| Permafrost is defined as a state when the ground, whether soil
or rock, remains at or below a temperature of 0°C for a minimum
period of two years. However, most permafrost has existed for much
longer. The thickness of the permafrost is controlled by the balance
between the heat emanating from the earth’s interior and cold atmospheric
conditions at the ground surface. Permafrost thickness ranges from
a few decimetres at the southern limit of the permafrost to over
700 metres in the Arctic Islands. The mean annual temperature of
permafrost in the upper 10 metres of the ground ranges approximately
from 0°C to –20°C. The upper part of the ground that thaws each
summer and refreezes each winter constitutes the active layer.
Distribution of Permafrost
Permafrost underlies about half of Canada’s landmass, as well as areas
of the seabed in the western Arctic and is also believed to exist
beneath the channels of the Arctic Islands. It develops wherever
the heat lost from the ground surface in winter exceeds that gained
in summer and where the resulting ground temperature remains below
0°C for the minimum time period of two consecutive winters and the
intervening summer. This situation prevails not only at high latitudes
but also at high altitudes, mainly in the mountains of western Canada.
The distribution of permafrost varies not only with latitude and altitude, but also with difference in climate,
surface capability to absorb heat, vegetation type, snowpack conditions, topography (both elevation and
slope aspect) and drainage. The successive Quaternary glaciations also had significant effects on the
current nature and distribution of permafrost and ground ice.
Ground Ice in Permafrost
Almost all the soil moisture in permafrost occurs in the form of
ground ice. Ground ice usually exists at temperatures close to its
melting point and so is liable to melt if the ground warms. Ground
ice occurs in several forms: as coating on soil particules and crystals
within the pores of sedimentary rocks and unconsolidated deposits;
as thin, lamellar lenses and veins of ice, and as larger bodies
of more-or-less pure ice, in the form of ice wedges, extensive sheets
of massive ice, and pingo ice. (Pingos are small hills with an ice
core growing by water injection; they are covered with soil and
vegetation.) The quantity of ice in the ground varies widely. At
one extreme, it can exceed 90% of the volume of the ground. In other
areas, permafrost contains essentially no ground ice and is termed
"dry". The distribution of ground ice is influenced by soil texture;
in general, fine-grained soils (rich in clay and silt) and organic
soils contain more ground ice than coarse-grained soils (sand and
gravels).
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Twenty-two metre high pingo
The Importance of Ground Ice Content in Permafrost
Ice-rich permafrost tends to be found within the upper layers of
the ground – the very area of greatest significance for engineering
and construction. Surface disturbances, whether caused by natural
phenomema such as erosion, flooding or wildfire, or by the activities
of society, can have strong influences on permafrost and ground
ice conditions. Also, in subarctic latitudes, where mean annual
ground temperature is only a few degrees below 0°C, permafrost is
most vulnerable.
For nearly all forms of economic or development activity in northern
regions, the temperature of the ground, as such, is less significant
than the occurrence of ground ice within the permafrost. This is
because of the ground stability problems associated with any disturbance
and subsequent thawing of ice-rich permafrost. If ice thaws, the
water from the melted ice occupies less space than the ice itself
and creates an empty space. The covering materials will then cave
in. Thus accurate information on the character, distribution and
form of frozen ground and ground ice, as well as on the geographical
and geological setting of its existance, is important for the rational
planning of the development of northern Canada.
Permafrost and ground ice have had significant effects on the economic
development of the North, not only for the energy and mining industries,
but also for the construction of modern settlements and infrastructure
elements such as roads, railways, airfields and utilities.
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![[D]](https://bac-lac.wayback.archive-it.org/web/20060203160147/http://atlas.gc.ca/site/english/maps/environment/land/pingo.jpg/image_longdesc_view){kind=link}
