In winter, permafrost undergoes contraction because of low temperature. The reduction of volumes due to this cause is seen in the development of roughly equal blocks separated by cracks. When ice melts in summer, it fills the cracks. Subsequently in autumn such water is frozen. Year after year, this crack-filling ice grows over centuries and is known as ice-wedge because of its downward tapering shape.
Lachenbruch has outlined the evolution of an ice wedge. During the initial winter, permafrost along with the active layer is in a frozen state. Thermal contraction causes the formation of cracks. Such a crack is filled with melt water during the following summer. During the succeeding autumn while, the active layer is still in a thawed state the water-filled crack has solidified into ice. Year after year, the ice wedge grows by the renewal of the crack during winter and it is filling by summer melt water and subsequent freezing of water.
Lachenbruch estimates that after a lapse of 500 years the wedge will be about 2 feet wide near the top. The depth of the wedge is about 2 meters. During the five centuries of ice-wedge evolution, the surface of the grotind has developed slight undulations with depressions (3 meter) above the wedge and welts, or wider depressions16 in the inter-wedge regions, of the same magnitude.
Other workers do not appear to have noted such regular undulations. Each year a film of ice is added to the wedge whence its age can be estimated. The polygons on the edges of which cracks and wedges are formed, may be 1 to 100 meters across. The width of the wedges may be as much as 10 meters at the top. The height of the wedges may be as much as about 10 meters.
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The width of the cracks is related to the size of the polygon. It may be nearly 1.5 mm in the case of polygons one meter across. The maximum width of annual crack observed in Alaska by Leffingwell (1915) who also believed in the contraction theory of the origin of ice-wedges and ice-veins, was about 10 mm.
According to Taber (1943), however, the cracks are not due to contraction but due to expansion in the ground caused by the formation of ground ice. The ice-wedges and other forms of ground ice were due to segregation, i.e. freezing of water held in sediments.
The ice wedges are active in the colder regions. Thus in Alaska active ice wedges are found in the northern continuous permafrost zone, and weakly active to inactive ice wedges occur further south. Still further south ice wedges disappear.