From Laura Klappenbach,
Your Guide to Animals / Wildlife.
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A. Permafrost is any soil or rock that remains frozen (below 0°C / 32°F) throughout the year. For a soil to be considered permafrost, it must be frozen for at least two consecutive years or longer. Permafrost can be found in cold climates where the mean annual temperature is less than the freezing point of water. Such climates are found near the North and South poles and in some alpine regions.

Some soils in areas that experience warmer temperatures thaw for a short time during warmer months. The thawing is restricted to the top layer of soil and a permafrost layer remains frozen several inches below the surface. In such areas, the top layer of soil (called the active layer) warms up enough so that plants are able to grow during the summer. The permafrost that lies below the active layer traps water close to the soil's surface thus making it rather soggy. The permafrost also ensures a cool soil temperature, slow plant growth, and slow decomposition.

The following soil formations are associated with permafrost habitats:

• Polygons—Tundra soils often form geometric shapes (or polygons) most noticeable from the air. The polygons form as soil contracts, cracks, and then collects water that is trapped by the permafrost layer.
• Pingos—Pingos form when the permafrost layer traps a large amount in the soil. When the water freezes, it expands and pushes the saturated earth upward into a large mound.
• Solifluction—Solifluction occurs when thawed soils slide down a slope over the permafrost layer. When this occurs, the soils form rippled, wave patterns.
• Thermokarst slumping—Thermkarst slumping occurs in areas that have been cleared of vegetation (usually due to human disturbance and land use). Such disturbance leads to the melting of the permafrost layer and as a result the ground collapses or 'slumps'.

Sources

• Luhr, J.F. 2003. Earth. London: Dorling Kindersley.
• Alaska Department of Fish and Game. Permafrost. Accessed September 28, 2007 at http://www.wildlife.alaska.gov/index.cfm?adfg=ecosystems.tundrapermafrost.