Permafrost: frozen ground

When the mean annual temperatures of the air remain below zero for long periods of time, the water in the ground is always in the solid state, and the land is permanently frozen. This state is named permafrost (i.e. permanent ice). The land, hardened and without any liquid water, is made up of mineral particles (particles of soil, grains and rock debris of various sizes) cemented together by ice. The depth of the ice depends on how cold the climate is, and can reach many dozens of metres (in some areas in Siberia and Alaska, with mean annual air temperature ranging between -7 and – 16°C, permafrost was found at a depth of 300-600 m, with a maximum depth of 1,500 m in an area of Northern Siberia).
In summer, a thin layer on the surface, the so-called active layer, is heated by the sun and the ice can melt. Since the permafrost below is impermeable, the melted water cannot be moved away and the thawed land becomes soft and drenched with water, often marshes and swamps form, and there may be serious problems with the stability of buildings that are built on this kind of terrain.
In order to build in these areas, particular construction techniques are required, where the buildings rest on poles dug into the ground till they reach the permanent frozen layer: similar to “pile dwellings” on the soft, loose active layer. Permafrost may be found in small areas in the high mountains (also in the Alps), but mainly in very vast areas in the high latitudes: approximately one fifth of the emersed land is affected by this phenomenon (half of the territory of the former Soviet Union, half of the Canadian territory, three quarters of Alaska, almost the whole of Greenland and the Antarctic).

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