What are periglacial processes?

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What are periglacial processes?

  1. 1. Learning destination: understand what permafrost, frost heave, groundwater freezing, nivation and solifluction are and what landforms they create Routes: Identify, describe and explain the formation of nivation hollows, ice wedges, patterned ground, pingos and solifluction lobes
  2. 2.  What glacial feature does this Wordsworth poem describe? “As a huge Stone is sometimes seen to lie Couched on the bald top of an eminence; Wonder to all who do the same espy, By what means it could thither come, and whence; So that it seems a thing endued with sense: Like a Sea-beast crawled forth, that on a shelf Of rock or sand reposeth, there to sun itself” W Wordsworth 1807 The Leech Gatherer
  3. 3.  Areas that experience a cold climate, with intense frost action and the development of permafrost  Aka tundra areas  20-25% of the Earth‟s surface is periglacial, e.g. northern North America and Russia above the Arctic Circle  Experience conditions close to glacial (near an ice sheet e.g. Greenland). Peri = near. Consistently below 0 degrees celsius.  Temporally close to glacials (e.g. Highland Scotland)  Very cold climate with distinctive landforms  May exhibit temperate climates today
  4. 4. Where are periglacial environments? • High altitudes – around ice masses in mountain ranges. Also in high altitude plateau areas e.g. the Tibetan plateau and Bolivian plateau • High latitudes e.g. northern Asia, North America and northern Europe • Continental interiors – due to continentality, e.g. Siberia, central Asia
  5. 5.  Permanently frozen ground  Impermeable  Subsoil temperatures must remain below zero for 2 years or more for permafrost to develop  The extent, depth and continuity of the permafrost layer varies through time according to fluctuations in climate  During summer, when air temperatures are above freezing, the surface layer thaws to form an active layer up to 4m deep.  The active layer gets easily waterlogged due to frozen ground beneath so will easily flow if there is a gradient - solifluction
  6. 6.  Continuous – found in coldest regions (mean below -5) e.g. the Arctic where there is little thawing even in summer. Affects soil and rock to a depth of 700m in Canada and twice that in Siberia  Discontinuous – found in slightly warmer regions where freezing conditions do not penetrate to such great depths (20-30m). Discontinuous due to breaks around rivers, lakes and the sea. Patches are frozen.  Sporadic – mean annual temperatures are around or just below freezing, so permafrost appears only in isolated spots
  7. 7.  Define the term „periglacial‟ – 2 marks (also in June 2011)  Explain the annual changes in the active layer above the permafrost – 4 marks
  8. 8. Describe the distribution of permafrost shown in Figure 3 – 4 marks June 2012
  9. 9. Blockfield Felsenmeer
  10. 10.  Provides a great deal of erosive material in glaciers, so already studied  In periglacial areas, screes develop at the foot of slopes due to frost shattering  On relatively flat areas, extensive spreads of angular boulders are left, known as blockfield or felsenmeer (sea of rocks)  E.g. the Glyders, North Wales
  11. 11. Patterned ground Stone polygons Stone stripes
  12. 12.  Results from the direct formation of ice crystals in the soil as it starts to refreeze  On freezing, fine-grained soils expand unevenly upwards to form domes.  As stones cool down faster than the surrounding soil, small amounts of moisture in the soil beneath the stones freeze and turn to ice, expanding by 9% as they do so.  By repeatedly freezing and thawing over time, these ice crystals and lenses heave stones upwards in the soil.
  13. 13.  In areas where temperatures fluctuate between 0 degrees and -4 degrees C, the frost heaving and subsequent thawing is able to sort material to form patterned ground.  The larger stones move outwards down to the very low slopes of smaller domes because of their weight.  On gentler slopes stone polygons are created, but where the ground is steeper (greater than 6 ̊) the stones are dragged downhill by gravity into more linear arrangements known as stone stripes.
  14. 14. Describe the patterned ground shown in Figure 2 and explain its formation – 6 marks
  15. 15. Pingos
  16. 16.  Freezing of water in upper layer of soil where permafrost is thin or discontinuous leads to the expansion of ice within the soil  This causes the overlying sediments to heave upwards into a dome-shaped feature known as a pingo  Less than 50m in height, 0.5km across, found in sandier soils = open-system or East Greenland type
  17. 17.  Closed-system pingos or Mackenzie type pingos are more typical of low-lying areas with continuous permafrost  On the site of small lakes, groundwater can be trapped by freezing from above and by the permafrost beneath as it moves in from the lakeside  Subsequent freezing and expansion of trapped water pushes the overlying sediments into a pingo form  If the centre collapses it may infill with water to form a small lake  Over a thousand of these pingos have been recorded in the Mackenzie delta (Canada)
  18. 18.  Draw 3 diagrams: 1. Formation of an open-system (East Greenland) type pingo 2. Formation of a closed-system (Mackenzie) type pingo 3. A ruptured pingo
  19. 19.  Draw a labelled sketch to show characteristics of a pingo and suggest an explanation for its formation – 7 marks
  20. 20. Ice wedges Ice wedge polygons
  21. 21.  Refreezing of the active layer during winter causes the soil to contract and cracks open up on the surface  During melting the following summer, the cracks open again and fill with meltwater and its associated fine sediment, which helps to partially fill the crack  Repetition widens and deepens the crack to form an ice wedge up to 1m wide and 3m deep  A near polygonal pattern is produced on the surface, similar to frost heave polygons  Forms ice wedge polygons
  22. 22. Nivation hollows
  23. 23.  Occurs mainly between north and east facing slopes beneath patches of snow in hollows  Frost action below snow which involves freeze-thaw and solifluction and meltwater  Freeze-thaw disintegrates underlying rock  During spring thaw, weathered particles moved downslope by meltwater and solifluction  Leads to nivation hollows which may be the start of corrie development
  24. 24. Solifluction lobes
  25. 25.  Summer thaw in the active layer releases a lot of meltwater  Water cannot percolate downwards due to frozen ground so it saturates the soil  It reduces internal friction between particles making the soil mobile  The soil flows even on slopes of only a few degrees  Leaves behind rounded tongue-like features forming terraces on the side of valleys – solifluction lobes
  26. 26.  Stepped features below vegetation, pushed forward and rolled under like Caterpillar truck  Where vegetation is sparse, stones heaved to the surface are pushed to the front of the advancing lobe and form a small stone bank at the front of the lobe  Many parts of southern Britain experienced these conditions during the Quaternary ice age and these deposits, which filled in valleys are known locally as head (coombe in chalky areas)
  27. 27.  Periglacial areas are often open and sparsely vegetated. This means that erosion by water and wind can be high.  Water erosion is seasonal, occurring in mainly spring and summer when the active layer melts.  This can bring short periods of high discharge in rivers bringing high levels of fluvial erosion.  Drainage is braided due to high amounts of debris being carried by meltwater streams.
  28. 28.  Unobstructed winds reach high velocities  Cause erosion through abrasion  They dislodge fine, unconsolidated materials.  Result in grooved and polished rock surfaces and in stones shaped by the wind, called ventifacts  Fine material of outwash plain is picked up and carried long distances  It is deposited elsewhere as areas of loess  Loess is found in many parts of North America and Eurasia, just south of the Pleistocene ice sheet limit
  29. 29.  Outline periglacial processes likely to be occurring around the glacier in Figure 2 – 5 marks

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