Everything about Permafrost totally explained
» This article is about frozen ground. For other meanings, 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 isn't 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 (for example land in close proximity to the North and South poles), but
alpine permafrost may exist at high
altitudes in much lower latitudes.
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 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's 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.
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 the
Urals, where snow acts as an insulating blanket. The bottoms of glaciers can also be free of permafrost, although this isn't common.
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.
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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 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 of the
Antarctic continent is overlain by glaciers.
Permafrost extent
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. This thawing has led to stands of trees falling, labelled
drunken trees, due to insufficient rooting. In the
Yukon, the zone of continuous permafrost has moved 100 kilometres 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. 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 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's some evidence for former permafrost from this period in central
Otago and
Argentine Patagonia, but was probably discontinuous.
Patterned ground
Patterned ground is a term used to describe the distinct, and often symmetrical geometric shapes formed by ground material in periglacial regions.
Image:Permafrost - polygon.jpg|Polygons on the ground
Image:Permafrost_stone-rings_hg.jpg|Stone rings on Spitsbergen
Image:Ice-wedge_hg.jpg|Ice wedges seen from top
Image:Permafrost_soil-flow_hg.jpg|Solifluction on Greenland
Image:Phoenix horizon view.jpg|Phoenix landing-day image near north pole of Mars showing flat terrain, containing what appears to be a polygonal pattern, stretching from the foreground to the horizon.
Time to form deep permafrost
| 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.
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
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 doesn't change with the seasons, remaining at about -5 °C (23F).
Further Information
Get more info on 'Permafrost'.
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