Periglacial Processes & Landforms in Cold Environments (DP IB Geography)
Revision Note
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
Action | Periglacial | Glacial |
---|---|---|
Altered by ice | Within the ground | Above ground |
Landforms created through | Freeze/thaw weathering and mass movement | Erosion and deposition |
Found | Outer margins of cold environments | High altitudes and latitudes |
Examiner Tip
It is important to differentiate between glaciated and periglacial landscapes
Remember that periglacial regions are dominated with permafrost and a seasonally changing active layer
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
Examiner Tip
Always remember that these processes are not one off occurrences. They are cyclical and take a number of years to complete the cycle.
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
Examiner Tip
Make sure you can draw and annotate simple sketches of periglacial landforms to help you explain their characteristics and formation in the exam.
Always give an indication of any timescale involved in their formation or changes.
Remember that processes in cold environments take a long time to happen due to the temperatures involved.
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