Explain how some physical environments present both challenges and opportunities
   
 
What are the challenging environments?
  • 4.2 Upland areas (either glaciated in the past and/or currently active)
  • 4.2 Periglacial and permafrost environments

Upland (Glaciated) Areas

Challenges

Opportunities
  • Steep slopes
  • Loose rocks/scree
  • Deep water
  • Poor soils
  • Hostile climate
  • Remote
  • Difficult access
  • Tourism
  • Energy - HEP
  • Quarrying
  • Transport (route-ways)
  • Agriculture - hill sheep and transhumance
  • Settlement

 

Periglacial Environments


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 0ยบ C

Permafrost

  • ground with a temperature perennially below 0o C
  • Water is usually present when ground freezes and ice is usually associated with permafrost
  • pore 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 (sporadic permafrost)

Origin and distribution of permafrost

  • with a mean annual temperature less than 0oC, the depth of frost penetration exceeds the depth of thaw. if this climate persists, permafrost depth  increases each year to thicknesses of several hundred metres, with maximum depths of about 1500 metres in parts of Siberia.
  • 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 55o 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

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 autumn freeze up
  • the growth and decay of segregated ground ice causes frost heave and subsidence

 

 

Challenges Opportunities

Frost Action

  • dominant set of periglacial processes given freeze thaw cycles and commonly wet conditions of the active layer

 

a. shattering (wedging, splitting)

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

 

b. 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)  with a core of segregated ice
  • 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 rock fragments (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

c. cracking

 

  • thermal contraction of sediments and ice at very  low soil 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 growth of ice creates ice wedges, segregated ice that is wedge-shaped in cross section
  • 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 5-20o
    • above 20o, 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
Human attempts to exploit periglacial regions commercially have only been made relatively recently.

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

 

  

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
  • there tends to be much sediment available during the melt season
  • with fluctuating discharge and high variable sediment loads, periglacial streams commonly are braided
 

Eolian (Wind) 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)
  • Loess deposits are unusually fertile (wheat belt in USA)
  • Used as a means for building dwellings

Human Action

Lowering the permafrost table

  • Removal of vegetation for construction means that in summer more heat penetrates the soil and so the depth of thaw increases, as does the likelihood of flooding
  • Construction of centrally heated buildings has warmed the ground underneath them, causing the buildings to subside
  • Siting of oil, sewerage and water pipes in the active zone has increased the rate of thaw, sometimes causing breaking of the pipes as the ground moves
  • Drilling for oil and gas poses problems because the heat from the drilling fluid melts the permafrost
  • Road construction upsets the delicate equilibium of many slopes produced by solifluction

Raising the permafrost table

  • Construction of unheated buildings causes upper surface of permafrost to rise in the summer and the buildings to tilt

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