PERMAFROST

perennially frozen earth, with a temperature below 0 C (32 F) continuously for two or more years. Permafrost is estimated to underlie 20 percent of the Earth's land surface and reaches depths of 1,500 m (5,000 feet) in northern Siberia. It occurs in 85 percent of Alaska, more than half of Russia and Canada, and probably all of Antarctica. In addition to the polar regions, permafrost is present in the highland regions of lower latitudes where, because of elevation, the mean annual air temperature is 0 C or colder. Permafrost is overlain by a surface layer that is subject to thawing during the warmer seasons of the year. This zone of seasonal freezing and thawing is termed the active layer, and the maximum depth of thaw penetration delimits the top of the permafrost (permafrost table). The depth to which soil is thawed will vary on an annual basis according to the vagaries of summer temperature. Hence a layer may occur above the permafrost table that may be subject to thaw only in certain years. Such a layer has been termed the pereletok, or supra-permafrost layer. Sporadic bodies of permafrost are often buried by perennially thawed ground that is itself overlain by the seasonally frozen active layer. Such unfrozen layers, as well as islands of thawed ground surrounded by permafrost, are called taliks. Permafrost is essentially the product of a negative heat balance at the ground surface. Where mean annual ground-surface temperatures are below 0 C, summer thaw depths will be exceeded by winter freezing depths, resulting in permafrost growth. If the condition is maintained, the progressive downward extension of the permafrost will ultimately be counterbalanced by the geothermal heat flow from the Earth's interior. The resultant equilibrium depth will determine the permafrost's base. Thus, the lower the mean surface temperature, the greater the depth at which the equilibrium position will lie. Although permafrost may exist without any ice content, it is usually associated with the presence of ground ice. Ice occurs in many forms and variable amounts in permafrost and often exceeds all other materials in quantity. It is present as crystals of diverse size and shape; as grains, films, veins, and lenses; and as large masses. Annual surface-temperature fluctuations affect the ice in the uppermost part of the permafrost, within about 10-20 m of the surface. Rapid cooling in winter may cause thermal contraction cracks to propagate down from the permafrost table and also up to the surface. These become infilled by melting snow and rainwater, which freezes. Repeated cracking and ice growth results in the growth of ice wedges in vertical section and polygons in horizontal form, two of the most widespread geomorphic features associated with permafrost. Complex stress forces operating within permafrost layers may produce such surface features as mud spots, frost blisters, ice mounds, and pingos. Pingos (hills composed largely of ice) originate in swampy lowlands and may attain heights exceeding 60 m and diameters of more than 800 m. Most authorities recognize a geographic subdivision of permafrost into two zones, continuous and discontinuous; some recognize an outer fringe to the latter called a sporadic zone. Within the continuous zone, permafrost everywhere underlies the ground surface and is usually continuous in depth down to its lower limit. In the discontinuous zone, the lateral continuity of the permafrost is broken to create alternating frozen and talik areas. As the sporadic zone is approached, the amount and thickness of the residual permafrost decreases. Beyond the main permafrost belt, limited areas of so-called alpine permafrost are present where elevation reduces the mean annual temperature regime, with the permafrost lying as cores to the mountain massifs. Permafrost is essentially continuous in polar regions, becomes increasingly discontinuous at lower latitudes, and occurs only sporadically toward its geographic limits. Permafrost and the geomorphic processes associated with it have a significant effect on the development and distribution of plant and animal life where it occurs. Its practical importance to human affairs is that it presents special problems in essentially all engineering projects. The buildup of ground ice is a response to a long and complex thermal history, and its preservation is dependent upon the insulatory properties of the overlying active layer. Because of this factor, land use in permafrost environments must take account of the special geomorphic sensitivity of the terrain. Where permafrost occurs, waste disposal, mining activities, well drilling, and road, airstrip, and building construction all pose critical problems. The delicate thermal balance within the active layer must be maintained; otherwise, extensive permafrost degradation may be induced and cause a loss in the weight-bearing strength of the permafrost. An interesting aspect of permafrost is the preservation in it of the carcasses of extinct Ice Age mammals; one or two almost complete, frozen mammoths dating from at least 10,000 years ago have been reported from Siberia. perennially frozen ground, a naturally occurring material with a temperature colder than 0 C (32 F) continuously for two or more years. Such a layer of frozen ground is designated exclusively on the basis of temperature. Part or all of its moisture may be unfrozen, depending on the chemical composition of the water or the depression of the freezing point by capillary forces. Permafrost with saline soil moisture, for example, may be colder than 0 C for several years but contain no ice and thus not be firmly cemented. Most permafrost, however, is consolidated by ice. Permafrost with no water, and thus no ice, is termed dry permafrost. The upper surface of permafrost is called the permafrost table. In permafrost areas the surface layer of ground that freezes in the winter (seasonally frozen ground) and thaws in summer is called the active layer. The thickness of the active layer depends mainly on the moisture content, varying from less than a foot in thickness in wet, organic sediments to several feet in well-drained gravels. Permafrost forms and exists in a climate where the mean annual air temperature is 0 C (32 F) or colder. Such a climate is generally characterized by long, cold winters with little snow and short, relatively dry, cool summers. Permafrost, therefore, is widespread in the Arctic, sub-Arctic, and Antarctica. It is estimated to underlie 20 percent of the world's land surface. Additional reading A.L. Washburn, Geocryology (1979), is the most thorough book in English on permafrost and periglacial processes. H.M. French, The Periglacial Environment (1976), clearly summarizes permafrost and periglacial processes, with emphasis on examples from Canada. The greatest source of permafrost information is the proceedings of the various International Conference on Permafrost meetings; each volume contains numerous up-to-date papers in English from many different countries. Wilfried Haeberli, Creep of Mountain Permafrost: Internal Structure and Flow of Alpine Rock Glaciers (1985), is a discussion of rock glaciers, a prominent feature of Alpine permafrost. Troy L. Pw, "Alpine Permafrost in the Contiguous United States: A Review," Arctic and Alpine Research, 15(2):145-156 (1983), summarizes in detail the character and distribution of mountain permafrost in this region.Arthur H. Lachenbruch, Mechanics of Thermal Contraction Cracks and Ice-Wedge Polygons in Permafrost (1962), is a classic paper on the quantitative interpretation of the formation of ice-wedge polygons in permafrost. Troy L. Pw, Richard E. Church, and Marvin J. Andresen, Origin and Paleoclimatic Significance of Large-Scale Patterned Ground in the Donnelly Dome Area, Alaska (1969), discusses the origin of ice-wedge casts and relict permafrost in central Alaska and offers paleoclimatic interpretations. R. Dale Guthrie, Frozen Fauna of the Mammoth Steppe (1990), discusses fossil carcasses of Ice Age mammals preserved in permafrost. Troy L. Pw, Geologic Hazards of the Fairbanks Area, Alaska (1982), a highly illustrated work, contains an up-to-date presentation of the greatest geologic hazard to life in polar areas: problems posed by seasonally and perennially frozen ground. G.H. Johnston (ed.), Permafrost: Engineering Design and Construction (1981), is a comprehensive book on construction problems in permafrost areas, with examples mainly from northern Canada. Troy L. Pw, "Permafrost," in George A. Kiersch et al. (eds.), The Heritage of Engineering Geology: The First Hundred Years (1991), pp. 277-298, provides an up-to-date, well-illustrated treatment of the origin, distribution, and ice content of permafrost and of engineering problems in permafrost regions. Troy L. Pw