Geology Reference
In-Depth Information
aggradation in saturated sand, pore-water expulsion in an open hydrological system, and
intrapermafrost groundwater fl ow of progressively increasing salinity and decreasing tem-
perature (Mackay, 1997, p. 31). By 1995, 17 years after drainage, freezing had penetrated
more than 7m downwards from the drained lake-bottom surface and, by 2005, the
maximum thickness of the progressively shrinking talik was about 30 m.
The Illisarvik experiment is highly informative. For instance, in the fi rst few years, the
saturated lake-bottom sediments gradually hardened from water loss by drainage and
evaporation in summer and repetitive freeze-thaw. However, after the fi rst 5 years, the
magnitude of frost heave and summer subsidence stabilized at about 3 cm/year (Mackay
and Burn, 2002). A second observation is that, following upon the decrease in thickness
of the active layer after the initial 5 years, ice began to accumulate in the upper 40 cm of
the newly-formed permafrost. This ice is termed aggradational ice (see pp. 157-158).
Other observations made at the Illisarvik drained-lake site have direct relevance to pingo
growth, thermal-contraction cracking and ice-wedge growth, active-layer development,
cryoturbation and patterned ground formation, and the growth and signifi cance of tundra
lakes (see below, and Chapter 4, 6, 7, and 8).
5.4. DISTRIBUTION OF PERMAFROST
Permafrost occurs in two contrasting but sometimes overlapping geographical regions,
namely, high latitudes and high altitudes. Accordingly, permafrost can be classifi ed into
one of the following categories: (1) latitudinal, or polar, permafrost (i.e. permafrost in
arctic regions), (2) alpine permafrost (i.e. permafrost in mountainous regions), and (3)
plateau or montane permafrost, (i.e. areas of permafrost at high elevation, such as on the
Qinghai-Xizang Plateau of China). In addition, sub-sea permafrost exists on the conti-
nental shelves of the Laptev, Siberian, and Beaufort Seas, and other permafrost bodies
occur in terrestrial sub-arctic locations that bear no relationship to current climatic condi-
tions. The latter constitute relict permafrost.
The importance of permafrost is best appreciated when it is realized that approximately
23-25% of the land surface area of the northern hemisphere is underlain by permafrost
(Table 5.2A). This does not include the permafrost in the ice-free areas of Antarctica, the
possible existence of permafrost beneath the Antarctic ice sheet, and the alpine perma-
frost that occurs in the Andes of South America. Excluding areas of frozen ground lying
beneath glaciers and ice sheets, the former Soviet Union (primarily Russia) possesses the
largest area of permafrost, followed by Canada, then China.
Permafrost varies in thickness from a few centimeters to several hundreds of meters.
In parts of Siberia and interior Alaska, permafrost has existed for several hundred thou-
sand years; in other areas, such as the modern Mackenzie Delta, permafrost is young and
currently forming. According to J. Brown et al. (1997) about 72% of the northern hemi-
sphere's permafrost occurs in mountains, uplands, and plateaus (Table 5.2B). Much of this
is low in ice content. The remaining 28% occupies lowlands, highlands, and inter-montane
depressions characterized by thick overburdens.
Permafrost is usually classifi ed in its extent as being continuous (90-100%), discontinu-
ous (50-90%), sporadic (10-50%), or isolated (0-10%) (Table 5.2B). In areas of continu-
ous permafrost, frozen ground is present at all localities except for locally unfrozen zones,
usually existing beneath lakes and river channels. In discontinuous permafrost terrain,
bodies of frozen ground are separated by areas of unfrozen ground. Where permafrost is
sporadic or isolated, it is usually restricted to isolated “islands,” often occurring beneath
peaty organic sediments.
