Periglacial Processes and Landforms

Periglacial Processes:

• Periglacial environments develop distinctive geomorphology because of four basic processes:
• Freeze-thaw weathering
• Contraction and cracking of rapidly freezing soils
• Migration of sub-surface water
• Mass movement
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Periglacial Processes: Mass Movement

• Solifuction:
• The slow downslope flow of soil and sediment that is saturated with water
• Can occur on very shallow grades
• Forms solifluction lobes, tongue like semi-mixed surface deposits
• Confined to times when temperatures are well above zero and free liquid water is available
• Very common when surface sediments are poorly drained
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• Gelifluction:
• A form of solifluction where the moving materials slide over a slick permafrost layer
• Frost Creep:
• The slow downslope movement of soil and sediment because of frost heaving and thawing
• Freezing of the ground surface elevatss particles at right angles to the slope due to expansion
• Thawing causes the ice to convert back to liquid water and the surface drops in elevation
• Influenced by gravity which causes particles to move slightly downslope
• Rockfalls:
• Exposed rock on slopes is subject to harsh conditions
• Caused by extreme freezing and thawing along the joints and bedding planes of rocks

Periglacial Processes: Ground Ice

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• Surface soils in periglacial environments are frequently influenced by a variety of different types of ground ice.
• Pore ice develops in the pores between soil and sediment particles where liquid water can accumulate and freeze.
• Needle ice consists of groups of narrow ice slivers that form in moist soils when temperatures drop below freezing overnight. On sloped surfaces, needle ice can also enhance soil creep.

Periglacial Processes: Nivation

• Nivation is the localized form of erosion associated with isolated patches of snow that remain through the summer season.
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• Nivation is mainly based on freeze-thaw weathering.
• Melted snow can also trigger mass movements such as solifluction and slope wash.
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Periglacial Landforms: Nivation Hollows

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• Nivation hollows form where fallen snow is protected from melting. This is common on slopes that face northeast in the Nothern Hemisphere.
• The fallen snow is compacted by further snow and melts into a névé (snow that has been melted and refrozen).
• Weathering can damage the soil and rock around the patch of snow, causing slope failure.
• Erosion from meltwater can wash sediment away from the snow patch.
• This causes a hollow to form around the snow patch.

Periglacial Landforms: Ice Wedge Polygons

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• 5-30m across
• Found on valley floors
• Formed by frost cracking
• When the active layer of ice thaws, ice wedges can begin to form as water flows into cracks in the surface
• This water freezes and expands, causing the crack to get bigger
• This separates the surface into polygons

Periglacial Landforms: Patterned Ground

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• Features including circles, nets, polygons, steps and stripes in the surface
• Formed by freeze-thaw action selectively moving coarse particles to the edge of the shape or to its surface
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• Usually start off as circles and change over time

Periglacial Landforms: Pingos

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• Pingos are ice-cored hills with a height between 30- 70m and a diameter between 100-500m
• Can be either conical or elongated
• The growth of ice cores forces up the overlying sediment, causing dilation cracks
• Once the ice core is exposed through the cracks, it melts and causes a crater to form in the centre

Periglacial Landforms: Palsas

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• Palsas are ice-cored peat mounds
• Range from 1-7m high, 10-30m wide and 15-150m long
• Most common at the southern margin of the permafrost zone
• Form when areas of reduced snow cover allow frost to penetrate more deeply into an unfrozen peat bog

Periglacial Landforms: Block Fields

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• Block fields are accumulations of angular, frost-shattered rock
• These pieces of rock pile up on flat surfaces
• Formed in situ

Periglacial Landforms: Tors

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• A tor is a mass of rock that caps a hill
• They form from more resistant rock
• Usually less than 5m high

Periglacial Landforms: Scree/Talus Slopes

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• Formed by frost-shattering
• Formed when rock fragments fall and accumulate at the base of hills
• The larger the rock material, the steeper the slope
• Slope is a reflection of rock type

Periglacial Landforms: Pro-Talus Ramparts

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• Created when a patch of snow settles at the base of a cliff
• The snow acts as a buffer for falling frost-shattered rock
• The rock settles at the bottom of the patch of snow
• A pile of rock is left when the snow melts