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Permafrost

Permafrost

About 20% of Earth's surface is covered by permafrost, land that is frozen year-round. Permafrost occurs at high latitudes or at very high altitudesanywhere 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. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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"Permafrost." World of Earth Science. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/permafrost

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.. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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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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"pergelic." A Dictionary of Earth Sciences. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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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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"pergelic." A Dictionary of Ecology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

"pergelic." A Dictionary of Ecology. . Encyclopedia.com. (August 16, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/pergelic-0

"pergelic." A Dictionary of Ecology. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/pergelic-0

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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"pergelic." A Dictionary of Plant Sciences. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

"pergelic." A Dictionary of Plant Sciences. . Encyclopedia.com. (August 16, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/pergelic-1

"pergelic." A Dictionary of Plant Sciences. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/pergelic-1

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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"permafrost." A Dictionary of Ecology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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"permafrost." A Dictionary of Ecology. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/permafrost-0

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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"permafrost." A Dictionary of Earth Sciences. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/permafrost

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. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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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. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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permafrost

permafrost permanently frozen subsoil. XX. f. next, FROST.

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"permafrost." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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"permafrost." The Concise Oxford Dictionary of English Etymology. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/permafrost-1

permafrost

permafrostaccost, cost, frost, lost, Prost, riposte •teleost • Pentecost • oncost • glasnost •compost • star-crossed • hoar frost •permafrost

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"permafrost." Oxford Dictionary of Rhymes. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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