Periglacial Processes & Periglacial Landforms – UPSC

• Periglacial literally means “around the ice”, but it now describes any cold, non-glacial environment dominated by frost action, whether or not a glacier lies nearby.
  • The Polish geologist Walery Łoziński (1909) coined the term for frost-shattered rubble beyond the former Pleistocene ice sheets.
• Periglacial areas are perennially frozen below the surface but carry no permanent ice cover; their hallmarks are permafrost, a seasonally thawing active layer, frequent freeze–thaw and sparse vegetation.
• Periglacial landforms result from intense frost, usually with permafrost, worked by frost weathering, frost heave, solifluction and nivation, aided by meltwater and wind.

Periglacial Environment

Periglacial Climate

• Louis C. Peltier (1950) defined the periglacial morphogenetic region by a mean annual temperature of about −15 °C to −1 °C, precipitation of roughly 120–1,400 mm (mostly snow), strong winds and weak fluvial action.
• The spring and autumn transitions are the most effective seasons: daytime thaw and night-time freezing give a diurnal freeze–thaw cycle.
  • Winter freezes everything; late summer is thawed and relatively dry.

Periglacial Areas

• About a quarter of Earth’s land surface is periglacial today, mostly in the Northern Hemisphere.
  • Present-day zones: tundra of Alaska, northern Canada, Greenland, northern Europe and Siberia; ice-free Antarctica; and high mountains in low latitudes (Alps, Andes, Tibetan Plateau).
  • Fossil (relict) zones: lands that were periglacial in the Pleistocene, such as southern England and central Europe.
• The zones shift with climate, and are now retreating poleward and upslope.
• India: periglacial belts lie above the tree line in the Himalaya, most widely in the cold, dry trans-Himalaya of Ladakh and Lahaul–Spiti, and at high altitude in Sikkim and Arunachal Pradesh.

Permafrost

• Permafrost is ground that stays at or below 0 °C for at least two consecutive years; it is defined by temperature, not ice content.
  • Siemon William Muller (1943) coined the term; Kirk Bryan (1946) proposed the synonym pergelisol.
• It underlies about 15% of the Northern Hemisphere’s exposed land; the wider permafrost region, including unfrozen gaps, covers about a quarter.
• Depth ranges from a few metres at its margins to nearly 1,500 m in the Lena and Yana basins of Siberia.
• A talik is an unfrozen layer or pocket within or below permafrost, common beneath deep lakes and rivers.
• Permafrost is not essential for every periglacial landform, but it controls drainage, heave and thermokarst wherever it exists.

• Himalayan permafrost: across the Hindu Kush Himalaya, permafrost covers more area than glaciers in nearly every country, yet it is little studied.
  • Rock glaciers are its best surface indicator; an inventory of Himachal Pradesh mapped 516 rock glaciers between about 3,050 m and 5,500 m, mostly on shaded, north-facing slopes.

Active Layer

• The active layer is the surface layer above permafrost that thaws in summer and refreezes in winter.
  • It ranges from under half a metre in north-eastern Siberia and Alaska to several metres on mountain plateaus, varying with latitude, aspect, snow, vegetation and moisture.
• All periglacial processes operate in it; the permafrost beneath is impermeable, so meltwater is trapped and the thawed layer becomes saturated and weak.
• Bryan named the active layer the mollisol (Latin mollis, soft), as against the permanently frozen pergelisol beneath it.

Periglacial Processes

• Early studies included equiplanation by D. D. Cairnes (1912), frost heave by Bertil Högbom (1914), altiplanation by Henry M. Eakin (1916) and cryoplanation, congelifraction and congeliturbation by Bryan.

Frost Weathering (Congelifraction)

• Congelifraction is the mechanical shattering of rock by freezing water, the dominant weathering process in cold regions.
  • Classical view: water freezing in joints expands by about 9%, prising cracks apart over repeated freeze–thaw cycles.
  • Current view: ice segregation does most of the work, as unfrozen water migrates to growing ice lenses in cracks, most effectively at about −3 °C to −8 °C (the “frost-cracking window”); sustained cold matters more than the number of cycles.
• Products: angular debris, scree, blockfields and frost-riven cliffs.

Frost Heave and Ice Segregation

• Frost heave is the upward bulging of the ground as ice lenses grow in it, followed by settlement on thaw.
  • In frost-susceptible fine soils (silt-sized), water is drawn to the freezing front and builds ice lenses parallel to the surface.
• Vertical thrust: repeated heave lifts stones to the surface, the ground seeming to “vomit stones”, and sorts them into patterned ground.
• Lateral thrust: in autumn the active layer freezes from the top down and from the permafrost up, squeezing the unfrozen layer between them and producing involutions and hummocks.
• Needle ice (pipkrake) lifts surface particles each night, a small-scale form of heave.

Solifluction and Gelifluction

• Solifluction, named by Johan Gunnar Andersson (1906), is the slow downslope flow of water-saturated regolith.
  • Jan Dylik (1951) proposed congelifluction for flow over permafrost; gelifluction is now the usual term.
• Mechanism:
  • Winter ice growth expands and loosens the active layer.
  • Spring meltwater cannot drain into the permafrost, so pore pressure rises and shear strength falls.
  • The saturated layer flows under gravity, even on slopes of a few degrees; frost creep adds to it.
• Rate: usually a few centimetres a year; fastest in the thaw season, halted in late summer and winter.
• Significance: smooths interfluves, aggrades valley floors, aids cryoplanation, and builds solifluction lobes, sheets and terraces.
• India: solifluction lobes and terraces are common on high pastures of Ladakh, Lahaul–Spiti and the Garhwal–Kumaon Himalaya.

Nivation

• Nivation is erosion around and beneath snow patches, first described by François E. Matthes (1900).
  • It combines freeze–thaw at the patch margins, meltwater erosion and solifluction of the loosened debris; the near-immobile snow does not abrade.
• It deepens nivation hollows and helps cut cryoplanation terraces.

Cryoturbation (Congeliturbation)

• Cryoturbation is the churning, heaving and thrusting of the active layer by frost action, also called frost churning or congeliturbation.
  • Repeated freezing and thawing, with differential frost heave and the squeezing of unfrozen soil between the downward and upward freezing fronts, mixes material from different soil horizons.
• Its products are involutions, sorted stones in patterned ground, and organic matter carried down into the mineral soil.
• Fossil cryoturbation structures beyond present permafrost limits are evidence of former periglacial climates.

Thermal Contraction Cracking

• In severe winters frozen ground contracts and cracks; meltwater fills the fissures and freezes as vertical ice veins.
• Repetition builds ice wedges, as Ernest de Koven Leffingwell (1915) showed in northern Alaska.

Running Water and Wind

• Streams are seasonal: winter stops flow, while spring thaw brings sudden, sometimes catastrophic floods.
  • J. L. Jenness (1952) found Arctic Canadian streams cutting ravines 60–90 m deep; Peltier and Anders Rapp saw them mainly as carriers of debris; Jean Corbel held that they erode in maritime climates but aggrade in continental ones, choked by solifluction debris.
• Wind matters once slopes are gentle and debris is fine, forming ventifacts, grooved surfaces, periglacial loess and dunes.
  • The loess belts of central Europe, Ukraine and China were blown from Pleistocene periglacial outwash.

Periglacial Landforms

• Landforms rarely have one cause, so genetic classification groups them by dominant process.

Erosional and Residual Landforms

Nivation Hollows

• Nivation hollows are depressions cut by snow-patch erosion on hillsides, from a few metres to about 1.5 km across.
• W. V. Lewis classified them as transverse (across the slope; the most common), longitudinal (along earlier gullies) and circular (on gentle slopes).
• Deepened hollows may resemble cirques, but true cirques need glacier ice.

Cryoplanation (Altiplanation) Terraces

• Cryoplanation terraces, also called altiplanation or goletz terraces, are near-horizontal bedrock benches backed by frost-shattered scarps, cut into summits and spurs like a giant staircase.
  • Scarps are typically 2–12 m high; treads may run for hundreds of metres.
• They form by scarp retreat through nivation and frost shattering, with solifluction carrying debris across the tread; the extreme product is a cryoplain.

Asymmetrical Valleys

• Asymmetrical valleys have one side markedly steeper than the other, common in present and relict periglacial terrain.
• The cause is aspect: unequal sunshine changes frost action, snow cover, active-layer depth and solifluction on the two sides.
  • One view: the sunnier slope has more freeze–thaw and solifluction, so it retreats and steepens.
  • The other view: the sunnier slope dries and stabilises, while the moister shaded slope is worn gentler.
• Structure and stream undercutting can also cause asymmetry.

Tors

• Tors are piles of exposed, jointed bedrock blocks about 6–30 m high on hilltops, flanks or platforms, often with clitter (block trains) on their sides.
  • They are commonest on granite but occur on many rocks, in climates from tropical to periglacial.

• In periglacial areas congelifraction along joints plus solifluction removal is the accepted mechanism, but most tors are polygenetic.
• India: granite tors of the Ranchi plateau, Palamau and Singhbhum, basalt tors of the Deccan and sandstone tors of the Rohtas and Rewa plateaus, all formed without a periglacial climate.

Frost-Heave and Ground-Ice Landforms

Patterned Ground

• Patterned ground is the geometric arrangement of stones and soil into circles, polygons, nets, garlands and stripes.
• A. Lincoln Washburn (1956) classified it by shape and by whether material is sorted (stones at the borders, fines in the centre) or unsorted.
  • Flat ground (under about 6°): circles, polygons and nets.
  • Slopes (about 6–30°): garlands and stone stripes, as solifluction draws the cells downslope into alternating coarse and fine lines.
• Mechanism: frost heave lifts stones and pushes them to the cell edges; Mark Kessler and Brad Werner (2003) modelled sorted circles as self-organising patterns.
• India: sorted circles, nets and stripes occur on the high plateaus of Ladakh (Changthang) and Spiti.

Ice Wedges and Frost-Crack Polygons

• Ice wedges are vertical, V-shaped bodies of foliated ice, up to 3–4 m wide at the top and several metres deep.
  • They need a mean annual air temperature of about −6 °C or colder, so they mark continuous permafrost.
• The cracks join into ice-wedge (frost-crack) polygons tens of metres across.
• When permafrost thaws, sediment fills the space as ice-wedge casts, key evidence of former permafrost in Pleistocene Europe.

Pingos

• Pingos are isolated, ice-cored, dome-shaped hills, a few metres to about 70 m high and 30–1,000 m across.
  • The word is Inuvialuit for a conical hill, introduced by Alf Erling Porsild (1938).
  • The Tuktoyaktuk Peninsula (Canada) has about 1,350 pingos, the densest group on Earth; its Ibyuk Pingo is about 50 m high.
• The old frost-heave hypothesis failed because most pingos stand on sands too coarse to heave; J. Ross Mackay (1962) showed that pressurised groundwater freezing into an ice core builds them.

• As a pingo grows its summit cracks; the exposed core melts to leave a crater (open-topped pingo), and collapse leaves a rampart-ringed pingo scar.

Hummocks and Palsas

• Hummocks are small mounds of the active layer raised by frost heave and lateral squeezing: earth hummocks (bare) and turf hummocks (vegetated).
• Palsas are peat mounds with a core of segregated ice, usually a few metres (up to about 10 m) high, in subarctic mires of Fennoscandia, Russia and Canada.
  • They decay when water tables rise or the surface cracks; warming is shrinking many palsa mires.

Involutions

• Involutions are contorted layers of unconsolidated sediment just below the surface, buckled by freezing in the active layer; by shape they are fold, pillar and amorphous.

Mass-Movement and Depositional Landforms

Blockfields (Felsenmeer)

• Blockfields (felsenmeer, “sea of rocks”) are spreads of large angular blocks on flat summits and gentle slopes, traditionally ascribed to in-situ frost shattering.
• Many are now thought to be old, inheriting weathered bedrock from warm pre-Quaternary climates, later reworked by frost.

Solifluction Lobes, Sheets and Terraces

• Solifluction lobes are tongue-shaped masses of flowing debris with a steep front (riser, up to about 5 m high) and a gentle upper surface (tread, up to about 50 m long), commonest on slopes of about 10–20°.
  • Turf-banked lobes have a vegetated riser; stone-banked lobes are fronted by a rampart of stones, where vegetation is sparse.
• On gentler slopes the debris spreads as solifluction sheets or forms solifluction terraces (benches) whose fronts run along the contour, again turf-banked or stone-banked.
• Their overlapping fronts give the slope a stepped, lobate surface that marks the differential movement of the active layer.

Stone Streams and Boulder Fields

• Stone streams (boulder fields) are tongues of coarse blocks along valley floors, moving slowly by gravity, frost heave and solifluction.
  • Frost heave sorts them vertically: coarse blocks on top, finer matrix below.
• Rock glaciers are tongue-shaped masses of angular debris with interstitial ice or an ice core creeping downslope; active ones indicate mountain permafrost.

Thermokarst

• Thermokarst is pitted, hummocky terrain formed by the thaw of ice-rich permafrost and collapse of the ground: thaw lakes, subsidence basins, thaw sinks, thaw slumps and dry valleys.
• Triggers: climate warming, forest clearance, fire, ploughing and construction; railways on permafrost, like the Qinghai–Tibet line, use crushed-rock embankments and thermosyphons to keep the ground frozen.
• Batagaika crater (Yakutia) is the world’s largest thaw slump, about 1 km long and up to 100 m deep, growing since forest clearance in the 1960s.
• Thermokarst resembles true karst in its pitted surface but differs in cause:

Periglacial Cycle of Erosion

• Louis C. Peltier (1950) proposed a periglacial cycle on the model of the geographical cycle of William Morris Davis (1899).
• Assumptions: a lasting periglacial climate, a stable land mass and sequential evolution.
• Mechanism: cryoplanation, the joint work of congelifraction and congeliturbation (solifluction), through parallel retreat of frost-riven scarps and extension of gentle slopes below them.

• The cryoplain (altiplain) is a low-relief surface not controlled by base level, the periglacial counterpart of the peneplain.

Evaluation

• Strength: scarp retreat by frost shattering usefully explains cryoplanation terraces, summit tors, nivation hollows and cryopediments.
• Criticism:
  • It orders forms into a sequence without explaining how frost and solifluction shape slope form.
  • It neglects running water, active in periglacial summers.
  • Quaternary climate changed too often for any cycle to run its course; most periglacial landscapes are polygenetic.
• Current view: periglacial landscapes are studied through process rates, ground-temperature monitoring and dating, not stages.

Permafrost Thaw and Himalayan Hazards

• Near-surface permafrost is projected to shrink by about a quarter for every 1 °C of global warming.
  • The permafrost region stores about 1,400–1,650 billion tonnes of organic carbon; thaw releases CO₂ and methane, a climate feedback.
• Himalayan consequences:
  • Rock-slope failure: warming weakens steep frozen rock walls; the 7 February 2021 Chamoli disaster began as a rock-and-ice avalanche of about 27 million m³ from Ronti peak, killing more than 200 people.
  • Glacial lake outburst floods: the October 2023 South Lhonak (Sikkim) flood began with a lateral moraine collapse into the lake.
  • A 2026 study of Kinnaur (Himachal Pradesh) put the mean 0 °C isotherm at about 5,060 m, with permafrost below it likely degrading, and modelled avalanche-triggered outburst floods from Kashang Lake reaching a downstream hydropower plant in 16–18 minutes.
  • Frost heave and thaw settlement damage roads and bridges in Ladakh and Lahaul–Spiti.