Presentation on theme: "Geography GE2011: Glacial and Periglacial Processes Periglacial Processes and Landforms Recommended reading Murray, T. (2005) Permafrost and periglaciation."— Presentation transcript:
Geography GE2011: Glacial and Periglacial Processes Periglacial Processes and Landforms Recommended reading Murray, T. (2005) Permafrost and periglaciation. In Holden, J. (ed.) Physical Geography and the Environment. Pearson, Harlow, Strahler, A.H. and Strahler, A.N. (2002) Physical Geography, 2nd Edition, Wiley, New York, Strahler, A.H. and Strahler, A.N. (2005) Physical Geography, 3rd Edition, Wiley, New York, Pingos (ice cored hills), 78° N Spitsbergen, March 2005
1.Introduction Periglacial: ‘the conditions, processes and landforms associated with cold nonglacial environments’ (Harris et al., 1988). Periglacial environments: 1.Active periglacial environments 2.Relict periglacial environments. Periglaciation: ‘the collective and cumulative effects of periglacial processes in modifying the landscape’ (Ballantyne and Harris, 1994).
2. Permafrost ‘Ground (soil or rock) that remains at or below 0°C’. Permafrost is a thermal condition - ice may not be present. Permafrost is overlain by a zone of seasonal freezing and thawing: the active layer:
Permafrost classification: Continuous permafrost Discontinuous permafrost Sporadic permafrost Permafrost underlies ~26% of the Earth’s land surface, including 82% of Alaska and 50% of Canada, much of northern Siberia.
Southern limit of continuous permafrost coincides approximately with -6°C to -9°C MAAT isotherms. Southern limit of discontinuous permafrost extends to -1°C isotherm. Resolute, 74°N: Permafrost depth = 396 m Active layer depth = 0.5 m Hay River, 61°N: Permafrost depth = m Active layer depth = m In North America:
Thermal regime of permafrost and the active layer
3. Ground ice ‘Ice formed in freezing and frozen ground’ 3.1 Ice lenses and massive ice beds Ice lenses are formed by ice segregation Massive ice beds form by: 1. Ice segregation 2. Hydrostatic pressure 3. Burial of glacier ice
Ice lens in a block of frozen soil Massive ground ice beds
Pingos formed by growth of massive ice by hydrostatic pressure Ground ice landforms: pingos and palsas Palsas (ice-cored peat mounds), Vallée des Trois Baies, Québec-Labrador. Pingos, Reindalen, Spitsbergen
3.2 Ice wedges Formed by thermal cracking of permafrost during winter. Meltwater trickles into the crack and freezes against the permafrost. Over long periods a thick ice wedge develops.
Ice-wedge polygons, Ellesmere Island, arctic Canada
Ice wedges in continuous permafrost, arctic Canada
Thermokarst landforms Thermokarst landforms are those produced by the thaw of ground ice, and are found in areas where continuous permafrost once existed (e.g. Great Britain).
4. Frost action in soils Freezing processes in soil: Frost-susceptible soils: ice segregation (lenses) Non-frost-susceptible soils: pore ice only formed Frost-susceptibility is determined by the size of voids in the soil and thus by grain-size distribution: Well-sorted sand (left) and well-sorted sand with 10% silt (right). Frost-susceptible soils generally contain 3-10% by weight finer than 20 µm.
5. Frost heave Definition: ‘the upward and outward movement of the ground surface caused by formation of ice in the soil’. 1.Active layer thawed 2.Active layer frozen - no ice segregation: heave < 3% 3.Active layer frozen with ice segregation: heave up to 50%. Large freezing pressures (up to 1 MPa) can develop as the ground freezes and heaves.
6. Frost weathering of rock = Breakdown of rock through repeated freezing and thawing Water in rocks expands by 9% on freezing Two effects: Macrogelivation: breakdown of rock into angular clasts by water freezing in joints Microgelivation: small-scale breakdown of rock into particles (silt, sand) by freezing of water in pores and by formation of ice lenses - often causes rounding of rock surfaces by granular disaggregation Landforms: frost-shattered bedrock, blockfields and other forms of frost-weathered detritus.
Blockfields: areas of frost- weathered bouldery debris, lacking surface fines. Quartzite blockfield, An Teallach, NW Scotland Gneiss blockfield with pegmatite vein, Jotunheimen, Norway NB: Many exposed boulder surfaces have been rounded by microgelivation.
Other types of frost- weathered detritus Sandy diamictons: clasts embedded in a sand-rich matrix (e.g. sandstones, most granites) Silty diamictons: clasts embedded in a silt-rich matrix (e.g. mica-schists, shales)
7. Frost Sorting and Patterned Ground Frost sorting: sorting of debris by freezing and thawing of the ground Patterned ground: Terrain that exhibits surface patterning Classification of patterned ground: Sorted patterns: defined by alternation of fines and clasts Nonsorted patterns: defined by vegetation or microrelief
Main sorten patterns: nets, circles, polygons and stripes. Secondary forms: steps, ovals and garlands
Many forms grade into others. For example, as gradient increases, nets become elongated forming garlands or ovals, then stripes.
Patterned ground varies in size from several centimetres to a few metres:
Formation of sorted patterns General model: Two main models of patterned ground formation 1.Freezing model: during freezing, frost penetration is uneven. Clasts are heaved upwards to the surface and outwards towards cell margins. 2.Thaw model: during thaw an unstable density configuration sets up convection cells in the soil.
Solifluction: ‘ slow downslope movement of soil due to freezing and thawing of the ground’ = frost creep + gelifluction (see Lecture 10: Soil Creep) 8. Periglacial mass-movement: solifluction
9. Periglacial Mass movement: Rock Glaciers Rock glacier: a thick lobate or tongue-shaped mass of debris that has moved slowly downslope through deformation of internal ice. Two types: Talus rock glaciers (form in permafrost areas) Glacigenic rock glaciers (form in glacial environments with a high debris supply) Movement is by ice creep (like glaciers, but much slower). Surface displacement: few cm yr -1 to 1 m yr -1
Talus rock glacier: Ferintosh Creek rock glacier, Ohau Range, NZ. Talus-derived debris is carried downslope through deformation of a massive ice bed in permafrost.
Glacigenic (glacially-derived) rock glacier, Himalayas; formed by burial of glacier under supraglacial debris.
Relict talus rock glaciers, Lyngen Peninsula, arctic Norway.
Talus rock glaciers formed in Scotland 12,000 years ago, implying permafrost conditions at that time. Relict talus rock glacier, Cairngorm Mountains