Frost Cracking

Frost cracking is a widespread process operating in regions underlain by permafrost, and occurs where thermal stresses exceed the tensile strength of ice-rich soils and regolith. (Mackay, 1986; Thorn, 1992). The lowering of the temperature of ice-rich sediments and soils leads to thermal contraction of the ground and the development of cracks. The cracks develop because pure ice has a coefficient of linear expansion that is higher at 0°C than at -30°C and the rates of expansion and concentration of the ice-rich soils and sediment are little different from that of pure ice. Conditions necessary for cracking vary, depending on the nature of the materials being affected and on their ice content. The rates of temperature change and the magnitude of the temperature change affect the degree of cracking. At very low temperatures, even soils and sediments with little ice undergo contraction and, conversely, a very rapid drop in temperature over a small temperature range below freezing can lead to cracking in ice-rich sediments.

Frost cracks divide the landscape into polygonal nets with junctions that commonly intersect at right angles, forming polygonal nets of patterned ground over large areas of the Arctic. Polygon dimensions typically range from 15 to 40 m. However, over large areas of the Arctic, hexagonal patterns dominate. It is widely believed that the hexagonal pattern reflects simultaneous crack propagation at a series of points, while orthogonal patterns imply evolution in a sequence of primary frost cracks that initially develop randomly and then secondary cracks divide the area into an orthogonal pattern. The infilling of frost cracks with water and snow, which percolates down the fractures, results in the development of ice wedges. With seasonal cracking and the addition of meltwater, the wedges progressively grow over time.

John C. Dixon

See also Patterned and Polygonal Ground; Periglacial Environments; Permafrost

Further Reading

Corte, A.E., "Particle sorting by repeated freezing and thawing." Biuletyn Peryglacjalny, 15 (1966): 175-240 French, H.M., The Periglacial Environment (2nd edition),

Harlow: Longman, 1996 Mackay, J.R., "The first seven years (1978-1985) of ice wedge growth Illisarvik experimental drained lake site, western Arctic coast." Canadian Journal of Earth Sciences, 23 (1986): 1782-1795 Thorn, C.E., "Perglacial geomorphology: What, Where and When?" In: Periglacial Geomorphology, edited by J.C. Dixon & A.D. Abrahams, Chichester: Wiley, 1992, pp. 1-30 Walder, J. & B. Hallet,"A theoretical model of the fracture of rock during freezing." Geological Society of America Bulletin, 67 (1985): 336-346 Washburn, A.L., Geocryology, London: Edward Arnold, 1980

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