Defining Periglacial Processses
Determining which processes that occur in northern regions ought to considered periglacial in nature is difficult. In general, however, Periglacial Processes can be thought of as those processes which involve the freezing and thawing of a surface and reult in one of several unique landforms such as Palsas, Pingos and Thermokarst. This dicussion of periglacial processes will provide only a broad overview of a few of these processes and landforms.
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Ice Wedges
As the name implies, Ice Wedges are large wedge-shaped ice formations that penetrate deeply into the ground. These wedges have been know to grow to more than 100 m in length and more than 30 m in width at their widest point.
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These large blocks of ice develop over
hundreds of years as water seeps into a fissure in the ground a freezes
there. Initially a narrow fissure develops in the permafrost layer,
perhaps due to the expansion and contraction of the layer resulting
from temperature changes. During the summer months, moisture from the
active layer seeps into the crack where it freezes. When water freezes
it expands producing a force of several hundred pounds per square |
| A view of an ice wedge
penetrating into the ground. Photo courtesy of the US Fish and Wildlife
Service. |
inch; this pressure forces the crack to
widen. During subsequent years, the process is repeated with water
flowing |
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Palsas and Pingos
Palsas are small mounds, typically ranging from one to three meters in diameter, found exclusively in the boggy regions of northern environments. These mounds are pushed up by ice that forms within the peat, i.e organic, material. Essentially, weter percolates downward through the organic material until it reaches a depth, refered to as the the freezing depth, that is cold enough to allow the water to freeze. As the ice freezes in a long horizontal sheet, which is known as an ice lens, it expands causing the overlying organic material to shift upward. This processes also results in an upward shift in the freezing depth. Now, when additional water seeps down through the peat material, it freezes at a level above the original ice lens. The end result is a series of segregated ice layers seperated by layers of peat. The mound created by the upward movement of the peat due to the formation of these ice layers is a palsa.
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Pingos are similar to palsa but develop on a much larger scale. Pingos are large mounds sometimes exeeding a kilometer in diameter and 100 m in height that develop around a massive core of ice. This ice core may develop as limited supply of water, such as a small lake, freezes. This type of pingo, commonly known as a closed-system pingo, is most typical of a region of continuous |
| An open-system pingo found in
the Mackenzie River Delta. Photo courtesy of Agriculture and Agri-foods
Canada. |
permafrost. An open-system pingo, which
would be foundin areas of discontinuous permafrost, is
characterized by continous |
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Frost Heave
Frost Heave, as the name suggests, is the shifting or heaving of the ground due to localized freezing and thawing. In the first case, if unfrozen ground were to freeze, the water in that ground would expand causing the overlying ground to be uplifted. In the second case, if the permafrost in an area were to thaw, the ground could sink or subside. In either case, the process is known as frost heave.
Also in either case, two conditions must be met in order for this process to occur. First, the ground must be composed of fine-grained materials such as silts and clays. Second, the ground must also contain a signicant amount of water. This combination, in unfrozen form, has little bearing strength and so shifts and moves readily when under pressure. When it freezes, however, the high water content results is substantial expansion which, in turn, generates a great deal of force.
Because of these characteristic of frost heave - the subsidence of thawing material and the upward pressure caused by freezing material - it is a major concern when undertaking large-scale development projects in the north. This is because it can result in significant and potentially catastrophic damage to buildings, roadways, and, of course, gas pipelines (Brown, 1970).
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Thermokarst
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Thermokarst
is actually a group of landforms and features that have their genesis
in the same process, the large-scale melting of ice-rich permafrost.
The result is the formation of strings of lakes such as those shown in
the photograph, subsidence, and landslides. This process is becoming an increasing concern due to global climate change. |
| An expanse of thermokarst lakes
in the Canadian Arctic. Photo courtesy of Natural Resources Canada. |
As boreal regions warm, more and more
permafrost will be degraded potentially resulting in vast regions
dominated by thermokarst landforms (Davis, 2001). |