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Permafrost and periglacial processes – what are the characteristics of periglacial areas?
The characteristics of a periglacial area are as follows:
In these environments, permafrost in some form is often present. Permafrost is ground that has been permanently frozen for over two years. The permafrost creates an impermeable layer which is an important factor that drives many processes. The permafrost can be continuous, discontinuous or sporadic permafrost.
A cold and dry climate (where the average annual temperature is in the range of -15⁰C to -1⁰C) but not permanently freezing.
These environments experience intense frost action or frost shattering.
Many processes result from the formation of ground ice.
Periglacial environments are not glaciated. Ice and snow are present, but crucially there will be a brief 'summer' when temperatures rise above 0⁰C.
The top layer of the ground thaws to form the active layer. It can be up to four metres in depth. This feature, together with permafrost, is important in many of the processes.
Where do periglacial processes occur?
These processes can occur anywhere periglacial environments exists, for example:
Around the edges of glaciers and polar ice areas.
Close to sea ice and snow.
Where permafrost in some form is present.
Far northern and southern latitude areas (e.g. Alaska, Northern Canada, Tundra areas in northern Russia and Scandinavia).
High altitude mountainous areas (e.g. European Alps, Himalayas).
High altitude plateau areas (e.g. Bolivian and Tibetan plateau).
Continental interiors (e.g. Siberia).
Permafrost covers approximately 20-25% of the earth’s surface. It is estimated that periglacial environments occur over approximately a third of the earth’s surface. Another word used to describe periglacial environments is tundra.
Fig. 1 - Permafrost off the coast of Canada
Importance of periglacial processes – how can we define and explain periglacial processes?
The processes we will define and explain can be separated into the categories:
Ground ice
Erosion
Periglacial Features and Processes
We will be looking at the periglacial features and processes arising from weathering and mass movement.
Periglacial features and processes arising from weathering
In periglacial environments, repeated freeze-thaw cycles and intense frost action cause frost shattering to occur. This mechanical weathering process occurs when temperatures rise above 0℃ during the day but drop below freezing during the night. Frost shattering can be described as follows:
When the temperature is above 0℃, water enters the cracks and joints of rock. This is often helped by capillary action.
As the temperature drops below freezing, the water turns to ice and expands by 9%, putting pressure on the sides of the joints and cracks, which causes them to widen over time.
Eventually, the rock breaks and smaller pieces of angular rock are produced.
On steep gradients (>20°), the features created by this process are scree (talus). On less steeply sloped ground, the features created are called blockfields, boulderfields or felsenmeer (sea of rocks).
Fig. 2 - Example of frost shattering in Iceland
Periglacial features and processes arising from mass movement
In periglacial environments, the processes of mass movement that occur are:
Solifluction, gelifluction and congelifluction
Frost creep and frost heave
Rock falls
Solifluction, gelifluction and congelifluction
Solifluction is the movement of waterlogged soil down a slope. The process of solifluction can be explained as follows:
As temperatures increase, any ice in the soil (pore and needle ice) melts. The soil becomes saturated.
Friction between the soil particles reduces.
If a slope of just a few degrees is present, the soil begins to flow downhill.
Fig. 3 - Example of solifluction in Alaska, USA
Gelifluction is similar to solifluction except that the mass movement occurs over a layer of permafrost. It can be explained as follows:
When the ground thaws and creates an active layer above the permafrost, large volumes of meltwater are released.
The permafrost below is impermeable. The meltwater cannot drain away.
The soil becomes mobile through the process of solifluction.
Gelifluction occurs as the soil moves over the slope created by the permafrost.
Any movement of earth that occurs within the frozen permafrost is called congelifluction.
The periglacial landforms that result from these processes are solifluction sheets, lobes and terraces.
Frost creep and frost heave
Frost creep occurs on periglacial slopes that experience repeated freeze-thaw cycles. These can occur annually or in repeated cycles when the diurnal range involves a fluctuation above and below freezing. The process can be explained as follows:
When the active layer freezes, particles heave upwards, perpendicular to the slope.
When the thawing occurs, the particles drop by gravity in a downward direction.
As a result, particles move down the slope.
Frost heave happens when freeze-thaw cycles occur in the active layer above the permafrost. Frost heave results in larger particles (stones and rocks) moving up through the active layer above the permafrost. The process can be described as follows:
Upward movement happens because stones and rocks have a lower specific heat capacity compared to that of the surrounding material.
As a result, they will heat up and cool down quicker than the surrounding finer material.
As the temperature drops, any moisture below a stone will freeze first.
The expansion that occurs when this ice forms pushes the stone up through the active layer above the permafrost.
These larger particles eventually become exposed on the surface.
Depending on the shape and slope of the surrounding periglacial landscape, the rocks and stones become sorted. This creates a number of periglacial features known as patterned ground. Examples of patterned ground are stone polygons, stone stripes, stone circles and stone nets.
Fig. 4 - Frost heave
Rock falls
Rock falls are the mass movement process that occurs when pieces of rock become detached through the process of weathering, specifically frost shattering.
Periglacial features and processes arising from ground ice
The climate in periglacial environments encourages many types of ground ice to form in the active layer above the permafrost. Examples of this are:
Pore ice – ice that forms between soil and sediment particles.
Needle ice – ice that forms in damp soils overnight, usually a few centimetres in length.
Ice lenses, cores and wedges.
The processes that are driven by the formation of ground ice are ground contraction and groundwater freezing.
Ground contraction
The process of ground contraction happens when the active layer above continuous permafrost freezes and contracts. The following process occurs:
Cracks open up in the soil and fine sediments of the active layer and continue down into the permafrost layer.
When temperatures rise above freezing the following summer, meltwater enters these cracks.
When the temperatures drop below 0℃, ice veins form.
Gradually, over many seasonal freeze-thaw cycles the ice vein widens to form an ice wedge.
Ice wedges can grow to over 1m wide and extend 3m down into the active layer.
If the ground is fairly level, these ice wedges create another type of patterned ground feature called ice-wedge polygons.
Fig. 5 - Exposed ice wedge in Canada
Groundwater freezing
When there is groundwater within an area where the permafrost is thin or discontinuous, an ice lens can form. This can then grow into an ice core. As the ice forms, it expands, making the material on top heave upwards to produce a small mound or dome. This will generally be around 500m in diameter (some are 1km in diameter) and will vary in height from 3m to 70m (most are under 50m in height). This periglacial landform is called a pingo.
There are two types of processes that result in pingos; closed and open systems.
Closed system pingos
The closed system can be explained as follows:
Permafrost is continuous except in the area underneath a lake which is deep enough (generally >2m depth) to remain ice-free.
The sediments (talik) under the lakebed are insulated by the lake. As temperatures drop below freezing, these sediments remain unfrozen.
If water begins to drain out of the lake or the lake fills up with sediment, the talik closest to the lakebed is no longer insulated, and a thin layer of permafrost forms under the lakebed.
Talik is now trapped within the permafrost layer. The liquid water within the talik is under hydrostatic pressure. It begins to gather towards the bottom of the talik layer.
When this trapped water freezes and an ice lens develops. This causes expansion, and the sediments above heave upwards.
The ice lens grows into an ice core as more water moves towards it through hydrostatic pressure, and more displacement occurs.
Fig. 6 - Formation of an ice lens
Open system pingos
The open system can be explained as follows:
Permafrost is discontinuous; the process often occurs on the valley floor of periglacial environments
Water travels through the unfrozen talik to the lower parts of the valley floor. This is driven by artesian/hydraulic pressure.
When the water accumulates and freezes, an ice lens develops.
An ice core forms and grows as more water continues to flow to this area.
The expansion of the water as the ice forms causes the permafrost above to dome upwards.
Periglacial features and processes arising from erosion
The process of erosion in periglacial environments is driven by the strong winds and freeze-thaw cycles, which cause frost action/frost shattering. The processes described here are:
Water (fluvial) and wind (aeolian) erosion and deposition.
Nivation.
Water (fluvial) and wind (aeolian) erosion
When the ice in the active layer above the permafrost thaws, large volumes of water can be released. Meltwater is responsible for moving significant amounts of debris. The amount of erosion will depend on the speed of the temperature change, the amount of ground ice that has thawed and the slope of the land. Drainage creates braided channels and significant fluvial erosion occurs in the rivers that run seasonally across periglacial environments.
Periglacial environments are largely empty of vegetation so winds can reach high speeds. When temperatures drop and needle ice forms, it cuts down through the soil and sediment and dislodges small particles. Loosened, fine material can be picked up by the wind and erodes the surfaces the wind blows against. Rocks become eroded by these particles. This abrasion results in ventifacts, grooves and polished surfaces. When the wind drops, the fine material and sediments are deposited. This creates a feature typical in periglacial environments called loess.
Nivation
Nivation is the name given to a combination of processes that result in the formation of hollows on the shaded and/or sheltered slopes in periglacial environments. The nivation process involves:
Snow and ice build-up on sheltered/shaded sloped areas. These patches often remain through the summer months. Compact névé snow can develop when snow melts and refreezes. Firn ice can develop over time in these shallow depressions, and when movement occurs, the ice can pluck small bits of loosened rock from the back surface of the area.
Freeze-thaw cycles around the edges and below the snow patch cause frost shattering.
When thawing occurs, the smaller pieces of frost shattered rock are moved down the slope by meltwater, solifluction and gelifluction.
As a result of these processes, nivation hollows are formed.
Over time these can develop into corries.
Periglacial Processes - Key takeaways
- Periglacial environments are defined by the presence of a cold climate with intense frost action.
- Permafrost is usually present. It can be continuous, discontinuous or sporadic permafrost.
- The climate enables the thawing of ground ice to form an active layer above permafrost.
- Periglacial processes occur on the edges of glaciers, polar ice caps and snow. These areas are in high latitudes, high altitudes and cold continental interiors.
- These processes are driven by the presence of ground ice, freeze-thaw cycles, intense frost action and the presence of permafrost.
- The weathering process that occurs in periglacial environments is frost shattering.
- Periglacial mass movement processes are solifluction, gelifluction, congelifluction, frost creep, frost heave and rock falls.
- Processes that occur because of ground ice are ground contraction and groundwater freezing.
- Erosion processes that occur in periglacial environments are water (fluvial) and wind (aeolian) action and nivation.
- Periglacial processes are important because they explain how periglacial landforms and features are created.
References
- Fig. 1: Permafrost off the coast of Canada (https://commons.wikimedia.org/wiki/File:Permafrost_in_Herschel_Island_002.jpg) by Boris Radosavljevic (https://www.flickr.com/photos/139918543@N06/24199065064/) licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/)
- Fig. 2: Frost shattering (https://commons.wikimedia.org/wiki/File:Weathering_freeze_thaw_action_iceland.jpg) by Till Niermann (https://commons.wikimedia.org/wiki/User:Till.niermann) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 3: Solifluction (https://en.wikipedia.org/wiki/File:Solifluctionterraceseaglesummit.jpg) by D.Sikes (https://www.flickr.com/people/48874808@N00) licensed by CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/deed.en)
- Fig. 4: Frost heave (https://commons.wikimedia.org/wiki/File:Wandern_Schweiz_auf_Cassons_bei_Flims.jpg) by Caumasee (https://commons.wikimedia.org/wiki/Special:Contributions/Caumasee) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 5: Ice wedge (https://commons.wikimedia.org/wiki/File:Ice_wedge.jpg) by Drpermafrost (https://commons.wikimedia.org/wiki/Special:Contributions/Drpermafrost) licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)
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Frequently Asked Questions about Periglacial Processes
What is permafrost?
Permafrost is ground that has been frozen for over two years.
What is the importance of periglacial processes?
Periglacial processes are important because they create periglacial landforms, for example, blockfields, pingos and solifluction lobes.
What are the characteristics of a periglacial area?
Periglacial areas have a cold climate but are not glaciated, areas of permafrost are present but seasonal thawing allows an active layer to exist for a period of time. Intense frost action, freeze-thaw cycles and the formation of ground ice occur in periglacial areas.
When do periglacial processes occur?
Periglacial processes occur when the top layer of the ground thaws to create an active layer above permafrost, when temperatures fluctuate to cause freeze-thaw cycles and intense frost action and when ground ice develops.
Where are periglacial environments found?
Periglacial environments are found adjacent to glaciers and polar ice sheets, in high latitudes, high altitudes and in cold interior continental areas. Examples are the European Alpine areas, northern Russia, Siberia and the Bolivian plateau.
What is the percentage of the area where periglacial activities are found?
Between 25-30% of the earth’s surface has the potential for periglacial activity.
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