Ice wedge
As ice wedges (also Frost wedges ) are located in the permafrost referred forming vertical (vertical) columns that are mainly filled with ice. These arise through thermal contraction : at low winter temperatures the soil contracts and tears open in various places. Hoar frost can form in the crevices , and snow and other material can also penetrate. In spring, the still open crevices can also be filled with meltwaterfill the thawing near-surface layers that freeze in the cold, deeper soil. In the warmer summer months the crevices close again. By repeating this process cyclically, the V-shaped wedges enlarge. The width of the ice wedges can be less than 10 centimeters and more than three meters. They typically reach a depth of one to ten meters, but it can also be up to 25 meters. The crevices do not necessarily have to be filled with ice, sometimes it is a mixture of ice and rubble, sometimes just sand. The latter are also known as "sand wedges".
On the surface there is usually a net-like pattern, the so-called ice wedge polygons , a special shape of a frost pattern floor . The cell diameter of these polygons is typically between 10 and 40 meters; there are also smaller ones with only one to three meters in diameter and larger ones with diameters of up to 150 meters. Mainly triangular to hexagonal shapes are formed in the plane. Such patterns are not only found on Earth, but also on the surface of Mars , whereby similar formation processes are assumed.
Depending on the chronological sequence of the development processes of the soil and ice wedges, a distinction is made between three different types: Epigenetic ice wedges develop in an already existing, stable permafrost soil, while syngeneic ice wedges develop simultaneously with the soil that is gaining in thickness due to alluvial material or the like. The counterpart to the latter are anti-syngeneic ice wedges that develop parallel to the erosion of the soil. Ice wedges can be active and continue to develop, or inactive if no further development is possible under the current climatic or other conditions. When the ice of formerly active ice wedges has melted and replaced by other material, these are known as ice wedge metamorphoses .
Form and spread
Ice wedges with their conspicuous polygonal patterns are one of the most characteristic manifestations of the periglacial landscape. After segregation ice and pore ice , it forms the third largest volume fraction of the ground ice there . They often occur in shallow loose sediment soils , but can also be found in rocky subsoil or slopes. In the loose sediment, the cell size of the polygons is typically between 15 and 40 meters, in the rocky subsoil they are smaller with a diameter of 5 to 15 meters. Badly drained deep tundra plains with permafrost, such as in northern Canada , in Alaska and in the northern Siberian lowlands, are particularly suitable for the formation of ice wedge polygons . They are also found on the Greenland and Antarctic coasts as well as on the Arctic islands and the highlands of Tibet , but are not as common there due to the dry climate.
In the dry areas of Antarctica and Greenland, as well as in the cold deserts of northern Canada, there are shapes whose crevices are filled with sand or other material instead of ice, the so-called "sand wedges". Strong winds and extreme drought play an important role here. These active forms are to be distinguished from the inactive ice wedge metamorphoses, in which the sand has replaced the melting ice. There are also shapes in which the filling material consists of a mixture of ice and other material.
The Antarctic dry valleys represent an interesting area for research. Ice wedge polygons: On the one hand, because there are all types of filler material in the various microclimate zones . On the other hand, because the conditions there are considered to be the most Mars-like on earth. This is of particular interest because there are corresponding polygonal patterns on Mars (see polygonal structures on Mars ).
Emergence
The now generally accepted theory that thermal contraction is the cause of the formation of ice wedges was scientifically worked out in the 1960s by Arthur Lachenbruch , who points out that Ernest de Koven Leffingwell clearly expressed this assumption at the beginning of the 20th century Has. Accordingly, there is an analogy to the formation of dry cracks in muddy ground, so the similarity of the resulting polygonal patterns is no coincidence.
When temperatures drop in winter, the cooling upper soil layers “try” to contract, but are held in place by the deeper, more stable layers, where there are hardly any annual temperature fluctuations. If the tension becomes too great, wedge-shaped cracks appear on the surface. It should be noted that the coefficient of thermal expansion of ice is five to ten times that of most minerals - the proportion of ice in the frozen ground therefore plays a decisive role. The crevices fill with snow and other material. In spring, the significantly colder deeper soil layers prevent the layers near the surface from expanding again and so meltwater can penetrate into the still open crevices and freeze. The additional material causes the typical bulging of the floor when it expands again in the summer months and the crevices close. It is often claimed that the approximately ten percent increase in volume of the freezing water is decisive for this process. However, this effect is an order of magnitude smaller than the contraction and expansion through which the crevice opens and closes completely - which is also evident from the fact that crevices filled exclusively with sand form similar shapes.
First of all, vertically oriented so-called "ice veins" are created in this way. This process is repeated in the following winters and since the predominantly ice filling of the former crevices can withstand less tension than the frozen ground, it tears open again in the same places. The latter, however, is not entirely self-evident if one takes into account the thawing soil, the layer that overlays the year-round frozen soil and covers the ice wedges. At least in the case of the initial cracks, it can be assumed that they will develop on the surface of the ground, since this is where the frost effect is greatest. In the following years, however, this can hardly happen because there would be no explanation as to how the cracks on the surface could “find” the former crevices in the permafrost area below. It is therefore assumed that in the following years the crack will be initiated at the upper limit of the permafrost area, although this is subject to lower temperature fluctuations than the surface. Another effect that speaks against cracking, which is the same every year, is that furrows often form over the cracks, which in winter are filled by an insulating layer of snow. The lower tear resistance of the ice compared to the frozen ground seems to overcompensate for these “disadvantages”.
Using numerical models, it can be understood that the ice wedges form polygon-like patterns, the shape and size of which mainly depends on the nature of the ground and the temperature differences. More decisive than the mean temperatures, however, are irregular rapid drops in temperature. This makes it difficult to draw conclusions about past climatic developments from the shape and size of the patterns.
Classification
Ice wedges can be classified in several ways. On the one hand, a distinction must be made between active and inactive. You can also differentiate between their filling material, this can be pure ice, but also icy mud or sand. Another important distinction differentiates according to the relationship between the growth of the ground level and that of the ice wedges. A distinction is made between the following types:
- Epigenetic ice wedges form and develop in an already existing permafrost. This is typically flat and there is hardly any erosion or sedimentation . By repeatedly breaking open and filling the wedge, it becomes wider over time, but hardly deeper.
- Syngeneic ice wedges grow with the permafrost that rises through sedimentation. They typically form in floodplains, beneath bogs, or beneath gelifluction deposits at the foot of a slope. Syngeneic ice wedges become both wider and higher, which predominantly depends on the relationship between the rate of ice growth and sedimentation.
- Antisyngenic ice wedges , the counterpart to syngeneic ones , form on surfaces that are exposed to severe erosion. They grow downwards, the upper part thaws, since the beginning of the permafrost area sinks as the ground level drops. The oldest ice is thus on top, in contrast to syngeneic ice wedges, where the lowest part of the ice wedge is the oldest.
Inactive ice wedges
Ice wedges that no longer periodically break and change in this way are inactive. These can remain unchanged for centuries. When the permafrost thaws, thermokarst structures initially form , and water collects in the channel system formed by the ice wedge polygons. Depending on the flow rate, the original structures can be severely eroded, and the formation of tube systems within the ice wedges also occurs frequently.
If the thawing ice in ice wedges is replaced by other filler material, ice wedge pseudomorphoses arise. This replacement process can be gradual. In the case of molds originally filled with material other than ice, the original filling can also be retained. Some such structures were found in the middle latitudes , they are an indication of former permafrost . In some cases, the polygonal pattern former ice wedges reflect in the vegetation, for example in the form of crop marks in cornfields. Usually only relatively young structures can be seen from the air, as older ones are covered by sediments. In this way one can infer an approximately 200 to 300 kilometer wide strip in Western and Central Europe , in which long-lasting permafrost occurred during the Pleistocene , in North America this zone was significantly narrower.
Age determination and climate
The banding of evenly grown ice wedges is reminiscent of annual rings. The air bubbles trapped in the milky-white ice of the ice wedges suggest that isotope analysis can be used to draw conclusions about previous climatic conditions. However, there are some complications to be considered when using ice wedge polygons as climate archives. First of all, it must be known whether epigenetic, syngenetic or anti-syngenetic growth is present - it is even possible that this is different within the period under consideration. There are other disturbing effects, for example diapir- like buoyancy forces can lead to deformation and periodic melting of the upper part.
Polygon structures on Mars
With the high-resolution images of the Mars Global Surveyor (MGS) and the Mars Reconnaissance Orbiter (MRO), as well as the images made by the Phoenix spacecraft in the landing area near the northern polar region, there is convincing confirmation of structures on Mars that correspond to ice wedges . Ground ice was also detected in these areas using a neutron spectrometer . Some of them appear to be wedges of sand, and some of the filling material may consist of a mixture of ice and other material. There does not seem to be any pure ice wedges.
The " sublimation polygons", a subtype of a sand wedge that also exist in the Antarctic dry valleys , are considered particularly common . These can arise when a layer of more massive ice - in which there is at least excess ice, so-called excess ice - is covered by sediment or rubble. In the contraction cracks that develop there, the more massive ice is relatively exposed, at best it is covered by coarse debris, which favors the sublimation of the ice in the dry areas. This leads to the clear deepening of the crevices, the spaces covered with sediment are significantly higher and can be rounded. In the Antarctic there are such polygons over moraines with ice cores and debris-covered dead ice or glaciers . There are also polygons on Mars that have the potential to have been shaped by liquid water, similar to the thermokarst processes on Earth.
literature
- Hugh M. French: The Periglacial Environment. 3. Edition. Wiley Publishing, Chichester 2007, ISBN 978-0-470-86588-0 .
- Albert L. Washburn: Geocryology. Edward Arnold Publishers, London 1979, ISBN 0-7131-6119-1 .
Web links
Individual evidence
- ^ Pratima Pandey: Active Ice Wedge. In: Vijay P. Singh, Pratap Singh, Umesh K. Haritashya (Eds.): Encyclopedia of Snow, Ice and Glaciers. Springer, Dordrecht 2011, ISBN 978-90-481-2641-5 , p. 4.
- ↑ geodz.com: Ice wedge. . Retrieved May 28, 2013.
- ^ AL Washburn: Classification of patterned ground and review of suggested origins. In: Bulletin of the Geological Society of America. Volume 67, 1956, pp. 823-865. ( Summary )
- ↑ Julia A. Jackson, James P. Mehl, Klaus KE. Neuendorf: Glossary of geology. Springer Verlag, Berlin 2005, ISBN 0-922152-76-4 . ( Google books )
- ↑ a b c d J. Ross Maxkay: Some Observations on the Growth and Deformation of Epigenetic, Syngenetic and Anti-Syngenetic Ice Wedges. In: Permafrost and Periglacial Processes. Volume 1, 1990, pp. 15-29, ( doi: 10.1002 / ppp.3430010104 )
- ^ A b French: The Periglacial Environment. 2007, pp. 176-181.
- ^ A b French: The Periglacial Environment. 2007, pp. 117-127.
- ^ A b David R. Marchant, James W. Head: Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars. In: Icarus. Volume 192, 2007, pp. 187-222. ( online ; PDF; 10.9 MB)
- ↑ a b c Arthur H. Lachenbruch: Contraction theory of ice-wedge polygons: A qualitative discussion. In: National Academy of Science – National Research Council: Proceedings of the Permafrost International Conference 1963. Washington, DC 1966. ( Google books )
- ^ LJ Plug, BT Werner: A numerical model for the organization of ice-wedge networks. In: Permafrost Seventh International Conference Proceedings. Vol. 55, 1998, pp. 897-902. ( online ; PDF; 1.1 MB)
- ^ LJ Plug, BT Werner: Nonlinear dynamics of ice-wedge networks and resulting sensitivity. In: Nature. Volume 417, 2002, pp. 929-932. ( online ( Memento of the original from October 18, 2012 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this note .; PDF; 476 kB)
- ↑ French: The Periglacial Environment. 2007, pp. 193-201.
- ↑ French: The Periglacial Environment. 2007, pp. 310-315.
- ^ Nicolas Mangold: High latitude patterned grounds on Mars: Classification, distribution and climatic control. In: Icarus. Volume 174, 2005, pp. 336-359. ( online ( Memento of the original from April 29, 2014 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this note .; PDF; 2.6 MB)
- ↑ a b Joseph S. Levy, David R. Marchant, James W. Head: Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies. In: Icarus. Volume 206, 2010, pp. 229-252. ( online ( Memento of the original from April 29, 2014 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this note .; PDF; 3.4 MB)
