Geology Reference
In-Depth Information
5.3.1. General Principles
The growth of permafrost can be examined in terms of heat-conduction theory. Assuming
an idealized homogeneous crust in thermal equilibrium, the distribution of ground tem-
perature is a linear function of depth. Thus, the temperature regime in permafrost can be
treated in a simple one-dimensional model:
=
(
)
Qg
K
dd
T
z
(5.10)
where
Qg
is heat conduction into the ground,
K
is thermal conductivity, and d
T
/d
z
is the
thermal gradient. Obviously, different earth materials have different thermal conductivi-
ties (see Table 5.1).
Perhaps the simplest illustration of the application of heat-conduction theory is in the
simulation of the geothermal disturbance that might result from the presence of a water
body (Smith, 1977; Smith and Hwang, 1973). Several models are available. One which
calculated the thermal contribution of a thaw-lake to the ground temperature regime in
the Sachs River lowlands of southwest Banks Island, Western Canadian Arctic, is illus-
trated in Figure 5.3. An aerial photograph (see Figure 8.12) illustrates the nature of the
terrain and the size and shape of the numerous lakes that are present. Given a mean
annual air temperature of approximately
14 °C (Ikaahuk: Sachs Harbour; see Table 3.1),
the model predicts maximum permafrost thickness to be 450-500 m beneath land and
50-60 m in the immediate offshore. However, the permafrost thickness on land varies
greatly due to the infl uence of the numerous water bodies. For instance, beneath lakes
approximately 50 m in radius (Lakes A and C in Figure 5.3), the thermal disturbance
extends downwards for only 10-15 m. As a result, only shallow taliks develop. However,
beneath Lake B, which has a radius of approximately 250 m, the upper permafrost bound-
ary is depressed to form a 40 m deep talik. The thermal infl uence of this lake is also suf-
fi cient to raise the lower permafrost boundary. Finally, the large lakes greater than 1.0 km
in radius (Lakes D and E in Figure 5.3) are predicted to form through-going taliks. As a
−
Figure 5.3.
Simple two-dimensional numerical simulation of geothermal disturbances resulting
from water bodies in the Sachs River lowlands, southwest Banks Island, Canada.
