Permafrost refers to a condition in which temperature below the ground surface has remained below 0°C continuously for more than two years. If the pore water is present in the subsoil, a larger percentage is frozen with the result that the minerals and organic particles are frozen.
In northern Alaska, Canada and Siberia the permafrost reaches to depths of 600 meters. It is only the top 1 meter of soil that thaws each summer and gets saturated.
The term 'permafrost' was coined by S.W. Muller in 1947 to describe permanently frozen
ground, but now permafrost has been subdivided in 3 subtypes: (i) Continuous permafrost, (ii) Discontinuous permafrost, and (iii) Sporadic permafrost.
Continuous permafrost covers about 20.85 km2xl06 of the earth's surface. Discontinuous permafrost refers to that condition in which the continuous permafrost breaks into different patches which merge into seasonally frozen ground. This second category of permafrost covers 17.30 knv'xlO6 of the land surface.
Most of the discontinuous permafrost is out of balance with the present climatic conditions. There is another sub-type called sporadic permafrost. Also known as alpine permafrost, this type extends to lower latitudes.
How a condition of permafrost develops has been described above. However, there are two additional factors that contribute to permafrost: (i) the presence of fossil permafrost from previous ice-age conditions and (ii) presence of snow cover and vegetation which have insulating effect that inhibits heat loss from the ground. Permafrost covers about 85% area of Alaska and 50% area of Canada and the erstwhile Soviet Union.
All that is required to produce permafrost is the mean annual soil temperature below the freezing point. A depth of one meter or two is generally used to define the permafrost zone.
However, the thickness of the frozen layer is largely determined by soil moisture, porosity, vegetation cover, dissolved components and ground-water pressure. There is wide variation in the depth of permafrost zone, but in the Arctic region wide areas have thickness of several hundred meters.
Only 1 or 2 meters of the top layer is subject to seasonal melt or thaw. This top layer of soil in a permafrost region is called the active layer.
Sometimes there may exist an unfrozen layer between the seasonally frozen active zone and permanently frozen ground. This unfrozen layer, and any unfrozen material within or below the permafrost, is called talik.
It is indeed due to geothermal heating that such an unfrozen ground is always found below permafrost. However, this unfrozen ground may be several hundred meters beneath the surface.
It may be pointed out that the presence of permafrost with an active layer overlying it obstructs the vertical movement of water and the result is that water movement takes place preferably in the surface layers.
Even where the ground is only temporarily frozen below, the surface layers become saturated and their strength then lowers due to reductions in interparticle friction.
As stated earlier, mass movement of materials saturated with water is called solifluction, but where it is associated with permafrost or seasonally frozen ground, the term 'gelifluction' is used.