Geology Reference
In-Depth Information
at a depth of
∼
1600 m, probably caused by super-cooled brine (Katasonov, in Brown,
1967b, p. 742).
Discontinuous permafrost is encountered in limited areas in northern Scandinavia, the
Kola Peninsula, and the tundra and boreal forest areas between the White Sea and the
Ural Mountains. East of the Urals, a broad zone of discontinuous permafrost exists across
western and eastern Siberia. Here, the transition from discontinuous to continuous per-
mafrost coincides approximately with the northern boundary of the boreal forest (taiga)
and is accompanied by a sharp increase in thickness of permafrost. For example, within
the boreal forest (taiga) discontinuous zone, an average thickness of 25-30 m is typical, as
compared with 300 m in the forest-tundra and 400 m in the tundra.
Given similar mean annual temperatures at the depth of zero-annual amplitude, the
Siberian permafrost is also colder than that in North American. This can be illustrated
by the geothermal gradient. In Yakutia, values of the geothermal gradient range from 40
to 178 m/°C in permafrost and from 30 to 135 m/°C in non-frozen rock (Melnikov, in
Brown, 1967b, p. 742). Observations in North America indicate values ranging from
20 m/°C at stations adjacent to large rivers or oceans to 55 m/°C at interior locations.
The greater extent, thickness, and coldness of permafrost in Siberia than in North
America refl ect differences in Quaternary glacial histories. During much of the Pleis-
tocene, ice sheets covered the majority of Arctic North America but in Siberia ice sheets
only formed in the principal mountain belts and uplands, leaving lowlands largely ice-free.
An additional factor is that retreat of Late-Pleistocene ice sheets in North America was
accompanied by development of extensive postglacial lakes and marine inundations.
These limited the land areas exposed to low sub-aerial air temperature and partially
explains the less-well developed Pleistocene periglacial zone in southern Canada and the
northern United States (see Part III). By contrast, ice-free areas occurred widely in
Siberia throughout the Pleistocene, and deep and continuous permafrost developed in
response to the low air temperatures in the interior of the continent (Gerasimov and
Markov, 1968). The lack of extensive glaciation in Siberia seems best explained by aridity
because the mountain belts and plateaus of central Asia and Tibet, and the vastness of
the landmass, effectively prevented the penetration of moisture-laden winds from either
the Pacifi c or Atlantic Oceans.
The importance of glacial limits as regards permafrost distribution and thickness
can be demonstrated with reference to known permafrost thicknesses in glaciated and
unglaciated terrain that currently experience similar mean annual air temperatures. For
example, 60 m of permafrost exist at Dawson City, in unglaciated Yukon Territory, Canada.
The mean annual air temperature is
5 °C. A similar permafrost thickness occurs in
southeast Siberia at Chita and Bomnak, which also experience mean annual air tempera-
tures of
−
5 °C (Brown, 1967b, p. 746). However, at Fort Simpson, NWT, and at Thompson,
Manitoba, both localities where air temperatures are similar to the other stations but
which are located in recently-glaciated terrain, the permafrost is only about 15 m thick
(Brown, 1970, p. 10).
−
5.4.2. Alpine (Mountain) Permafrost
Permafrost exists at high elevations in both mid- and low latitudes. However, since moun-
tains also exist in polar regions and some mid-latitude mountain chains extend into polar
regions, such as the Western Cordillera of North America and the Ural Mountains in
Russia, it is sometimes diffi cult to distinguish between alpine and polar (high-latitude)
permafrost. Although the term “mountain permafrost” is increasingly used to refer to
