Periglacial Processes and Landforms

Nivation is the umbrella term given to periglacial processes caused by snowfall. Snow can build up and have an erosional effect, creating depressions in the sides of mountains Meltwater can penetrate cracks made by snow and freeze-thaw weathering can then occur When the snow melts, the meltwater can remove weathered rocks Snow accumulates in cracks and erodes a deep hollow through erosional slip which can develop into a corrie

Loess is the name given to fine-grained material found in the outwash plain of a glaciated region, blown by strong winds and deposited somewhere geographically different.  Extensive loess deposits can be found in northern China, the Great Plains of North America, central Europe and parts of Russia and Kazakhstan.

Also known as freeze-thaw action or frost shattering, freeze-thaw weathering occurs due to seasonal climatic variations.  Weathering is the breakdown of rocks in situ.  In this instance, meltwater enters cracks in the mountainside where it freezes and expands by 9% as the temperature drops, putting pressure on the surrounding rock.  The water then melts as the temperatures increase and the cycle repeats, putting increasing pressure on the rocks until they crack, sending broken-off scree moving down the mountain. A blockfield is a large expanse of flat land covered in broken pieces of rock, also known as a felsenmeer.

Saturated soil flows downhill due to the effects of gravity.  In summer, the active layer of permafrost thaws, releasing water which saturates the soil and causes it to move downhill.  The greater the gradient, the longer the solifluction lobe which terminates at the base of a hill as the gradient is too low to support the slump.  The solifluction lobe becomes a solifluction terrace, creating the appearance of a natural staircase. The geological term for solifluction deposits is 'head'.  Head infills valleys in periglacial areas and can be seen on BGS maps.

Thermal contraction creates ice wedges which are permanent features because they are 3D.  The ground contracts in winter due to low temperatures and cracks.  As the temperature increases in summer, the cracks become infilled by sediment carried by saturated soil.  This freeze-thaw cycle repeats over many years, enlarging these cracks over time.

Freezing progresses down from the surface.  Meanwhile, the ground expands upwards, lifting a stone by first freezing onto the upper surface.  The freezing front progresses downwards and space is created below the stone as it is lifted.  Ice crystals grow into this space and push up the stone.  Thawing progresses down from the surface causing contraction and lowering of the surface level but the stone is still supported by the ice crystals which have collected beneath it.  Thawed sediment collapses around the stone supporting it.  The stone is held in thawed sediment while ice crystals melt and space is filled by collapsing sediment. Patterned ground is formed as stones collect, or stone polygons when ice wedges (see thermal contraction) are present.  These stones remain in place when the gradient is lower than 6.  Shapes elongate on steeper slopes to form stone stripes or garlands.

Pingos are huge landforms, roughly 500m in diameter and 50m in height.  Pingos are often located in Canada and Greenland.  There are two types of pingos: Closed system pingos are known as Canada or Mackenzie pingos due to their location.  Canada pingos occur in areas of continuous permafrost.  On the site of a lake, groundwater is trapper by freezing from above and permafrost below expanding inwards.  The water freezes and expands, pushing overlying sediment upwards.  If the pressure is too great, the top of the sediment heap can rupture and collapse.  It may then infill with water. Open system pingos are known as Greenland pingos and occur in areas of discontinuous permafrost.  Water infiltrates from the surface to the layer below during the summer.  Freezing and expansion cycles force sediment up into a dome shape which can rupture if the pressure becomes too great. When a pingo ruptures, the collapsed structure is known as an ognip.