Permafrost is defined on the basis of temperature, as soil or rock that remains below 0°C throughout the year, and forms when the ground cools sufficiently in winter to produce a frozen layer that persists throughout the following summer.

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Temperature at the ground surface.	Temperature at the ground surface.
Temperature at the ground surface.	Temperature at the ground surface.

The atmospheric climate is the main factor determining the existence of permafrost. However, the spatial distribution, thickness and temperature of permafrost is highly dependent on the temperature at the ground surface. The temperature at the ground surface, although strongly related to climate, is influenced by several other environmental factors such as vegetation type and density, snow cover, drainage, and soil type.

A typical example of ground temperatures within permafrost in the Yellowknife region is shown in the figure below. The annual range in ground temperatures is shown by the warmest and coolest temperatures occurring at depth. With increasing depth in the ground, the seasonal difference in temperature decreases. The point at which there is no discernable change in temperature is termed the "depth of zero annual amplitude". In Yellowknife, this depth occurs at about 15 m. Below this depth, temperatures change very little during the year. Each year a portion of the ground at the surface rises above 0°C for part of the year. This part of the ground, termed the active layer, freezes and thaws with the changing seasons.

An illustration of the range in temperatures experienced at different depths in the ground during the year. The active layer (shown in grey) thaws each summer and freezes each winter, while the permafrost layer remains below 0°C. larger image [JPEG, 87.2 kb, 506 X 482, notice]

Both the thickness of permafrost and the active layer depend on local climatic conditions, vegetation cover and soil properties. The thickness of permafrost can be altered by changes in the climate or disturbance of the surface. Permafrost thins and the active layer thickens when ground temperatures increase. Permafrost thickness is also a function of a number of factors, including ground surface temperatures and the rate of temperature increase at depth. Because rock deep beneath the earth's crust is hot and molten, the temperature beneath the earth's surface increases with depth. This change of temperature is known as the geothermal gradient.

The figure below illustrates how both the air and ground temperatures vary with latitude in the Mackenzie Valley. Although both air and ground temperatures generally decrease with increasing latitude, ground temperatures are usually higher (by 2 to 4°C) than air temperatures. Ground temperatures show considerable variability for a given latitude due to the environmental factors listed above. There is an exception to this general trend in the Mackenzie Delta region. On the delta, ground temperatures are considerably warmer than adjacent tundra uplands. In fact, ground temperature here is generally warmer than -4°C which is similar to that observed in the Mackenzie Valley 100 km to the south. Ground temperatures are about -8°C at a similar latitude on Richards Island. Vegetation which leads to deeper snow cover, and the large amount of water cover maintained by lakes and river channels are the main factors contributing towards maintaining the warmer ground temperatures in the Mackenzie Delta.

Mean Annual Temperature vs Latitude

The ground temperature, therefore, may show considerable variation over a small area even though air temperature may show little variation. In the area around Fort Simpson for example, the mean annual air temperature is approximately -4°C but near-surface ground temperatures are between -2 and 2°C.

The figure below shows the variability of permafrost in mineral soils in the vicinity of Fort Simpson, which is within the discontinuous permafrost zone. In this area, the presence of a surface organic layer is one of the key factors necessary for the existence of permafrost. Peat changes thermal conductivity during the year, becoming a particularly good insulator (low conductivity) in the summer when it is dry, and a good conductor in the fall/winter when it is wet. This characteristic enables permafrost to be preserved in an otherwise non-permafrost environment. Site 84-4A is located in sandy soil at the base of a sand dune in eolian terrain east of the Mackenzie River and north of Fort Simpson. Soils at site 85-8A consist of 5 m of sands and silts overlying ice-rich clay and covered by 0.7 m of peat. At site 85-9, coarse granular soils have essentially no insulating surface organic layer, whereas at site 85-11, a 30 cm cover of peat overlies 1-2 m of gravel and sand which in turn rest on clay. Sites 84-4A and 85-9, with no surface peat, have no permafrost and mean ground temperatures are around 1.5°C. Sites 85-8A and 85-11, both with a layer of peat at the surface, have warm permafrost with temperatures greater than -1°C at the former site, and warmer than -0.5°C at the latter. Permafrost thickness is also greater at 85-8A (11 m) than at 85-11 (2 m).

Temperature vs Depth

At temperatures below 0°C almost all soil moisture occurs in the form of ground ice. In permafrost regions ground ice is one of the most important attributes of the terrain. Its presence influences topography, geomorphic processes, vegetation, and the response of landscape to environmental changes whether natural or anthropogenic. A key concept in understanding the influence of ground ice conditions is that of excess ice, that is, ice which, when melted, exceeds the void volume or pore volume of the enclosing sediments. Thawing of ice-rich soils can result in weakening or settlement of the soil. Consideration of ground ice contents and ground thermal conditions are important aspects of northern engineering. Throughout the North, ground ice is generally within a few degrees of the melting point, and often, in the discontinuous permafrost zone, within a fraction of a degree of 0°C. Surface disturbance such as clearing of vegetation or removal of organic cover for construction, or forest fires, cause warming and thawing of permafrost. This in turn may lead to thaw settlement or slope instabilities.

Cross-section of a pingo with exposed ice. larger image [JPEG, 98.1 kb, 600 X 396, notice]

Ground ice occurs in two main forms, as structure-forming ice, bonding the enclosing sediments, and as large bodies of more or less pure ice. The structure-forming ice comprises segregated ice, intrusive ice, reticulate vein ice, ice crystals, and icy coatings on soil particles. The large bodies of more or less pure ice, which exist mainly in the upper part of the ground, occur as pingo cores, massive icy beds, and ice wedges.

Sediment core showing reticulated ice. larger image [JPEG, 79.3 kb, 573 X 375, notice]

The distinction between structure-forming ice and bodies of massive ground ice is important for engineering and construction purposes. The design, construction and operational plans for engineering works need to accommodate structure-forming ice beneath the foundations, as the presence of such ice is ubiquitous. The larger ground ice bodies can often be avoided provided their presence can be identified and their location detected. Landform analysis and geophysical exploration techniques can be of great value in addressing this problem.

Massive ice near Tuktoyaktuk, NWT. larger image [JPEG, 64.7 kb, 528 X 343, notice]

Thus sufficient and accurate information on the character, distribution and form of frozen ground and ground ice, as well as the geographical, geological and geothermal setting of its existence, is very important for rational planning of development in northern Canada. In various ways, permafrost has always had significant effects on the economic development of the North, particularly for the energy and mining industries, but also for the construction of modern settlements and infrastructure elements such as roads, railways, pipelines, airfields and utilities.

Massive ice exposed in glaciofluvial deposits being quarried for granular resources, BHP Ekati Mine, NWT (June 1998). larger image [JPEG, 98.1 kb, 378 X 569, notice]

Text and diagrams extracted from Burgess & Smith (2000), Heginbottom (2000), and Wolfe (1998a).