. In the last 10 million years ago also came inclinations up to 50 °, which extreme differences between summer and winter temperatures meant.

In the area near the northern polar region, where the Phoenix landed in 2008, to find polygons with a diameter of three to six meters, as they often are in the northern plain. For this it is assumed that they arise as the Eiskeilpolygone on Earth by thermal contraction. It is assumed that they are active, since the structures are quite pronounced. The columns are similar to those to be found in the Antarctic dry valleys polygons, mostly filled with sand and debris. According to calculations by a numerical model to polygons of this size result when a two- to six -inch-thick layer of ground ice is assumed. Handed it deeper, the polygons would be greater. In these latitudes there are also larger polygons with diameters of 20 to 25 meters. This can be explained in the same way, if one assumes an earlier emergence of these polygons at a time more extreme climate with a greater inclination of the axis of Mars, when the Bodeneisschicht had a thickness of ten to twelve centimeters. There are also regularly arranged clusters of rocks and boulders, through which a pattern with corresponding approximately to the polygons dimensions yields. It is believed that this sort of material with the development of the polygons related. Unlike the earthly sorted patterned ground frost this material sorting is obviously not due to any freezing and thawing nor frost heave.