3.1.4.3, 3.1.4.4 - Periglacial processes and landscapes

Permafrost and the active layer

Permafrost is ground remaining at or below 0°C for at least two consecutive years. It can be soil, sediment or rock and need not contain much ice. Continuous permafrost occupies nearly all ground within a region; discontinuous permafrost has unfrozen gaps. Seasonal frost alone does not meet the two-year definition, and a glacier is moving land ice rather than permafrost.

The active layer above permafrost thaws in summer and freezes in winter. Its depth responds to summer heat, snow insulation, vegetation, soil properties and moisture. Where ice-rich frozen ground restricts drainage, the thawed layer may become saturated. A thicker active layer can change rooting, water flow and slope stability, but does not necessarily mean all deeper permafrost has vanished.

MOSJ monitoring at Janssonhaugen, about 20 km from Longyearbyen in Svalbard, reports that active-layer thickness has increased by about 35 cm since measurements began in 1998. Its 10 m ground-temperature trend is around +0.8°C per decade over the monitoring period. These are site-specific ground observations, reported on the monitoring page updated 1 April 2025, rather than an air-temperature forecast or uniform value for all Svalbard. Borehole measurements distinguish annual surface thaw from slower change at depth.

Patterned ground and blockfields

Repeated frost heave and thaw settling can sort and rearrange material in the active layer. Differential movement of stones and finer sediment forms sorted circles or polygons on gentle ground; gravity stretches patterns into stripes on slopes. Not all patterned ground is sorted: polygon outlines can also reflect contraction cracks and ice wedges. Pattern describes the form, so material and subsurface structure are needed to identify its mechanism.

Frost weathering of jointed rock produces angular fragments. Where fragments accumulate on relatively level upland surfaces, they can form blockfields; on steeper slopes, rockfall and movement can carry them away as scree or other deposits. Blockfields can have complex histories involving older weathering and selective glacial preservation, so a spread of angular blocks does not date itself or prove an entirely frost-created origin.

BGS mapping identifies relict polygons and stripes in East Anglia, demonstrating that periglacial landscapes extend beyond present mountain and polar settings. Some patterns are partly buried or altered by later farming. Their interpretation requires geological context, rather than assuming that a modern circular vegetation patch proves current permafrost.

Ice wedges and pingos

Ice wedges develop when severe winter cooling contracts frozen ground and opens cracks. Water entering them refreezes; repeated cracking and infilling enlarge wedge-shaped ice bodies and can produce polygon networks. When the ice later melts, sediment-filled wedge casts may preserve evidence of former cold conditions. An ice wedge differs from a sorted stone polygon even if both create surface patterns.

A pingo is an ice-cored hill. In a closed-system example near Tuktoyaktuk in Arctic Canada, a lake drains and its formerly unfrozen saturated bed begins freezing. Growing permafrost expels and traps water towards the centre; pressure lifts overlying ground, and freezing builds the ice core. Open-system pingos instead involve groundwater supplied under pressure, so one mechanism should not be imposed on every pingo. Cracking and thaw can eventually collapse the hill.