Periglacial

• originally defined as the zone peripheral to glaciers
• now defined as near-glacial in the sense of either location or conditions:
  • perennially frozen ground (permafrost)
  • seasonally-thawed ground (active layer)
  • incomplete vegetation cover of herbaceous plants and dwarf trees
  • ground is snow free for part of the year
  • frequent fluctuations of air temperature across 0^o C

Permafrost

• ground with a temperature perennially below 0^o C
• by definition does not require the presence of ice
• for example, there is dry permafrost in northeast Greenland (Perryland) and parts of Antarctica
• however, water is usually present when ground freezes and thus ice is usually associated with permafrost
• pore or interstitial ice
  • ice that forms in pore spaces and fractures and cements the soil matrix
  • forms as freezing plane descends into the ground without displacing soil
• segregated ground ice
  • bodies of pure ice (lenses, veins, wedges) that form as liquid water and vapour are attracted to the lower vapour pressure (cold air) at the freezing plane
  • intermediate porosity and permeability (i.e. silt) permit the optimal storage and diffusion of water and thus formation of segregated ground ice

Origin and distribution of permafrost

• with a mean annual temperature less than 0^oC, the depth of frost penetration exceeds the depth of thaw. if this climate persists, an increment of permanently frozen ground is created each year and permafrost aggrades by cm per year to thicknesses of several hundred metres, with maximum depths of about 1500 in parts of siberia.
• permafrost thickness represents an equilibrium between heat loss to the atmosphere and the increase in geothermal with depth (ca. 1^o/30 m)
• thus distribution of permafrost depends on mean annual air temperature and the thermal properties of earth materials
• permafrost underlies about 20% of the earth's land surface or about 50% of canada, in three zones:
  
  

  continuous

  permafrost is everywhere except under deep lakes
  

  discontinuous

  permafrost absent under water bodies and warmer sites (e.g. south-facing slopes), north of about 55^o N in Canada
  

  sporadic

  permafrost is preserved at scattered sites, e.g. northern-facing slopes or peat bogs, where the peat prevents melting (insulates) in summer

  
  
• permafrost also occurs in submerged sea bottoms (e.g. Beaufort Sea) and at high altitudes at all latitudes
• much mid-latitude permafrost is relict, that is, inherited from the pleistocene, when permafrost overlay up to 40% during the earth's land surface
• thus periglacial conditions have affected a large proportion of the continents in recent geological time

Geomorphic significance of permafrost

• confines water and frost to the active layer between the permafrost table and the ground surface
• descent of the freezing plane from the surface pressurizes the soil water, reducing the freezing temperature and maintaining the thawed (active) during fall freeze up
• the growth and decay of segregated ground ice causes heave and subsidence

Periglacial processes and landforms

1. frost action
   
   
   • dominant set of periglacial processes given freeze thaw cycles and commonly wet conditions of the active layer
   • produces characteristic periglacial landforms and deposits, such as the mantle of coarse angular debris, which can become relict and diagnostic features with a change in climate
   
   
   
   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
   
   
   • frost creep
     • soil creep is enhanced by expansion and contraction of the active layer with freezing and thawing
   • solifluction (gelifluction)
     • slow flow of the active layer over the permafrost table
     • occurs on slope of 5-20^o
     • above 20^o, periglacial slopes are subject to more rapid mass wasting
   • rockfall and rock avalanches on steep rock slopes
   • earthflow and debris flow in unconsolidated materials
   • massive landsliding in thawing permafrost and ground ice
   
   
   
3. nivation
   
   
   • geomorphic activity enhanced by snow that persists into the melt season
   • periglacial environments commonly have less snowfall than warmer climates, especially temperate mountains, but the duration of snow cover is long and thus snow has much ecological and geomorphic significance
   • wet snow and slush avalanches tend to be dense and full depth (unlike mid-winter powder avalanches) and thus can be effective geomorphic agents on arctic and alpine slopes creating, under extreme conditions, avalanche plunge pools
   • beneath and near the snow pack margins, meltwater erodes fines and, with depressed temperatures, favours intensified frost action
   • erosive creep of water-saturated or refrozen snow can generate basal shear stresses that are comparable to glacier bed stresses
   
   
   
4. fluvial processes
   
   
   • much of the year, water is stored as snow and ice, however, water is released violently during a short melt season
   • ice jams floods characterize north-flowing rivers in canadian and russian arctic, as melt proceed from the headwaters downstream; floodwater passing over ice tend to erode laterally producing wide shallow river channels
   • bank erosion of frozen sediments involves mechanical (hydraulic) and thermal erosion
   • smaller streams commonly have a diurnal cycle with freezing nightly temperatures
   • there also tends to be much sediment available during the melt season
   • with fluctuating discharge and high variable sediment loads, periglacial streams commonly are braided
   
   
   
5. eolian processes
   
   
   • favoured by incomplete vegetation cover, braided stream deposits, cryoturbation and dessication (freeze drying) of surface sediments, and exposure to strong winds
   • thus many present and former periglacial environments are mantled with loess (e.g. northern China; upper Mississippi basin, mid-western US)

Characteristic periglacial landforms and deposits

1. blockfields (felsenmeer)
   
   
   • fields coarse angular blocks on high level summits
   
   
2. rock streams
   
   
   • sloping (3-12^o) blockfields
   • fines at depth permits gelifluction and thus small terraces and oriented blocks
   
   
3. talus
   
   
   • slopes in coarse angular debris that falls to the foot of steep slopes
   • not unique to periglacial environments, but is characteristic given rates of frost shattering and rockfall on periglacial cliffs
   
   
4. rock glacier
   
   
   • thick deposits of rock debris that move as the result of an ice core or interstitial ice
   
   
5. gelifluction lobes
   
   • tongue-shaped masses of active layer with a gentle terrace and steep frontal scarp (1-6 m) where reisistance from adjacent materials causes the mass to bulge
   • occur in fields where lobes overlaps one another to from a staircase slope profile
   • composed of colluvium, angular debris from fines to boulders
   
   
6. cryoplanation terraces and nivation hollows
   
   
   • flattened summits and benched hillslopes from a combination of periglacial weathering, erosion and deposition, especially nivation
   
   
7. asymmetric valleys
   
   
   • in high arctic, north-facing slopes are perennially frozen and thus commonly steeper than south-facing slopes which have an active layer and thus decline by gelifluction
   • in the subarctic, the north-facing slopes tend to be more active and thus lower, because permafrost and thus the active layer can be confined to these slopes in the zone of discontinuous permafrost
   
   
8. patterned ground
   
   
   • various geometric patterns of sediment and vegetation caused by the cracking, heaving and size sorting of sediments
   • e.g. frost-crack polygons, ice-wedge polygons, sorted and non-sorted circles and nets, stone stripes, soil stripes, frost boils, stone garlands
   
   
9. pingos (Inuit: hill)
   
   
   • large ice-cored hills, 10s m high and up to 1200 m in diameter
   • as the ground is heaved by the growth of the segregated ice, the tensile stress causes it to crack, exposing the ice core and leading to the degradation of the pingo; thus they a crater is a common feature
   • closed system
     • progressive inward and downward freezing of a talik, typically under a drained lake
     • 1450 closed-system pingos in the mackenzie delta
   • open-system
     • lacolith of intruded ice
     • develop above a talik in permafrost (i.e. discontinuous permafrost) , where water escapes from a confined aquifer underlying the permafrost
   
   
10. thermokarst topography
    
    
    • sinkholes, thaw depressions and ponds, slumps., etc. that form with the degradation of segregated ground ice
    • initiated by climate change or disruption of the thermal regime by fire, or vegetation or hydrologic changes