Used to indicate all environments which develop in cold climates; except glaciers and their related features. For example, the Alps, the Russian tundra and Canada.
Permafrost.Perpetually frozen layer where the temperature remains below 0°C.It may be regolith or bedrock. It may contain ice.
Currently, around 1/5 of the worlds land are is underlain by permafrost. Most of this is in the Northern hemisphere.The thickness of the permafrost layer varies, according to the type of permafrost. This is regulated by the average annual temperature.
̃ Temperatures very low in winter, summer very short.
̃ Annual average temp >-5°C.
̃ Can be up to 600m thick.
̃ Annual average temperature approx. between -5 and -1.5°C
̃ Annual temperature between -1.5 - 0°C.
̃ found in small isolated patches, where local climate is relatively cold for the latitude.
̃ Extends further due to effect of continent.
Landforms Produced By Movements of the Active Layer
The active layer is the layer above the permafrost which has a temperature above freezing in summer. The penetration of the heat in to the ground is slow, and deep temperature may not rise until September after an entire summers input. The melting of the active layer generates large volumes of water which is undrainable, and so the permafrost becomes saturated.
· Occurs on slopes with min angle of 2°.
· Wet active layer moves like a mud flow.
· Produces solifluction terraces, or lobes;
· Step like features, with treads and risers.
· Treads can be 500m long, and the risers can be up to 15m high.
· If solifluction occurs under vegetation, it rolls the vegetation cover up. This produces a turf banked lobe.
· Stone banked terraces occur where vegetation cover is less continuous, and stones more frequent. Stones accumulate along the lobate front of the terrace.
· Fossilised deposits are widespread in Britain. Known by a number of names, e.g. head in areas of Palaeozoic rock in SW England, and coombe rock in chalk areas of SE England.
· Long axes are usually orientated with the downslope movement, as opposed to glacial, which are orientated in the direction of glacial flow.
Resulting from Freezing
Ice wedge polygons
· Requires winter temp of <-20°C, and continuous permafrost.
· Intense winter cold produces contraction cracks.
· When surface temp rises above 0°C, water flows in to the cracks, and freezes on the ground beneath.
· Permafrost retains the ice. Cyclic action, with ice accretion p.a. of <10mm.
Ice wedge cast
· Depressions in active layer, water filled.
· Depressions have a raised edge, and the centre of the polygons are covered by water.
· Raised rims produced by expansion in the summer pushing up the sediments in contact with the ice wedge.
· Fossilised ice wedge casts are shown by crop ripening patterns, where the small differences in the textures of the soils within the polygon and along the edge cause differential ripening. Wedge is dark (green) and the centre of the polygon is light (golden).