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

Periglacial Processes

• 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

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

• 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

Periglacial Landforms

• 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)

• 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

• 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

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