As ice wedges (also frost wedges ) forming in the permafrost vertical (vertical ) columns are called, which are primarily filled with ice. These are caused by thermal contraction: at low winter temperatures, the soil shrinks and pulls on at various points. In the columns to frost can form, also can snow as well as other material from entering. In the spring, the outstanding columns can also fill with melt water thawing of near-surface layers from freezing in the cold, deep soil. In the warmer summer months, the columns close again. By cyclically repeating this process, the V-shaped wedges enlarge. The width of the ice wedges may be less than 10 cm and more than three meters. Typically they range in depths of one to ten meters, but it can also be up to 25 meters. The columns do not necessarily fill with ice, sometimes it's a mix of ice and debris, sometimes just sand. The latter are also referred to as " sand wedges ".
At the surface resulting in a generally net-like pattern, the so-called Eiskeilpolygone, a special form of a frost pattern soil. The cell diameter of this polygon is typically between 10 and 40 meters; there are also smaller with only one to three meters in diameter and larger with diameters of up to 150 meters. In the plane is mainly based on three -to six- cornered shapes. Such patterns can be found not only on Earth, but also on the surface of Mars, where similar development processes are suspected.
Depending on the timing of the development processes of soil and ice wedges, three different types can be distinguished: Epigenetic ice wedges develop in an existing, stable permafrost, whereas syngeneic simultaneously with the ground, which increases in thickness by alluvial material or the like. The counterpart of the latter are anti- syngentische ice wedges that develop parallel to the erosion of the soil. Ice wedges can be active and continue to develop, or inactive if under current climatic or other conditions no further development is possible. When the ice formerly active ice wedges was melted and replaced by other material, these are known as Eiskeilmetamorphosen.
Shape and distribution
Ice wedges are with the striking polygonal patterns one of the most characteristic manifestations of periglacial landscape. After Segregationseis and Poreneis it is there, the third largest volume fraction of ground ice. They often occur in shallow unconsolidated sediment soils, but can also be found in rocky terrain or slopes. In unconsolidated sediment the cell size of the polygons is typically between 15 and 40 meters, in the rocky ground, they are smaller with 5 to 15 meters in diameter. Particularly suitable for the formation of Eiskeilpolygonen are poorly drained tundra plains with permafrost, such as in northern Canada, Alaska and the North Siberian Lowland. Also at the Greenland and Antarctic coastline and on the Arctic islands and the highlands of Tibet they occur, are there but not as common due to the dry climate.
In the arid regions of Antarctica and Greenland and in cold deserts of northern Canada, there are forms whose columns are filled instead with ice with sand or other material, the so-called " sand wedges ". In this case, strong winds and extreme drought play a significant role. These active forms can be distinguished from the inactive Eiskeilmetamorphosen in which the sand has to replace the consumable ice. There are also molds, in which the filling material consists of a mixture of ice and other material.
The Antarctic dry valleys represent an interesting area for research is Eiskeilpolygone is: First, because it, in the different micro- climates are all types there, as far as the filler material. On the other hand, because conditions than the marsähnlichsten the earth shall apply. This is of particular interest because there are corresponding on Mars polygonal pattern (see polygon structures on Mars ).
The now generally accepted theory that thermal contraction is the cause of the formation of ice wedges, was worked out scientifically in the 1960s by Arthur Lachenbruch, this indicates that Ernest de Koven Leffingwell already expressed this assumption the beginning of the 20th century clearly has. Accordingly, there is an analogy to the formation of desiccation cracks in muddy soil, the similarity of the resulting polygon pattern is no accident.
In winter, when the temperatures drop, "try" the a slowing upper soil layers to contract, but are held by the deeper, more stable layers in which hardly annual temperature fluctuations occur in their position. If the voltage is too high, cracks result in the wedge-shaped surface. It should be noted that the thermal expansion coefficient of ice five to ten times as large as that of most minerals - the ice fraction in the frozen ground therefore plays a crucial role. The columns are filled with snow and other material. In spring, still significantly colder deeper soil layers prevent the near-surface layers to expand again and so can melt water penetrate the outstanding columns and freeze. The additional material causes the typical swelling of the soil, if this expands in the summer months again and close the gaps. It is often claimed that about ten percent increase in volume of freezing water was decisive in this process. However, this effect is an order of magnitude smaller than the contraction and expansion, through the column completely opens and closes - which also this will be seen not least that exclusively sand-filled columns form similar forms.
First created in this way vertically oriented so-called " Eisvenen ". This process is repeated in the following winter and as existing primarily of ice fill of the former columns to withstand tensile stress less than the frozen soil, the soil breaks at the same positions again. However, the latter is not entirely self-evident when considering the Auftauboden, the layer that overlies the year-round frozen ground and ice wedges covered in the consideration involving with. At least with the initial crack is assumed that these arise at the soil surface, since there is the action of frost is greatest. In subsequent years, however, this can hardly run so because there would be no explanation of how the form on the surface cracks could "find" the present in the underlying permafrost former columns. Therefore, it is assumed to be initiated in the following years of the crack at the upper limit of the permafrost area, although this lower temperature swan customer is exposed to the surface. Another speaking against the same every year cracking effect is that often form ruts over the cracks, which are filled by an insulating layer of snow in winter. The lower tensile strength of the ice over the frozen ground seems these " disadvantages " but overcompensate.
By means of numerical models can understand that the ice wedges 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.
Ice wedges can be classified in various ways. On the one hand are active and distinguish inactive. Also, they differ in their filler material, this may be pure ice, mud or sand as well as freezing. Another important distinction differentiated according to relation in which the growth of the ground level at which the ice wedges is. Here, the following types are distinguished:
- Epigenetic ice wedges form and develop in an existing permafrost. This is typically flat and there is little erosion, or sedimentation. By the repeated breaking and filling of the wedge, it is wider in time, but no lower.
- Syngenetic ice wedges grow with by lifting by sedimentation permafrost. They are typically located in floodplains, marshes below or below Gelifluktionsablagerungen at the foot of a slope. Syngenetic ice wedges are both wider and higher, which predominates depends on in what relation do the Eiszuwachs and Sedimenationsrate.
- Antisyngenetische ice wedges to syngeneic counterpart to form on surfaces that are exposed to severe erosion. They grow downward, the upper compartment defrosts because lowers according to the lowering of the ground level and the beginning of the permafrost area. The oldest ice is thus located above, in contrast to syngeneic ice wedges, where the lowest part of Eiskeils is the oldest.
Inactive ice wedges
Ice wedges that are no longer break periodically and change in this way, are inactive. These can remain unchanged for centuries. Taut permafrost on first form thermokarst structures in the gutter system formed by the Eiskeilepolygone water collects. Depending on the flow rate of the original structures can be severely eroded, and the formation of tubular systems within the ice wedges is often the case.
If during the thawing ice wedges ice replaced by other filler material, caused ice wedge casts. This replacement process may well be gradual. When originally filled with other material as ice forms, the original filling are also preserved. In the mid-latitudes some of these structures have been found, they are an indication of former permafrost. In some cases, the polygonal pattern of former ice wedges reflect in vegetation, for example in the form of crop marks in cornfields. From the air, only relatively young structures are usually but to discover, as older are covered by sediments. In this way, it can be concluded on a 200 to 300 kilometer wide strip, occurred in the long-lasting permafrost during the Pleistocene in North America, this zone was significantly narrower in Western and Central Europe.
Age determination and climate
The harness evenly grown ice wedges reminiscent of annual rings. The trapped in milky white ice of the ice wedges bubbles suggest that using isotope analysis to draw conclusions about past climatic conditions can be drawn. However, in the use of Eiskeilpolygonen as climate archives some complications are observed. First of all it must be known whether epigenetic syngeneic or anti- syngeneic growth is present - it is even possible that this is different within the considered period. There are other disturbing effects, for example, can diapir -like buoyancy forces lead to deformation and periodic melting of the upper part.
Polygonal features on Mars
With the high-resolution images of Mars Global Surveyor (MGS ) and Mars Reconnaissance Orbiter (MRO ) and the recordings that were made by the Phoenix spacecraft in the landing area near the northern polar region, there is convincing confirmations of the ice wedges corresponding structures on Mars. In these areas also ground ice was determined by neutron spectrometer. In part, it seems to be to sand wedges, partly made the filling material may be a mixture of ice and other material. Pure ice wedges it does not seem to give.
Particularly often the " sublimation polygons ", a subtype of a sand wedge, which it also is in the Antarctic dry valleys apply. This can occur when a layer of massive ice - in which there are at least excess ice, so-called excess ice, - is covered by sediment or debris. In the resulting contraction cracks where the thicker ice is relatively free, is covered by coarse debris at best, which favors the sublimation of ice in the dry areas. This leads to a significant deepening of columns covered with sediment interstices are much higher and may be spherical shaped. In Antarctica, there are those polygons on moraines and ice core schuttbedecktem dead ice or glaciers. On Mars there are polygons, which has the possibility exists that they have been shaped by liquid water, similar to thermokarst processes on Earth.