Periglacial Processes & Landforms in Cold Environments (SL IB Geography)

• A periglacial landscape is characterised by permanently frozen ground, or permafrost and is defined as:

      ‘An area where soil and rock has not risen above 0°C for at least 2 consecutive years'

• The major process that contributes to the production of a periglacial landscape is:
  • Freeze-thaw 
    • The action of freeze-thaw weathering (frost shattering) results in rough, angular, broken rocks 
    • At the foot of a slope, these rocks are known as scree
    • Periglacial regions are also characterised by blockfields or felsenmeer, caused by the quick freeze-thaw action of temperatures ranging between -5°C to -14°C
• This leads to vast plains of permafrost with low-growing marsh vegetation and exposed rocks

Comparison of Glacial and Periglacial Areas

Image showing main features of a typical periglacial landscape

Other periglacial processes

• Solifluction
  • This is a form of mass movement
  • There are two types of solifluction:
    • Fast 
    • Slow
  • Fast 
    • When an active layer on a slope becomes heavily waterlogged due to melting, gravity acts to pull it downhill
    • This flow of material is called solifluction and produces characteristic lobes on the sides of the slopes
    • The steeper the slope, the faster and further the material will travel, and the larger the lobe will be
    • Movement can be up to 10 cm/yr
  • Slow 
    
    • Also known as solifluction or soil/frost creep, it occurs when water in the soil expands as it freezes
    • This expansion forces soil particles to rise perpendicular to the ground (frost heave)
    • As the ground thaws, the particles are dropped vertically downward (due to gravity)
    • With each repeated cycle, soil particles gradually 'creep' downslope at a rate of a few cm/yr

Frost action

• Frost-action periglacial processes include:
  • Ground ice
    • The most common form of ground ice is pore ice
    • It develops in the pore spaces between soil and rock particles, where meltwater has accumulated and frozen
    • Needle ice are thin slivers of ice and can be several cm long
    • Found mostly in moist soil where temperatures drop below freezing at night
    • Needle ice helps with loosening material for erosion and moving soil particles in soil creep
  • Frost contraction and ice wedging
    • As temperatures drop, the active layer freezes and contracts
    • Crack begin to form in the permafrost as a result
    • When the active layer thaws, meltwater will fill the cracks
    • The cold of the permafrost freezes the water in the crack, forming ice-wedges
    • Continued melting and thawing can enlarge the crack to sizes of 3m wide and 10m deep

Ice wedge formation over time

• • Frost heave
    • When ground ice forces soil or small stones to the surface, frost heave occurs
    • Freezing occurs from the surface downwards, which helps ice crystals form either within the soil pores or as ice needles
    • As the ground ice expands, these crystals force soil and stones to the surface

• Typical landforms found within a periglacial landscape are:
  • Permafrost
  • Solifluction lobes and terracettes
  • Ice wedges and ice lenses
  • Patterned ground
  • Blockfields/felsenmeer
  • Thermokarsts
  • Pingos

Permafrost

• Most permafrost water stays frozen as ground ice
• Seasonal melting at the surface produces the active layer  of between 2cm and 5 metres in depth 
• However, meltwater cannot drain through the impermeable permafrost below and sits on the surface as thaw lakes
• Thaw lakes are common in these poorly drained areas and as the water continues to absorb solar radiation, so the depth and size of these lakes increase in size
• Permafrost can be subdivided into:
  • Continuous: large, unbroken stretches of permafrost, that reach depths of up 1 500 metres—the largest areas are located in Canada, Alaska and Siberia
  • Discontinuous: mostly permafrost, with some small, localised unfrozen ground (talik)
  • Sporadic: where small patches of frozen ground occur in talik (unfrozen ground)

Image showing types of permafrost found in periglacial areas

• Areas of unfrozen ground within the permafrost is known as talik
• Talik can be:
  • Open: a small area of unfrozen ground exposed to the surface
  • Through: a large mass of unfrozen ground beneath a small open area
  • Closed: unfrozen ground completely surrounded by permafrost

Blockfields or felsenmeer

• The periglacial landscape is littered with angular rocks across its surface
• Quickly weathered through freeze-thaw processes, these areas are called felsenmeer, meaning 'field of rocks' in German
• In mountainous/alpine regions, extensive freeze-thaw weathering of the bedrock leaves broken, angular fragments of rock strewn across the landscape
• These areas are subject to intensive, repeated cycles of freezing and thawing

Solifluction lobes and terracettes

• Solifluction is the downward movement of rock and soil under gravity, resulting in lobed-shaped features called solifluction lobes
• Occurs during the summer melt period, when the permafrost's active layer becomes saturated with meltwater and 'slips' downslope
• Terracettes are formed when saturated soil freezes and expands, which forces the soil to shift (heave) upwards towards the surface
• During the spring/summer melt, the soil dries and collapses back vertically
• Each cycle of frost heave and thaw moves the soil downslope, slowly forming a terraced environment

Ice wedges and ice lenses

• Ice wedges form when cracks in the surface fill with summer meltwater and freeze during the winter
• Temperatures have to remain low for cracks to form initially but also to prevent evaporation of water during the melt phase
• Continued freezing and thawing cycles increase the size of the ice wedges each year
• Ice lenses begin to form when moisture in the soil pools and freezes
• Ice lenses grow with subsequent thawing and refreezing, forming a lens-shaped block of ice
• As ice lenses increase in size, they cause soil heave, patterned ground and pingos

Image showing the formation of an ice lens and wedge.

• Note that lenses form when moisture within the soil pool freezes
• Ice wedges initially form when temperatures are low enough for the surface to contract and crack
• Meltwater fills these cracks and subsequently freezes, expanding the crack
• Further cycles of melting provide water to the forming wedges and lenses, helping them to grow and expand to further 'heave' the ground upwards 

Patterned ground

• The ordered pattern is created through the sorting of sediment, stones and ice wedges
• Shapes include polygons, circles, and stripes 
• The repeated freezing, heaving and thawing of the active layer produces the pattern
• Initially freezing sorts material from the rock, and when thawing occurs, redistributes the rock particles into a system of shapes
• Frost heave then pushes larger stones to the surface, which due to its uplift, moves the stones sideways
• Smaller particles are removed via meltwater or wind, which leaves the larger material lying on top of ice wedges, which in turn, marks the polygon pattern
• The sloping ground and gravity, force rocks to move downhill forming elongated stone stripes instead of polygons or circles

Image showing formation of stone patterned ground. Ice wedges form the outer demarcation line of polygon patterned ground, where lighter material is removed by meltwater, leaving the heavier stones behind at the point of the ice wedge during summer melt.

Thermokarsts

• Thermokarsts result from melted ground ice settling unevenly to form marshy ground of hummocks and hollows
• Found mostly in the flat, lowland plains of the Arctic

Pingos

• Found in the Arctic and sub-Arctic region
• These landforms can reach heights of up to 90m
• They have a core of ice and are surrounded on the outside by green vegetation 
• There are two forms of pingos:
  • Closed system
  • Open system
• Closed-system
  • Form in areas of continuous permafrost with a lake on the surface
  • Lake sediments act as an insulator to the ground beneath, which remains unfrozen, and the permafrost layer
  • Liquid water is contained in this unfrozen ground/talik
  • When the lake retreats, the ground is no longer insulated and the residual water freezes into a core/lens
  • As the permafrost advances, it squeezes the talik and pushes the ice lens and lake sediments towards the surface
  • During the summer the ice lens may melt and collapse, leaving a hollow, called an ognip, that fills with water
• Open-system
  
  • Form in areas of discontinuous permafrost
  • Groundwater is forced through gaps in the permafrost
  • Water rises, accumulates and freezes in the active layer of permafrost, to form an ice lens
  • As the groundwater continues to feed the ice lens, the surface domes to form a pingo

Image showing formation of the open and closed pingo system. Note the importance of ice lens/block formation.