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permafrost
permafrost Permafrost is ground that remains at or below 0°C for at least 2 years. About one-quarter of the Earth's land surface is underlain by permafrost, including over half of Canada and Alaska, the Tibetan Plateau, and north-eastern Russia. Permafrost that formed during the last glacial period when the sea level was lower is also found on the continental shelf of the Arctic Ocean in the Beaufort, Kara, Laptev, and East Siberian Seas. Continental permafrost regions are usually divided into zones of continuous, widespread discontinuous, and scattered discontinuous permafrost, where respectively over 80 per cent, 30–80 per cent, and less than 30 per cent of the terrain is underlain by perennially frozen ground. In the northern hemisphere, the southerly boundaries of these zones are defined by the mean annual air temperature isotherms of –5, –4, and 0 °C, respectively. The zones not only indicate the spatial extent of permafrost, but also its temporal stability, for short-term climatic changes may lead to its eradication or development in the scattered zone, while, further north, permafrost is continuous in space and time. Alpine, or plateau, permafrost is found at high elevations in mountain regions throughout the world.
Permafrost terrain comprises a seasonally thawed active layer underlain by perennially frozen ground. The thickness of the active layer varies from a few centimetres to several metres, depending on summer conditions and the thermal properties of ground materials. The thickness is greatest in dry sand and bedrock, and least in moist, organic soils. The total thickness of permafrost depends on the annual mean temperature at the base of the active layer, the geothermal flux, and the thermal conductivity of ground materials. The heat flux is equal to the product of conductivity and geothermal gradient, represented by the near-surface temperature divided by the depth to the base of permafrost. Permafrost thicknesses vary from a metre or so at the southerly margins of the scattered permafrost zone to over 1000m in Arctic Canada and Russia. In the discontinuous zones, snow depth, soil moisture, and soil organic matter content dominate environmental variables responsible for the presence or absence and temperature of permafrost. The geotechnical significance of permafrost is derived from the occurrence of ground ice, often close to its melting point. The ice may be buried relict glacier ice, or intrasedimental ice formed either by segregation during permafrost development or by intrusion of water under pressure into frozen ground. Ice wedges may form at the base of the active layer after snow melt infiltrates thermal contraction cracks. Characteristically the uppermost layers of permafrost are ice-rich owing to these wedges, and to water incorporated into permafrost since the Early Holocene, when active layers were generally deeper. The presence of such near-surface ground ice renders the terrain liable to subsidence and ponding or to hill-slope failure if vegetation is disturbed and the active layer deepens. Exceptional care is therefore taken to preserve the integrity of permafrost during construction activities. Since permafrost is a climatic phenomenon, climatic changes lead to an altered permafrost regime. The depth of permafrost development and degradation follows the square root of time since the change occurred. Ice-rich ground over 10 m thick requires centuries to millennia for complete development or thawing, and for other adjustments to permafrost thickness. The response of permafrost temperatures to climate warming is usually rapid until the ground reaches –2 to −1 °C, at which point thawing of pore water has begun. The latent heat required for further increases in ground temperature then slows the warming. The terrain response to climate warming may, however, be rapid, since thickening of the active layer will lead to melting of the abundant near-surface ground ice. This is a significant issue in considering the response of permafrost terrain to potential global warming. C. R. Burn |
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Cite this article
PAUL HANCOCK and BRIAN J. SKINNER. "permafrost." The Oxford Companion to the Earth. 2000. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. PAUL HANCOCK and BRIAN J. SKINNER. "permafrost." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O112-permafrost.html PAUL HANCOCK and BRIAN J. SKINNER. "permafrost." The Oxford Companion to the Earth. 2000. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-permafrost.html |
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Permafrost
PermafrostAbout 20% of Earth's surface is covered by permafrost, land that is frozen year-round. Permafrost occurs at high latitudes or at very high altitudes—anywhere the mean annual soil temperature is below freezing . About half of Canada and Russia, much of northern China, most of Greenland and Alaska, and probably all of Antarctica are underlain by permafrost. Areas underlain by permafrost are classified as belonging to either the continuous zone or the discontinuous zone. Permafrost occurs everywhere within the continuous zone, except under large bodies of water , and underlies the discontinuous zone in irregular zones of varying size. Fairbanks, Alaska, lies within the discontinuous zone, while Greenland is in the continuous zone. The surface layer of soil in a permafrost zone may thaw during the warmer months, and the upper layer of the frozen zone is known as the permafrost table. Like the water table , it may rise and fall according to environmental conditions. When the surface layer thaws, it often becomes waterlogged because the meltwater can only permeate slowly, or not at all, into the frozen layer below. Partial melting coupled with irregular drainage leads to the creation of hummocky topography . Walking on permafrost is extremely difficult, because the surface is spongy, irregular, and often wet. Waterlogging of the surface layer also causes slopes in permafrost areas to be unstable and prone to failure. Permafrost provides a stable base for construction only if the ground remains frozen. Unfortunately, construction often warms the ground, thawing the upper layers. Special care must be taken when building in permafrost regions, and structures are often elevated above the land surface on stilts. The Trans-Alaska Pipeline, along much of its length, is elevated on artificially cooled posts, and communities in permafrost regions often must place pipes and wires in above-ground conduits rather than burying them. Even roads can contribute to warming and thawing of permafrost, and are generally built atop a thick bed of gravel and dirt. See also Creep |
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"Permafrost." World of Earth Science. 2003. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. "Permafrost." World of Earth Science. 2003. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1G2-3437800454.html "Permafrost." World of Earth Science. 2003. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437800454.html |
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permafrost
permafrost permanently frozen soil, subsoil, or other deposit, characteristic of arctic and some subarctic regions; similar conditions are also found at very high altitudes in mountain ranges. In 1962 measurements in a borehole drilled on Melville Island, Northwest Territories, Canada, showed that the ground was frozen to a depth of at least 1,475 ft (450 m); comparable thicknesses have been found in other far north regions. Tundras , though underlaid by permafrost, today support centers of population in Alaska, Canada, and Siberia. Permafrost is a very fragile system that may easily be damaged or destroyed by the presence of man-made heat. A controversy developed in the late 1960s and early 70s over the construction of an oil pipeline from the Alaska North Slope to the southern part of the state. Critics of the project argued that if the pipeline containing hot oil ever came into contact with the permafrost, it would melt the permafrost; the pipeline would then sink and eventually break. The oil spilled during the breakage would result in a major ecological disaster. It was decided to build the pipeline with insulated pipe raised above the permafrost or on gravel beds in order to prevent melting and thus preserve both the pipeline and the ecosystem. |
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"permafrost." The Columbia Encyclopedia, 6th ed.. 2008. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. "permafrost." The Columbia Encyclopedia, 6th ed.. 2008. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1E1-permafro.html "permafrost." The Columbia Encyclopedia, 6th ed.. 2008. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-permafro.html |
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pergelic
pergelic The lowest of the soil-temperature classes for family groupings of soils in the soil taxonomy system, applied to soils in temperate regions. The assessment of soil temperature is based on the mean annual temperature, and on the difference between mean summer and mean winter temperature, measured at a depth of 50 cm or at the surface of the underlying rock, whichever is shallower. In order of ascending temperature, the higher-temperature classes in temperate-region soils are called cryic, frigid, mesic, thermic, and hyperthermic, and in tropical regions the scale from cold to hot is isofrigid, isomesic, isothermic, and isohyperthermic.
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AILSA ALLABY and MICHAEL ALLABY. "pergelic." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "pergelic." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O13-pergelic.html AILSA ALLABY and MICHAEL ALLABY. "pergelic." A Dictionary of Earth Sciences. 1999. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-pergelic.html |
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pergelic
pergelic The lowest of the soil-temperature classes for family groupings of soils in the USDA Soil Taxonomy system, applied to soils in temperate regions. The assessment of soil temperature is based on mean annual temperatures, and on differences between mean summer and mean winter temperatures, measured at a depth of 50 cm or at the surface of the underlying rock, whichever is shallower. Higher temperature classes in temperate region soils are called cryic, frigid, mesic, thermic, and hyperthermic, and in tropical regions the scale from cold to hot is isofrigid, isomesic, isothermic, and isohyperthermic.
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MICHAEL ALLABY. "pergelic." A Dictionary of Ecology. 2004. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "pergelic." A Dictionary of Ecology. 2004. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O14-pergelic.html MICHAEL ALLABY. "pergelic." A Dictionary of Ecology. 2004. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-pergelic.html |
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pergelic
pergelic The lowest of the soil-temperature classes for family groupings of soils in the USDA Soil Taxonomy system, applied to soils in temperate regions. The assessment of soil temperature is based on mean annual temperatures, and on differences between mean summer and mean winter temperatures, measured at a depth of 50 cm or at the surface of the underlying rock, whichever is shallower. Higher temperature classes in temperate region soils are called cryic, frigid, mesic, thermic, and hyperthermic, and in tropical regions the scale from cold to hot is isofrigid, isomesic, isothermic, and isohyperthermic.
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MICHAEL ALLABY. "pergelic." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "pergelic." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O7-pergelic.html MICHAEL ALLABY. "pergelic." A Dictionary of Plant Sciences. 1998. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O7-pergelic.html |
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permafrost
permafrost(pergelisol) The permanently frozen ground which occupies some 26 per cent of the Earth's land surface under thermal conditions where temperatures below 0°C have persisted for at least two consecutive winters and the intervening summer. Considerable thicknesses may develop (e.g. 600 m on the North Slope of Alaska and 1400 m in Siberia, but these are partly relicts of the last glaciation). Permafrost may contain an unfrozen unit, called ‘talik’, and may be overlain by an active layer. The permafrost may be continuous, discontinuous, intermittent, or sporadic.
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Cite this article
MICHAEL ALLABY. "permafrost." A Dictionary of Ecology. 2004. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "permafrost." A Dictionary of Ecology. 2004. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O14-permafrost.html MICHAEL ALLABY. "permafrost." A Dictionary of Ecology. 2004. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-permafrost.html |
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permafrost
permafrost (pergelisol) Permanently frozen ground which occupies some 26% of the Earth's land surface under thermal conditions where temperatures below 0°C have persisted for at least two consecutive winters and the intervening summer. Considerable thicknesses may develop, e.g. 600 m on the North Slope of Alaska, and 1400 m in Siberia, but these are partly relict from the last glaciation. Permafrost may contain an unfrozen unit, called ‘talik’, and may be overlain by an active layer.
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Cite this article
AILSA ALLABY and MICHAEL ALLABY. "permafrost." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "permafrost." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O13-permafrost.html AILSA ALLABY and MICHAEL ALLABY. "permafrost." A Dictionary of Earth Sciences. 1999. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-permafrost.html |
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permafrost
permafrost Land that is permanently frozen, often to a considerable depth. The top few centimetres generally thaw in the summer, but the meltwater is not able to sink into the ground because of the frozen subsoil. If the landscape is fairly flat, surface water lies on the ground throughout the summer. Construction work is very difficult, and many methods have been employed in Russia, Canada and Alaska to overcome the problems. See also tundra
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"permafrost." World Encyclopedia. 2005. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. "permafrost." World Encyclopedia. 2005. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O142-permafrost.html "permafrost." World Encyclopedia. 2005. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-permafrost.html |
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permafrost
per·ma·frost / ˈpərməˌfrôst; -ˌfräst/ • n. a thick subsurface layer of soil that remains frozen throughout the year, occurring chiefly in polar regions. |
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"permafrost." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. "permafrost." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O999-permafrost.html "permafrost." The Oxford Pocket Dictionary of Current English. 2009. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-permafrost.html |
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permafrost
permafrost permanently frozen subsoil. XX. f. next, FROST.
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T. F. HOAD. "permafrost." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. T. F. HOAD. "permafrost." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O27-permafrost.html T. F. HOAD. "permafrost." The Concise Oxford Dictionary of English Etymology. 1996. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O27-permafrost.html |
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permafrost
permafrost •accost, cost, frost, lost, Prost, riposte
•teleost • Pentecost • oncost • glasnost
•compost • star-crossed • hoar frost
•permafrost
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"permafrost." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 14 Feb. 2012 <http://www.encyclopedia.com>. "permafrost." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. (February 14, 2012). http://www.encyclopedia.com/doc/1O233-permafrost.html "permafrost." Oxford Dictionary of Rhymes. 2007. Retrieved February 14, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O233-permafrost.html |
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