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In geology, permafrost is soil that stays in a frozen state for more that two years in a row.

The extent of permafrost can vary as the climate changes, approximately 20% of the Earth's land mass is underlain by permafrost. Seasonal frost commonly overlays permafrost and is called the active layer as it will thaw during the summer. The active layer can support plant life, permafrost will not. Thickness of the active layer varies by year and location but is typically 2 to 12 feet thick. Thickness of the permafrost can go to hundreds of feet (1330 feet measured at Barrow, Alaska)

Construction on Permafrost

Building on permafrost is difficult due to the heat of the building (or pipeline) melting the permafrost and sinking downwards. This sinking problem has three common solutions: building up on wood piles, building on a thick gravel pad (usually 4 to 6 feet thick), or using anhydrous ammonia heat pipes. The Alaskan Oil Pipeline uses insulated heat pipes to keep the pipeline from sinking into the permafrost.

Permafrost Extent

Measurement of the depth and extent of permafrost maybe an indicator of global warming and recent years (1998 and 2001) have seen record thawing of permafrost in Alaska and Siberia. However accurate records only go back 30 years.

See Also global warming controversy, Tundra

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