Permafrost

  Permafrost is ground remaining at or below 0°C continuously for at least 2 years. It may consist of cold, dry earth; cold, wet, saline earth; icy lenses and masses; or ice-cemented earth or rock. Although GLACIERS fit this definition, they are a special case dealt with elsewhere. Most permafrost includes ice cement, lenses or masses that result from the freezing of the bulk of the subsurface water. At most surface temperatures some capillary water will be unfrozen. Saline water in soil freezes below 0°C; hence talik (unfrozen zones in permafrost) occurs below parts of the shores of the Arctic Ocean and in cold brine pockets in the ground. If the ground is dry, no ice will be present despite the temperature.

In permafrost regions, the upper layer that undergoes seasonal freezing and thawing is called the active layer. Below this lies the permafrost, the upper surface of which is called the permafrost table. Permafrost is mapped as follows: where over 80% of the ground is underlain by permafrost, it is continuous; 30-80%, discontinuous, with the rest being talik; under 30%, sporadic, although the exact boundaries vary from author to author. Taliks occur beneath water bodies and where water flows through the ground, keeping it warm.

Permafrost is produced by a surface sub-zero winter heat flux, which is greater than the sum of the surface above-zero summer heat flux; the heat added by water moving laterally through the lower part of the active layer; and the annual geothermal heat flux. Its development is favoured by high latitude and altitude, by long, cold winters and thin winter snow cover. Cold-air drainage in mountain valleys or through rock caves may cause pockets of permafrost in the sporadic zone.

Permafrost underlies 20-25% of Earth's land area, including about 99% of Greenland, 80% of Alaska, 50% of Russia, 40-50% of Canada and 20% of China. The greatest thicknesses in Canada are over 1000 m at high elevations in parts of BAFFIN and Ellesmere islands, ranging down to 60-90 m at the southern limit of the continuous permafrost zone. Active layer thicknesses range from under 10 cm in Ellesmere in porous sediments to 15 m at high altitudes in fissured impermeable rocks in the mountains of southwestern Alberta on the outer margin of continuous permafrost. The coldest ground temperatures in permafrost worldwide are to be found on ELLESMERE ISLAND (about -15°C).

The permafrost in Canada developed largely after the last GLACIATION, and its distribution adjusts to small climate changes. Thus the continuous permafrost zone extended to south of TULITA in the mid-19th century. A 4°C warming of the winter air temperature resulted in the migration of its southern boundary nearly to INUVIK. Since 1940, a 2°C cooling in the mean winter temperature has resulted in the boundary moving southwards to TSIIGEHTCHIC. Now the southern limit of permafrost is moving north again in response to the climatic warming along the Mackenzie Valley. Similar changes can be demonstrated in alpine areas and in Alaska, northern Russia, etc.

In Western Canada, long-term monitoring of air and ground temperatures shows that the air temperature has behaved differently at different sites, and this is also the case in Asia. In the Alps and around the Arctic Ocean, the climate is warming and the ground ice is tending to melt. This causes considerable problems with stability of foundations and slopes in areas of permafrost.

Ice

 The ice in permafrost may be interstitial or segregated, and is concentrated in the few metres immediately below the permafrost table. Water tends to move through the pores to the coldest part of the ground. This will be the freezing plane during the fall freeze-up, and freezing results in ice contents as high as 1600% by weight of the mineral matter in some silts and PEATS in low-lying areas, eg, the Mackenzie Delta (seeMACKENZIE RIVER). The segregated ice may be in isolated masses, in distinct lenses parallel to the surface or in wedges tapering downwards. Particularly in peaty soils, there is usually a layer of rotten ice at the base of the active layer through which water moves laterally in summer. This layer may be similar in thickness to the ice-free active layer in summer.

Landforms

 Distinctive landforms are associated with permafrost (seePERIGLACIAL LANDFORM). The ice wedges form a polygonal pattern and are the result of winter contraction cracks that have become infilled with ice from percolating summer meltwater. In dry areas, sand may fill in the cracks, forming sand wedges within the permafrost table. On steep slopes, high ice content may permit unconsolidated deposits to move slowly downslope as rock glaciers. In taliks, the freezing of water may cause the localized growth of PINGOS, ice-cored mounds up to 100 m high. Localized ice segregation in wet lowlands can form mounds domed up by small lenses of ice (lithalsas in mineral soils or palsas in peats). The lower part of the active layer accumulates as peat plateaus develop, producing extensive flat plateaus with low density porous ice in their core. Two types occur: floating and anchored. The latter eventually drown as the water level rises.

Seasonal freezing in the active layer can cause the turning up of stones, heaving of the ground, and sorting of coarse and fine rock material. These seasonal changes also cause physical weathering of the bedrock and can reduce mudstones to silts, eg, along the DEMPSTER HIGHWAY in the Rat Pass, NWT. The sorting processes can form patterned ground in only 5 years. Once sorted, the pattern remains indefinitely. Thawing of the permafrost often results in subsidence where ice was present; and in the formation of thaw lakes or hummocky terrain. This terrain, called thermokarst, can be induced by human activities or by climactic change.

Development Challenges

The presence of ice and the consequences of an alteration in the thermal regime make economic development of permafrost terrain difficult. All materials, except gravels or clean sand, heave or become unstable during seasonal freezing and thawing. Normal agriculture is impossible over permafrost due to subsidence, while heated structures must be separated from the frozen ground by adequate insulation. Paved roads or runways increase summer heating and require a suitable insulating subgrade. Even a slight alteration of the vegetation cover or the hydrology may cause thermokarst subsidence. Artificial refrigeration of the ground is too expensive for all except special cases. However, rock has been used to cool the railbed along the Tibetan railway, making use of the cooler temperatures (up to 5º C) found beneath stone or rock-covered slopes.

Water supplies are difficult to obtain except where natural springs or deep lakes occur. Artificial impoundments tend to result in substantial development of thermokarst and are difficult to maintain. Garbage and sewage disposal is a major problem. Construction of dams for HYDROELECTRICITY requires special techniques, as do mining and drilling for oil and natural gas. The elasticity of ice reduces the effectiveness of explosive charges. Ice-rich ore is more difficult to process because it must be thawed. Drilling without disturbing the environment is practically impossible in Canada. Grounding of electrical devices is a problem. Electrical cables are usually placed above ground, and water and sewage pipes must be insulated and are usually placed in insulated boxes on piles called utilidors. Similarly, oil and natural gas PIPELINES are usually carried on piles in permafrost areas.