Defining Permafrost
At the most fundamental level, permafrost may be defined as any ground that is permanantly frozen. In other words, ground that has remained below 0°C for at least two consecutive years. Notice that the definition of permafrost is based soley on temperature and not at all on the composition of the frozen ground. Permafrost may be frozen rock, mineral soils, or organic material such as peat. Additionally, permafrost may or may not contain water, typically in the form of ice. While these factors will certainly impact the physical characteristics of the permafrost, they are at the most fundamental level irrelevant to defining a region as permafrost (Brown, 1970).
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Because the
definition of permafrost is very broad, it is necessary to further
subdivided permafrost into a number of different types. Some of the
more important types in the context of human activities in the north
include warm and cold permafrost. Cold
Permafrost is permafrost that has a mean temperature below -1°C. Because it is so cold, at times below -10°C, cold permafrost is not suceptible to modest changes
in environmental conditions. In contrast, Warm Permafrost is defined as
permafrost |
| The exposed permafrost is
clearly visible between a vegetated surface layer and an underlying
layer of rock. Photo courtesy of the Department of Energy. |
with a mean temperature between -1°C and 0°C. Since the temperature of this permafrost is so close to the 0°C threshold fo defining a region as permafrost,even modest changes in the environment, such as moderately warmer air temperatures, may |
Another method of classifying permafrost is based on its horizontal extend within a region. Using this system, permafrost may be classed into one of three classes: continuous, discontinuous, and sporadic. As the name implies, Continuous Permafrost is permafrost that is essentially everywhere; in other words, it forms a horizontally contiguous layer throughout the region of interest. In regions of Discontinuous Permafrost the area is still dominated by permafrost but there are regions of unfrozen ground that interupt the permafrost. Finally, Sporadic Permafrost is an isolated mass of permafrost in a region of otherwise unfrozen ground (Brown, 1970).
Regardless of the type of permafrost that exist within a permafrost zone, three distinct components will be found and only one of these is the actual permafrost. At the surface, a layer that is typically between one and three meters deep will be found. This layer is refered to as the Active Layer because it undergoes a seasonal freeze-thaw cycle - it thaws during the warmer summer months and refreezes during the colder winter months - and various processes take place here. For example, this is the region that supports vegetation. Beneath the active layer is the Permafrost Layer which can be hundreds of meters thick. As described above, this is a layer of permanantly frozen ground. The third component is regions of unfrozen ground that are interspersed within and above the perafrost layer. These "warm" regions are refered to as Taliks (Davis, 2001).
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Causes of Permafrost
Permafrost forms when there is an inbalance between the amount of heat loss from the ground to the atmosphere and the amount of heat grained by the ground from the atmosphere over a long period of time. Basically, because the overlaying air is very cold, the heat loss from the ground is quite significant. Furthermore, as the ground disapates heat to the atmosphere, it cools. Eventually it will reach 0°C, the threshold for defining a region as permafrost. If this temperature is maintained for a long enough period of time, two years, the ground is classified as permafrost (Brown, 1970).
While it is clear from the above discussion that air temperature is a key factor in determining if a region will become permafroost, air temperature is certainly not the only factor. Anything that would impact the flow of heat over a long period of time would influence the process. For example, the amount of snow cover can be an important factor. While we often consider snow to be cold in everyday life, when it comes to permafrost formation, snow actually works to insulate the underlying ground minimizing heat loss. In other words, a thick layer of snow prevents the formation of permafrost. Other important factors include the amount of vegetation cover and the physical characteristics of the ground, i.e the type of material it is composed of and the thermal properties of that material.
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Extent of Permafrost
| As can be seen
from the map on the right, permafrost of one type or another extends
over a large portion of the planet. Indeed, nearly 25% of the Earth's
terrestrial surface is covered by permafrost. This includes
approximately half of the land surface of Canada, nearly two-thirds of
the area of Russia, and one-fifth of China. In addition to these large
areas, smaller regions of permafrost can be found many of the nations
of northern Europe and Asia. For example, permafrost can be found in
the northern regions of Japan. Additionally, permafrost can be found in smaller isolated regions beneath the oceans. For example it can be found beneath the |
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| Beaufort Sea in the Arctic Ocean. This type of permafrost is commonly paleo-permafrost. In other words, it is a remnant of permafrost that formed thousands of years ago that still has not thawed. | The major regions of permafrost brocken down by class are show. Clearly, permafrost is an important component of the global environment. Image courtesy of the National Snow and Ice Data Center |
Finally, permafrost may be found at higher elevations in mountainous regions. However, this type of permafrost, which is refered to as Alpine Permafrost, will not be discussed here.
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