Geography A2 Unit 4 Periglacial, Landforms and Landscapes

Periglacial Processes, Landforms and Landscapes 

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  • Created by: James
  • Created on: 11-06-11 11:33

Periglacial Regions

  • Polar Lowlands e.g Southern Greenland & Labrador 
    • Mean temp of coldest month -3oC. The area is characterised by ice caps, bare rock and tundra vegitation.    
  • Subpolar Lowlands e,g Northern Canada & Siberia 
    • Mean temp of the coldest month is below -3oC and of the warmest month above 10oC. Taiga Vegitation. The 10oC isotherm of the waremest month roughly coinsides with the tree line in the northern hemishpere.
  • Mid-latitude Lowlands e.g Finland, Northen Great Lakes & Central Siberia 
    • Mean temp of the coldest month is below -3oC but the mean temp is over 10oC for atleast four months of the year. 
  • Highlands e.g Rockies, Alps, Himalayas, Andes
    • Climate influnces by altitude as well as latitude. Aspects causes great variability over short distances. Daily temp ranges are larger.   
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  • Active layer - The zone of greatest temp fluctuation which freezes and thaws seasonally.  From a few cm to 4m.
  • Permafrost Layer
    • Open Talik - Unfrozen water trapped as freezing moves downwards from the ground surface and upwards from the permafrost. These may last for several months.  
    • Closed Talik - Water pocket due to the release of latent heat as water changes from liquid to solid state due to the volume changes of freezing water.  
    • Intra-Permafrost Talik - Water pocket which may last for a long time before freezing.
  • Sub-Permafrost Talik - Below the permafrost zone 
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Frost-Heaving and Thrusting

  • The freezing water in the active layer is unevenly distributed. thus, as it freezes, so the expansion associated with this change to the solid state will be uneven. 
  • Spatial variations in sediment particle size, and pressure variations in and around trapped pockets of unfrozen soil and water also help to generate differential movement.
  • Differential expansion results in movement of material by the process of horizontal frost thrusting and vertical frost-heaving. The vertical movement is the more pronounced,mainly because there is less resistance from the ground surface. 
  • Landforms from heaving and thrusting - Hummocks and Lobes, which occur both singly and in clusters.(Mackenzie Delta, Canada)
  • The internal structure of theses distributed active layers shows a complex churning pattern caused by interaction of variables of water ,sediment and pressure. 
  • the churning motions create displacements of the materials, called cryoturbations. As the movements recur each thaw season, so details of the topography steadily change.
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Landforms- Active Layer: Hummocks and Mounds

  • Thufur and Palsas

Thufurs are Earth hummocks up to 0.5m high & 1-2m diameter.

  • They occur in clusters, often with a regular spacing, to give form of patterned ground which may cover extensive areas.
  • Their shape is influenced by slope angle. On gentle slopes the're circular, but become elongated as gradients increase.
  • They develop best in soils with uniform particle size.

Palsas are larger mounds between 1-6m high & 10-30m wide.

  • They develop. best on unfrozen peat bogs in areas of sporadic permafrost
  • The broad dome shape of Palsas is caused by the formation of segregated ice and differential frost heave.
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Landforms- Active Layer: Hummocks and Mounds

  • Pingos - Extreme form of mound, from Inuit word for 'Hill'.
    • Pingos are large mounds with permanent ice cores.
    • Roughly circular, 30-600m in diameter and 3-70m high.
    • Two types, closed-system Pingos and open-system Pingos, due to character of  the underlying permafrost.
    • Pingos can grow as much as 1m a year some maybe thousands of years old. e.g radiocarbon dating of 2 Pingos in Canadian Arctic aged at 4500 and 7000 years.
    • Largest concentration of Pingos is 1450 and found in Mackenzie delta Canada. 98% have developed in or near to lake basins(alases)
    • The abundant water, the uneven distribution of freezing and expansion, and recurrent freeze-thaw cycles which progressively add to the ice core.
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Landforms- Active Layer: Frost-Cracking & Ice Wedg

  • When ground temp continues to  fall well below zero, the frozne ground materials may contract. this causes sets of cracks to appear, often polygonal in arrangements, known as Frost-Crack Polygons.
  • They are up to 10mm wide and 8m deep, with each polygon 5-30m in diameter.
  • As the active layer thaws in summer, cracks fill with water. In autumn refreezing causes expansion and widening of the cracks.
  • Overtime a crack may grow to become an Ice Wedge.  
  • An example of the evolution of frost-crack polygons occurred in the highland area of Maelifellssandur, Southern Iceland
  • An expanse of vegetation-free, drumlinised ground moraine has been exposed over the past 40 years. and during 1984 and 1989 permafrost formed. 
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Landforms- Active Layer: Patterned Ground

  • Repeated freeze-thaw cycles gradually rearranges soil surface sediment into a variety of patterned shapes: Stripes,Lobes and Polygons. 
  • For patterned ground to develop, active layer materials must contain a range of particle sizes
  • Then sorted by the freeze-thaw action, which generates cracking, wedging and heaving. 
  • The coarser fragments are moved slowly towards the surface, where they cluster to form the patterns.
  • The type of patterning is influenced by the thickness of the active layer, particle dimensions and slope angle.
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Landforms- Active Layer: Patterned Ground

  • Stone stripes are defined as 'Patterned ground with a striped pattern and sorted appearance due to parallel lines of stones, and vegetation cover'.
  • Cost of the clasts settle with their long axes aligned down slope, which increases their ability to slip. Once at the valley bottom or slope base the materials build up as masses of unsorted, angular debris which may be tens of meters thick.  
  • These slope-foot deposits are known as head. Dependent on local conditions, they may occur as sheets, benches or lobes, and may remain as landforms long after the periglacial conditions have ended. 
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Landforms- Active Layer: Thermokarst Scenery

Thermokarst: Surface depressions produced by thawing of ground ice in periglacial areas. 

  • It is characterised by extensive areas of irregular, hummocky ground interspersed with waterlogged hollows
  • Thermokarst depressions can take on a variety of dimensions depending on the extent and pattern of ground ice thaw.
  • Depressions may fill with water to form Thaw Lakes that are relatively shallow, usually less than 5m deep, and generally less than 2km across. 
  • Narrow depressions of a polygonal pattern can occur because of the melting of ice wedge polygons.
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Landforms- Active Layer: Thermokarst Scenery

  • Alases are flat-floored and steep-sided thermokarst depressions that range in size from 3 to 40m deep and from100m to 15km long. They develop from numerous ice wedges and other ground ice causing large ground subsidence, & they often contain lakes.  
  • The coalescence of individual alases results in Alas Valleys that can be tens of km in length. Lena rivers in central YAKUTIA RUSSIA. 
  • The long-term thawing of ground ice that creates a theromokarst landscape can be initiated by climatic warming or changes in insulating characteristics of the surface.
  • For instance, vegetation distribution decreases the insulation of ground ice causing it to thaw to greater depths during spring and summer.  
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Landforms- Active Layer: Nivation

Nivation is a process of localised weathering and erosion beneath and around a snow patch that creates and enlarges a hollow on a hillside.

  • Nivation refers to enlargement of a hollow by a combination of freeze-thaw weathering to loosen the rock, and meltwater from the snow in warmer months to help transport the rock and debris away.  
  • The presence of a snow patch abundant moisture accelerating frost action, frost creep and gelifluction
  • Furthermore meltwater from the snow patch in spring and summer helps to transport weathered material away from the developing hollow. 
  • Snow patches do not always develop into glaciers, and in the absence of glaciation the nivation hollow remains considerably smaller than a glacial cirque.
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Landforms- Slope Processes: Scree Slopes

  • Along cliffs, rock fragments fall and accumulate at the base to form a SCREE SLOPE.
  • The upper part of a scree slope typically rests at an angle between 30 and 38 degrees, and the lower part rests at an angle between 25 and 30 degrees.
  • A scree slope with a larger avg particle size rests at a steeper angle than one made with smaller particles. 
  • the largest boulders tend to be at the base of the scree slope because greater mass and momentum allows them to travel further downslope. 
  • Wastwater Screes found in the lake district rises up more than 60m from the shoreline.
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Landforms- Slope Processes: Solifluction

Occurs in 2 distinctive and closely related forms in such environments.

  • Frost Heave causes the gradual downslope movements of particles through the operation of each freeze thaw cycle.
  • Gelifluction is the downslope creep caused as the active layer thaws during the summer. 

Both occur above permafrost and generate migration of up to 10cm a year at the surface. the movement decreases rapidly with depth, and generally ceases by 2m below the surface. Rate of movement is related to Angle of Slope, the amount of Surface Snowmelt and Infiltration, the ice content of the active layer, and the vegetation cover.

 Most of the clasts settle with long axes down slope increasing slip ability. Once at valley bottom materials build up as masses of unsorted angular debris may be tens of meters thick known as head. 

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Landforms- Slope Processes: factors

  • Insolation - N-facing slopes will remain in shadow for longer. thus remaining frozen for longer with less gelifluction, this maintaining slope steepness.
  • Prevailing Wind - Snow will accumulate to greater depths on more sheltered slopes, and by providing more meltwater encourage downhill movement of debris, increases mass movement, and slope angle declines. 
  • Stream Action - Streams undercut valley sides, causing collapse & steepening of slopes. If a stream is flowing towards 1 side of a valley due to debris supply pushing its course over. 
  • Plant Cover - Vegetation cover can be influenced by aspect, since this affects the rate of ground freeze thawing. RUBY RANGE ALASKA, this SE facing slope had shallow active layer & less Gelifluction, thicker vegetation provided better insulation.
  • Animals - of minor importance, but borrowing animals can be significant locally and can accelerate mass movement. 
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Geomorphological Processes

  • Cryoturbation - As water  freezes its volume expands by about 9%. repeated freezing and thawing within the active layer therefore causes expansion and contraction of the surrounding material beneath the ground. the resultant breaking, churning & mixing of soil, rock & sediment is known as CRYOTURBATION
  • This can alter beds of sediment that were originally laid down horizontally by twisting and contorting them into folded formations called involutions. Fine-grained, unconsolidated sediment is most susceptible to this process due to a relatively high pore water content that makes it slow to freeze. allows coarser material to freeze and exert pressure.
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Geomorphological Processes

  • Segregated Ice - The rate of freezing down through the active layer is unequal, and this results in pockets of ice forming more quickly within parts of sediment.
  • Once started, an ice pocket becomes enlarged into an ice mass SEGREGATED ICE as surrounding water is drawn to it and freezes on to it.
  • Masses of segregated ice range in thickness from millimetres to several meters.
  • Sediments consisting mainly of slit-sized particles are most susceptible to the formation of segregated ice.
  • Coarse material lacks necessary capillarity for water to draw towards freezing pocket, whereas low permeability of clay restricts water movement. 
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Geomorphological Processes

Frost Cracking -the ground is already frozen, further subzero temp decline of the ground during the winter cause contraction and cracking. This creates fractures in the ground that extend from the surface through the active layer and into the permafrost. 

Frost Heave - is a process in the creation of sorted patterned ground because it provides a mechanism for separating stones from finer material. frost heave is the upward movement of sediment and soil due to expansion as water freezes within the material. over many cycles this causes stones to rise to rise to the surface and finer material tends to move downward. 

Frost Thrust - causes stones to move downwards from the centre to form the circle. frost thrust occurs because the doming of the ground produced by frost heave causes heavy stones brought to the surface to roll outwards leaving finer material in centre.

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