Permafrost is estimated to occur in areas totaling about 26 per cent of the land area of the earth. Permafrost is as much as 2000 feet thick in Siberia and 1300 feet thick in Alaska, where its minimum temperature is −12°C and −10°C respectively. As it is defined on the basis of temperature, its composition varies over wide limits. Ice is one of the most important components, and large masses commonly are in the form of ice wedges. Structures in ice wedges are generally smaller and more complex than those in glacial ice. Dimensional and optic-axis lineations and foliations of inclusions of air bubbles, organic matter, and clastic material occur in all wedges. Not all structures and orientations of ice crystals in ice wedges can be explained.
Permafrost results when the net heat balance of the surface of the earth over a period of several years produces a temperature continuously below 0°C. Although the general thesis of the problem is relatively simple, it is extremely complex in detail. Complete freezing of bedrock for long periods of time has little geologic aftereffect, except through control of movement of ground water. Freezing of mantle completely eliminates ground-water movement, preserves organic remains indefinitely, reduces or prevents mass movements within the frozen material, and promotes frost action in the overlying active layer. Most emphasis has been given the engineering aspects of permafrost. The volume of literature pertaining directly or indirectly to permafrost is becoming so great that individual competence cannot cover the various fields represented.