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permafrost

 
Dictionary: per·ma·frost   (pûr'mə-frôst', -frŏst') pronunciation
n.
Permanently frozen subsoil, occurring throughout the Polar Regions and locally in perennially frigid areas.

[PERMA(NENT) + FROST.]


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Sci-Tech Encyclopedia: Permafrost
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Perennially frozen ground, occurring wherever the temperature remains below 32°F (0°C) for several years, whether the ground is actually consolidated by ice or not and regardless of the nature of the rock and soil particles of which the earth is composed. Perhaps 25% of the total land area of the Earth contains permafrost; it is continuous in the polar regions and becomes discontinuous and sporadic toward the Equator. During glacial times permafrost extended hundreds of miles south of its present limits in the Northern Hemisphere.

Temperature of permafrost at the depth of no annual change, about 30–100 ft (10–30 m), crudely approximates mean annual air temperature. It is below 23°F (−5°C) in the continuous zone, between 23–30°F (−5 and −1°C) in the discontinuous zone, and above 30°F (−1°C) in the sporadic zone. Temperature gradients vary horizontally and vertically from place to place and from time to time.

Ice is one of the most important components of permafrost, being especially important where it exceeds pore space. Physical properties of permafrost vary widely from those of ice to those of normal rock types and soil. The cold reserve, that is, the number of calories required to bring the material to the melting point and melt the contained ice, is determined largely by moisture content.

Permafrost develops today where the net heat balance of the surface of the Earth is negative for several years. Much permafrost was formed thousands of years ago but remains in equilibrium with present climates. Permafrost eliminates most groundwater movement, preserves organic remains, restricts or inhibits plant growth, and aids frost action. It is one of the primary factors in engineering and transportation in the polar regions.


Geography Dictionary: permafrost
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Areas of rock and soil where temperatures have been below freezing point for at least two years. Permafrost need contain no ice; a sub-zero temperature is the sole qualification. Any water present need not be frozen since the presence of dissolved minerals lowers the freezing point of water. In epigenetic permafrost ground ice develops mainly in upper parts, vertical freezing dominates, and cryogenic textures develop as water migrates under pressure to the freezing front, so that pressure is exerted on the ground leading to deformation. See involution. In syngenic permafrost the ground ice is regularly distributed throughout the whole thickness of the permafrost; cryogenic textures develop as permafrost grows upwards, and sediments above are not contorted. The growth of permafrost is permafrost aggradation, which decreases the thickness of the active layer and may be caused by the freezing of taliks. It is responsible for the formation of pingos. The decline of permafrost is permafrost degradation, which plays a key role in the development of thermokarst. Permafrost is a very sensitive system; small mistakes in constructing buildings in this environment can have catastrophic effects because of thermo-erosion and thermo-abrasion. Permafrost features are well preserved in the chalklands of southern England.

The permafrost zones of periglacial areas are of two types: continuous permafrost is present in all localities apart from small, localized thawed zones, while discontinuous permafrost exists as small, scattered areas of permanently frozen ground. A frost table marks the upper limit of permafrost, which is overlain by the active layer. See also talik.


Perennially frozen earth, with a temperature below 32 °F (0 °C) continuously for two years or more. Permafrost is estimated to underlie 20% of the Earth's land surface and reaches depths of 5,000 ft (1,500 m) in northern Siberia. It occurs in 85% of Alaska, more than half of Russia and Canada, and probably all of Antarctica. Permafrost has a significant effect on plant and animal life, and it presents special problems in engineering projects. All land use in permafrost environments must take into account the terrain's special sensitivity; if the delicate natural balance is not maintained, extensive degradation and ecological damage may result.

For more information on permafrost, visit Britannica.com.

Architecture: permafrost
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Permanently frozen soil, subsoil, or other deposits in arctic or subarctic regions.


Archaeology Dictionary: permafrost
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[De]

More or less permanently frozen ground represented as a zone up to 3m deep the surface of which undergoes seasonal freezing and thawing. Common in periglacial conditions, the effects of permafrost in the creation of geomorphological features such as ice-wedges, involutions, and pingos have important implications for archaeology because they are sometimes confused with anthropogenic features.

 
Columbia Encyclopedia: permafrost
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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.


Military Dictionary: permafrost
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(DOD) Permanently frozen subsoil.

Wikipedia: Permafrost
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This article is about frozen ground. For other uses, see Permafrost (disambiguation).

In geology, permafrost or permafrost soil is soil at or below the freezing point of water (0 °C or 32 °F) for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Most permafrost is located in high latitudes (i.e. land in close proximity to the North and South poles), but alpine permafrost may exist at high altitudes in much lower latitudes.

Map showing extent and types of permafrost in the Northern Hemisphere

The extent of permafrost can vary as the climate changes. Today, a considerable area of the Arctic is covered by permafrost (including discontinuous permafrost). Overlying permafrost is a thin active layer that seasonally thaws during the summer. Plant life can be supported only within the active layer since 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 as much as 1493 m (4510 ft) in the northern Lena and Yana River basins in Siberia. Permafrost can also be a storage of carbon. One estimate[citation needed] is that 1700 Gt of carbon are stored within the permafrost worldwide.

Contents

Continuous and discontinuous permafrost

While these two men dig in Alaska to study soil, the hard permafrost requires the use of a jackhammer
The Storflaket permafrost plateau bog near Abisko in northern Sweden shows cracks at its borders due to melting of the 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 the Urals, where snow acts as an insulating blanket. The bottoms of many glaciers can also be free of permafrost.

Typically, the below-ground temperature will be less variable from season to season than the air temperature, with temperatures tending 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. Discontinuous permafrost is often further divided into extensive discontinuous permafrost, where permafrost covers between 50 and 90 percent of the landscape and is usually found in areas with mean annual temperatures between −2˚ and −4˚C (28˚ and 25˚ F), and sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between 0˚ and −2˚C (32˚ and 28˚F).

In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ.

There are exceptions in un-glaciated 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 (abbreviated to CPZ) 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 Suwałki 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 most northerly points at which permafrost sometimes thaws 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 of the Antarctic continent is overlain by glaciers.

In the Andes at Atacama Desert permafrost extends down to an altitude of 4,400 metres and is continuous above 5,600 metres.

Changes in permafrost extent

Cryosphere atlas northleg122159163110472.pngLocation of Permafrost in the Northern Hemisphere. Glaciers and the Greenland Ice Sheet are violet, and Arctic Sea Ice is light blue. from NSIDC

In Yukon, the zone of continuous permafrost has moved 100 kilometers poleward since 1899, but accurate records only go back 30 years. It is thought that permafrost thawing could exacerbate global warming by releasing methane and other hydrocarbons, which are powerful greenhouse gases.[1] It also could encourage erosion because permafrost lends stability to barren Arctic slopes.

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 (southeastern Hungary) 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 was probably discontinuous.

Ecological consequences

Formation of permafrost has significant consequences for ecological systems, primarily due to constraints imposed upon rooting zones, but also due to limitations on den and burrow geometries for fauna requiring subsurface homes. Of course, there are secondary effects from species dependent on plants and animals whose habitat is constrained by the permafrost. One of the most widespread examples is the dominance of Black Spruce in extensive permafrost areas, since this species can tolerate a more near surface constrained rooting pattern.[2]

Patterned ground

Patterned ground is a term used to describe the distinct, and often symmetrical geometric shapes formed by ground material in periglacial regions.

Time to form deep permafrost

Time taken for permafrost to reach depth
Time (yr) Permafrost Depth (m)
1 4.44 (0.002759 mi.)
350 79.9 (0.049648 mi.)
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.[citation needed] The table to the right shows that the first hundred metres of permafrost forms relatively quickly but that deeper levels take progressively longer.

Construction on permafrost

Utility lines in a permafrost zone must be above ground

Building on permafrost is difficult due to the heat of the building (or pipeline) melting the permafrost and sinking. This problem has three common solutions: using foundations on wood piles; building on a thick gravel pad (usually 1-2 meters/3.3-6.6 feet thick); or using anhydrous ammonia heat pipes. The Trans-Alaska Pipeline System uses insulated heat pipes to prevent the pipeline from sinking. 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 the sinking of large buildings into the ground (known to the Yakuts before Yakutsk was founded) can be prevented by using stilts extending down to about fifteen metres or more. At this depth the temperature does not change with the seasons, remaining at about -5 °C (23F).

See also

References

External links


Translations: Permafrost
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Dansk (Danish)
n. - permafrost

Nederlands (Dutch)
permanent bevroren ondergrond

Français (French)
n. - permagel, permafrost

Deutsch (German)
n. - Dauerfrostboden

Ελληνική (Greek)
n. - (γεωγρ.) μόνιμα παγωμένο έδαφος ή υπέδαφος

Italiano (Italian)
permagelo

Português (Portuguese)
n. - região polar permanentemente congelada (f)

Русский (Russian)
вечная мерзлота

Español (Spanish)
n. - permagel

Svenska (Swedish)
n. - ständig tjäle

中文(简体)(Chinese (Simplified))
永久冻土层

中文(繁體)(Chinese (Traditional))
n. - 永久凍土層

한국어 (Korean)
n. - (북극 지방의) 영구 동토층

日本語 (Japanese)
n. - 永久凍土層

العربيه (Arabic)
‏(الاسم) تربه متصقعه‏

עברית (Hebrew)
n. - ‮שכבת אדמה קפואת-עד (בחוג הקוטב)‬


 
 
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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
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Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved.  Read more
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Architecture. McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc. All rights reserved.  Read more
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Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
Military Dictionary. US Department of Defense Dictionary of Military and Associated Words, 2003.  Read more
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