Periglacial Landforms: Terminology for UPSC Geography Optional

Periglacial geomorphology deals with cold, non-glacial landscapes: ground that freezes deeply or permanently but is not buried under glacier ice. Its vocabulary covers frozen ground (permafrost, active layer, talik), the frost-driven processes that work it, and the mounds, polygons, terraces and hollows those processes leave. Examiners reward precise terms here because the landforms look deceptively similar.

Read each entry definition first, then mechanism, examples and the sketch line. UPSC has asked this family twice as a single named concept: altiplanation (2021) and Peltier’s periglacial cycle of erosion (2017). Both are treated in full below, with the processes they depend on explained first so the ★ answers can be built directly.

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The Periglacial Environment and Frozen Ground

Periglacial (periglacial environment)

Periglacial describes cold, non-glacial environments in which frost action and frozen ground, not glacier ice, are the dominant geomorphic agents. Such areas experience intense freezing and thawing, usually over permafrost, with sparse vegetation, and include both present-day high-latitude and high-altitude zones and relict areas that were periglacial during Pleistocene cold stages.

• Coined by: Walery von Łoziński in 1909, while studying frost-shattered sandstone in the Carpathians; the “periglacial zone” concept was developed after the 1910 International Geological Congress in Stockholm.
• Key features: mean annual air temperature generally below about +3°C; freeze–thaw cycles; snow patches; little chemical weathering; strong wind and snowmelt action.
• Types: present-day areas (Arctic Alaska, Canada, Greenland, Siberia, ice-free Antarctica, high mountains); relict or fossil areas of the Pleistocene (southern England, central Europe).
• Examples: the tundra of the Canadian Arctic; in India, the cold-arid trans-Himalaya of Ladakh and the high Spiti and Changthang plateaus.
• Don’t confuse with: proglacial (in front of an ice margin, fed by meltwater — see proglacial lakes) and paraglacial (landscapes adjusting after deglaciation). Peltier’s periglacial morphogenetic region is classified under morphogenetic regions.

Permafrost (continuous, discontinuous & sporadic permafrost)

Permafrost is ground — soil, sediment or rock — that remains at or below 0°C for at least two consecutive years. It is defined by temperature, not ice content, so dry frozen rock counts. It underlies much of the Arctic and high mountains and controls hydrology, slope stability and engineering in cold regions.

• Coined by: Siemon William Muller in 1943 (a contraction of “permanently frozen ground”); Kirk Bryan proposed pergelisol as an alternative in 1946.
• Types: continuous (over 90 per cent of the area frozen), discontinuous (50–90 per cent), sporadic (10–50 per cent) and isolated patches (under 10 per cent); also alpine or mountain permafrost, and relict subsea permafrost on Arctic shelves.
• Key features: thickness ranges from a few metres at its southern edge to about 1,500 m in parts of Yakutia; ground ice occurs as pore ice, ice lenses, wedges and massive ice.
• Examples: roughly 21 million km² of the Northern Hemisphere lies in permafrost zones, and about 1.5 million km² of this falls in the Hindu Kush–Himalaya region; in India, permafrost has been measured at South Pullu (4,727 m) in the upper Ganglass catchment, Ladakh.
• Significance / Hazard link: thaw of ice-rich permafrost causes subsidence, slope failure and carbon release (see permafrost thaw below).

Active layer

The active layer is the uppermost layer of ground above permafrost that thaws every summer and refreezes every winter. Its thickness — from a few centimetres to a few metres — sets the depth at which all periglacial processes operate, since frost heave, cryoturbation and solifluction are confined to this seasonally mobile zone.

• Mechanism: summer thaw advances downward from the surface; autumn freeze-back proceeds both downward from the surface and upward from the permafrost table, trapping an unfrozen, pressurised layer between them.
• Key features: thicker under bare, dry, south-facing ground; thinner beneath peat, moss and snow-poor sites; its base is the permafrost table.
• Examples: long-term thickness records come from the Circumpolar Active Layer Monitoring (CALM) network across Alaska, Canada and Siberia; the Tuktoyaktuk Peninsula in Arctic Canada is a classic study area.
• Significance / Hazard link: a deepening active layer is the first measurable sign of permafrost degradation and weakens building foundations.
• Don’t confuse with: seasonally frozen ground, which freezes each winter but has no permafrost beneath it.

Talik

A talik is a body of unfrozen ground within, beneath or through permafrost, kept thawed by heat from water bodies, groundwater flow or local warming. Taliks form the plumbing of permafrost regions, carrying groundwater that feeds springs, icings and pingos.

• Types: closed talik (a thawed bulb beneath a lake, surrounded by permafrost); open or through talik (penetrating the full permafrost thickness, beneath large lakes and rivers); lateral and isolated taliks.
• Formation: a water body deeper than the winter ice thickness does not freeze to its bed, so heat stored in the water keeps the ground below unfrozen.
• Examples: thaw-lake taliks of the Mackenzie Delta, Canada; through-taliks beneath the Lena river, Yakutia.
• Significance / Hazard link: freezing of a closed talik after a lake drains squeezes pore water upward and builds closed-system pingos.

Periglacial Processes

Congelifraction (frost shattering, frost wedging)

Congelifraction is the mechanical break-up of rock by the repeated freezing of water in joints, bedding planes and pores. Water expands by about 9 per cent on freezing, and ice that grows in a crack prises it open, detaching angular fragments. It is the dominant weathering process of periglacial and high-mountain environments.

• Mechanism: two models operate. Volumetric expansion needs saturated, sealed cracks and rapid freezing; the ice-segregation model, now favoured by experimental work, holds that water migrates towards growing ice lenses at sustained sub-zero temperatures of roughly –3°C to –8°C, cracking rock even when it is not saturated.
• Key features: angular, unweathered debris; frost-riven cliffs; scree and blockfields.
• Coined by: Kirk Bryan (1946), who also named congeliturbation and cryoplanation.
• Examples: scree cones below frost-shattered crags of the Canadian Rockies; in India, the scree-mantled slopes of the Spiti and Zanskar valleys.
• Don’t confuse with: insolation weathering (heating and cooling of dry rock — see physical weathering); talus accumulations as a mass-movement form belong to talus and scree.

Frost heave & frost thrust

Frost heave is the upward displacement of the ground surface caused by the growth of segregated ice lenses in frost-susceptible soil as it freezes. Frost thrust is the corresponding lateral push. Together they lift stones, fence posts and foundations, and they drive the sorting that builds patterned ground.

• Mechanism: fine silty soils draw water by suction towards the freezing front, where it forms lenses of clear ice; the ground rises far more than the 9 per cent expansion of pore water alone would allow.
• Key features: stones are jacked upward because ice grips and lifts them and fine soil slumps beneath them on thaw; needle ice (pipkrake) forms at the surface on cold nights.
• Examples: heaved roads and airstrips of interior Alaska; the Qinghai–Tibet railway (opened 2006) uses crushed-rock embankments and thermosyphons to keep permafrost beneath the track frozen.
• Significance / Hazard link: a leading cause of damage to roads, pipelines and buildings in cold regions.

Cryoturbation & involutions

Cryoturbation is the churning, mixing and deformation of the active layer by frost heave, freeze–thaw and differential loading. Involutions are the contorted, festoon-shaped structures it leaves in layered sediments — folds, pillars and pockets of one layer injected into another.

• Mechanism: during autumn freeze-back the unfrozen layer trapped between the downward-freezing surface and the permafrost table is pressurised and deforms; density contrasts between saturated layers cause loading and diapirs.
• Types: fold, pillar (flame) and amorphous involutions.
• Coined by: Kirk Bryan introduced congeliturbation (1946); cryoturbation is the shortened form now standard.
• Examples: involutions in Pleistocene gravels of southern England and the Netherlands are key evidence of former permafrost.
• Significance / Hazard link: involutions and churned soils (turbels) are used as palaeoclimatic indicators.

Congelifluction (gelifluction, solifluction) & solifluction lobes

Congelifluction (now usually gelifluction) is the slow downslope flow of water-saturated, thawed debris over frozen ground. Solifluction is the broader term, coined in 1906 for any saturated soil flow, and is often used for the periglacial case. Its most distinctive landform is the tongue-shaped solifluction lobe.

• Coined by: Johan Gunnar Andersson introduced solifluction in 1906 from observations on Bear Island (Bjørnøya); the congeli- form restricted it to frozen-ground conditions.
• Mechanism: meltwater from thawing ice lenses cannot drain into the permafrost below, so pore pressure rises and shear strength falls; combined with frost creep, material moves a few millimetres to several decimetres a year.
• Types: stone-banked and turf-banked lobes; solifluction terraces and sheets.
• Examples: lobes on the slopes of the Scandinavian mountains and Svalbard; relict “head” deposits in south-west England.
• Sketch: a slope with lobes having steep, bulging fronts 0.5–3 m high; label the active layer, permafrost table and flow arrows.
• Don’t confuse with: non-frozen creep and earthflow — see creep and earthflow.

Nivation & nivation hollow

Nivation is the group of processes — intensified freeze–thaw, meltwater wash and solifluction — that operate beneath and around the edges of a snow patch that lingers into summer. A nivation hollow is the shallow, often crescent-shaped depression it gradually enlarges on a hillside.

• Coined by: François Émile Matthes (1900), from the Bighorn Mountains, Wyoming.
• Mechanism: the snow patch supplies meltwater daily to its margins, sustaining frost shattering there, while meltwater and solifluction remove the debris; the hollow deepens and the snow patch grows larger each year.
• Types: transverse, longitudinal and circular hollows, depending on the shape of the snow patch.
• Examples: snow-patch hollows in the Rocky Mountains of Colorado and the Scandinavian mountains.
• Significance / Hazard link: nivation drives altiplanation terraces; its real erosive power is debated, since a 2002 review found little direct measurement of it.
• Don’t confuse with: a cirque, which is cut by moving glacier ice. A large nivation hollow may become the nucleus of a cirque if climate cools further.

Periglacial Landforms

Patterned ground (circles, polygons, nets, stripes)

Patterned ground is the geometric arrangement of stones and fine soil into circles, polygons, nets, steps and stripes, produced by frost sorting and frost cracking in the active layer. Pattern shape is controlled mainly by slope angle, grain size and moisture.

• Types: Albert Lincoln Washburn’s 1956 classification: circles, polygons, nets, steps and stripes, each sorted (stone borders around fine centres) or non-sorted.
• Mechanism: frost heave lifts stones and pushes them outward from fine-grained, heave-prone centres; convective cells in the active layer concentrate stones at the borders; on steeper slopes, solifluction stretches circles into garlands and stripes.
• Key features: circles and polygons on flat ground (under about 2–6°); steps and stripes on slopes up to about 30°; sorted circles reach several metres across.
• Examples: sorted circles of Spitsbergen (Svalbard); relict stone stripes on Breckland, eastern England.
• Sketch: plan view of sorted polygons grading downslope into stone stripes; label fine centres, stone borders and slope direction.

Ice wedge & ice-wedge polygon

An ice wedge is a vertical, wedge-shaped body of foliated ice that grows in a thermal contraction crack in permafrost. Repeated cracking and filling over centuries builds wedges up to a few metres wide. An ice-wedge polygon is the network of wedges, usually tens of metres across, seen in plan on tundra plains.

• Mechanism: intense winter cooling contracts the frozen ground, which cracks; spring meltwater enters the crack and freezes, and each year adds a thin vein of ice.
• Types: low-centred polygons (raised rims, wet centres) and high-centred polygons (formed when wedges thaw and troughs deepen).
• Key features: an ice-wedge cast forms when the ice melts and sediment fills the void — proof of former permafrost in regions now temperate.
• Examples: the Arctic coastal plain near Utqiaġvik (Barrow), Alaska; ice-wedge casts in Pleistocene gravels of the Netherlands.
• Sketch: a cross-section of a V-shaped wedge with vertical foliation beneath upturned strata, plus a plan-view polygon net.

Earth hummock (thufur)

An earth hummock is a small dome of fine, frost-heaved soil, typically under 1 m high and 1–2 m across, usually with a turf or moss cover, occurring in dense fields on poorly drained periglacial ground. In Iceland these are called thúfur (thufur).

• Formation: differential frost heave and cryoturbation lift fine soil into domes; vegetation and uneven snow cover then reinforce the pattern.
• Types: turf hummocks (vegetated) and earth hummocks (bare); some classifications also group palsas with them as frost mounds.
• Examples: hummock fields of Iceland and of the tundra near the Mackenzie Delta, Canada.
• Don’t confuse with: palsas, which have a peat body and an ice core and are much larger.

Pingo (open-system & closed-system pingo)

A pingo is a perennial, ice-cored hill that rises above flat permafrost terrain, formed when pressurised groundwater freezes to create a massive ice core. Pingos range from a few metres to about 70 m in height and can reach several hundred metres across.

• Coined by: Alf Erling Porsild (1938), from the Inuvialuit word for a small hill.
• Types: closed-system (hydrostatic) pingos, formed where a drained lake’s talik freezes and expels water upward (Mackenzie Delta type) — a model established by John Ross Mackay; open-system (hydraulic) pingos, fed by artesian groundwater from outside the permafrost (East Greenland and interior Alaska type).
• Key features: summit dilation cracks; collapse craters and a rampart ring once the ice core thaws (“pingo scars”).
• Examples: Ibyuk Pingo near Tuktoyaktuk, 49 m high and still growing about 2 cm a year — one of roughly 1,350 pingos on the Tuktoyaktuk Peninsula, about a quarter of the world’s total.
• Sketch: a cross-section showing drained-lake basin, freezing front, pressurised talik and the ice core lifting the overburden.

Palsa

A palsa is a low mound or ridge of peat with a permanently frozen core of peat and mineral soil containing segregated ice lenses, found in bogs of the discontinuous permafrost zone. Palsas are typically 1–7 m high and 10–30 m wide, and they grow and decay in cycles.

• Formation: where snow cover is thin, winter frost penetrates deeper; dry summer peat insulates the frozen core, which grows by ice segregation and heaves the surface.
• Types: dome palsas, palsa plateaus and string (ridge) palsas; lithalsas are the peat-free equivalents.
• Examples: palsa mires of Finnish Lapland and northern Norway; in Norway’s Dovrefjell, entire palsa bogs have melted since warming began.
• Significance / Hazard link: palsas are sensitive climate indicators; models suggest Fennoscandia could become climatically unsuitable for them by about 2040.
• Don’t confuse with: a pingo, which has a massive ice core built by injected water, not a peat body with segregated ice lenses.

Thermokarst & alas

Thermokarst is irregular, hummocky and pitted terrain produced when ice-rich permafrost thaws and the ground surface collapses into the space the ice occupied. An alas is a large, steep-sided, flat-floored thermokarst depression, often holding a lake, typical of central Yakutia.

• Mechanism: thermally, not chemically, controlled — warming, fire, vegetation clearance or drainage change melts ground ice; subsidence then pools water, which absorbs more heat and speeds further thaw.
• Types: thaw (thermokarst) lakes, alases, retrogressive thaw slumps, thermo-erosion gullies, beaded streams, high-centred polygons.
• Examples: the Batagay megaslump, Yakutia — about 990 m wide by 2023, with a 55 m headwall retreating about 12 m a year; alases of the Lena–Aldan interfluve.
• Significance / Hazard link: a direct hazard to settlements and pipelines, and a pathway for permafrost carbon release.
• Don’t confuse with: karst sinkholes, which are formed by the solution of limestone. Thermokarst only resembles karst in form.

Blockfield (felsenmeer)

A blockfield or felsenmeer (“sea of rocks”) is a continuous sheet of angular, frost-shattered boulders covering flat or gently sloping summits and plateaus. It is produced by in-situ congelifraction of well-jointed bedrock, with fines washed out.

• Formation: frost shattering along joints produces blocks; frost heave lifts them to the surface while meltwater removes fine material, leaving an open-work boulder carpet.
• Key features: lichen-covered, angular blocks; little movement on flat ground; many blockfields are relict, and some may carry an older, pre-Quaternary weathering mantle.
• Examples: Hickory Run Boulder Field, Pennsylvania (relict Pleistocene); summit blockfields of the Scandinavian mountains.
• Don’t confuse with: talus, which is debris accumulated at a slope foot by rockfall — see talus.

Block stream (stone stream)

A block stream or stone stream is an elongated tongue of boulders filling a valley floor or running down a slope, fed from blockfields or frost-riven cliffs and moved by frost creep and gelifluction of the underlying fines.

• Formation: boulders migrate downslope with the saturated matrix beneath them; fines are later flushed out by meltwater, leaving an open boulder train with water audible underneath.
• Key features: crude sorting with coarse blocks at the surface; the long axes of blocks are often aligned downslope.
• Examples: the stone runs of the Falkland Islands, some several kilometres long; River of Rocks at Hawk Mountain, Pennsylvania.
• Don’t confuse with: a rock glacier, which contains interstitial ice and moves as a coherent, lobate mass.

Rock glacier

A rock glacier is a tongue- or lobe-shaped mass of angular debris cemented or underlain by ice, which creeps slowly downslope under gravity like a glacier. It is the most visible landform of mountain permafrost and forms below talus slopes or debris-covered cirques.

• Types: active (ice-rich and moving), inactive (ice present, not moving) and relict (ice melted); talus-derived and glacier-derived forms.
• Key features: steep front at about the angle of repose; transverse ridges and furrows; typical creep rates range from centimetres to a few metres a year, and many Alpine examples accelerated after 2000.
• Examples: the rock glaciers of the Swiss Alps; in India, thousands mapped in Ladakh, Kashmir and Lahaul–Spiti, Himachal Pradesh, by recent satellite inventories.
• Significance / Hazard link: rock glaciers store ground ice that may become an important water reserve in the cold-arid Himalaya as glaciers shrink.
• Don’t confuse with: debris-covered glaciers — see glacier types.

Asymmetric valley (periglacial valley asymmetry)

An asymmetric valley in periglacial terrain has one valley side markedly steeper than the other, because the two slopes receive unequal solar radiation and snow. As a result, they experience different intensities of freeze–thaw, meltwater supply and solifluction.

• Formation: in the Northern Hemisphere, sun-facing slopes thaw more often, so frost shattering and gelifluction remove debris and push the stream towards the opposite side; shaded slopes stay frozen or snow-covered.
• Key features: the direction of steepening varies between regions, which is why two competing explanations persist.
• Examples: relict asymmetric dry valleys on the chalk of southern England; active examples in the Canadian Arctic.

Planation, the Periglacial Cycle and Permafrost Thaw

Altiplanation terrace (cryoplanation terrace, goletz terrace)

Altiplanation is the levelling of high summits and upper slopes in periglacial regions by frost action, producing altiplanation terraces. These are bedrock benches, each a gently inclined tread backed by a steeper frost-riven scarp, which retreat parallel to themselves under nivation, frost shattering and solifluction.

• Coined by: Henry Miner Eakin in 1916, from the Yukon–Koyukuk region of interior Alaska. He described altiplanation as a special phase of solifluction that flattens summits and cuts terraces. Kirk Bryan later (1946) used cryoplanation for the same family of processes. Russian workers call the benches goletz terraces, after the bald summits (goltsy) of Siberia and the Urals.
• Mechanism: a snowbank lodges at the foot of a scarp; nivation and congelifraction attack the scarp base and drive it back; gelifluction and slopewash carry the shattered debris across the tread. Repeated retreat widens the tread, and a staircase develops as several scarps retreat at different levels.
• Key features: treads usually slope 1–5° and are 10 to several hundred metres wide and long; risers are 1–75 m high; a 1–2 m rubble veneer covers the tread. The terraces prefer closely jointed, fine-grained rock, and a 1976 Alaskan study linked their formation to permafrost with mean annual temperatures near –12°C or colder.
• Recent research: cosmogenic-nuclide dating (2020) at Eagle Summit and Mount Fairplay, Yukon–Tanana Upland, showed that the scarps were last actively retreating between about 49,000 and 22,400 years ago, at 0.11–0.56 cm a year. The terraces are therefore diachronous, cut during several cold stages rather than in one.
• Examples: Yukon–Tanana Upland, Alaska; the goltsy of the Urals and Yakutia; relict terraces on Cox Tor, Dartmoor, England. In India, the cold-arid trans-Himalaya of Ladakh has the permafrost and frost regime these benches need, but well-documented altiplanation terraces there are still few.
• Significance / Hazard link: a palaeoclimatic indicator of former permafrost. The long-running debate on whether climate (nivation) or rock structure (jointing and bedding) controls terrace position is now usually treated as complementary rather than a contest.
• Sketch: a stepped ridge profile with two or three treads and risers; mark the snowbank at the scarp foot, frost-shattered debris, the direction of scarp retreat and the permafrost table.
• Don’t confuse with: pediments, which are cut by scarp retreat under arid wash; and structural benches, which follow resistant beds.

Cryoplanation & cryoplain

Cryoplanation is the reduction and levelling of relief in periglacial climates by the combined action of congelifraction, nivation, cryoturbation and gelifluction, working mainly through parallel scarp retreat. Its theoretical end-form, a surface of low relief cut across varied rock, is the cryoplain.

• Coined by: Kirk Bryan (1946). Earlier equivalents include “equiplanation” and Eakin’s altiplanation; Carl Troll’s Solifluktionsrumpf and Jean Tricart’s pénéplaine périglaciaire express the same idea.
• Mechanism: frost-riven scarps retreat and the terraces below them coalesce; debris is spread by gelifluction rather than removed by rivers, so the surface is not graded to a sea-level base level.
• Key features: gentle summit flats, blockfields and tor-like residuals.
• Examples: planation surfaces of the uplands of north-eastern Siberia and interior Alaska.
• Don’t confuse with: the peneplain (fluvial, base-level controlled) and the pediplain (arid, scarp retreat by wash). Most workers now doubt that cryoplanation alone can produce large surfaces, and see it mainly as a modifier of older ones.

Periglacial cycle of erosion (Peltier’s cycle)

The periglacial cycle of erosion is the model proposed in 1950 by the American geographer Louis C. Peltier. It traces the evolution of a landscape under a persistent periglacial climate through stages of youth, maturity and old age, ending in a low-relief cryoplain. It is driven by congelifraction and congeliturbation, with gelifluction as the main transporting process.

Stages

1. Initial stage — periglacial climate sets in and permafrost forms; congelifraction attacks bare rock on pre-existing slopes and blockfields appear on summits.
2. Youth — slopes of bare, frost-riven rock at about 25–30° retreat parallel to themselves; frost-riven cliffs rise above small cryoplanation benches at their base; valleys aggrade with gelifluction debris because seasonal streams cannot carry it all away.
3. Maturity — divides are consumed by continued scarp retreat; residual crags and tors survive on uplands; cryoplanation surfaces expand and coalesce while valley floors keep filling.
4. Old age — hills are reduced to a nearly level cryoplain with slopes of 5° or less; comminuted fine debris is reworked by wind into loess and dunes, and ventifacts form by deflation.

• Significance: the model applied the Davisian architecture of the geographical cycle to cold climates, and the scarp-retreat logic of pediplanation to frost-riven slopes. It also forms part of Peltier’s scheme of morphogenetic regions.

Critique

• Climatic stability: Quaternary climate oscillated every few tens of thousands of years, so no periglacial region has had the uninterrupted cold climate a full cycle needs; most periglacial landscapes are polygenetic and inherited.
• Rates: measured solifluction (millimetres to decimetres a year) and dated scarp retreat (0.1–0.6 cm a year in Alaska) are too slow to create a regional cryoplain within a single cold stage.
• Neglected agents: snowmelt floods are often the most effective sediment transporters in periglacial basins, and wind is important earlier than old age; the model underplays both.
• Method: the stages were built by arranging observed forms into a sequence, not from process measurement — the same form-before-process criticism levelled at William Morris Davis. Post-1950 periglacial research turned to measuring processes instead.
• Sketch: four profiles from initial to old stage; label frost-riven cliffs, cryoplanation terraces, aggraded valleys, residual tors and the final cryoplain.

Permafrost thaw (Arctic & Himalayan, recent findings)

Permafrost thaw is the warming-driven deepening of the active layer and loss of ground ice. It turns stable frozen ground into subsiding thermokarst, unstable slopes and moraines, and it releases stored carbon. It is now the fastest-changing part of periglacial geomorphology, in both Arctic lowlands and high Asian mountains.

• Arctic findings: the Arctic warmed nearly four times faster than the global average between 1979 and 2021, according to a 2022 analysis. NOAA’s Arctic Report Card of December 2024 found that the tundra, with its wildfires, had become a net emitter of carbon dioxide. The Batagay megaslump in Yakutia still grows by about a million cubic metres a year.
• Infrastructure: in May 2020 the collapse of a fuel tank near Norilsk, Siberia, which the operator blamed on subsiding permafrost foundations, spilled about 21,000 tonnes of diesel.
• Himalayan findings: the Hindu Kush–Himalaya holds about 1.5 million km² of permafrost, more than its glacier area in nearly every country, yet it is rarely monitored. At South Lhonak, Sikkim, on 3–4 October 2023, about 14.7 million m³ of frozen lateral moraine that had been creeping more than 15 m a year collapsed into the lake. The collapse raised a 20 m wave and drove the outburst, and a February 2025 study concluded that permafrost thaw probably contributed to the instability — see glacial lake outburst flood.
• Uncertainty: the Himalayan permafrost extent rests largely on models and rock-glacier inventories, with very few boreholes (one example is South Pullu, Ladakh), so figures carry wide error margins.
• Significance / Hazard link: subsidence under roads and army posts in Ladakh and Sikkim, rockfall from thawing rock walls, and mobilisation of debris for GLOFs. The wider Himalayan hazard picture is covered under geomorphic hazard.

PYQs Built on These Terms

• Describe the concept of Altiplanation. (2021)
• Discuss the concept of Periglacial cycle as propounded by Peltier. (2017)

Frequently Asked Questions

What is the difference between periglacial and glacial landforms?

Glacial landforms are cut or built by moving ice, like cirques, U-shaped troughs and moraines. Periglacial landforms are made by frost action and frozen ground without a glacier cover: patterned ground, pingos, ice-wedge polygons, solifluction lobes and altiplanation terraces. Periglacial areas may lie beside ice sheets, but most today are unglaciated Arctic lowlands and high mountains.

What is the difference between a pingo and a palsa?

A pingo is an ice-cored hill up to about 70 m high. Its core is a lens of massive ice formed by freezing, pressurised groundwater, either from a drained lake’s talik (closed system) or from an artesian supply (open system). A palsa is a lower peat mound, usually under 7 m, whose frozen core grows by ice segregation beneath insulating peat.

Is solifluction the same as gelifluction?

Not exactly. Solifluction, named in 1906, means any slow flow of water-saturated soil down a slope. Gelifluction, the modern form of congelifluction, is the specifically periglacial type, in which thawed debris moves over frozen ground that stops meltwater from draining. In periglacial writing the two are often used interchangeably, but gelifluction is the precise term.

Who coined the term altiplanation and what are goletz terraces?

Henry Miner Eakin coined altiplanation in 1916 while mapping the Yukon–Koyukuk region of Alaska, describing summit flattening and terracing by frost-driven solifluction. Goletz terraces are the same bedrock benches as described by Russian geomorphologists on the bald summits (goltsy) of Siberia and the Urals. Cryoplanation terrace is the name most widely used today.

Why is thermokarst not true karst?

Karst forms by the chemical solution of limestone and dolomite, so it is controlled by lithology. Thermokarst forms when ground ice in permafrost melts and the surface collapses, so it is controlled by temperature. The pits, lakes and depressions look like karst, but no rock is dissolved, and thermokarst can develop in any ice-rich sediment.

Are there periglacial landforms in India?

Yes. The cold-arid trans-Himalaya of Ladakh, Zanskar, Spiti and northern Sikkim has permafrost, frost-shattered scree, solifluction and thousands of rock glaciers mapped by recent satellite inventories. Permafrost has been confirmed by ground temperature measurements at about 4,700 m in the Ganglass catchment near Leh. These areas are India’s best field examples for periglacial answers.