Permafrost
In
geology, permafrost or permafrost soil is a thermal condition where ground material stays at or below 0
C for two or more years. Permafrost is where water is permanently frozen in the soil resulting in minimal or no plant growth. 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 , but alpine permafrost exists at high altitudes.
The extent of permafrost can vary as the
climate changes.
Encyclopedia
In
geology,
permafrost or
permafrost soil is a thermal condition where ground material stays at or below 0°
C for two or more years. Permafrost is where water is permanently frozen in the soil resulting in minimal or no plant growth. 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 , 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 or
glacial ice. Seasonal frost commonly overlays permafrost and is called the
active layer as it will thaw 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 thick. In areas of continuous permafrost and harsh winters the depth of the permafrost can be very great: 440m at
Barrow, Alaska,
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 , 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 . 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 colder than those of today. At mean annual soil surface temperatures below -5°C 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.
A
line of continuous permafrost in the
Northern Hemisphere 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.
Permafrost extent
Measurement of the depth and extent of permafrost may be an indicator of
global warming as recent years 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 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.
Time to form deep permafrost
Time taken for permafrost to reach depth| Time | Permafrost Depth |
| 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.
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 melting the permafrost and sinking downwards. This sinking problem has three common solutions: using foundations on wood
piles, building on a thick
gravel pad , 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 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.
See also
The Permafrost Young Researchers Network is a network formed in 2005 to formally facilitate and st...
External links
- , Geological Survey of Canada
- from *
- - ENS
- — BBC
skPermafrost
