English Language Glossary of Permafrost and Related Ground-Ice Terms
This is the English version of the Multi-Language Glossary of Permafrost and Related Ground-Ice Terms compiled by the IPA's Terminology Working Group and edited by Robert van Everdingen first published in 1998. Dr. van Everdingen made some minor updates and corrections to the Glossary in 2002 and made the full multi-language version available in 2005.
Please cite this version of the glossary as follows:
van Everdingen, Robert, ed. 1998 revised May 2005. Multi-language glossary of permafrost and related ground-ice terms. Boulder, CO: National Snow and Ice Data Center/World Data Center for Glaciology.
A foundation pile on which a cold air refrigeration system has been installed to remove heat from the ground.
COMMENT:
The refrigeration system may be fastened to, or form part of the pile, or, if the pile is a sealed unit (e.g. steel pipe), the pile itself may serve as both a heat-removal device and a structural unit.
REFERENCES: Heuer, 1979; Johnston, 1981.
3. active construction methods in permafrost
Special design and construction methods used for engineering works in permafrost areas where permafrost degradation cannot be prevented.
COMMENT:
Two main approaches are possible. Unfavourable foundation materials can be thawed and compacted or replaced with more suitable materials before the structure is erected. Alternatively, the foundations and structure may be designed to accommodate any thaw settlement that will occur.
REFERENCES: Andersland and Anderson, 1978; Linell and Lobacz, 1980; Johnston, 1981.
5. active ice wedge
An ice wedge that is growing as a result of repeated (but not necessarily annual) winter cracking.
COMMENT:
Active ice wedges developed in mineral soil occur primarily in areas of continuous permafrost.
6. active layer
The layer of ground that is subject to annual thawing and freezing in areas underlain by permafrost.
COMMENT:
In the zone of continuous permafrost the active layer generally reaches the permafrost table; in the zone of discontinuous permafrost it often does not.
The active layer includes the uppermost part of the permafrost wherever either the salinity or clay content of the permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0° C).
The active layer is sometimes referred to as the "active zone"; the term "zone," however, should be reserved for the zones of discontinuous and continuous permafrost.
In Russian and Chinese literature, the term active layer covers two distinct types: (1) the seasonally thawed layer overlying permafrost, and (2) the seasonally frozen layer overlying unfrozen ground inside or outside permafrost areas.
REFERENCES: Muller, 1943; Williams, 1965; Brown, 1971; van Everdingen, 1985.
7. active-layer failure
A general term referring to several forms of slope failures or failure mechanisms commonly occurring in the active layer overlying permafrost.
COMMENT:
These include detachment failures but not retrogressive thaw slumps and thaw slumping since these involve permafrost.
SYNONYM: (not recommended) skin flow.
9. active-layer thickness
The thickness of the layer of the ground that is subject to annual thawing and freeing in areas underlain by permafrost.
COMMENT:
The thickness of the active layer depends on such factors as the ambient air temperature, vegetation, drainage, soil or rock type and total water content, snowcover, and degree and orientation of slope. As a rule, the active layer is thin in the High Arctic (it can be less than 15 cm) and becomes thicker farther south (1 m or more).
The thickness of the active layer can vary from year to year, primarily due to variations in the mean annual air temperature, distribution of soil moisture, and snowcover.
The thickness if the active layer includes the uppermost part of the permafrost wherever either the salinity or clay content of the permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0° C).
Use of the term "depth to permafrost" as a synonym for the thickness of the active layer is misleading, especially in areas where the active layer is separated from the permafrost by a residual thaw layer, that is, by a thawed or noncryotic (T > 0° C) layer of ground.
REFERENCES: Muller, 1943; Williams, 1965; van Everdingen, 1985.
10. active liquid refrigerant pile
A foundation pile on which a liquid coolant refrigeration system has been installed to remove heat from the ground.
COMMENT:
The refrigeration system may be fastened to, or form part of the pile, or, if the pile is a sealed unit (e.g. steel pipe), the pile itself may serve as both a heat-removal device and a structural unit.
REFERENCES: Heuer, 1979; Johnston, 1981.
11. active rock glacier
A mass of rock fragments and finer material, on a slope, that contains either an ice core or interstitial ice, and shows evidence of present movement.
COMMENT:
Active rock glaciers possess steep fronts with slope angles that may exceed the angle of repose. Gradual elimination of such oversteepening of the frontal portion may suggest some movement long after the ice has melted and the main body of the rock glacier has stopped moving.
REFERENCES: White, 1976b; Washburn, 1979.
12. active thermokarst
The process by which characteristic landforms are currently developing as a result of thawing of ice-rich permafrost or melting of massive ice.
COMMENT:
The occurrence of active thermokarst indicates the presence of disequilibrium permafrost.
REFERENCES: French, 1976; Washburn, 1979.
14. adfreeze/adfreezing
The process by which two objects are bonded together by ice formed between them.
COMMENT:
Adfreezing is the process that anchors piles or buried foundations in permafrost, so that they can be unaffected by frost heave occurring in the active layer.
SYNONYM: frost grip.
REFERENCE: Muller, 1943.
15. adfreeze strength
The tensile or shear strength which has to be overcome to separate two objects that are bonded together by ice.
COMMENT:
The term is usually used to describe the resistance to the force that is required to separate frozen ground or an ice mass from an object (frequently a foundation unit) to which it is frozen. The shear stress required to separate an object from frozen ground is frequently referred to as the "tangential adfreeze strength".
REFERENCES: Muller, 1943; Johnston, 1981.
18. aggradational ice
The additional ground ice formed as a direct result of permafrost aggradation.
COMMENT:
Ice lenses form seasonally, especially in the lower part of the active layer, and can be incorporated into the permafrost if they do not melt over a period of years.
REFERENCES: Mackay, 1972b, 1983; Cheng, 1983.
19. air freezing index
The cumulative number of degree-days below 0° C for the air temperature during a given time period.
COMMENT:
The air freezing index differs from the corresponding surface freezing index (see n-factor).
20. air thawing index
The cumulative number of degree-days above 0°C for the air temperature during a given period.
COMMENT:
The air thawing index differs from the corresponding surface thawing index (see n-factor).
21. alas/alass
A large depression of the ground surface produced by thawing of a large area of very thick and exceedingly ice-rich permafrost.
COMMENT:
Typically, alasses are large depressions ranging from 0.5 to more than 100 km2 in area and from 5 to 20 m in depth. In the early stages of formation, a shallow (appx. 2 m) circular "alas lake" forms in a steep-sided depression. Enlargement and ultimate drainage of a number of such lakes may leave low interalas plateaus (termed "mezhalasye" in Russian). Ultimately, the plateaus disappear and mass wasting produces gentle side slopes. The term is of Yakut origin. An analogous term of Nenets (northwestern Siberia) origin is "khasyrey".
REFERENCES: Czudek and Demek, 1970; Soloviev, 1973.
22. albedo
Albedo is a measure of the reflecting power of a surface, expressed as the fraction of the incoming solar radiation reflected by the surface.
COMMENT:
The albedo of natural land surfaces varies over a wide range, and it changes with the seasons, primarily due to changes in vegetation and snowcover.
25. altitudinal limit of permafrost
The lowest altitude at which mountain permafrost occurs in a given highland area outside the general permafrost region.
COMMENT:
The altitudinal limit of mountain permafrost generally rises progressively with decreasing latitude.
REFERENCE: Brown, 1967b.
26. altitudinal zonation of permafrost
The vertical subdivision of an area of mountain permafrost into permafrost zones, based on the proportion of the ground that is perennially cryotic.
COMMENT:
As mean annual temperatures decrease with increasing elevation, mountain permafrost can normally be expected to be more extensive, thicker and colder at higher elevations, although exposure and the extent of vegetation and snow cover will moderate this effect. A noteable exception is found in the southwestern portion of Yukon, where (1) the high St. Elias Mountains block the circulation of moist and warm maritime air from the Pacific Ocean, particularly in winter, and (2) the deeply dissected nature of the terrain promotes cold-air drainage and ponding in the valleys. As a result, permafrost is more extensive, thicker and colder in valley bottoms than at higher elevations.
REFERENCE: Burn, 1994.
29. anti-syngenetic ice wedge
An ice wedge that grows progressively downwards into a receding slope, in a direction normal to the surface .
COMMENT:
The younger portions of the wedge extend farthest down. The downward rate of growth is a function of the rate of slope recession and the rate of formation of new ice veinlets.
REFERENCE: Mackay, 1990.
30. apparent heat capacity
The amount of heat required to raise the temperature of a unit mass of frozen ground by one degree.
COMMENT:
Because the phase change in frozen ground often occurs gradually over a range of temperatures, the apparent heat capacity (which is the sum of the specific heat capacity and the latent heat released) may vary significantly with temperature. Apparent heat capacity is commonly expressed in Joules per kg per degree K.
31. approximate freezing index
The cumulative number of degree-days below 0°C for a given time period, calculated from the mean monthly temperatures for a specific station without making corrections for positive degree-days in spring and fall.
REFERENCES: Boyd, 1973, 1979.
32. approximate thawing index
The cumulative number of degree-days above 0°C for a given time period, calculated from the mean monthly temperatures for a specific station without making corrections for negative degree-days in spring and fall.
REFERENCES: Boyd, 1973, 1979.
33. artificial ground freezing
The process of inducing or maintaining a frozen condition in earth materials by artificial means.
COMMENT:
The frozen condition can be induced or maintained by natural convection movement of air, gases or liquids (e.g., through or in ventilation ducts, thermal piles, thermosyphons), or by forced (mechanical) circulation of cold air or a refrigerant through a system of ducts or freeze pipes in the ground.
REFERENCES: Jumikis, 1977; Johnston, 1981.
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form subhorizontal layers.
COMMENT:
This fabric, found in cryosols, results from freeze-thaw processes accompanied by eluviation.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
36. barrens
Areas of discontinuous vegetation cover in the polar semi-desert of the High Arctic.
COMMENT:
Unvegetated areas of polar desert may be caused by climatic factors (too cold and/or too dry), or edaphic factors (low soil nutrients or toxic substrate), or a combination of those.
37. basal cryopeg
A layer of unfrozen ground that is perennially cryotic (T < 0° C), forming the basal portion of the permafrost .
COMMENT:
In a basal cryopeg, freezing is prevented by freezing-point depression due to the dissolved-solids content of its pore water.
REFERENCE: Tolstikhin and Tolstikhin, 1974.
38. basal cryostructure
The cryostructure of a frozen deposit of boulders that is saturated with ice.
REFERENCE: Kudryavtsev, 1978.
40. basal-layered cryostructure
The cryostructure of a frozen layered deposit of gravel and boulders that is saturated with ice.
REFERENCE: Kudryavtsev, 1978.
44. beaded stream
A stream characterized by narrow reaches linking pools or small lakes.
COMMENT:
A characteristic pattern of small streams in areas underlain by ice wedges. The course of the stream channel is controlled by the pattern of the wedges, with the beads (pools) occurring at the junctions of the wedges. When the intervening channels are dry, they may be called "beaded channels".
SYNONYMS: (not recommended) beaded drainage, button drainage.
REFERENCES: PJwJ, 1954; Hopkins et al., 1955; Ferrians et al., 1969; Brown, 1970b; Lawson and Brown, 1978.
47. block field
A surficial layer of angular shattered rocks formed in either modern or Pleistocene periglacial environments.
SYNONYMS: stone field and (not recommended) felsenmeer, blockmeer.
REFERENCES: French, 1976; Washburn, 1979.
49. bottom temperature of snow cover
Temperature measured at the base of the snow cover during mid- to late-winter (February/March).
COMMENT:
Such measurements are used in the BTS method to predict the presence or absence of permafrost in mountain areas where the winter snow cover is thicker than 0.8 m
REFERENCES: Haeberli, 1973; Hoelzle et al., 1993.
51. BTS method
Method to predict the presence or absence of permafrost in a mountain area, using measurements of the bottom temperature of snow cover mid- to late-winter.
COMMENT:
REFERENCES: Haeberli, 1973; Hoelzle et al., 1993.
54. buried ice
Ice formed or deposited on the ground surface and later covered by sediments.
COMMENT:
Buried ice likely represents buried glacier ice or snowbanks; or less likely, lake, river or sea ice, or icings.
Ice formed in a closed or open cave.
COMMENT:
Cave ice can form and persist in an area of temperate climatic conditions where the configuration of the cave or cave system permits an influx of cold winter air by gravity flow but limits access of warm summer air.
REFERENCE: Harris, 1979.
61. closed-cavity ice
Ice formed in a closed space, cavity or cave in permafrost.
COMMENT:
Along the western Arctic coast of Canada for example, underground cavities, apparently formed by pockets of methane gas, have been found filled with ice crystals. The water from which the crystals have grown probably entered the cavities through vapour diffusion.
REFERENCES: Mackay, 1965, 1972b.
62. closed-system freezing
Freezing that occurs under conditions that preclude the gain or loss of any water by the system.
COMMENT:
Pure closed-system conditions, in which water is neither added to nor removed from the system, are sometimes referred to as "in situ freezing". Closed-system freezing of water-saturated soil causes expansion equal to about 9 percent of the original pore volume of the unfrozen soil. Inside closed systems, some redistribution of water may take place during freezing.
63. closed-system pingo
A pingo formed by doming of frozen ground due to freezing of injected water supplied by expulsion of pore water during permafrost aggradation in the closed talik under a former water body.
COMMENT:
Most closed-system pingos are found in flat, poorly-drained terrain in the zone of continuous permafrost. Repeated injections of expelled water into the overlying permafrost, followed by freezing of the injected water, cause progressive doming, and produce the massive ice forming the core of the pingo. Progressive formation of segregated ice and dilation crack ice can also contribute to the process.
SYNONYMS: hydrostatic pingo and (not recommended) Mackenzie Delta pingo.
REFERENCES: Mackay, 1973b, 1979, 1985.
64. closed talik
A layer or body of unfrozen ground occupying a depression in the permafrost table below a lake or river.
COMMENT:
The temperature of closed lake taliks and river taliks remains above 0°C because of the heat storage effect of the surface water.
REFERENCES: Williams, 1965; Washburn,1973; van Everdingen, 1976.
65. coefficient of compressibility
The volume change per unit volume of a substance per unit increase in effective compressive stress, under isothermal conditions.
COMMENT:
Frozen soils are usually considered to be practically incompressible, and volume-change deformations can therefore be neglected. Numerous investigations have shown, however, that the compressibility of frozen soils can be significant and should not be neglected in some cases, especially when large areas are to be loaded.
Compressibility and its time dependence in frozen soils are due to several causes, such as instantaneous compression of the gaseous phase, creep of the pore ice due to shear stresses at the grain contacts, and hydrodynamic consolidation due to the expulsion of unfrozen water under stress, the amount of which varies with the applied pressure.
66. collapse scar
That portion of a peatland where the whole or part of a palsa or peat plateau has thawed and collapsed to the level of the surrounding peatland.
COMMENT:
A collapse scar is not a depression but is marked by vegetation different from the peatland that did not contain permafrost. Irregular topography (hence thermokarst terrain) may be present on the peatland as a whole but the collapse scars are only part of that thermokarst terrain, marked by the absence of permafrost, and by vegetation different from that on both the previously unfrozen peatlands and the remnant permafrost peat landforms.
REFERENCE: Zoltai, 1971.
68. composite wedge
A wedge showing evidence of both primary and secondary filling.
COMMENT:
When used with reference to thermal contraction crack phenomena in permafrost, it describes a wedge filled by a combination of ice and soil (usually sand). The term is also frequently used to describe Pleistocene sand wedges and ice-wedge casts. Typically, composite wedges are filled with material very similar in structure and texture to sands in a typical fissure of primary filling (i.e., sand wedge). However, there may be inclusions of material from the fissure walls and some distortion of the adjacent sediments bordering the wedge, reflecting the presence of ice in the original fill material.
REFERENCES: Black and Berg, 1966; Gozdzik, 1973; French, 1976; Washburn, 1979.
72. conglomeric cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which coarser soil particles form compound arrangements.
COMMENT:
This fabric, found in cryosols, results from freeze-thaw processes probably accompanied by cryoturbation.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
73. construction methods in permafrost
Special design and construction procedures required when engineering works are undertaken in permafrost areas.
COMMENT:
Design and construction of engineering works on permafrost normally follow one of two broad principles which are based on whether or not the frozen foundation soil or rock is thaw-stable (thaw-stable permafrost) or thaw-unstable (thaw-sensitive permafrost).
Permafrost conditions can be neglected, and conventional designs and construction methods used when foundation materials are stable upon thawing. When the foundation materials are thaw-sensitive, however, then either the "passive" or "active" design and construction method is selected.
The passive method maintains the foundation materials in a frozen state. Preservation of the frozen condition or permafrost aggradation can be accomplished using thermal piles or thermosyphons, or by using either a ventilation or an insulation construction technique; often a combination of these techniques is used.
The ventilation technique requires that there is a clear space left between the bottom of the structure and the ground surface, or that the structure is placed on a fill pad with ducts incorporated in the pad or the floor system. Movement of cold air through the air space or ducts dissipates heat from the structure above and removes heat from the ground below.
Using the insulation technique, a relatively thick fill (sometimes containing a layer of insulation) is placed on the ground surface to prevent thawing or to reduce or control the depth of thaw below the original ground surface.
When permafrost degradation cannot be prevented, then one of the active methods must be considered. Two main approaches are possible. Unfavourable foundation materials are thawed and compacted or replaced with more suitable materials before the structure is erected. Alternatively, the foundations and structure are designed to accommodate the thaw settlements that will occur.
REFERENCES: Andersland and Anderson, 1978; Linell and Lobacz, 1980; Johnston, 1981.
76. continuous permafrost
Permafrost occurring everywhere beneath the exposed land surface throughout a geographic region with the exception of widely scattered sites, such as newly deposited unconsolidated sediments, where the climate has just begun to impose its influence on the thermal regime of the ground, causing the development of continuous permafrost.
COMMENT:
For practical purposes, the existence of small taliks within continuous permafrost has to be recognized. The term, therefore, generally refers to areas where more than 90 percent of the ground surface is underlain by permafrost.
REFERENCE: Brown, 1970b.
77. continuous permafrost zone
The major subdivision of a permafrost region in which permafrost occurs everywhere beneath the exposed land surface with the exception of widely scattered sites.
COMMENT:
Taliks associated with rivers and lakes may occur in the continuous permafrost zone.
REFERENCE: Brown, 1970b.
79. creep of frozen ground
The slow deformation (or time-dependent shear strain) that results from long-term application of a stress too small to produce failure in the frozen material.
COMMENT:
In frozen soils, creep deformations are due mainly to the creep of pore ice and the migration of unfrozen pore water. In ice-saturated frozen soils, most creep deformations are distortional with little or no volume change. In frozen soils with large unfrozen water contents or in unsaturated frozen soils, slow deformations due to consolidation, and creep due to volume change, may also occur. Usually, a large portion of the creep deformation is permanent.
REFERENCES: Vyalov, 1959; Ladanyi, 1972, 1981.
80. creep strength
The failure strength of a material at a given strain rate or after a given period under deviatoric stress.
COMMENT:
Under high confining stress and at relatively high freezing temperatures, most frozen soils creep and eventually fail in a plastic manner.
83. crust-like cryostructure
The cryostructure of a frozen deposit of angular blocks that are coated with ice, whereas large spaces between the blocks are not filled with ice.
COMMENT:
A deposit with a crust-like cryostructure has a relatively low ice content.
REFERENCE: Kudryavtsev, 1978.
87. cryofront
The boundary between cryotic and noncryotic ground as indicated by the position of the 0°C isotherm in the ground.
COMMENT:
The permafrost base and the boundaries between noncryotic and cryotic portions of the active layer constitute cryofronts. As a result of freezing-point depression, the freezing front usually lags behind the cryofront as it moves downwards during annual freezing of the active layer.
REFERENCE: van Everdingen, 1976.
88. cryogenesis
The combination of thermophysical, physico-chemical and physico-mechanical processes occurring in freezing, frozen and thawing earth materials.
COMMENT:
Specific processes of cryogenesis include water migration during freezing and thawing of the ground, frost heave, heat and mass (moisture) exchange, regelation and gelifluction. In the Russian permafrost literature, cryogenesis includes mainly processes that lead to development of permafrost cryostructure and landforms.
REFERENCE: Poppe and Brown, 1976.
89. cryogenic aquiclude
A layer of ground which, because of its frozen state, has a low enough permeability to act as a confining bed for an aquifer.
COMMENT:
Annual freezing can turn the active layer into a cryogenic aquiclude.
REFERENCE: Fotiev, 1978.
90. cryogenic fabric
The distinct soil micromorphology resulting from the effects of freezing and thawing processes.
COMMENT:
The following cryogenic fabrics, found in cryosols, can be distinguished under a microscope:
1. granic and granoidic fabrics - soil particles form discrete loosely packed units. These fabrics are generally attributed to freeze-thaw processes and the formation of needle ice near the ground surface.
2. fragmic and fragmoidal fabrics - soil particles form discrete units that are either densely packed or coalescing. These fabrics commonly occur in subsurface soil horizons, usually close to the freezing front where soil material is subject to ice lens formation.
3. banded and isoband fabrics - soil particles form subhorizontal layers which result from freeze-thaw processes accompanied by eluviation.
4. orbiculic, suscitic and conglomeric fabrics - coarser soil particles form circular to ellipsoidal patterns (orbiculic), vertical or near-vertical orientation (suscitic), and compound arrangements (conglomeric), probably as a result of cryoturbation activity.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
95. cryogenic temperature
In international materials science, this term refers to temperatures generally below -50°C, but usually to temperatures within a few degrees of absolute zero (-273°C).
101. cryolithology
The study of the genesis, structure and lithology of frozen earth materials.
COMMENT:
This is a Russian term not widely used in North America. It is a branch of geocryology in which lithological and ground-ice conditions are emphasized.
106. cryopedology
The study of soils at temperatures below 0°C, with particular reference to soils subject to intensive frost action, and to soils overlying permafrost.
COMMENT:
As originally defined (Bryan, 1946), the term comprised the whole science of geocryology, including the civil engineering methods used to overcome difficulties resulting from intensive frost action and the existence of permafrost.
REFERENCES: Bryan, 1946; Canada Soil Survey Committee, 1978.
107. cryopeg
A layer of unfrozen ground that is perennially cryotic (forming part of the permafrost), in which freezing is prevented by freezing-point depression due to the dissolved-solids content of the pore water.
COMMENT:
Three types of cryopeg can be distinguished on the basis of their position with respect to permafrost:
1. a basal cryopeg forms the basal portion of the permafrost;
2. an isolated cryopeg is entirely surrounded by perennially frozen ground;
3. a marine cryopeg is found in coastal or subsea perennially frozen ground; marine cryopegs may also be basal and/or isolated. In Russian literature cryopegs are defined as lenses of "cryosaline water" (supercooled brine) found within saline cryotic soils or rock.
REFERENCE: Tolstikhin and Tolstikhin, 1974.
108. cryoplanation
The process through which cryoplanation terraces form.
COMMENT:
Cryoplanation occurs most frequently in periglacial areas of moderate aridity underlain by permafrost, under conditions of intense frost wedging associated with snowbanks.
109. cryoplanation terrace
A step-like or table-like bench cut in bedrock in cold climate regions.
COMMENT:
Cryoplanation terraces may occur as both hillside benches or bevelled summit surfaces and often lack structural control. They are thought to form under conditions of intense frost wedging associated with snowbanks. Cryoplanation terraces are more frequently reported from periglacial areas of moderate aridity. As these areas are usually underlain by permafrost, cryoplanation terraces are regarded by some as diagnostic landforms of permafrost terrain.
REFERENCES: Eakin, 1916; Demek, 1969; French, 1976; Reger and PJwJ, 1976; Washburn, 1979.
110. cryosol
Soil formed in either mineral or organic materials having permafrost either within 1 m below the surface or, if the soil is strongly cryoturbated, within 2 m below the surface, and having a mean annual ground temperature below 0°C.
COMMENT:
This is a pedological term for a major soil order. Cryosols are divided into three groups:
1. organic cryosols developed in organic (peat) materials;2. static cryosols developed in mineral soils and showing little or no evidence of cryoturbation;
3. turbic cryosols developed in mineral soils and showing strong evidence of cryoturbation.
SYNONYMS: (not recommended) cryosolic soil, pergelic soil.
REFERENCE: Canada Soil Survey Committee, 1978.
112. cryosphere
That part of the earth's crust, hydrosphere and atmosphere subject to temperatures below 0EC for at least part of each year.
COMMENT:
The cryosphere may be divided into the cryoatmosphere, the cryohydrosphere (snowcover, glaciers, and river, lake and sea ice) and the cryolithosphere (perennially and seasonally cryotic ground). Some authorities exclude the earth's atmosphere from the cryosphere; others restrict the term "cryosphere" to the regions of the earth's crust where permafrost exists.
REFERENCE: Baranov, 1978.
114. cryostructure
The structural characteristics of frozen earth materials.
COMMENT:
The cryostructure is determined by the amount and distribution of pore ice (or ice cement) and lenses of segregated ice. The type and arrangement of ice in the frozen material will depend largely on the initial total water content of the material and the extent of moisture migration during subsequent freezing.
For engineering purposes, the structure of frozen soil may be described as massive, layered or reticulate, based on the type and distribution of ice in the soil. A massive structure (not to be confused with massive ground-ice forms) is characterized by the predominant presence of pore ice and by a relatively low total ice content. In soils with a reticulate structure, ice veins generally form a random net, whereas in those with a layered structure, well-oriented horizontal ice lenses alternate with soil layers having a massive structure. In both cases their total ice content is relatively high.
Russian permafrost scientists identify ten cryogenic structures or cryostructures: basal, basal-layered, crust-like, layered, lens-type, massive-agglomerate, massive, massive-porous, reticulate and reticulate-blocky.
REFERENCES: U.S.S.R., 1969, 1973; Poppe and Brown, 1976; Kudryavtsev, 1978.
115. cryosuction
A suction developed in freezing or partially frozen fine-grained materials as a result of temperature-dependent differences in unfrozen water content.
COMMENT:
Cryosuction occurs where gradients of the temperature-dependent unfrozen water content in a freezing or partially frozen fine-grained earth material cause hydraulic gradients large enough to induce migration of pore water from unfrozen soil into the partially frozen soil via unfrozen water films.
SYNONYM: (not recommended) frost suction.
REFERENCES: Blanchard and FrJmond, 1982.
117. cryotexture
The textural characteristics of frozen, fine-grained organic and mineral earth materials cemented together with ice.
COMMENT:
Cryotextures may be useful in determining the nature of the freezing process and the conditions under which the sediments accumulated.
REFERENCES: Poppe and Brown, 1976; Kudryavtsev, 1978.
118. cryotic ground
Soil or rock at temperatures of 0°C or lower.
COMMENT:
The terms "cryotic" and "noncryotic" were introduced to solve a major semantic problem identified by Brown and Kupsch (1974), namely the lack of specific separate terms to designate "above 0°C" and "below 0°C" as opposed to "unfrozen" (not containing ice) and "frozen" (containing ice). Cryotic and noncryotic refer solely to the temperature of the material described, independent of its water or ice content. Perennially cryotic ground refers to ground that remains at or below 0°C continuously for two or more years and is therefore synonymous with permafrost.
REFERENCES: Brown and Kupsch, 1974; van Everdingen, 1976.
119. cryoturbate
A body of earth material moved or disturbed by frost action.
COMMENT:
The process of cryoturbation results in the disruption and distortion of soil horizons and structures, the formation of patterned ground, the growth of involutions, and the redistribution of organic-rich subsurface masses and layers. Downslope soil movements are more properly termed solifluction, gelifluction and frost creep, although many authors include the products of solifluction within the term cryoturbate.
SYNONYMS: cryoturbations and (not recommended) congeliturbate.
120. cryoturbation
1. (Singular) A collective term used to describe all soil movements due to frost action.
2. (Plural) Irregular structures formed in earth materials by deep frost penetration and frost action processes, and characterized by folded, broken and dislocated beds and lenses of unconsolidated deposits, included organic horizons and even bedrock.
COMMENT:
Cryoturbation encompasses frost heave, thaw settlement and all differential movements, including expansion and contraction due to temperature changes and the growth and disappearance of ground ice bodies, whether perennial or seasonal. Low temperatures alone are not enough to produce cryoturbation; the water-ice phase change is necessary. Cryoturbation is an important process in the development of patterned ground.
SYNONYMS: 1. (not recommended) congeliturbation, frost churning, frost stirring; 2. cryoturbates.
REFERENCES: French, 1976; Washburn, 1979.
A sudden and destructive variety of landslide, in which loose material on a slope, with more than 50 percent of particles larger than sand size, is mobilized by saturation and flows down a channel or canyon.
COMMENT:
The water content of debris flows may range from 10 to over 50 percent by volume. They may transport huge boulders for great distances down gentle slopes. Debris flows also occur in non-permafrost areas.
122. deformability
The ability of a material to change its shape or size under the influence of an external or internal agency, such as stress, temperature, or pore pressure.
123. degree-day (C or F)
A derived unit of measurement used to express the departure of the mean temperature for a day from a given reference (or base) temperature.
COMMENT:
The freezing index and the thawing index are expressed in degree-days with respect to a reference temperature of 0°C (32EF); units: degree-day Celsius or degree-day Fahrenheit.
REFERENCE: Boyd, 1979.
124. degree of saturation
1. The total degree of saturation of frozen soil is the ratio of the volume of ice and unfrozen water in the soil pores to the volume of the pores.
2. The degree of saturation of frozen soil by ice is the ratio of the volume of ice in the soil pores to the volume of the pores.
COMMENT:
Neither definition is the equivalent of the standard definition used in soil mechanics where the degree of saturation is the ratio of the volume of water in a soil to the volume of the pores.
125. delayed strength
The failure strength of a material at a given strain rate or after a given period under deviatoric stress.
126. density of frozen ground
The mass of a unit volume of frozen soil or rock.
129. depth of seasonal frost penetration
The maximum thickness of the seasonally frozen layer.
COMMENT:
This term is primarily used in areas without permafrost, but also in areas with permafrost where the seasonal frost does not reach the permafrost table.
130. depth of thaw
The minimum distance between the ground surface and frozen ground at any time during the thawing season in an area subject to seasonal freezing and thawing.
COMMENT:
When no frozen ground remains, thawing is complete, and depth of thaw cannot be determined.
131. depth of zero annual amplitude
The distance from the ground surface downward to the level beneath which there is practically no annual fluctuation in ground temperature.
COMMENT:
A change of no more than 0.1°C throughout the year is arbitrarily considered as "practically no annual fluctuation". The temperature at the depth (or level) of zero annual amplitude ranges from about -0.1°C at the southern limit of the permafrost region to about -15°C in the extreme polar reaches of the zone of continuous permafrost. The depth of zero annual amplitude varies widely but generally lies between 10 and 20 m below the ground surface, depending on climatic and terrain conditions such as amplitude of annual surface temperature variation, vegetation, snowcover and characteristics of the soils and rocks including thermal diffusivity (see thermal properties of frozen ground).
SYNONYMS: (not recommended) zone of minimum annual amplitude, zone of zero annual amplitude.
REFERENCE: Muller, 1943.
133. desiccation crack
Crack or fissure developed in fine-grained soil material as a result of shrinkage during drying.
134. desiccation polygon
Closed, multi-sided patterned-ground feature formed by desiccation cracks in fine-grained soil material. Usually less than 2 m in diameter.
135. design depth of frost penetration
1. (North-American usage) The mean of the three largest depths of seasonal frost penetration measured during the past thirty years, or the largest depth of seasonal frost penetration beneath a snow-free soil surface measured during the past ten years.
2. (Russian usage) The mean of the depths of seasonal frost penetration measured during at least the last ten years with the ground surface free of snow and the groundwater level below the depth of seasonal frost penetration.
136. design freezing index
The cumulative number of degree-days below 0°C, calculated by taking the average of the seasonal freezing indices for the three coldest winters in the most recent 30 years of record.
COMMENT:
If data for 30 years are not available, then the index is based on the coldest winter in the latest 10-year period of record.
REFERENCE: U.S. Army/Air Force, 1966.
137. design thawing index
The cumulative number of degree-days above 0°C, calculated by taking the average of the seasonal thawing indices for the three warmest summers in the most recent 30 years of record.
COMMENT:
If data for 30 years are not available, then the index is based on the warmest summer in the latest 10-year period of record.
REFERENCE: U.S. Army/Air Force, 1966.
138. detachment failure
A slope failure in which the thawed or thawing portion of the active layer detaches from the underlying frozen material.
COMMENT:
Detachment failures are common on colluvial slopes in areas of fine-grained, ice-rich deposits. They occur more frequently during warm summers or following disturbance of the vegetation or ground surface by, for example, tundra or forest fires or engineering activity, when the depth of thaw is greater than normal. Detachment failures that expose massive ice or icy sediments can develop into retrogressive thaw slumps.
SYNONYMS: (not recommended) skin flow, active-layer glide.
REFERENCES: Hughes, 1972; McRoberts and Morgenstern, 1974.
139. dielectric constant
The dielectric constant of a soil is a measure of the ability of the soil to store electrical energy in the presence of an electrostatic field.
COMMENT:
The dielectric constant is calculated as the ratio of the permittivity of the soil to the permittivity of a vacuum. It varies with soil type, temperature, unfrozen water content, and the dissolved-solids content of the pore water.
SYNONYM: relative permittivity.
140. dilation crack
A tensile fracture in a frozen material due to surface extension caused by doming.
COMMENT:
This usually occurs during the growth of frost mounds.
141. dilation crack ice
Ice that forms in dilation cracks.
COMMENT:
Dilation crack ice may form a significant component of the total ice content of features such as pingos. The ice is vertically banded and may be discoloured by inclusions of mineral soil and organic matter. Individual bands may be up to 20 cm wide.
REFERENCES: Mackay, 1979, 1985.
142. discontinuous permafrost
Permafrost occurring in some areas beneath the exposed land surface throughout a geographic region where other areas are free of permafrost.
COMMENT:
Discontinuous permafrost occurs between the continuous permafrost zone and the southern latitudinal limit of permafrost in lowlands. Depending on the scale of mapping, several subzones can often be distinguished, based on the percentage of the land surface underlain by permafrost. The following table present details regarding recent North-American and Russian usage.
| English usage |
Permafrost |
Russian Usage |
Permafrost |
| Extensive |
65-90% |
Massive Island |
70-80% |
| Intermediate |
35-65% |
Island |
40-60% |
| Sporadic |
10-35% |
Sporadic |
5-30% |
| Isolated Patches |
0-10% |
? |
0-5% |
SYNONYMS: (not recommended) insular permafrost; island permafrost; scattered permafrost.
REFERENCES: Brown, 1970b; Heginbottom and Radburn, 1992; Kudryavtsev, 1978.
143. discontinuous permafrost zone
The major subdivision of a permafrost region in which permafrost occurs in some areas beneath the exposed land surface, whereas other areas are free of permafrost.
COMMENT:
The zone of discontinuous permafrost lies between the continuous permafrost zone and the southern latitudinal limit of permafrost in lowlands. Near its northern boundary, discontinuous permafrost is extensive, whereas near its southern boundary it occurs as isolated patches of permafrost, and sporadic permafrost. There is no sharp distinction, or boundary, between the continuous and discontinuous permafrost zones.
REFERENCE: Brown, 1970b.
144. disequilibrium permafrost
Permafrost that is not in thermal equilibrium with the existing mean annual surface or sea-bottom temperature and the geothermal heat flux.
REFERENCE: Mackay, 1972a.
145. drunken forest
Trees leaning in random directions.
COMMENT:
A descriptive term for trees usually growing on ice-rich terrain and subject to repeated differential frost heave or thermokarst subsidence. Active, forested rock glaciers may also exhibit this phenomenon due to differential movements.
REFERENCES: Muller, 1943; Zoltai, 1975.
146. dry density
The mass of a unit volume of dried material (e.g. soil).
COMMENT:
To determine the dry density of frozen ground, a measured volume of the material has to be thawed and then dried and weighed.
147. dry frozen ground
Frozen ground with a very low total water content consisting almost completely of interfacial water, and not cemented by ice.
COMMENT:
The compressibility of dry frozen ground is the same as that of unfrozen ground of the same composition, total water content, and density.
REFERENCE: van Everdingen, 1976.
148. dry permafrost
Permafrost containing neither free water nor ice.
COMMENT:
A negligible quantity of moisture in the form of interfacial water may be present. Dry permafrost is thaw-stable.
REFERENCE: van Everdingen, 1976.
149. dynamic modulus of elasticity
The ratio of stress to strain for a material under dynamic loading conditions.
150. dynamic Poisson's ratio
The absolute value of the ratio between the linear strain changes, perpendicular to and in the direction of a given uniaxial stress change, respectively, under dynamic loading conditions.
A hummock having a core of silty and clayey mineral soil which may show evidence of cryoturbation.
COMMENT:
Earth hummocks are a type of nonsorted circle (see also patterned ground) commonly found in the zone of continuous permafrost. They develop in materials of a high silt and clay content and/or of high ice content. Earth hummocks found outside the southern limit of present-day permafrost are believed to have formed during a previous period of cooler climate when the area was underlain by permafrost.
SYNONYMS: mud hummock and (not recommended) earth mound, tundra hummock.
REFERENCES: Tarnocai and Zoltai, 1978; Washburn, 1979.
154. electrical conductivity
The inverse of electrical resistivity.
COMMENT:
The electrical conductivity of unfrozen soils decreases slightly with a decrease in temperature. A significant decrease in conductivity occurs when a soil freezes, but this change is directly related to the decreasing unfrozen water content of the frozen ground, and it may be affected by cryostructure. Available data show that the conductivities of frozen ground may be from 1 to more than 2 orders of magnitude smaller than those of the same material when unfrozen. The conductivity of frozen soils decreases further with further decreases in temperature.
155. electrical properties of frozen ground
The dielectric constant (or relative permittivity), electrical conductivity and electrical resistivity are the major electrical properties governing the flow of electric current through frozen ground.
COMMENT:
The dielectric constant of a soil is a measure of the ability of the soil to store electrical energy in the presence of an electrostatic field; it is the ratio of the soil's permittivity to the permittivity of a vacuum. The electrical conductance of a soil is the inverse of the resistance offered by a soil to current flow.
Current flow under an electrical gradient in a frozen soil occurs almost entirely through the unfrozen water films. Electrical conduction is related to the thickness of these water films and their degree of interconnection; it decreases with decreasing temperature and increases with increasing pressure. As a consequence, all these electrical properties are influenced by soil type, density, salinity, temperature and, in particular, the unfrozen water content.
For example, the resistivity of unfrozen soils increases slightly with decrease in temperature. A significant increase in resistivity occurs when the soil freezes; this change is directly related to the decreasing unfrozen water content of the frozen ground. Available data show that the resistivities of frozen soils and rocks may be from 10 to more than 100 times larger than those of the same materials when unfrozen.
Electromagnetic geophysical techniques for mapping permafrost, and the electrical grounding of various types of machines and electrical equipment, power transmission systems and radio transmitting antennae in permafrost areas, require a knowledge of the electrical properties of frozen ground.
REFERENCE: Johnston, 1981.
156. electrical resistivity
The property of a material that determines the electrical current flowing through a centimetre cube of the material when an electrical potential is applied to opposite faces of the cube.
COMMENT:
The electrical resistivity of unfrozen soils increases slightly with a decrease in temperature. A significant increase in resistivity occurs when a soil freezes, but this change is directly related to the decreasing unfrozen water content of the frozen ground, and it may be affected by cryostructure. Available data show that the resistivities of frozen ground may be from 10 to more than 100 times larger than those of the same material when unfrozen. The resistivity of frozen soils increases further with further decreases in temperature.
157. epigenetic ice
Ground ice developed in epigenetic permafrost, or in previously formed syngenetic permafrost.
COMMENT:
If epigenetic ice occurs in the form of ice lenses in which the volume of ice to soil is large, the more descriptive term segregated ice is preferred. Examples of epigenetic ice also include wedge ice and intrusive ice.
REFERENCE: Mackay, 1972b.
158. epigenetic ice wedge
An ice wedge developed in epigenetic permafrost, or in previously formed syngenetic permafrost.
COMMENT:
Epigenetic ice wedges grow progressively wider, rather than deeper, and are characteristically wedge-shaped. The ice of an epigenetic ice wedge is oldest on the outside edges.
REFERENCES: Romanovskii, 1973; Mackay, 1990.
159. epigenetic permafrost
Permafrost that formed through lowering of the permafrost base in previously deposited sediment or other earth material.
160. equilibrium permafrost
Permafrost that is in thermal equilibrium with the existing mean annual surface or sea-bottom temperature and with the geothermal heat flux.
SYNONYM: contemporary permafrost.
REFERENCE: Mackay, 1972a.
161. excess ice
The volume of ice in the ground which exceeds the total pore volume that the ground would have under natural unfrozen conditions.
COMMENT:
In standard geotechnical terminology, a soil is considered normally consolidated when its total pore volume or its total water content is in equilibrium with the acting gravity stresses. Due to the presence of ground ice, the total water content of a frozen soil may exceed that corresponding to its normally consolidated state when unfrozen. As a result, upon thawing, a soil containing excess ice will settle under its own weight until it attains its consolidated state.
162. extensive discontinuous permafrost
1. (North-American usage) Permafrost underlying 65 to 90 percent of the area of exposed land surface.
2. (Russian usage) Permafrost underlying 70 to 80 percent of the area of exposed land surface.
COMMENT:
It is suggested that this term be used as a replacement for "widespread" discontinuous permafrost, because the word "widespread" can mean either "extensively distributed" or "widely spaced".
SYNONYM: widespread discontinuous permafrost.
REFERENCE: Heginbottom and Radburn, 1992.
Soil micromorphology.
175. fragmic cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form discrete units that are densely packed.
COMMENT:
This fabric, found in cryosols, commonly occurs in soil horizons close to the freezing front, where soil material is subject to ice lense formation.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
176. fragmoidal cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form discrete units that are coalescing.
COMMENT:
This fabric, found in cryosols, commonly occurs in soil horizons close to the freezing front, where soil material is subject to ice lense formation.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
177. free water
Free water is that portion of the pore water that is free to move between interconnected pores under the influence of gravity.
COMMENT:
The term free water also covers water in fissures, solution channels, and other openings in soils or rocks. The temperature at which free water will change phase depends primarily on its dissolved-solids content, which determines the freezing-point depression.
179. freeze-thaw cycle
Freezing of a material followed by thawing.
COMMENT:
Natural freeze-thaw cycles consist of the freezing and subsequent thawing during a freezing season and the following thawing season. In laboratory experiments and geotechnical testing, freeze-thaw cycles are usually much shorter (several days to a few months).
180. freezeback
Refreezing of thawed materials.
COMMENT:
This term is used to describe:
1. seasonal refreezing of the thawed active layer, or
2. refreezing of soil thawed as a result of construction activity or drilling of a well in permafrost, and of soil placed as backfill or a slurry around foundations or engineering facilities buried or embedded in frozen ground, e.g., pipelines, piles or shallow foundations in permafrost.
REFERENCE: Johnston, 1981.
181. freezing (of ground)
The changing of phase from water to ice in soil or rock.
COMMENT:
The temperature at which ground freezing starts may be lower than 0°C as a result of freezing-point depression.
183. freezing front
The advancing boundary between frozen (or partially frozen) ground and unfrozen ground.
COMMENT:
In the usual case, where the active layer extends to the permafrost table, two freezing fronts will be present during annual freezing of the ground, one moving downward from the ground surface, the other moving upward from the permafrost table.
The freezing front may not coincide with the 0°C isotherm (cryofront).
SYNONYMS: (not recommended) freezing plane, frost front.
REFERENCES: Corte, 1962; Mackay, 1974a; van Everdingen, 1976.
184. freezing index
The cumulative number of degree-days below 0°C for a given time period.
COMMENT:
Four main types of air freezing indices have been used:
1. approximate freezing index - calculated from the mean monthly air temperatures for a specific station without making corrections for positive degree-days (T > 0°C) in spring and fall (Boyd, 1973, 1979);
2. total annual freezing index - calculated by adding all the negative mean daily air temperatures (°C) for a specific station during a calendar year (Harris, 1981);
3. seasonal freezing index - calculated as the arithmetic sum of all the negative and positive mean daily air temperatures (°C) for a specific station during the time period between the highest point in the fall and the lowest point the next spring on the cumulative degree-day time curve (Huschke, 1959);
4. design freezing index - calculated by taking the average of the seasonal freezing indices for the three coldest winters in the most recent 30 years of record. If data for 30 years are not available, then the index is based on the coldest winter in the latest 10-year period of record (U.S. Army/Air Force, 1966).
The total annual freezing index has been used to predict permafrost distribution, and the design freezing index is commonly used in engineering to estimate the maximum thickness of lake ice and the maximum depth of frost penetration in the ground.
A surface (ground, pavement, etc.) freezing index differs from the air freezing index (see n-factor).
187. freezing point
1. The temperature at which a pure liquid solidifies under atmospheric pressure.
2. The temperature at which a ground material starts to freeze.
COMMENT:
See also freezing-point depression.
188. freezing-point depression
The number of degrees by which the freezing point of an earth material is depressed below 0°C.
COMMENT:
The highest temperature at which soil, water, ice and air can coexist at equilibrium. In soils, the freezing-point depression is due mainly to capillarity and surface adsorption. It depends on soil particle effects (curvature and nature of soil particle surfaces), pressure, and the effects of dissolved solids in the pore water.
The freezing-point depression can often be determined from the ground temperature profile where it is the difference between the temperature at the base of the ice-bearing permafrost and 0°C.
REFERENCES: Anderson and Morgenstern, 1973; van Everdingen, 1976; Osterkamp and Payne, 1981.
189. freezing pressure
The positive pressure developed at ice-water interfaces in a soil as it freezes.
COMMENT:
It is also known to result in a heaving pressure or frost-heave pressure that is responsible for the heaving of utilities, foundations and pavements. Ice-water interfaces occur at the contact between ice lenses and the frozen fringe.
REFERENCES: Jumikis, 1977; Andersland and Anderson, 1978; Johnston, 1981; Gilpin, 1982.
190. friable permafrost
Permafrost in which the soil particles are not held together by ice.
191. frost
The occurrence of air temperatures below 0°C.
COMMENT:
An alternative definition states that "frost is the condition existing when the surface temperature falls below freezing (0°C)". In British usage, "ground frost" is recorded when a minimum thermometer just above a grass surface reads less than 30.4EF.
192. frost action
The process of alternate freezing and thawing of moisture in soil, rock and other materials, and the resulting effects on materials and on structures placed on, or in, the ground.
COMMENT:
Frost action in soils describes the detrimental processes of frost heave that occurs in the ground during the freezing period, and thaw weakening (followed by thaw settlement) that occurs as the seasonally frozen ground thaws.
Although it normally refers to seasonal freezing and thawing processes and effects, the term "frost action" has also been used to describe the long-term heaving that occurs when soils are subjected continuously to a freezing temperature over a long period of time (years), e.g., under cold storage plants and buried chilled pipelines.
Frost action contributes to the mechanical weathering (i.e., disintegration or breakdown) of soil and rock materials, by frost wedging, cryoturbation activity, and to the development of cryotexture and cryogenic fabric in soils.
REFERENCES: Hennion, 1955; Washburn, 1979; Johnston, 1981.
193. frost blister
A seasonal frost mound produced through doming of seasonally frozen ground by a subsurface accumulation of water under elevated hydraulic potential during progressive freezing of the active layer.
COMMENT:
Freezing of the accumulated subsurface water produces a domed layer of clear ice beneath the overlying frozen ground. Frost blisters are formed in a single winter; their decay may take more than a year. They are distinguished from icing blisters by the layer of seasonally frozen ground overlying the ice layer.
SYNONYMS: (not recommended) cryolaccolith, hydrolaccolith, seasonal pingo, winter pingo.
REFERENCES: Muller, 1943; van Everdingen, 1978; Pollard and French, 1984.
194. frost boil
A small mound of soil material, presumed to have been formed by frost action.
COMMENT:
A type of nonsorted circle; they are commonly found in fine-grained sediments underlain by permafrost, but also occur in non-permafrost areas.
SYNONYM: mud boil.
REFERENCES: Thorn, 1976; Shilts, 1978.
195. frost bulb
A more or less symmetrical zone of frozen ground formed around a buried chilled pipeline or beneath or around a structure maintained at temperatures below 0°C.
COMMENT:
Heaving of the ground and/or of a structure or facility may occur as the frost bulb forms.
REFERENCES: Andersland and Anderson, 1978; Johnston, 1981.
200. frost creep
The net downslope displacement that occurs when a soil, during a freeze-thaw cycle, expands normal to the ground surface and settles in a nearly vertical direction.
REFERENCES: Benedict, 1970; Washburn, 1979.
204. frost heave
The upward or outward movement of the ground surface (or objects on, or in, the ground) caused by the formation of ice in the soil.
COMMENT:
Frost action in fine-grained soils increases the volume of the soil not only by freezing of in situ pore water (. 9% expansion) but also by drawing water to the freezing front where ice lenses form. Soils that have undergone substantial heaving may consist of alternate layers of ice-saturated soil and relatively clear ice lenses. The lenses are formed normal to the direction of heat flow and when freezing penetrates from the ground surface (which may be horizontal, sloping or vertical), they form parallel to that surface. When unrestrained, the amount of surface heave may be almost as much as the total thickness of the ice lenses. Frost heave can occur seasonally or continuously if freezing of the ground proceeds without interruption over a period of years.
Differential, or non-uniform, frost heaving is one of the main detrimental aspects of the frost action process and reflects the heterogeneous nature of most soils, or variations in heat removal rate and groundwater supply over short distances.
Depending on the degree of restraint, large freezing pressures (up to 1 MPa) can be developed as the ground freezes. These can be transmitted to a foundation, structure or other object placed on the ground surface, or embedded or buried in the ground, as basal (i.e., vertical) forces acting on their underside, or through adfreezing of the soil to the sides of the foundation, structure or object.
REFERENCES: Penner, 1967, 1968; Washburn, 1979; Linell and Lobacz, 1980; Chamberlain, 1981; Johnston, 1981.
205. frost-heave extent
The difference between the elevations of the ground surface before and after the occurrence of frost heave.
208. frost jacking
Cumulative upward displacement of objects embedded in the ground, caused by frost action.
COMMENT:
Frost jacking results from basal or vertical freezing pressures acting on the underside of a foundation, structure or object, or from adfreezing of soil to the sides of these objects. The cumulative upward movement over a period of time (one or several freeze-thaw cycles) may result in the foundation unit or object being ejected from the ground.
Fence posts and utility poles or towers are commonly affected, and both deep (pile) and shallow (post) foundations (e.g., used for bridges, wharves, unheated or lightweight buildings) have been seriously affected by frost jacking.
Blocks of jointed or fractured bedrock have also been displaced upward by frost jacking.
SYNONYM: (not recommended) upfreezing.
REFERENCE: Linell and Lobacz, 1980.
209. frost mound
Any mound-shaped landform produced by ground freezing combined with accumulation of ground ice due to groundwater movement or the migration of soil moisture.
COMMENT:
Various types of frost mounds, (e.g., frost blisters, icing blisters, palsas and pingos) can be distinguished on the basis of their structure and duration, and the character of the ice contained in them.
SYNONYMS: (not recommended) bugor, bulgunniakh, cryogenic mound, earth mound, frost hummock, ground-ice mound, tundra hummock.
REFERENCES: Porsild, 1938; Muller, 1943; van Everdingen, 1978; Pollard and French, 1984.
210. frost penetration
The movement of the freezing front into the ground during freezing.
211. frost phenomena
Effects on earth materials and on structures placed in or on the ground, resulting from frost action.
216. frost shattering
The mechanical disintegration of rock by the pressure of the freezing of water in pores and along grain boundaries.
COMMENT:
Frost shattering is the process of grain loosening and rock disintegration by the freezing pressure of water in films of varying thickness on the surfaces of individual mineral grains. Freezing of the water drawn between the grains by various particle surface forces exerts sufficient differential pressure to loosen and separate the grains.
217. frost sorting
The differential movement of soil particles of different size ranges as a result of frost action.
COMMENT:
Frost sorting often accompanies cryoturbation.
REFERENCE: Washburn, 1979.
219. frost-stable ground
Ground (soil or rock) in which little or no segregated ice forms during seasonal freezing.
COMMENT:
Significant cumulative ice segregation and frost heave may occur even in seasonally frost-stable ground (e.g., gravels and rock) under conditions of continuous freezing and plentiful water supply.
SYNONYM: (not recommended) non-frost-susceptible ground.
REFERENCES: van Everdingen, 1976; Chamberlain, 1981; Konrad and Morgenstern, 1983.
220. frost-stable soil
Soil in which little or no segregated ice forms during seasonal freezing.
SYNONYM: (not recommended) non-frost-susceptible soil.
223. frost-susceptible ground
Ground (soil or rock) in which segregated ice will form (causing frost heave) under the required conditions of moisture supply and temperature.
COMMENT:
Frost-susceptible ground will eventually become ice-rich, regardless of its initial total water content, if the appropriate moisture supply and temperature conditions persist. By implication, frost-susceptible ground may also be susceptible to thaw weakening effects when it thaws.
SYNONYM: (not recommended) frost-sensitive ground.
REFERENCES: van Everdingen, 1976; Chamberlain, 1981; Konrad and Morgenstern, 1983.
224. frost-susceptible soil
Soil in which segregated ice will form (causing frost heave) under the required conditions of moisture supply and temperature.
SYNONYM: (not recommended) frost-sensitive soil.
226. frost weathering
The disintegration and break-up of soil or rock by the combined action of frost shattering, frost wedging and hydration shattering.
COMMENT:
Hydration shattering is the process of grain loosening and soil or rock disintegration by the wedging pressure of water in films of varying thickness on the surfaces of silicate minerals. Water is drawn between the grains by various particle surface forces and exerts sufficient differential pressure to loosen and separate the grains. The process can act in all climates without the aid of freezing and thawing. When combined with freezing and thawing (frost shattering and frost wedging), however, the resulting process of frost weathering can be a very efficient mechanism for the break-up of soil or rock.
SYNONYM: (not recommended) rock shattering.
REFERENCES: White, 1976a; Washburn, 1979.
228. frost wedging
The mechanical disintegration, splitting or break-up of rock by the pressure of the freezing of water in cracks, crevices, pores, joints or bedding planes.
COMMENT:
A wide variety of terms has been used to describe what appears to be a single process.
SYNONYMS: congelifraction, frost bursting, frost prying, frost riving, frost splitting, gelifraction.
REFERENCE: Washburn, 1979.
229. frozen fringe
The zone in a freezing, frost-susceptible soil between the warmest isotherm at which ice exists in pores and the isotherm at which the warmest ice lens is growing.
COMMENT:
The temperature at the growing ice lens is slightly below 0°C.
REFERENCES: Miller, 1972; Konrad and Morgenstern, 1983.
230. frozen ground
Soil or rock in which part or all of the pore water has turned into ice.
COMMENT:
Perennially and seasonally frozen ground can vary from being partially to extensively frozen depending on the extent of the phase change. It may be described as hard frozen ground, plastic frozen ground, or dry frozen ground, depending on the pore ice and unfrozen water contents and its compressibility under load. Hard-frozen soils are firmly cemented by ice, are subject to brittle failure, and exhibit practically no consolidation under load. Plastic-frozen soils are cemented by ice but have viscous properties due to their high unfrozen water content and therefore will compress under load. Dry-, or friable-frozen, soils have a very low total water content and are not cemented by ice; their compressibility is the same as for unfrozen soils having the same composition, total water content and density.
REFERENCES: U.S.S.R., 1969, 1973; van Everdingen, 1976.
A special form of solid clathrate compound in which crystal lattice cages or chambers, consisting of host molecules, enclose guest molecules.
COMMENT:
Gas hydrates are formed under special temperature and pressure conditions. The host molecules are water, and the guest molecules may be a variety of gases, including argon, nitrogen, carbon dioxide, hydrogen sulfide, methane, ethane, halogens and other small molecules. In permafrost regions, gas hydrates are commonly found near the permafrost base and may lead to unexpected problems during drilling for hydrocarbons.
REFERENCES: Bily and Dick, 1974; Kaplan, 1974; Judge, 1982.
233. gelifluction
The slow downslope flow of unfrozen earth materials on a frozen substrate.
COMMENT:
Gelifluction is a type of solifluction implying the presence of either seasonal frost or permafrost.
REFERENCE: Washburn, 1979.
235. geocryology
The study of earth materials having a temperature below 0°C.
COMMENT:
The term is derived from the Russian word "geokriologiya". Although glaciers are not excluded, the term is usually applied to the study of frozen ground, including seasonally frozen ground as well as permafrost.
REFERENCES: Fyodorov and Ivanov, 1974; Poppe and Brown, 1976; Washburn, 1979.
236. geothermal gradient
The rate of temperature increase with depth in the subsurface.
COMMENT:
Commonly expressed as °C per metre depth.
237. geothermal heat flux
The amount of heat moving steadily outward from the interior of the earth through a unit area in unit time.
COMMENT:
Commonly expressed as joules per square metre per second.
238. granic cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form discrete loosely packed units.
COMMENT:
This fabric, found in cryosols, is generally attributed to freeze-thaw processes and the formation of needle ice near the ground surface.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
239. granoidic cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form more or less discrete loosely packed units.
COMMENT:
This fabric, found in cryosols, is generally attributed to freeze-thaw processes and the formation of needle ice near the ground surface.
REFERENCES: Brewer and Pawluk, 1975; Pawluk and Brewer, 1975; Fox, 1983.
241. gravimetric (total) water content
The ratio of the mass of the water and ice in a sample to the dry mass of the sample, commonly expressed as a percentage.
COMMENT:
Because of the way it is defined, the gravimetric total water content can greatly exceed 100 percent. During thawing of a sample of frozen ground the gravimetric total water content remains constant, unless excess water is allowed to drain out of the sample.
REFERENCE: Anderson and Morgenstern, 1973.
243. ground ice
A general term referring to all types of ice contained in freezing and frozen ground.
COMMENT:
Ground ice occurs in pores, cavities, voids or other openings in soil or rock and includes massive ice. It generally excludes buried ice, except in Russian usage. Ground ice may be epigenetic or syngenetic, contemporaneous or relict, aggrading or degrading, perennial or seasonal. It may occur as lenses, wedges, veins, sheets, seams, irregular masses, or as individual crystals or coatings on mineral or organic particles. Perennial ground ice can only occur within permafrost bodies.
REFERENCES: Mackay, 1972b; Pollard and French, 1980.
Frozen ground (soil or rock) which is firmly cemented by ice.
COMMENT:
Hard frozen ground is subject to brittle failure, and exhibits practically no consolidation under load.
REFERENCE: U.S.S.R., 1969, 1973.
252. heat capacity
The amount of heat required to raise the temperature of a unit mass of a substance by one degree.
COMMENT:
This term is a commonly used abbreviation of specific heat capacity, which does not include the effects of changes in latent heat due to the melting of ice or the freezing of water with changes in temperature. Because the phase change in a frozen soil often occurs gradually over a range of temperatures, the apparent heat capacity (covering both sensible and latent heat contents) may vary significantly with temperature. Heat capacity is commonly expressed in Joules per kg per degree K.
254. heaving pressure
Upward pressure developed during freezing of the ground.
COMMENT:
Heaving pressure develops during the formation of both pore ice and ice lenses. It is responsible for the heaving of utilities, foundations, pavements etc.
256. high-centre polygon
An ice-wedge polygon in which melting of the surrounding ice wedges has left the central area in a relatively elevated position.
258. hydraulic conductivity
The volume of fluid passing through a unit cross section in unit time under the action of a unit hydraulic potential gradient.
COMMENT:
The hydraulic conductivity is a function of the viscosity and density of the fluid, which in turn depend on the fluid temperature. In practice it is commonly expressed in cm per second or m per day.
259. hydraulic diffusivity
The ratio of the hydraulic conductivity and the storage capacity of a groundwater aquifer.
COMMENT:
The hydraulic diffusivity expresses the facility with which an aquifer will undergo a pressure change.
261. hydraulic thawing
Artificial thawing (and removal) of frozen ground by the use of a stream or jet of water under high pressure.
COMMENT:
Hydraulic thawing (hydraulicking) is a common method of working frozen placer deposits in North America.
262. hydrochemical talik
A layer or body of cryotic (but unfrozen) ground in a permafrost area, maintained by moving mineralized groundwater.
COMMENT:
In a hydrochemical talik, freezing is prevented by freezing-point depression due to the dissolved-solids content of mineralized groundwater moving through the talik.
REFERENCE: van Everdingen, 1976.
267. hydrothermal talik
A layer or body of noncryotic unfrozen ground in a permafrost area, maintained by moving groundwater.
COMMENT:
In a hydrothermal talik, the temperature is maintained above 0°C by the heat supplied by moving groundwater.
REFERENCE: van Everdingen, 1976.
Water in the solid state.
COMMENT:
Ice commonly occurs as hexagonal crystals. In permafrost regions, ice may occupy voids in soils and rocks and may develop in a variety of forms. Ice may be colourless to pale blue or greenish-blue. It may appear white due to included gas bubbles; in exposures, ground ice may also appear black. Various types of ground ice are defined elsewhere in this Glossary.
269. ice-bearing permafrost
Permafrost that contains ice.
271. ice-bonded permafrost
Ice-bearing permafrost in which the soil particles are cemented together by ice.
COMMENT:
Frozen soils may be partially-bonded, poorly-bonded or friable, if the soil particles are held together (cemented) weakly by the ice. If ice bonding is strong, the soil is said to be well-bonded. The distinction between ice-bonded permafrost and permafrost that contains ice but in which the ice does not act as a cement, is particularly important in subsea permafrost, where the dissolved-solids content of the pore water affects the ability of ice to act as a cement. Acoustic geophysical methods can be used to delineate ice-bonded permafrost, but use of the term "acoustically-defined permafrost" is not recommended except as a modifier to describe the method used to determine the permafrost conditions.
REFERENCES: Pihlainen and Johnston, 1963; Linell and Kaplar, 1966; Johnston, 1981; Hunter, 1984; Sellmann and Hopkins, 1984.
273. ice content
The amount of ice contained in frozen or partially frozen soil or rock.
COMMENT:
Ice content is normally expressed in one of two ways:
1. on a dry-weight basis (gravimetric), as the ratio of the mass of the ice in a sample to the mass of the dry sample, expressed as a percentage, or
2. on a volume basis (volumetric), as the ratio of the volume of ice in a sample to the volume of the whole sample, expressed as a fraction.
The volumetric ice content cannot exceed unity whereas the gravimetric ice content can greatly exceed 100 percent.
REFERENCES: Penner, 1970; Anderson and Morgenstern, 1973; Johnston, 1981.
274. ice-cored topography
Topography that is due almost solely to differences in the amount of excess ice underlying its surface.
COMMENT:
The relief may be totally or partially due to thermokarst, or to irregular development of ground ice, primarily segregated ice, or to buried glacier ice. An example is the "involuted hill" near Tuktoyaktuk, N.W.T.
REFERENCES: Rampton, 1974; Rampton and Walcott, 1974.
276. ice lens
A dominantly horizontal, lens-shaped body of ice of any dimension.
COMMENT:
The term is commonly used for layers of segregated ice. Ice lenses may range in thickness from hairline to more than 10 m. Very thick and extensive ice lenses are better termed massive ice beds.
REFERENCES: Mackay, 1971, 1973a; Rampton and Walcott, 1974.
278. ice-nucleation temperature
The temperature at which ice first forms during freezing of a soil/water system that does not initially contain ice.
COMMENT:
The ice-nucleation temperature may be lower than 0°C due to freezing-point depression caused by the dissolved-solids content of the pore water.
279. ice-rich permafrost
Permafrost containing excess ice.
COMMENT:
A qualitative term. Ice-rich permafrost is thaw-sensitive.
280. ice segregation
The formation of discrete layers or lenses of segregated ice in freezing mineral or organic soils, as a result of the migration (and subsequent freezing) of pore water.
COMMENT:
The migration of pore water to the frozen fringe is caused by cryosuction.
REFERENCES: Miller, 1972; Penner, 1972.
282. ice vein
An ice-filled crack or fissure in the ground.
284. ice wedge
A massive, generally wedge-shaped body with its apex pointing downward, composed of foliated or vertically banded, commonly white, ice.
COMMENT:
The size of ice wedges varies from less than 10 cm to more than 3 m in width at the top, commonly tapering to a feather edge at a depth of 1 to more than 10 m. Some ice wedges may extend downward as much as 25 m and may have shapes dissimilar to wedges. Epigenetic ice wedges are characteristically wedge-shaped, whereas syngenetic ice wedges are generally wedge-shaped but with more irregular sides (see also epigenetic permafrost, syngenetic permafrost).
Ice wedges are formed in thermal contraction cracks in which hoar frost (see open-cavity ice) forms and into which water from melting snow penetrates in the spring. Repeated annual contraction cracking of the ice in the wedge, followed by freezing of water in the crack, gradually increases the width (and possibly the depth) of the wedge and causes vertical banding of the ice mass. The surface expression of ice wedges is generally a network of polygons. Ice wedges growing as a result of repeated (but not necessarily annual) winter cracking are called active ice wedges. They occur primarily in areas of continuous permafrost when developed in mineral soil. Inactive ice wedges can be stable and remain for many centuries without changing.
REFERENCES: Dostovalov and Popov, 1966; Lachenbruch, 1966; Mackay and Black, 1973; French, 1976; Washburn, 1979; French et al., 1982; Mackay and Matthews, 1983.
285. ice-wedge cast
A filling of sediment in the space formerly occupied by an ice wedge.
COMMENT:
An ice-wedge cast is a wedge of secondary filling. When the permafrost thawed, the ice wedge melted and the enclosing and overlying sediments collapsed into the resulting trough.
An ice-wedge cast is one of the few acceptable criteria indicating the earlier presence of permafrost.
The term "fossil ice wedge" is not recommended because ice is no longer present.
SYNONYM: (not recommended) ice-wedge pseudomorph.
REFERENCES: Washburn, 1979, 1980.
286. ice-wedge polygon
A polygon outlined by ice wedges underlying its boundaries.
COMMENT:
Ice-wedge polygons occur in both mineral terrain and peatland, commonly in poorly drained permafrost areas.
288. iciness
A qualitative term describing the quantity of ice in frozen ground.
COMMENT:
In Russian literature, the equivalent term is used to describe volumetric ice content of frozen, or partially frozen, soil or rock. The "relative iciness" is expressed as the ratio of the mass of ice to the total mass of water (ice and unfrozen water) present.
REFERENCE: U.S.S.R., 1969.
289. icing
A sheetlike mass of layered ice formed on the ground surface, or on river or lake ice, by freezing of successive flows of water that may seep from the ground, flow from a spring or emerge from below river or lake ice through fractures.
COMMENT:
Icings also occur in non-permafrost areas. Many icings incorporate snow. In North America the term "icing" is gradually replacing a variety of terms used in the past. Aufeis (German), flood ice, flood-plain icing, ice field, naled (Russian) and overflow ice usually indicated icings formed on river ice and floodplains. Chrystocrene (or crystocrene), ground icing, groundwater icing and spring icing usually indicated icings formed by freezing of ground-water discharge. Use of the term "glacier" to describe icings, which is common in Alaska and the Yukon, is inappropriate and should be avoided.
REFERENCES: Muller, 1943; Carey, 1970, 1973.
290. icing blister
A seasonal frost mound consisting of ice only and formed at least in part through lifting of one or more layers of an icing by injected water.
COMMENT:
Freezing of the injected water will produce a layer of clear ice, contrasting with the overlying thinly layered ice of the icing. Rupture and draining of an icing blister may leave an empty cavity. Icing blisters are distinguished from frost blisters by the absence of a covering layer of seasonally frozen ground; they are distinguished from icing mounds by the layer of clear ice, and in some cases by the presence of an empty cavity. They have also been termed "ice blisters" (not recommended).
REFERENCE: van Everdingen, 1978.
291. icing glade
An area kept clear of trees and shrubs by the annual occurrence of icings.
COMMENT:
Icing glades along streams may also show the effects of lateral erosion occurring along the edges of the icingss during and after the snowmelt.
292. icing mound
A seasonal frost mound consisting exclusively of thinly layered ice, formed by freezing of successive flows of water issuing from the ground or from below river ice.
COMMENT:
Icing mounds may incorporate snow.
SYNONYM: (not recommended) ice mound.
REFERENCES: Muller, 1943; van Everdingen, 1978.
294. inactive ice wedge
An ice wedge that is no longer growing.
COMMENT:
Inactive ice wedges can be stable and remain unchanged for decades or centuries.
295. inactive rock glacier
A mass of rock fragments and finer material, on a slope, that contains either an ice core or interstitial ice, and shows evidence of past, but not present, movement.
COMMENT:
Rock glaciers are said to be inactive when their main body ceases to move, and they show no evidence of very recent movement.
REFERENCES: White, 1976b; Washburn, 1979.
298. interfacial water
Interfacial water forms transition layers at mineral/water and mineral/water/ice interfaces in frozen ground.
COMMENTS:
The intermolecular forces involved are such that interfacial water does not move under the influence of gravity. The temperature at which any portion of the interfacial water will change phase depends on the total energy of the various adsorption forces, which in turn depends on the distance from the mineral surface, the type of mineral, and the solute content of the water.
REFERENCE: Anderson and Morgenstern, 1973.
299. intermediate discontinuous permafrost
1. (North-American usage) Permafrost underlying 35 to 65 percent of the area of exposed land surface.
2. (Russian usage) Permafrost underlying 40 to 60 percent of the area of exposed land surface.
REFERENCE: Heginbottom and Radburn, 1992.
301. intrapermafrost water
Water occurring in unfrozen zones (taliks and cryopegs) within permafrost.
COMMENT:
Intrapermafrost water includes water in open, lateral and transient taliks and in basal, isolated and marine cryopegs. Sometimes erroneously called interpermafrost water.
REFERENCES: Williams, 1965, 1970; Tolstikhin and Tolstikhin, 1974.
302. intrusive ice
Ice formed from water injected into soils or rocks.
COMMENT:
Freezing of injected water will normally lift the ground above it, thereby producing topographic forms somewhat resembling those of igneous intrusions in rocks. Thus, a tabular mass of intrusive ice is analogous to a sill or a dyke (see ice vein) whereas the domed form is analogous to a laccolith (see frost blister, pingo). Use of terms such as "sill ice" or "hydrolaccolith," however, is not recommended. Intrusive ice may develop in porous unconsolidated sediments and in jointed or fractured bedrock. Fractures may become enlarged by hydraulic fracturing or frost wedging processes. For the greatest effect, water must become trapped in the joints or fractures and be subjected to relatively rapid freezing.
SYNOMYM: (not recommended) injection ice.
REFERENCES: Mackay, 1972b; Dyke, 1981, 1984.
304. isoband cryogenic fabric
A distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which soil particles form subhorizontal layers of similar thickness.
COMMENT:
This fabric, found in cryosols, results from freeze-thaw processes accompanied by eluviation.
305. isolated cryopeg
A body of unfrozen ground, that is perennially cryotic (T < 0°C) and entirely surrounded by perennially frozen ground.
COMMENT:
In an isolated cryopeg, freezing is prevented by freezing-point depression due to the dissolved-solids content of its pore water.
REFERENCE: Tolstikhin and Tolstikhin, 1974.
306. isolated patches of permafrost
Permafrost underlying less than 10 percent of the exposed land surface.
COMMENT:
Individual areas of permafrost are of limited areal extent, widely separated, and are completely surrounded by unfrozen ground.
SYNONYMS: (not recommended) insular permafrost; island permafrost; scattered permafrost.
REFERENCES: Heginbottom and Radburn, 1992.
307. isolated talik
A layer or body of unfrozen ground entirely surrounded by perennially frozen ground.
COMMENT:
Isolated taliks are commonly cryotic (see isolated cryopeg), but they may be noncryotic (see transient talik).
REFERENCE: van Everdingen, 1976.
A general term for all types of coarse clastic formations on slopes of 2-3 to 40 degrees, moving downslope mainly due to creep.
COMMENT:
This Russian term, used extensively in Siberia, covers "stone streams", "rock streams", "rock covers", and "rock fields".
REFERENCE: Tiurin, Romanovskii and Poltev, 1982.
A layer or body of unfrozen ground occupying a depression in the permafrost table beneath a lake.
COMMENT:
The temperature of a lake talik remains above 0°C due to the heat storage effect of the lake water. Depending on the permafrost thickness and the size of the lake, such taliks may be either open or closed.
REFERENCES: Williams, 1965; Washburn, 1973; van Everdingen, 1976.
310. latent heat (of fusion)
The amount of heat required to melt all the ice (or freeze all the pore water) in a unit mass of soil or rock.
311. lateral talik
A layer or body of unfrozen ground, overlain and underlain by perennially frozen ground.
COMMENT:
Lateral taliks may be cryotic (see hydrochemical talik) or noncryotic (see hydrothermal talik).
REFERENCE: van Everdingen, 1976.
312. latitudinal limit of permafrost
The southernmost (northernmost) latitude at which permafrost occurs in a lowland region in the northern (southern) hemisphere.
313. latitudinal zonation of permafrost
The subdivision of a permafrost region into permafrost zones, based on the percentage of the area that is underlain by permafrost.
314. layered cryostructure
The cryostructure of frozen silt or loam in which ice layers alternate with mineral layers that have a massive cryostructure.
COMMENT:
Silt or loam with a layered cryostructure has a relatively high ice content.
REFERENCE: Kudryavtsev, 1978.
315. lens ice
Ground ice occurring as ice lenses.
316. lens-type cryostructure
The cryostructure of frozen silt or loam containing numerous ice lenses.
COMMENT:
Silt or loam with a lens-type cryostructure has a relatively high ice content.
REFERENCE: Kudryavtsev, 1978.
319. long-term strength
The failure strength of a material after a long period of creep deformation.
COMMENT:
It is sometimes defined in practice as the creep strength (or the delayed strength) related to the service life of a structure, but its general definition is the failure strength attained when the strain rate tends to zero or time tends to infinity.
320. low-centre polygon
An ice-wedge polygon in which thawing of ice-rich permafrost has left the central area in a relatively depressed position.
Macro-scale polygons are closed, multi-sided, roughly equidimensional patterned-ground features, typically 15 to 30 m across, commonly resulting from thermal contraction cracking of the ground.
COMMENT:
Macro-scale polygons form random or oriented polygonal patterns in both mineral terrain and peatland in permafrost areas.
REFERENCES: Rapp and Clark, 1971; Washburn, 1979.
323. marine cryopeg
A layer or body of unfrozen ground, that is perennially cryotic (T < 0°C), forming part of coastal or subsea permafrost.
COMMENT:
Freezing of a marine cryopeg is prevented by freezing-point depression due to the high dissolved-solids content of its pore water.
REFERENCE: Tolstikhin and Tolstikhin, 1974.
324. mass wasting
Downslope movement of soil or rock on, or near, the earth's surface under the influence of gravity.
COMMENT:
Mass wasting includes slow displacements such as frost creep, gelifluction or solifluction, and more rapid movements such as earthflows or active-layer failures. It does not include crustal displacements resulting from tectonic activity or those movements where material is carried directly by an active transporting medium, such as glacial ice, snow, water or air. In permafrost areas, mass wasting is not limited to the active layer; it can include displacements caused by the formation and creep of ground ice within permafrost. When frost heave is a component of frost creep, it is a mass wasting process.
REFERENCES: Hutchinson, 1968; Savage, 1968; Washburn, 1979.
325. massive-agglomerate cryostructure
The cryostructure of frozen silt or loam in which ice veins form an irregular three-dimensional network.
COMMENT:
Massive-agglomerate cryostructure is characterized by a relative high ice content.
REFERENCE: Kudryavtsev, 1978.
326. massive cryostructure
The cryostructure of frozen sand in which all mineral particles are bonded together with ice.
COMMENT:
Massive cryostructure is characterized by the predominance of pore ice.
REFERENCE: Kudryavtsev, 1978.
327. massive ice
A comprehensive term used to describe large masses of ground ice, including ice wedges, pingo ice, buried ice and large ice lenses.
COMMENT:
Massive ice beds typically have an ice content of at least 250 percent (on an ice-to-dry-soil weight basis). If the ice content is less than 250 percent, the beds are better termed "massive icy beds". Some massive ice beds are more than 40 m thick and 2 km in horizontal extent, and some are responsible for prominent topographic rises.
REFERENCES: Rampton and Mackay, 1971; Mackay, 1971, 1973a; Rampton and Walcott, 1974.
328. massive-porous cryostructure
The cryostructure of frozen sand and gravel in which all mineral particles are bonded together with ice, but larger pore spaces are not completely filled with ice.
COMMENT:
Massive-porous cryostructure is characterized by the predominance of pore ice and a relatively low ice content.
REFERENCE: Kudryavtsev, 1978.
329. mean annual ground-surface temperature (MAGST)
Mean annual temperature of the surface of the ground.
COMMENT:
Permafrost exists if the mean annual ground-surface temperature is perennially below 0°C. Although the mean annual surface temperature may be below 0°C, the surface temperature will fluctuate during the year, causing a layer of ground immediately beneath the surface to thaw in the summer and freeze in the winter (the active layer). Small changes in the annual range of surface temperature and in the mean annual surface temperature from year to year, or over a period of a few years, may cause a layer of ground between the bottom of the active layer and the permafrost table to remain at a temperature above 0°C, creating a talik or residual thaw layer.
330. mean annual ground temperature (MAGT)
Mean annual temperature of the ground at a particular depth.
COMMENT:
The mean annual temperature of the ground usually increases with depth below the surface. In some northern areas, however, it is not uncommon to find that the mean annual ground temperature decreases in the upper 50 to 100 metres below the ground surface as a result of past changes in surface and climate conditions. Below that depth, it will increase as a result of the geothermal heat flux from the interior of the earth. The mean annual ground temperature at the depth of zero annual amplitude is often used to assess the thermal regime of the ground at various locations.
331. mechanical properties of frozen ground
The properties of frozen ground governing its deformability and strength.
COMMENT:
1. Properties under quasi-static loading:
The behaviour of frozen soils under quasi-static loading is usually different from that of unfrozen soils because of the presence of ice and unfrozen water films. In particular, frozen soils are more susceptible to creep and relaxation effects, and their behaviour is strongly affected by temperature change. In addition to creep, volumetric consolidation may also develop in frozen soils with large unfrozen water contents.
As with unfrozen soils, the strength of frozen soils depends on interparticle friction, particle interlocking and cohesion. In frozen soil, however, bonding of particles by ice is the dominant strength factor. This is complicated by the unfrozen water films surrounding the soil particles, which reduce interparticle bonding. The strength of ice in frozen soil is dependent on many factors, such as temperature, pressure, strain rate, grain size, crystal orientation and density. At very high ice contents, frozen soil behaviour under load is similar to that of ice. At low ice contents, however, when interparticle forces begin to contribute, the unfrozen water films play an important role, especially in fine-grained frozen soils.
(a) Deformability and strength:
The deformability and strength of a frozen soil can be studied by specially designed tests either in a cold room or in situ. Quasi-static elastic parameters usually determined in such tests are: Young's modulus and Poisson's ratio for short-term response, and creep parameters (used in a creep equation) for long-term response. In addition, the variation of strength with time or with strain rate is also determined from the tests. The strength is usually found to vary from a high short-term value to a much smaller long-term value, which is considered to govern the behaviour of frozen soil under sustained loading. Under high confining stresses and at relatively high freezing temperatures, most frozen soils creep and eventually fail in a plastic manner. On the other hand, under compression at low confining stresses, or at tensile stresses combined with low temperatures, many frozen soils fail in a brittle manner by tensile crack propagation.
(b) Compressibility of frozen soils:
Although frozen soils are usually considered to be practically incompressible, and volume change deformations can therefore be neglected, investigations show, however, that the compressibility of frozen soils can be significant and should not be neglected in some cases, especially when large areas are loaded.
Compressibility and its time dependence in frozen soils are due to several causes, such as instantaneous compression of the gaseous phase, creep of the pore ice due to shear stresses at the grain contacts, and hydrodynamic consolidation due to the expulsion of unfrozen water under stress, the amount of which varies with the applied pressure.
2. Dynamic properties of frozen ground:
Information on the dynamic properties of frozen soils is important with regard to the behaviour of structures subjected to seismic or vibratory loads, and the evaluation of results of seismic field surveys in permafrost areas.
Dynamic properties are expressed either in terms of two dynamic elastic parameters: the dynamic modulus of elasticity and the dynamic Poisson's ratio, or in terms of the propagation velocities of compressional w | 
