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This article is about frozen ground. For other meanings, see Permafrost (disambiguation).

In geology, permafrost or permafrost soil is a thermal condition where ground material stays at or below 0°C for two or more years. The presence of ice is not necessary, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts excess of the potential hydraulic saturation of the ground material. Most permafrost is located in high latitudes (e.g. North and South poles), but alpine permafrost exists at high altitudes.

The extent of permafrost can vary as the climate changes. Today, approximately 20% of the Earth's land mass is covered by permafrost (including discontinuous permafrost) or glacial ice. Overlying permafrost is a the layer of ground called the active layer that seasonally thaws during the summer. Plant life can be supported only within the active layer because growth can occur only in soil that is fully thawed for some part of the year. Thickness of the active layer varies by year and location but is typically 0.6 - 4 m (2 to 12 feet) thick. In areas of continuous permafrost and harsh winters the depth of the permafrost can be very great: 440m (1330 feet) at Barrow, Alaska, (600m (1970 feet) at Prudhoe Bay, Alaska, up to 726m (2382 feet) in the Canadian Arctic islands and as much as 1493m (4510 feet) in the northern Lena and Yana River basins in Siberia.


[edit] Continuous and discontinuous permafrost

Permafrost will typically form in any climate where the mean annual air temperature is less than the freezing point of water. Exceptions are found in moist-wintered forest climates, such as in Northern Scandinavia and North-Eastern Russia west of Ural, where winter snow acts as an insulating blanket. The bottoms of glaciers can also be free of permafrost, even though this is not the most common situation.

Typically the temperature of the ground will be on average less variable from season to season than the air temperature, and temperatures also tend to increase with depth. Thus, if the mean annual air temperature is only slightly below 0°C (32°F), permafrost will form only in spots that are sheltered — usually with a northerly aspect. This creates what is known as discontinuous permafrost. Usually, permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between -5 and 0 °C (23 to 32°F). In the moist-wintered areas mentioned before, there may not be even discontinuous permafrost down to -2 °C.

There are exceptions in unglaciated Siberia and Alaska where the present depth of permafrost is a relic of climatic conditions during glacial ages where winters were up to 11°C (20°F) colder than those of today. At mean annual soil surface temperatures below -5°C (23°F) the influence of aspect can never be sufficient to thaw permafrost and a zone of continuous permafrost forms. There are also "fossil" cold anomalies in the Geothermal gradient in areas where deep permafrost developed during the Pleistocene that still persists down to several hundred metres. The Suwalki cold anomaly in Poland led to the recognition that similar thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes throughout Poland. [1]

A line of continuous permafrost in the Northern Hemisphere (Frozen Ground 28, 2004, p5) is formed from the furthest-northward points at which permafrost sometimes melts or is interrupted by regions without permafrost; north of this line all land is covered by permafrost or glacial ice. The "line" of continuous permafrost lies further north at some longitudes than others and can gradually move northward or southward due to regional climatic changes. In the southern hemisphere, most of the equivalent line would fall within the Southern Ocean if there were land there; most if not all of the Antarctic continent is covered not with frozen soil but with glacial ice.

[edit] Permafrost extent

Image:Permafrost - polygon.jpg
Polygons on the ground.

Measurement of the depth and extent of permafrost may be an indicator of global warming as recent years (1998 and 2001) have seen record thawing of permafrost in Alaska and Siberia. In the Yukon, the zone of continuous permafrost has moved 100 kilometres poleward since 1899. However accurate records only go back 30 years. It is thought that the thawing of permafrost could exacerbate global warming through the release of methane and other hydrocarbons which are powerful greenhouse gases. It also could encourage erosion because permafrost lends considerable stability to the barren slopes in the Arctic.

At the Last Glacial Maximum, continuous permafrost covered a much greater area than it does today, covering all of ice-free Europe south to about Szeged and the Sea of Azov (then dry land) and China south to Beijing. In North America, only an extremely narrow belt of permafrost existed south of the ice sheet at about the latitude of New Jersey through southern Iowa and northern Missouri. In the southern hemisphere, there is some evidence for former permafrost from this period in central Otago and Argentine Patagonia, but it was probably discontinuous.

[edit] Time to form deep permafrost

Time taken for permafrost to reach depth
Time (yr) Permafrost Depth (m)
1 4.44
350 79.9
3500 219.3
35000 461.4
100000 567.8
225000 626.5
775000 687.7

It has been calculated that the time required to form the deep permafrost underlying Prudhoe Bay, Alaska is 500,740 years. This time extends over several glacial and interglacial cycles of the Pleistocene and suggests that the present climate of Prudhoe Bay is probably considerably warmer than it has been on average over that period. Such warming over the past 15,000 years is widely accepted. [2] The table to the right shows that the first hundred metres of permafrost forms relatively quickly but that deeper levels take progressively longer.

[edit] Construction on permafrost

Building on permafrost is difficult due to the heat of the building (or pipeline) melting the permafrost and sinking downwards. This sinking problem has three common solutions: using foundations on wood piles, building on a thick gravel pad (usually 1 to 2 meters (about 4 to 5 feet) thick), or using anhydrous ammonia heat pipes. The Trans-Alaska Pipeline System uses insulated heat pipes to keep the pipeline from sinking into the permafrost. Qingzang railway in Tibet was built using a variety of methods to keep the ground cool.

At the Permafrost Research Institute in Yakutsk, it has been found that sinking of large buildings into the frozen earth (known to the Yakuts before Yakutsk was even founded) can be prevented effectively by means of stilts extended down to a depth of about fifteen metres or more. At this depth the temperature does not change with the seasons but remains at about -5°C.

[edit] See also

[edit] External links

ca:Permafrost cs:Permafrost da:Permafrost de:Permafrostboden et:Igikelts es:Permafrost eo:Multjare frostiĝinta tero fr:Pergélisol it:Permafrost hu:Permafroszt nl:Permafrost ja:永久凍土 nn:Permafrost pl:Wieczna zmarzlina pt:Permafrost ru:Вечная мерзлота sk::Permafrost fi:Ikirouta sv:Permafrost zh:永久冻土


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