Geology Reference
In-Depth Information
received. The effects of insolation are particularly clear in mountainous regions, such as
northern British Columbia and Yukon Territory, where permafrost occurs on north-facing
slopes and not upon adjacent south-facing slopes. Similarly, in continuous permafrost, the
active layer is usually thinner on north-facing slopes. However, in certain instances, ex-
posure to local weather conditions assumes greater importance. For example, on the
Beaufort Plain of northwest Banks Island, Arctic Canada, the active layer is thinnest on
southwest-facing slopes (French, 1970). This is attributed to the infl uence of the dominant
southwest winds in this part of the Arctic, which promote evaporation and latent heat loss
from exposed slopes during the summer months.
5.7.2. Rock Type
Variations in the nature of rock and soil express themselves in differing albedo and
thermal conductivity values. These controls attain their greatest signifi cance in the con-
tinuous permafrost zone, where climate is suffi ciently cool to produce permafrost regard-
less of the type of terrain. Average albedo values for bare rock and soil can also vary
between 10% and 40%. Thus, signifi cant variations in active-layer thickness and the per-
mafrost thermal regime can be expected in different rock and soil types. Table 5.1 provides
typical thermal conductivity values for various materials. Because the thermal conductiv-
ity of ice is much higher than for water, frozen icy soils have higher thermal conductivities
then unfrozen soils. Another important observation is that loose fresh snow and dry,
organic (peaty) material have low thermal conductivity values, and hence are good insula-
tors of the ground.
5.7.3. Vegetation
Probably the most complex terrain factor is vegetation. It affects permafrost in a variety
of ways and is signifi cant in all areas of discontinuous and continuous permafrost with the
exception of the vegetation-free polar deserts. Its most fundamental infl uence is to shield
the underlying permafrost from solar heat. The insulating property of vegetation and
organic (peaty) material (see Table 5.1) is probably the single most important factor in
determining the magnitude of the thermal offset (see Chapter 3) and the thickness of the
active layer. Numerous observations from a wide variety of permafrost environments
indicate that the active layer is thinnest beneath poorly-drained and well-vegetated areas,
and thickest beneath well-drained bare soil or rock.
Much permafrost lies within the boreal forest and taiga regions of North America and
Eurasia. In these regions, trees are important controls over local permafrost conditions
because they shade the ground from direct solar radiation and intercept snowfall in winter.
Thus, the winter cold often penetrates more deeply beneath trees than beneath areas of
thick snow cover, and the amount of summer solar radiation received at the surface is
reduced. Different tree species further complicate the situation. For example, the Siberian
taiga is composed predominantly of pine (
Pinus silvestris
) and tamarack (
Larix dahurica
)
whereas the spruce (
Picea glauca
and
Picea mariana
) is more common in North America.
Since the spruce forest provides a denser canopy and more shade, a surface moss cover is
widely developed; other things being equal, this promotes a thinner active layer in the
North American boreal forest than in the Siberian taiga. In northern Finland, where birch
(
Betula
) dominates the northern forest, the episodic occurrence of insect infestations, such
as
Oporinia autumnata
, can reduce the leaf canopy, thereby increasing summer solar
radiation and causing change in the near-surface thermal regime. In North America, the
