Geography 323 -- Geomorphology

Periglacial processes and landforms



Origin and distribution of permafrost

Geomorphic significance of permafrost

Periglacial processes and landforms

  1. frost action

    1. shattering (wedging, splitting)

      • mechanical weathering (disintegration) caused by force of ice and dense water in fractures
    2. heave

      • displacement of soil and rock with the growth of segregated ground ice as free water migrates to the freezing plane (lower vapour pressure)
      • produces hills (pingos) and mounds (palsas, thufurs) with a core of segregated ice
      • also results in the size sorting of heterogenous materials, as larger clasts migrate to the surface at rates up to 5 cm/yr
      • frost pull
        • the entire soil mantle expands, or is pulled up, as the freezing plane descends from the surface
        • with thawing, the cohesive matrix of fine material retracts, filling the space beneath clasts and leaving them in a slightly elevated position relative to the preceding thaw season
        • this process can pull clasts towards the surface, but not through it
      • frost push
        • ice forms beneath clasts because of their higher thermal conductivity (more rapid heat loss)
        • thus the clasts are pushed towards and eventually through the surface, because the cohesive matrix retracts into the spaces under the clasts when the ice melts in the spring
      • needle ice
        • slender ice crystals that form at night in moist loamy periglacial soils
        • typically 1-3 cm in length, but up to 40 cm
        • as the ice needles growth, the soil is dessicated and disturbed and thus becomes more susceptible to wind and water erosion
    3. cracking

      • thermal contraction of sediments and ice at very soil low temperatures that occur with low air temperatures and a lack of snow and vegetation cover
      • water seeps from the active layer into vertical cracks up to a metre or more in depth. this water freezes and then cracks, because ice has less tensile strength than frozen ground
      • the repeated cracking and incremental accretion of ice creates ice wedges, segregated ice that is wedge-shaped in cross section and occupies the polygonal network of thermal contraction cracks
      • thus ice wedge polygons are preserved until climate change when the ice wedges melt and are replaced by ice wedge casts, fine sediments washed and blown into the cracks

  2. mass wasting

  3. nivation

  4. fluvial processes

  5. eolian processes

Characteristic periglacial landforms and deposits

  1. blockfields (felsenmeer)

  2. rock streams

  3. talus

  4. rock glacier

  5. gelifluction lobes

  6. cryoplanation terraces and nivation hollows

  7. asymmetric valleys

  8. patterned ground

  9. pingos (Inuit: hill)

  10. thermokarst topography

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