2A.3 - Permafrost and periglacial landscapes

Permafrost and the active layer

Periglacial environments are shaped by cold-climate processes, and many contain permafrost rather than a glacier covering the ground.

The active layer above permafrost freezes in winter and thaws in summer. Its thickness varies with air temperature, soil moisture, snow insulation, vegetation, aspect and material. Water released during thaw may be unable to infiltrate through the frozen ground below, encouraging saturated soils even where annual precipitation is low.

Continuous permafrost underlies almost all ground in a region, conventionally over 90%. In warmer marginal regions, discontinuous permafrost is interrupted by unfrozen ground, commonly 50–90% coverage. Sporadic permafrost occurs in scattered patches, commonly 10–50%. These are broad regional classes: individual slopes and soils may differ substantially within a mapped zone. Isolated patches may occur farther towards the climatic margin.

Present permafrost is widespread in northern Canada, Alaska and northern Russia and also occurs in high mountains. Northern tundra commonly has treeless vegetation, a short growing season and an active layer. Tundra and permafrost overlap extensively, but vegetation type is not itself a measurement of ground temperature.

During Pleistocene cold phases, periglacial conditions extended well beyond ice margins into much of ice-free Britain and Europe. Relict ice-wedge casts and disturbed soils can reveal former ground freezing even where no permafrost survives. “Periglacial” therefore does not mean that the location must be beside a present glacier, and some seasonal frost processes operate without permafrost.

Frost, snow patches and slope movement

Repeated freezing and thawing can break rock, lift sediment and move saturated material downslope.

Freeze–thaw weathering acts where water enters cracks and freezing conditions promote ice growth. Expansion and, particularly under suitable sustained conditions, water migration towards growing ice can generate stresses exceeding rock strength. Repetition enlarges joints and releases angular debris. Water availability, jointing and an appropriate temperature regime matter; extremely cold, dry rock need not weather fastest.

Frost heave raises soil and stones as ice lenses grow. Water is drawn towards the freezing front, adding to lens growth. The uplift is therefore not merely the expansion of water already occupying one pore. During thaw, particles settle differently from their positions before freezing. Repeated cycles can sort and displace material.

Solifluction is the slow downslope movement of water-saturated soil, commonly when thawed material overlies frozen or otherwise impermeable ground. Thaw reduces support and saturation lowers resistance; gravity moves the material into lobes or sheets. Frost creep also contributes when particles rise roughly perpendicular to a slope during freezing and settle more vertically during thaw. Solifluction is mass movement, not meltwater flowing as a river.

Nivation combines weathering, movement and meltwater removal around a persistent snow patch. Persistent snow maintains cool, moist summer margins and supplies meltwater, while insulating ground beneath from short temperature extremes. Moisture and temperature variations at its margins aid weathering. Meltwater and slope movement remove debris, gradually enlarging a hollow. Nivation is a group of processes around snow patches, not a small glacier eroding by basal sliding.

Cold environments may have sparse protective vegetation and loose fine sediment. Strong winds deflate dry surfaces and transport dust. Seasonal meltwater erosion cuts channels and moves sediment when runoff rises sharply; frozen ground can restrict infiltration. These processes connect the winter production of debris with its summer removal.

Ice wedges and patterned ground

Periglacial landforms develop through repeated small changes that accumulate over many cold seasons.

In very cold winters, contraction of frozen ground can open thermal contraction cracks. Water from snowmelt enters and freezes. If a crack reopens in subsequent winters, further ice is added, gradually forming an ice wedge that is broad near the top and narrows downwards. A network can outline polygons. If the ice later melts, sediment may fill the space to form an ice-wedge cast, evidence of former conditions rather than an active wedge.

Patterned ground includes circles, polygons, nets and stripes. It can be sorted, with coarse stones concentrated along edges and finer material within, or unsorted, distinguished by vegetation and relief. Differential frost heave, thaw settlement and repeated particle displacement help organise the surface. Thermal contraction can produce polygon networks, but not every stone circle is an ice-wedge polygon.

On level ground, movements may produce roughly circular or polygonal patterns. On sloping ground, gravity gives movement a preferred direction, stretching patterns into stripes running approximately downslope. The visible pattern reflects sediment size, slope, moisture and freeze–thaw history; it is not a universal indicator of one temperature.

Pingos, loess and a Canadian tundra landscape

Groundwater pressure and wind transport create landforms with very different shapes within and beyond permafrost regions.

A pingo is a mound with a core of ground ice. In a closed-system example, a lake drains and exposes previously unfrozen saturated sediment. As permafrost grows into that sediment, water is trapped and pressurised; freezing water and accumulating ice lift the overlying ground into a mound. In an open-system pingo, groundwater supplied under pressure can freeze and build an ice core. Growth can crack the summit; later thaw or erosion may expose the core and produce collapse. The mound is raised by ice growth, not by a volcanic eruption or sediment piled by a glacier.

Loess is mainly wind-deposited silt. Glaciers and meltwater can create abundant fine sediment, which becomes available for wind transport when outwash or floodplain surfaces dry. Wind carries the fines and deposits them where speed falls or vegetation traps them, building a blanket of silt. Thus loess can accumulate outside the frozen source area and is not proof that the deposit itself formed on permafrost.

The Tuktoyaktuk Peninsula in the western Canadian Arctic supplies a concrete tundra example. Parks Canada's 2018 management statement records eight pingos in the 16.4-square-kilometre Pingo Canadian Landmark, including Ibyuk at about 49 metres high. This is a defined protected area, not the count for the whole Canadian Arctic. The combination of permafrost, former lake basins, groundwater and seasonal thaw supports ice-cored mounds alongside polygonal ground and shallow surface drainage.

A tundra landscape is an assemblage: wedges reflect winter contraction, surface patterns reflect repeated frost action, slope lobes reflect summer saturation, and pingos require suitable water and freezing conditions. They need not occur together everywhere. Britain may preserve relict periglacial deposits and structures while northern Canada still supports active processes. Interpreting the difference requires present ground-temperature evidence as well as landform shape.