The active layer refers to the layer of ground in areas underlain by permafrost which
is subject to annual thawing and freezing (Permafrost Science, 1981; French, 1996). Most
cryogenic relief-forming processes occur in the active layer, and changes in the active
layer, forced by both natural and anthropogenic factors, affect almost every aspect of the
permafrost landscape. A seasonally thawed layer (STL) develops in summer in the
perennially frozen ground. From an ecological viewpoint, STL is of great importance since
it is a 'life-containing' layer in which soil-forming processes take place. Seasonally,
frozen layer (SFL) forms in winter in unfrozen or thawed grounds. The SFL occurs in thawed
ground in the south of the continuous permafrost zone, in taliks within the discontinuous
permafrost zone, and in unfrozen grounds beyond the cryolithozone. Thus, SFL occupies most
of Russia's territory in winter.
The depth (thickness) of the active layer varies temporally and spatially between 0.1 m
and 4 m for STL and between 0.1 m and 7 m for SFL, its variability controlled by such
factors as climate, terrain, and geology (Kudryavtsev, 1959). STL forms in summer, and
therefore its depth depends mainly on summer air temperature, duration of warm season, and
type of vegetation cover. As SFL occurs in winter, it is winter temperature, duration of
cold season, and depth of snow cover that are the main controls. Thickness of active layer
varies between types of soil and rock since it is closely related to their composition,
thermal properties, and moisture content (Kudryavtsev et al., 1978a). Shallow active
layers are observed in peat; their thickness increases from clay to silt, sand and debris,
with maximum depths being registered in solid rocks. Geographically, STL depth decreases
northwards; the thinnest STL is observed in the arctic desert. Here, variations in STL
depth are marginal (normally between 0.1 m and 0.5 m); as a result of an exceptionally
warm summer STL depth can reach 1 m in sands (Regional Cryolithology, 1989).
Further south, STL thickness increases and so does its variability. In tundra and taiga
STL thickness ranges between 0.3 m and 2.0 m. The smallest depth is observed in
moss-covered, poorly drained peatlands; typical moderately drained moss-covered shrub
tundra and taiga is characterized by an STL depth of 0.8-1.5 m. The deepest STL develops
in well-drained sites which are composed of sand and are sparsely vegetated. Taiga is
characterized by a wide spread of SFL. In northern taiga, SFL forms in peatlands and in
thermal taliks beneath depressions and gullies with extensive snowbank formation. SFL
depth is controlled by snow cover; its typical values vary between 1 m and 2 m under the
permafrost-free forests and on sandy sites. In the southernmost part of the permafrost
zone SFL is a typical winter phenomenon. Its depth varies greatly in response to local
conditions and the depth of snow cover. In the western part of the cryolithozone, it does
not exceed 1 m in clays and silts under forests where snow cover is about 0.7m thick,
while in hills where the snow cover is reduced it may reach 2.5 m (Geocryology of the
USSR: Western Siberia, 1989; Regional Cryolithology, 1989). In the south-eastern part of
the cryolithozone (Transbaikalia), due to the extreme continentality of climate, the snow
cover seldom exceeds 0.2 m-0.4 m. Here, SFL depth ranges between 2 m and 4 m, reaching 7 m
on bare rocks (Geocryology of the USSR: Southern Mountains of the USSR, 1989).
