PEAK FLOOD GLACIER DISCHARGE

Monohar AroraNational Institute of Hydrology (NIH), Roorkee, UA,India

Sudden release of glacially impounded water causes cata-strophic floods (sometimes called by the Icelandic term“jôkulhlaup”), known as outburst floods, and occasionallyspawn debris flow that pose significant hazards in moun-tainous areas. Commonly, the peak flow of an outburstflood may substantially exceed local conventional bench-marks, such as the 100-year flood peak, but predicting thepeak discharge of these subglacial outburst floods is a verydifficult task. Increasing human habitation and recrea-tional use of alpine regions has significantly increasedthe hazard posed by such floods. Outburst floods releasedin steep, mountainous terrain commonly entrain loose sed-iment and transform into destructive debris flows.

PERCOLATION ZONE

Prem DattResearch and Development Center (RDC),Snow and Avalanche Study Establishment, Himparisar,Chandigarh, India

The area on a glacier or ice sheet or in a snowpack wherea meltwater percolates are known as percolation zone.In case of glaciers, the upper part of the glacier (accumula-tion zone) where ice is covered by snow representsthe percolation zone. As such water percolates throughthe snowpack because snow behaves like a porous media,while in the lower part of glacier (ablation zone) waterflows over the ice because ice is not permeable and hardlyallows any percolation. This is the reason the water chan-nels are found in the ablation part of the glaciers whereexposed ice surface is available.

PERENNIALLY FROZEN GROUND

Monohar AroraNational Institute of Hydrology (NIH), Roorkee, UA,India

Perennially frozen ground occurs wherever the groundtemperatures remain continuously below 0�C for 2 ormore years. Most permafrost is located in high latitudes(i.e., land in close proximity to the North and South poles),but alpine permafrost may exist at high altitudes in muchlower latitudes. The extent of permafrost can vary as theclimate changes. Permafrost, or perennially frozenground, is a critical component of the cryosphere and the

Arctic system. Permafrost regions occupy approximately24% of the terrestrial surface of the Northern Hemisphere.Today, a considerable area of the Arctic is covered by per-mafrost (including discontinuous permafrost).

PERIGLACIAL

H. M. FrenchUniversity of Ottawa (retired), North Saanich, BC,Canada

SynonymsCryogenic

Definition“Periglacial”: an adjective used to refer to cold, non-glacial landforms, climates, geomorphic processes, orenvironments.“Periglaciation”: the degree or intensity towhich periglacialconditions either dominate or affect a specific landscape orenvironment.

OriginThe term periglacial was first used by a Polish geologist,Walery von Łozinski, in the context of the mechanical dis-integration of sandstones in the Gorgany Range of thesouthern Carpathian Mountains, a region now part ofcentral Romania. Łozinski described the angular rock-rubble surfaces that characterize the mountain summitsas periglacial facies formed by the previous action ofintense frost (Łozinzki, 1909). Following the XI Geologi-cal Congress in Stockholm in 1910 and the subsequentfield excursion to Svalbard in 1911 (Łozinzki, 1912), theconcept of a periglacial zone was introduced to refer tothe climatic and geomorphic conditions of areas periph-eral to Pleistocene ice sheets and glaciers. Theoretically,this was a tundra zone that extended as far south as thetree-line. In the mountains, it was a zone between timber-line and snow line (Figure 1).

Today, Łozinski’s original definition is regarded asunnecessarily restricting; few, if any, modern analogs exist(French, 2000). There are two main reasons. First, frostaction phenomena are known to occur at great distancesfrom both present-day and Pleistocene ice margins. Infact, frost action phenomena can be completely unrelatedto ice-marginal conditions. For example, parts of centralSiberia and interior central Yukon remained unglaciatedduring the Pleistocene, yet these are regions in which frostaction was, and is, very important. Second, althoughŁozinski used the term to refer primarily to areas ratherthan processes, the term has increasingly been understoodto refer to a complex of cold-dominated geomorphic pro-cesses. These include not only unique frost action and per-mafrost-related processes but also the range of azonalprocesses, such as those associated with snow, running

PERIGLACIAL 827

water and wind, which demand neither a peripheral ice-marginal location nor excessive cold. Instead, they assumedistinctive or extreme characteristics under cold, non-glacial (i.e., periglacial) conditions.

Historical contextPeriglacial geomorphology developed in a relativelyrapid fashion in the 2 decades after 1945 as a branch ofa European-dominated, but somewhat unscientific, cli-matic geomorphology. It was aimed largely at Late-Pleistocene paleo-climatic reconstruction. This changedin the latter half of the twentieth century when isotopicdating techniques and the explosion of the Quaternary sci-ences came to dominate paleo-environmental reconstruc-tion. At the same time, the growth of permafrost studiesin Arctic North America and the emergence of Russiangeocryology liberated periglacial geomorphology fromits Pleistocene heritage.

Modern periglacial geomorphology is a branch not onlyof mainstream geomorphology but also of permafrost sci-ence or geocryology (Washburn, 1979; Romanovskii,1980; Williams and Smith, 1989; Yershov, 1990; Zhouet al., 2000). Periglacial areas are regarded as cold-climate“zones” in which seasonal and perennial frost, snow, andnormal azonal processes are all present to greater or lesserdegrees (French, 2007). The reality is that many so-calledperiglacial landscapes inherit the imprint, in varyingdegrees, of previous glacial conditions.

Extent and significance of periglacialenvironmentsPeriglacial environments are restricted to areas that experi-ence cold, but essentially non-glacial, climates. They occurnot only as tundra zones in the high latitudes, as definedby Łozinski`s concept, but also as forested areas south oftree-line and in the high-altitude (i.e., alpine) regions of

Permafrost

Sporadic

Discontinuous

(Timberline)

Continuous

Per

igla

cial

zon

e

Snow and ice

Patchy

Widespread

Discontinuous

Continuous

Timberline

Per

igla

cial

zon

e

∗

∗∗∗

∗

(1) Theoretical limit of periglacial zone, as determined by climate

(2) Pleistocene periglacial zone, displaced southward and peripheral to ice sheets

(3) Present-day periglacial zone includes (a) climatically induced periglacial zone and (b) relict periglacial zone: northern part of boreal forest zones

Periglacial zoneIce

X7

X1

X2

X3

X4

X5

X6

Relictperiglacial zone

X1–X7 = Climate zonesX4 = TreelineX6 = snowlinea

b

Periglacial, Figure 1 Schematic diagram illustrating the concept of the periglacial zone in (a) high-latitude and (b) high-altitude(alpine) areas. (From French, 2007.)

828 PERIGLACIAL

mid-latitudes (Figure 2). They include (a) the polar desertsand semi-deserts of the High Arctic, (b) the extensive tun-dra zones of high northern latitudes, (c) the northern partsof the boreal forests of North America and Eurasia, and(d) the alpine zones that lie above timberline and belowsnowline in mid-latitude and low-latitude mountains. Tothese must be added: (a) the ice-free areas of Antarctica,(b) the high-elevation montane environments of centralAsia, the largest of which is the Qinghai-Xizang (Tibet)Plateau of China, and (c) small oceanic islands in the highlatitudes of both Polar Regions.

Periglacial environments occur over approximatelyone quarter of the Earth’s land surface. During the Pleisto-cene glacial periods, large areas of now-temperate mid-latitude experienced reduced temperatures because oftheir proximity to the continental ice sheets and glaciers.

Permafrost and/or intense frost action would have charac-terized an additional 20–25% of the earth’s land surface atsome time during the Pleistocene.

As regards human occupance, the periglacial environ-ments are relatively sparsely populated. A reasonable esti-mate is just seven to nine million people, mostly living inRussia, or only 0.3% of the world’s population. Thus, thelarger importance of periglacial environments lies not intheir spatial extent, their snow and ice, or their proximityto glaciers but in their environment and their naturalresources. For example, the Precambrian basement rocksthat outcrop as huge tablelands in both Canada and Siberiacontain precious minerals, such as gold and diamonds, andsizable deposits of lead, zinc, and copper, while the sedi-mentary basins of western Siberia, northern Alaska, andthe Canadian High Arctic contain large hydrocarbon

Limit of continuouspermafrost

Treeline

Glaciers

Alpine periglacialzone

Boreal periglacialzone

Tundra zone

Arctic frost-debris zone

0 500 1000 1500 km

High arctic frost-debris zone

Subarctic-continentalperiglacial zone

Subarctic-maritimeperiglacial zone

Limit of sporadicpermafrost

Limit of discontinuouspermafrost

Periglacial, Figure 2 Map showing the extent of the current periglacial domain in the northern hemisphere. Not included arethe alpine areas of mid-latitude mountains and the high-altitude montane environment of central Asia. (From Karte and Liedtke,1981. Reproduced in French, 2007.)

PERIGLACIAL 829

reserves. In the more distant future, the exploitation of gashydrates that occur within permafrost and the freshwaterresources associated with the large northern lakes and riv-ers will become important. A second reason whyperiglacial environments are of significance is their placewithin the cryosphere (snow, ice, frozen ground, sea ice)and the critical role which the cryosphere plays in theglobal climate system.

Periglacial climatesPeriglacial environments experience mean annual air tem-peratures of less than +3�C. They can be subdivided by the�2�C mean annual air temperature into environments inwhich frost action dominates (mean annual air tempera-ture less than�2�C) and those in which frost action occursbut does not necessarily dominate (mean annual air tem-perature between �2�C and +3�C). Fundamental to mostperiglacial environments is the freezing of water and itsassociated frost heaving and ice segregation.

Based upon temperature and solar radiation charac-teristics, the majority of periglacial environments can becategorized as being either (1) High Arctic (polar),(2) Continental, or (3) Alpine in nature. In both HighArctic and Continental environments, temperatures aredominated by a seasonal rhythm; summer temperaturesrange between 10�C and 30�C and winter temperaturesmay fall as low as �30�C. Perennially frozen ground(permafrost) is widespread. By contrast, the Alpine mid-latitude environment experiences both diurnal and sea-sonal rhythms. Permafrost may, or may not, be present.

Periglacial environments that do not fit the above clas-sification are (1) the extensive high-altitude montane envi-ronments of central Asia that experience a mix of bothseasonal and continental temperature rhythms, (2) Iceland,and other smaller islands in the subarctic oceans of bothpolar regions such as Jan Mayen, Kerguelen, and SouthGeorgia that experience diurnal, seasonal and/or perennialfrost, and (3) the high elevations and summits of moun-tains in South America and Africa that, lying near theequator, experience low annual temperature range andstrong diurnal rhythms. The freezing and thawing condi-tions experienced by these different periglacial environ-ments are summarized in Figure 3.

In terms of periglacial landscape dynamics, groundtemperature is probably more important than air tempera-ture. Typically, the depth of ground freezing varies fromas little as 10–20 cm beneath organic materials to over500 cm in areas of exposed bedrock. It is important tostress that relatively few freeze-thaw cycles occur atdepths in excess of 30 cm; there, only the annual tempera-ture cycle usually occurs. It is important to differentiatebetween the mean annual air temperature (MAAT) andthe mean annual ground surface temperature (MAGST)that results in the so-called surface offset and the meanannual ground surface temperature (MAGST) and the tem-perature at the top of permafrost (TTOP) that results in theso-called thermal offset (Smith and Riseborough, 2002).

The surface offset reflects primarily the influence of snowcover and vegetation, while the thermal offset is condi-tioned largely by the physical properties of the active layer(thermal conductivity and moisture content).

Periglacial ecosystemsPeriglacial environments contain a range of ecosystems.The most extensive are those of the high northern lati-tudes. They can be regarded as being either arctic or sub-arctic in nature (Table 1). The boundary between the twoapproximates the northern limit of trees, the so-calledtree-line. This is a zone, 30–150 km in extent, north ofwhich trees are no longer able to survive. Ecologistsrefer to the barren, treeless Arctic as tundra. The tundraprogressively changes into polar desert at extreme highlatitude as climate becomes increasingly colder and drier.The tree-line also approximates the southern boundary ofthe zone of continuous permafrost; i.e., north of the tree-line, the terrain is perennially frozen and the surface thawsfor a period of only 2–3 months each summer (see above).

The mid-latitude alpine environments are a localizedand specialized periglacial environment. They are domi-nated by both diurnal and seasonal climatic effects, bysteep slopes, tundra (alpine) plants, rocky outcrops, andsnow and ice. In such environments, the timberline consti-tutes the boundary between the alpine and sub-alpine. Themontane environments of central Asia are also distinct andconsist of extensive steppe grasslands and interveningdesert-like uplands. Finally, the ice-free areas of Antarc-tica and northeast Greenland are essentially polar desertsor rock-rubble surfaces.

The tundra and polar desert regions of both NorthAmerica and Eurasia contain a surprisingly large numberof plant and animal species. Plant cover varies from 5%in the polar deserts to over 60–75% in meadow tundraterrain (Figure 4a). In the western Canadian Arctic Archi-pelago, the diversity of flora and vascular plants is espe-cially well documented (e.g., Porsild, 1957). Largemammals such as the polar bear, musk-oxen, and fox allmanage to survive in the extreme high northern latitudes.

In the subarctic, two major ecological zones can be rec-ognized. Near the tree-line is a zone of transition fromtundra to forest, consisting of either open woodland or for-est-tundra. Here, the trees are stunted and deformed, oftenbeing less than 3–4-m high (Figure 4b). Woodland cari-bou and grizzly bear replace polar bear and musk-oxen.This zone merges into the boreal forest, or taiga, animmense zone of almost continuous coniferous forestextending across both North America and Eurasia. It isregarded as a fire climax community (Figure 4c). In NorthAmerica, the dominant tree is spruce (Picea glauca andPicea mariana) and in central Siberia, both pine (Pinussilvestris) and tamarack (Larix dahurica) are dominant.In northern Scandinavia, on account of the warm GulfStream, stunted birch forest (Betula nana) forms thetree-line (Figure 4d). The southern boundary of the sub-arctic is less clearly defined than its northern boundary;

830 PERIGLACIAL

typically, coniferous species begin to be replaced byothers of either local or temperate distribution, such asoak, hemlock, and beech, or by steppe, grassland, andsemi-arid woodland in more continental areas. Ungulates

such as bison and yak take advantage of these grasslands.Because these ecosystems experience deep seasonal frost,they represent the outer spatial extent of the periglacialenvironment.

0a

10No.

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ays 20

30

Yakutsk, Russia

0

10No.

of d

ays 20

30

Tuktoyaktuk, Canada

0

10No.

of d

ays 20

30

Spitsbergen

0J D

Days > 0°C

J D

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of d

ays 20

30

Sonnblick, Alps

0

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of d

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30

Fenghuo Shan, Tibet Plateau

0

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of d

ays 20

30

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of d

ays 20

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Summit station, Peru

0

120

225

323

42

337

27

11

354

35

63

267

21591

108

214

126197

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ays 20

30

Kerguelen Island, South Atlantic

Freeze-thaw days

(a)–(c) High-latitude, low elevation(d)–(f) Low-latitude, high elevation(g) Midlatitude, high elevation(h) Subarctic oceanic, low elevation

Days < 0°C

b

c

d

e

f

g

h

Periglacial, Figure 3 Freezing and thawing conditions in various periglacial environments of the world. (From French, 2007.)

PERIGLACIAL 831

The harsh, climatic environments of the ice-free areasof Antarctica support little life. However, these areas areadjacent to a highly productive marine ecosystem thatresults from the nutrients associated with upwelling ofwater along the Antarctic Divergence. Not surprisingly,a number of marine mammals (e.g., Antarctic elephantseal, southern fur seal) and birds (e.g., penguins, wonder-ing albatross) use the ice-free areas for critical breedingpurposes. However, the terrestrial flora and fauna arefew; mainly mosses and lichens. There are no land mam-mals in Antarctica.

Periglacial landscapesThe geomorphic footprint of periglacial environments isnot always achieved and most periglacial landscapes pos-sess some degree of inherited paraglacial or proglacialcharacteristics. The reality is that periglacial landscapes

range between those in which the entire landscape is fash-ioned by permafrost and frost-action processes and thosein which frost-action processes are subservient to others.This diversity is accentuated by the fact that (1) certainrock types are more prone to frost weathering than othersand (2) many regions currently experiencing periglacialconditions have only recently emerged from beneath con-tinental ice sheets and are largely glacial landscapes. Forexample, certain areas of western Siberia and the north-western Canadian Arctic possess large bodies of relictglacier ice, of Pleistocene age, partially preserved beneathablation till (e.g., Astakov et al., 1996; Murton et al.,2005). It is clear that these so-called periglacial landscapesare largely relict and that periglacial processes are slowlymodifying the landscape.

The only periglacial landscapes that are probably ingeomorphic equilibrium are those that have protracted his-tories of cold non-glacial conditions. In the northern

Periglacial, Table 1 Summary characteristics of high-latitude periglacial ecosystems

Arctic Antarctic

Low Arctic High Arctic Continent, not Peninsula

Climate: Very cold winters, cold summers,low precipitation, 3.5–5 months>0�C

Very cold winters, cold summers,very low precipitation, 2–3months >0�C

Extremely cold, short summers, very lowprecipitation, strong winds � 1 month>0�C

Snow-freeperiod

�3–4 months �1–1.5 months �1–2 months

Length ofgrowingseason

�3.5–5 months �1–2 months Negligible

Permafrost: Continuous: temperature is ��3 to�4�C at 10–30-m depth

Continuous: temperature is��10 to�14�C at 10–30-m depth

Continuous: temperature is��8 to�18�Cat 10–30-m depth

Active-layerdepth

�30–50 cm in silt/clay�2–5m in sand

�30–50 cm in silt/clay�70–120 cm in sand

30–50 cm in gravel and ablation till�1–2 m in bedrock

Vascularplants:

400–600 species 50–350 species Hair-grass, pearlwort

Mosses Sphagnum common Sphagnum minor 30+ typesLichens Foliose species abundant Fruticose and crustose species

common125+ types

Total plantcover

80–100% 1–5% polar deserts20–100% polar semi-deserts80–100% sedge-moss tundra

<5% in most areas

Total plantproduction

200–500 g/m² 0.5 g/m² polar deserts20–50 g/m² polar semi-deserts150–300 g/m² sedge-moss tundra

0.5 g/m² in most areas

Vegetation: Tundra types dominate Tundra types minor Mosses, lichensTall shrubs, 2.4 m Polar semi-desert commonLow shrubs, 0.5 m Cushion plant – mossCottongrass tussock- Cushion plant – lichenDwarf shrub heath Herb-mossDwarf shrub heath wet-edge sedge Polar desert common

Herb-mossMammals: 10–15 species 8 species (1) Terrestrial: none

(2) Southern Ocean: numerous marinemammals

Nesting Birds: 30–60 species 10–20 species Penguins, skuasLargeherbivores:

Barren-ground caribou, musk-oxen,moose, Polar bear, fox, wolf

Peary’s caribou, musk-oxen, Polarbear, wolf

None

Fishes: (lakesand rivers)

4–6+ species 1–2 species (Arctic char, trout) (1) Rivers: none(2) Southern Ocean: numerous

832 PERIGLACIAL

hemisphere, these include (1) parts of central Alaska andinterior Yukon, (2) much of central and eastern Siberia,and (3) much of the montane and steppe environmentson, and surrounding, the Qinghai-Xizang (Tibet) Plateau.In the southern hemisphere, some of the ice-free areas ofVictoria Land are thought to have been free of ice for sev-eral million years.

In all these areas, it is clear that geological structureand lithology largely control the macroscale periglaciallandscape. For example, in areas of resistant igneous,metamorphic and sedimentary bedrock, the higher eleva-tions consist of structurally-controlled rock outcrops.Everywhere, the upland surfaces and upper valley-sideslopes are covered by angular rock-rubble accumulations(variously termed “mountain-top detritus,” blockfields,or kurums). Bedrock is frequently disrupted by joint andfissure widening, the frost-jacking of blocks, and by brec-ciation. Typically, uplands are bordered by low-angle,pediment-like surfaces. In many ways, these landscapesresemble those of the hot deserts of the world. By contrast,areas of poorly-lithified bedrock and unconsolidatedTertiary- and Quaternary-age sediments form more undu-lating, poorly-drained, lowland terrain. Typically, thelandscape is characterized by large-scale tundra polygons,thaw lakes and depressions, and widespread mass-wastingand patterned-ground phenomena.

Frost action and cold-climate weatheringThe weathering of bedrock in periglacial areas is generallyassumed to be mechanical in nature and the result of freez-ing and thawing of water within rock or mineral soil. Ratesof cold-climate rock weathering are usually assumed to beas great, if not greater, than those in warmer environmentsbut this has yet to be convincingly demonstrated.

Rock disintegration by frost action is generallyassumed to be the result of either (1) volumetric expansionof ice or (2) ice segregation.

Volumetric expansionThe freezing of water is accompanied by a volumetricexpansion of approximately 9%. In theory, this can gener-ate pressures as high as 270 MPa inside cracks in a rockstrong enough to withstand such pressures. While volu-metric expansion was probably the mechanism thatŁozinski envisaged when he talked of “periglacial facies,”the dominant role attributed to simple volumetric expan-sion is probably incorrect. This is because the conditionsnecessary for frost weathering by volumetric expansionare somewhat unusual. Not only must the rock be watersaturated but also freezing must occur rapidly from allside. On the other hand, there is no doubt that volumetricexpansion of water within existing joints and other lines

a b

c d

Periglacial, Figure 4 Examples of typical periglacial ecosystems: (a) Lowland tundra, Kellett River, southern Banks Island, Canada,showing ice-wedge polygon terrain, thaw lakes, an active-layer detachment (in foreground) and grazing muskoxen; (b) Northernboreal forest near the tree-line, just south of Inuvik, NWT, Canada, showing stunted black spruce with non-sorted circles (mud/earthhummocks); (c) A recently burned area of taiga forest, Lena River valley, central Siberia, now subject to thermokarst erosion andwillow/shrub revegetation; (d) Birch trees constitute the northern boreal forest in northern Finland.

PERIGLACIAL 833

of weakness within bedrock outcrops can lead to bedrockheave and joint widening, and that near-surface frostwedging in fissile sedimentary rocks is a common occur-rence (Figure 5a).

A related mechanism is hydrofracturing, in which rockdisintegration results from pressures generated by pore-water expulsion. For this to happen, the water-saturatedrock must possess large interconnected pores, the expelledpore water is unable to drain away as quickly as it isexpelled, and pore-water pressures must rise sufficientlyto deform or “hydrofracture” the rock. Rapid inward freez-ing is the ideal circumstance; for example, there have beenoccasional instances where Arctic field observations indi-cate that boulders or rocks on the ground surface haveburst or “exploded” during periods of rapid temperaturedrop during midwinter.

A third possible mechanism relates to the breakup ofindividual mineral particles by the wedging effect of iceformed in micro-cracks or by the freezing of water withingas–liquid inclusions at cryogenic (i.e., subzero) tempera-tures. For example, certain Russian laboratory experi-ments (e.g., Konishchev and Rogov, 1993) indicate that,during repeated freeze-thaw cycles, quartz sand breaksdown more readily than feldspar and produces finer parti-cles; this may also reflect the increasing brittleness ofquartz at very low temperatures when compared to otherminerals.

Ice segregationThere is increasing acceptance that the progressive growthof ice lenses as liquid water migrates to the freezing planeis the most likely cause of the widespread fracture of moistporous rocks (Walder and Hallet, 1986). This is because itis now understood that moisture migrates within freezingor frozen ground. It is the result of a temperature gradi-ent-induced suction (dPw) that affects the unfrozen waterheld in capillaries and adsorbed on the surfaces of mineralparticles. In theory, a temperature drop of 1�C inducesa cryosuction of 1.2 MPa (12 atm). According to Williamsand Smith (1989):

dPw ¼ dTl=VT (1)

where, dT is the lowering of the freezing point, l is thelatent heat of fusion, V is the specific volume of water,and T is the absolute temperature.

The conditions needed for ice segregation are slowrates of freezing and sustained subzero temperatures.These are relatively common in most periglacial environ-ments. In frost-susceptible sedimentary bedrock, long-continued ice segregation can lead to the brecciation ofbedrock to a depth of several meters (Figure 5b). Icesegregation and rock fracture has also been verified inlaboratory experiments that simulate natural uni- andbi-directional freezing; the most susceptible rock types

a b

c d

Periglacial, Figure 5 Examples of frost action and cold-climate rock weathering: (a) In situ bedrock disintegration and frost jacking ofDevonian-age siltstone and sandstone, Rea Point, Melville Island, Arctic Canada; (b) An exposure of Late-Cretaceous shale in thewall of a drilling sump illustrates near-surface brecciation due to ice lensing, Sabine Peninsula, Melville Island, Arctic Canada;(c) Fractured, fine-grained diorite boulder lying on ablation till surface (“Younger Drift”), Simpson Crags, northern Victoria Land,Antarctica; (d) Taffoni weathering of coarse-grained monzogranite boulder, Terra Nova Bay, northern Victoria Land, Antarctica.

834 PERIGLACIAL

appear to be fine-grained porous rocks such as chalk andshale (Murton et al., 2006).

Other mechanismsA number of other weathering mechanisms are alsothought to operate in periglacial environments. These arebriefly discussed below.

First, insolation weathering, or spalling, refers to crack-ing in bedrock thought to be caused by temperature-inducedvolume changes such as expansion and contraction. Formany years these thermally-induced stresses were thoughtmore appropriate for rock weathering in hot arid regionsthan for cold regions. However, laboratory studies suggestthe threshold value for thermal shock approximates toa rate of temperature change of 2�C/min and experimentalstudies, using cold room facilities, have established thatdifferent minerals have varying coefficients of linear ther-mal expansion in the range of +10 to�10�C. These param-eters certainly apply to many periglacial environments. Forexample, in parts of Antarctica, field studies documentdaily temperature ranges of 40�C�42�C, and rates ofheating and cooling of 0.8�C/min and of 15–20�C/h.These measurements suggest that thermal stress, or fatigue,may be a viable rock-weathering process (Hall, 1999)(Figure 5c). Unfortunately, until further field, laboratory,and experimental studies are undertaken, this importantmechanism is still largely speculative.

Second, equally perplexing is the relationship betweensalt, present in the snow in areas adjacent to marine envi-ronments and the granular disintegration of coarse-grainedigneous rocks that results in cavernous weathering in theice-free polar deserts of Antarctica (French andGuglielmin, 2000) (Figure 5d). These phenomena are

similar to the more well-known “taffoni” of mediterraneanand tropical regions.

Third, the efficacy of chemical weathering at low tem-peratures is unclear, despite a number of recent detailedstudies in northern Scandinavia (e.g., Dixon and Thorn,2005), while the nature of the biological and biochemicalweathering processes associated with rock-colonizingorganisms and the formation of phenomena such as rockvarnish are extremely poorly understood (e.g., Guglielminet al., 2005; Etienne, 2002).

It would appear that cold-climate weathering is com-plex, frost action takes many forms, certain processes actalone, others in combination, and some may be physico-chemical in nature.

Frozen groundPeriglacial environments experience either seasonally fro-zen or perennially frozen ground. The latter, if it persistsfor more than 2 years, is termed permafrost (Muller,1943) (Permafrost). In areas underlain by permafrost, theactive layer refers to the near-surface layer of groundwhich thaws during summer.Where discontinuous perma-frost is present, the active layer may be separated fromunderlying permafrost by an unfrozen layer (tálik), orby a residual thaw layer if permafrost is relict. If nopermafrost is present, the active layer no longer existsand the near-surface layer is one of seasonal freezingand thawing.

The typical ground thermal regime of an area underlainby permafrost is illustrated in Figure 6. Thawing begins inearly summer and the depth of thaw reaches a maximum inlate summer at which point, freeze-back occurs. Thefreeze-back is slower than the thaw because the releaseof latent heat offsets the temperature drop. This gives rise

4.0–0.5° –0.5°

0.0°

–0.5°–1.0°–2.0°–3.0°–4.0°–5.0°

–10.0°–15.0°

0.5°1.0°2.0°

3.0°4.0°

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–1.0°

–2.0°–3.0°

–4.0°

–5.0°

–10.0°

–15.0°

3.5

3.0

Depth in m

eters

Depth in feet

2.5

5.0

6.5 2.0

1.5

1.0

0.5

0.0Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov.

Active layers

Perm

afrost

Thermal regime of ground atSkovorodino, Siberia

(1928–1930)

All temperatures are gentigrade

Dec.

Periglacial, Figure 6 Diagram illustrating the typical ground thermal regime of a permafrost area, Skovorodino, Siberia, 1928–1930.(From Muller, 1943.)

PERIGLACIAL 835

to the so-called zero-curtain effect, in which near isother-mal conditions persist in the active layer for several weeks.Both thawing and freezing are one-sided processes, fromthe surface downward. However, if permafrost was pre-sent, freezing is a two-sided process, occurring bothdownward from the surface and upward from the perenni-ally frozen ground.

The Stefan equation is sometimes used to approximatethe thickness of the active layer:

Z ¼ p2TKt=Qi (2)

where Z = thickness of the active layer (m or cm),T = ground surface temperature during thaw season(�C), K = thermal conductivity of unfrozen soil (W/m Kor kcal/m �C h), t = duration of the thawing season(day, h, s), and Qi = volumetric latent heat of fusion (kJ/m³). The Stefan equation can also be used to calculatethe depth of seasonal frost penetration. In this case, timet is the duration of the freezing season (T< �C) and K rep-resents the thermal conductivity of frozen soil.

The base of the active layer represents an unconformitybetween frozen and unfrozen earth material. Because theannual depth of thaw may vary from year to year,depending upon the variability of summer climate, theconcept of the transient layer recognizes the different peri-odicities at which near-surface permafrost cycles through0�C (Shur et al., 2005). The active-layer permafrost inter-face is commonly ice-rich. This is because, in summer, asthe active layer thaws, moisture migrates downward andrefreezes at the base while, during winter, unfrozen watermigrates upward in response to the colder temperatures atthe surface. Thus, the active layer not only limits the depthto which freeze-thaw action occurs but also its base acts asa slip plane for solifluction and other gravity-inducednear-surface movements such as active-layer detachmentsand for slope instability.

Ground ice is an important component of permafrost.Many of the human occupance problems of periglacialenvironments relate to either frost heaving of the groundor thaw subsidence (thermokarst) of ice-rich material. Poreand segregated ice are themost widespread forms of groundice but other types include vein ice and intrusive ice. Inparts of western Siberia and the western North AmericanArctic, massive icy bodies of either an intra-sedimental(i.e., segregated ice) or glacier-ice origin are present(Astakov et al., 1996; Mackay and Dallimore, 1992). Ingeneral, fine-grained materials are often ice rich and frost-susceptible, whereas coarse-grained materials are ice poorand generally regarded as non-frost-susceptible. Typically,the base of the active layer and the upper 1–3 m of perma-frost contain the highest amounts of ice, often exceeding50% by volume. Ground ice is discussed more fully underPermafrost.

Periglacial processes and landformsGeomorphological processes clearly unique to periglacialenvironments are those related to the formation and

degradation of perennially frozen ground. Other pro-cesses, not necessarily restricted to periglacial environ-ments, are important on account of their high magnitudeor frequency in cold environments. They relate to the pres-ence of snow, or lake and river ice. Other azonal processes,such as fluvial, eolian, and coastal processes, assumespecial characteristics in cold environments.

Permafrost-related processesPermafrost-related processes include (1) the aggradationof permafrost and ground-ice bodies, (2) thermal contrac-tion cracking of frozen ground, (3) thawing of permafrost(thermokarst), and (4) the creep of ice-rich permafrost. Allare intimately related to the presence of ice withinpermafrost.

Much of our understanding of permafrost-related land-forms and geomorphic processes is derived from the50 years of field investigations undertaken by J. R. Mackayin the Mackenzie Delta region of the western CanadianArctic (e.g., Mackay, 1963, 1998, 2000; Mackay andBurn, 2002).

Frost mounds, reflecting the growth of ground ice bod-ies, are aggradational permafrost features. A range offorms exists. The largest and most dramatic is the pingo,a perennial ice-cored mound. Pingos are of two types:either hydaulic (open) or hydrostatic (closed) in nature.Both cases require specific hydrologic conditions fortheir formation. Pingos are relatively rare in the majorityof periglacial landscapes; however, the largest concentra-tion of closed-system pingos, over 1,350, occurs in theMackenzie Delta region of Canada (Figure 7a), whilemore isolated open-system pingos are found in centralYukon and interior Alaska, Svalbard, and central andnorthern Siberia. In many subarctic areas, the preferentialgrowth of permafrost beneath organic material resultsin the formation of peat plateaus and palsas. A numberof smaller frost mounds, mostly seasonal in nature,also occur.

The most widespread surface feature that is characteris-tic of permafrost is a network of thermal-contractioncracks, typically 15–30 m in dimensions, which dividethe ground surface up into orthogonal or random-orthogonal patterns or polygons (Figure 7b). The cracksare caused by thermal contraction cracking of the groundduring winter. In summer, the cracks fill with water fromsnow melt that subsequently freezes to form wedge-shaped bodies of foliated ice (Figure 7c). In lowland tun-dra terrain, ice-wedge ice may constitute between 10%and 20% by volume of the upper 5–10 m of permafrost.

The thaw of ice-rich permafrost gives rise to distinctfeatures. These, and the complex of processes associatedwith thawing permafrost, are generally termedthermokarst. For example, snowmelt-induced runoff inspring results in gully erosion along the lines of ice wedgesand where massive icy bodies become exposed, as alongriverbanks and at coastal locations, retrogressive ground-ice slumps may develop (Figure 7d). On terrain underlain

836 PERIGLACIAL

by fine-grained and ice-rich sediments, numerous shallowponds or thaw lakes can develop (see Figure 4a). On gentleslopes, active-layer-detachment failures may occur inyears of enhanced summer thaw (see Figure 4a). On bed-rock outcrops, the thaw of ice within joints can lead toinstability and enhanced rockfall activity.

The creep of permafrost refers to the long-term defor-mation of frozen ground under the influence of gravity.Fine-grained frozen sediments, such as silt and clay, whichcontain large unfrozen water content amounts, are espe-cially suitable to frozen creep deformation. Rates of move-ment are slow, usually less than 0.5 cm/year. The mostrapid creep deformation is recorded in rock glaciers, espe-cially those that occur in mid-latitude mountains. Forexample, the Muragle rock glacier in Switzerland isreported to be deforming at a rate of 50 cm/year (Kaaband Kneisel, 2006).

Azonal processesMass-wasting processes are not unique to cold environ-ments but can assume distinctive characteristics andenhanced importance. For example, in non-permafrostregions in summer there is slow mass wasting of thewater-saturated thawed layer. This is termed solifluction.Where it occurs in permafrost regions, the process is calledgelifluction (Figure 8a). The result is heterogeneous slopedeposits, or diamicts, that mantle gentle and lower valley-side slopes. These may form lobes and terraces, usuallywith risers between 0.5 and 3 m in height (Figure 8b).More rapid movements involve rockfalls, debris flows,and avalanches. These are particularly common in humidalpine environments where steep bedrock outcrops arepresent. Frost-induced movements within the active layer,or in the zone of seasonal frost, leads to the formation ofstone nets that, as slope angle increases, turn into stone

a

c

b

d

Periglacial, Figure 7 Features of permafrost terrain: (a) Closed-system pingo near Tuktoyaktuk, NWT, Canada; (b) Poorly drainedtundra lowland, northern Alaska, showing low-centered ice-wedge polygons; (c) Epigenetic ice wedge, southern Banks Island, ArcticCanada; (d) Ground- ice slump, northern Yukon coast, Canada.

PERIGLACIAL 837

stripes (Figure 8c). Even in non-permafrost environments,frost action and ice segregation within the seasonally fro-zen layer gives rise to the formation of small-scale sortedand non-sorted patterned ground (circles, nets, stripes),frost heaving of bedrock, and the formation of smallhummocks or frost mounds (thufurs, earth hummocks)(Figure 8d).

Wind plays an especially important geomorphic role inthe tundra and polar desert environments (Seppala, 2004).For example, the depth and coverage of snow is deter-mined by the prevailing wind regime. Typically, uplandsurfaces are blown clear of snow, while lee slopes andlower valley-side slopes are sites of snow-bank accumula-tion. In spring, the melt of snow banks promotes runoff orsurface wash that transports sediment down slope. Soli-fluction or gelifluction is enhanced immediately belowsnow banks because of the saturated near-surface thawlayer. In some regions, preferential snow distributionresults in enhanced solifluction on lee slopes and thedevelopment of asymmetrical valleys, with streams beingprogressively “pushed” toward the windward (steeper)slope. Elsewhere, localized wind erosion can occur inweakly consolidated sedimentary bedrock and deflationoperates on fine-grained sediments.

In the absence of vegetation, deflation assumes localimportance in many periglacial environments (Figure 9a).In more continental periglacial environments, loess andcover-sand deposition is widespread. For example, in

parts of northwestern North America, long-continuedmass wasting of loessic materials has led to the partialinfilling of valleys with heterogenous organic-rich andice-rich sediments known locally as “muck” while inparts of central Siberia, similar mass wasting combinedwith long-continued aggradation of alluvial sedimentshas created similar ice-rich sediments known locally as“Yedoma complex.”

A special characteristic of periglacial areas immedi-ately adjacent to the Antarctic and Greenland ice sheets,and to a lesser degree, the ice-marginal areas peripheralto all glaciers, is the presence of persistent and stronggravity-driven (katabatic) winds that flow outward fromthe ice. At extremely low temperatures, snow crystalsbecome effective abrasive agents with MOH hardnessvalues exceeding 4 at �40�C; as a result, wind-polishedand fluted rocks and bedrock outcrops and wind-abradedcobbles (ventifacts) are common (Figure 9b). In the dryvalleys of southern Victoria land and Antarctica, thesewinds promote sublimation to such an extent that peren-nial snow and ice is unable to form.

In spite of apparent aridity, fluvial activity is anotherimportant component of periglacial environments. Thisis because losses through evaporation and infiltrationare minimized by low temperatures and frozen ground,respectively. The result is a highly seasonal dischargeregime, dominated by a nival (snowmelt) peak in earlyspring. The fluvial dynamics are no different to other

a b

c d

Periglacial, Figure 8 Examples of periglacial patterned-ground phenomena: (a) Oblique air view of mass wasting (non-sortedstripes) and gelifluction movement, Sachs Harbour, Banks Island, Canada; (b) Turf-banked gelifluction lobe, Holman, Victoria Island,Arctic Canada; (c) Stone stripes on low angled slopes (3–7�) are separated from each other by vegetated stripes of finer material; thepatterned ground is relict, Mont Jacques-Cartier, Gaspesie, Quebec, Canada; (d) Small earth or frost hummocks (thufur), centralIceland.

838 PERIGLACIAL

environments, but high sediment loads and highly variableseasonal discharges lead to a dominance of braidedchannels as opposed to straight, meandering, andanastomozing patterns. In fact, the well-developed drain-age networks and large-scale organization of periglaciallandscapes are not dissimilar to those elsewhere. Riverand lake ice persists for several months of the year; atspring melt, ice jams may develop, causing flooding anddamage to structures such as bridges. Where powerfulperennial springs emerge, the freezing of discharge inthe downstream direction forms tabular ice bodies (rivericings) that may cover several square kilometers.

In the northern hemisphere, periglacial coasts experi-ence wave action for a restricted period of the yearand are often protected from erosion by an ice foot. Icepushing is a common feature of many arctic coastlines.Where the coast is formed in ice-rich unconsolidated sed-iments, as along much of the Beaufort, Laptev, and EastSiberian Seas, wave action and thermal erosion result incoastal retreat of several meters per year. The rapid forma-tion of spits and offshore bars is characteristic. In Antarc-tica, wave action and coastal processes are relativelyunimportant on account of the hard bedrock that formsthe coastline and the extremely short open-water season.

Periglacial environments, environmentalchallenges, and global climate changePeriglacial environments constitute part of the cryosphere.As such, they play a critical role in global climate change(Lemke et al., 2007). It is now understood that the hydro-logical cycle of the big northern rivers of North Americaand Eurasia links snowmelt and precipitation with riverrunoff, sea ice, and ocean circulation in a single system.This influences deep water formation in the Arctic basinand the corresponding global thermohaline circulation ofthe oceans (Peterson et al., 2002). At the same time, anyreduction in the extent of sea ice or snow cover reduces

albedo, or reflectivity, of land or ocean surface and allowsmore solar radiation to be absorbed.

Periglacial environments are thought to act, therefore,as a positive feedback mechanism for climate warming.Already, there is speculation that this is the cause ofrecent reductions in snow cover and sea-ice extent. Inthe future, any thaw of the organic-rich upper layers ofpermafrost, especially in the subarctic, will releasesignificant quantities of carbon dioxide and methane, bothof which are important greenhouse gases. Finally, the sig-nificance of gas hydrates, which exist frozen withinpermafrost, is still not widely recognized. Yet, their even-tual potential release to the atmosphere will lead toa dramatic increase in greenhouse gases and this could fur-ther accelerate any climate warming.

If climate warming proceeds as predicted, periglacialenvironments will be among the first to be affected.Warming will be enhanced because of (1) increased meth-ane flux due to decomposition of organic matter frozen innear-surface permafrost and release of methane hydratesas permafrost bodies degrade, (2) increased biomassproduction and decay in tundra and taiga zones, and(3) decreased surface albedo as snow-cover extent andduration decrease. Already, there is evidence that warmingof permafrost has been ongoing for over 30 years (e.g.,Osterkamp, 2008; Brown and Romanovsky, 2008). Thismay lead to long-term changes. For example, at the south-ern (warm) limits of the discontinuous permafrostzone, permafrost bodies will progressively disappear, thetree-line will advance northward, and the active-layerthickness will increase. The latter, monitored by theCALM program of the International Permafrost Associa-tion (Brown et al., 2000), will probably lead to an increasein the frequency of active-layer detachment failures andslope instability, to changing snow-melt and hydrologicalregimes, and to enhanced mass wasting and landscapemodification in high-latitude permafrost environments.In alpine periglacial environments, thawing permafrost

a b

Periglacial, Figure 9 Examples of wind action in periglacial environments: (a) Deflation caused by strong winds and an absence ofsurface vegetation, central Iceland; (b) Wind-sculpted bedrock blocks of welded volcanic ash, Brown Bluffs, Antarctic Peninsula.

PERIGLACIAL 839

may lead to instability of rock outcrops that could threatenthe foundations of ski lifts and other recreational installa-tions at high elevation (Gruber and Haeberli, 2007;Haeberli, 1992).

A number of environmental concerns relate to global cli-mate change and the impact of human activity in periglacialenvironments. Many centre around the various problemsassociated with natural resource management, exploitation,and ownership (e.g., Young, 2009; Tin et al., 2009). Forexample, the marine and terrestrial ecosystems are increas-ingly being subject to environmental stress. In the arctic,the marine food chain is linked to sea ice, nutrient availabil-ity, and water density. Any changes to these may inducechanges to the marine ecosystem and the associatedbiochemical cycling of essential nutrients. The terrestrialfood chain is limited by the short growing season, lowtemperatures, and low rates of nutrient cycling. Thus, cli-mate warming in high latitudes will change plant and ani-mal communities. This will affect the hunting andharvesting of animals and plants by northern indigenouspeoples in Canada, Greenland, and northern Scandinavia.A second environmental concern for the northern high lat-itudes is the recent increase in industrial air pollution frommid-latitudes. Small particles, such as sulfur dioxide, aretransported by atmospheric circulation toward high lati-tudes where they appear as “arctic haze.” The harsh realityis that the northern high latitudes act as an atmospheric“sink” for industrial pollutants generated in the mid-latitudes, especially those of northern Europe and EuropeanRussia. Third, in Antarctica, the increasing number ofcruise ships and related tourism activities will soon inadver-tently impact upon certain of the critical marine- and bird-breeding localities in South Georgia and the AntarcticPeninsula. There is also the possibility of a major marineenvironmental disaster if a cruise ship were to hit an icebergand sink in Antarctic waters. Finally, Chinese upgrading ofthe Qinghai-Xizang (Tibet) Highway in recent years andcompletion of the Qinghai-Tibet Railway in 2003 hasopened the large montane periglacial environment of theTibet Plateau to a potential increase in human occupancyand economic activity with, as yet, unknown consequences.

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Brown, J., and Romanovsky, V. E., 2008. Report from the Interna-tional Permafrost Association: State of permafrost in the firstdecade of the 21st century. Permafrost and Periglacial Pro-cesses, 19, 255–260.

Brown, J., Hinkel, K. M., and Nelson, F. E., 2000. The CircumpolarActive Layer Monitoring (CALM) program. Research designand initial results. Polar Geography, 24, 165–258.

Dixon, J. C., and Thorn, C. E., 2005. Chemical weathering andlandscape development in mid-latitude alpine environments.Geomorphology, 67, 85–106.

Etienne, S., 2002. The role of biological weathering in periglacialareas: a study of weathering rind in south Iceland. Geomorphol-ogy, 47, 75–86.

French, H. M., 2000. Does Łozinzki’s periglacial realm exist today?A discussion relevant to modern usage of the term “periglacial”.Permafrost and Periglacial Processes, 11, 35–42.

French, H. M., 2007. The Periglacial Environment, 3rd edn.Chichester: Wiley, 458 pp.

French, H. M., and Guglielmin, M., 2000. Cryogenic weathering ofgranite, Northern Victoria Land, Antarctica. Permafrost andPeriglacial Processes, 11, 305–314.

Guglielmin, M., Cannone, N., Strini, A., and Lewkowicz, A. G.,2005. Biotic and abiotic processes on granite weatheringlandforms in a cryotic environment, Northern Victoria Land,Antarctica. Permafrost and Periglacial Processes, 16, 69–85.

Gruber, S., and Haeberli, W., 2007. Permafrost in steep bedrockslopes and its temperature-related destabilization following cli-mate change. Journal of Geophysical Research, 112, F02S18,doi:10.1029/2006JF000547.

Haeberli, W., 1992. Construction, environmental problems andnatural hazards in periglacial mountain belts. Permafrost andPeriglacial Processes, 3, 111–124.

Hall, K., 1999. The role of thermal stress fatigue in the breakdownof rock in cold regions. Geomorphology, 31, 47–63.

Kaab, A., and Kneisel, C., 2006. Permafrost creep within a recentlydeglaciated glacier forefield: Muragl, Swiss Alps. Permafrostand Periglacial Processes, 17, 79–85.

Karte, J., and Liedtke, H., 1981. The theoretical and practical defi-nition of the term “periglacial” in its geographical and geologicalmeaning. Biuletyn Peryglacjalny, 28, 123–135.

Konishchev, V. N., and Rogov, V. V., 1993. Investigations of cryo-genic weathering in Europe and Northern Asia. Permafrost andPeriglacial Processes, 4, 49–64.

Lemke, P., Ren, J., Alley, R. B., Allison, I., Carrasco, J., Flato, G.,Fujii, Y., Kaser, G., Mote, P., Thomas, R. H., and Zgang, T.,2007. Observations: changes in snow, ice and frozen ground.In Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M.,Averyt, K. B., Tignor, M., and Miller, H. L. (eds.), ClimateChange 2007: The Physical Science Basis. Contribution ofWorking Group 1 to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. Cambridge, UK/New York: Cambridge University Press.

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PERMACRETE

Ashok Kumar VermaDepartment of Geography and Environmental Studies,Cold Regions Research Center, Wilfrid LaurierUniversity, West Waterloo, ON, Canada

SynonymsDuracrete

DefinitionPermacrete is a versatile and unique resurfacing materialthat can be applied to concrete, masonry, foam, steel,stucco, and aggregate surfaces. Permacrete is an architec-tural, chemical-concrete with twice the strength of stan-dard concrete (What is Permacrete, 2009). Permacrete isa very strong and heat-resistant material that can be usedin a variety of applications.

FormationPermacrete is a three part, acrylic polymer cementationssystem with strength of over 6,000 PSI (pounds forceper square inch). These three parts include a matrix mix(early-strength concrete mixture), chemical bonding addi-tive, and a stain sealer. It is sealed and nonporous, resistschemicals, and withstands freeze-thaw cycles as well asintense heat and ultraviolet rays (What is Permacrete,2009; Kirk, 1998).

Thermal disturbance and exposure to solar heat consid-erably affect the physical and mechanical properties of thepermafrost. Preservation of the thermal properties in thepermafrost is the major challenge in building the engineer-ing and construction work (Jumikis, 1983). Since thepermacrete has the high insulating and heat-resistant prop-erties, it is extensively used in permafrost regions to buildthe residential properties (houses) and commercial proper-ties (oil, gas pipelines, and tunnels) in combination of soiland ice material.

The ancient form of similar material can be comparedwith use of snow by Eskimos where they used the blockof ice to build the igloos for their shelters in high arcticenvironment. The application of permacrete as buildingmaterial can be found when the US Army developed theartificial aggregates by mixing silicates and aluminum-silicate with snow to form “permacrete” while performingexperiments in Greenland (Kirk, 1998).

BibliographyKirk, R., 1998. Snow. University of Washington Press: Seattle,

p. 150.Jumikis, A. R., 1983. Rock Mechanics. Trans Tech: Clausthal,

pp. 87–88.Permacrete, 2009. http://www.permacrete.com/commercial/faq.phpWhat is Permacrete, 2009. http://www.bulifant.com/HFBulifant/

permacrete.htm

PERMAFROST

Yuri Shur1, M. Torre Jorgenson2, M. Z. Kanevskiy11Department of Civil and Environmental Engineering,University of Alaska Fairbanks, Fairbanks, AK, USA2Alaska Ecoscience, Fairbanks, AK, USA

SynonymsPerennially cryotic ground

PERMAFROST 841