Geology Reference
In-Depth Information
(Dylik, 1972; Kachurin, 1962) but fi eld studies have yet to substantiate this claim. If sub-
surface fl ow is concentrated at a locality, and if lithological conditions are favorable, col-
lapse hollows or soil pipes may form by subsurface erosion and the transport of fi nes
(Carey and Woo, 20 0 0). This process also occurs in central Yakutia, where springs, emerg-
ing from the foot of terraces and fed by supra- and sub-permafrost waters, excavate large
amounts of fi ne sand (Anisimova et al., 1973).
Few landforms can be attributed directly to slopewash activity but subtle features may
be associated with seepage of supra-permafrost groundwater or subsurface fl ow. For
example, on concave slopes underlain by coarse sediment possessing a high hydraulic
conductivity and where the frost table is shallow, the topography may intersect the water
table and seepage produces local saturated zones (“wet” spots) and seepage lines (Woo
and Xia, 1995).
9.7. FROZEN AND THAWING SLOPES
Frozen terrain introduces at least two complications when considering slope evolution.
These relate to the creep of frozen ground and thaw consolidation. The latter is often
associated with thaw subsidence.
9.7.1. Permafrost Creep
Permafrost creep refers to the long-term deformation of frozen ground under the infl uence
of gravity. Deformation is due mainly to the presence of pore ice, the migration of unfro-
zen pore water, and thaw consolidation (see below). Essentially, the warmer the perma-
frost and the greater the amount of ground ice, the greater will be the deformation.
Fine-grained frozen sediments, such as silt and clay, which contains large unfrozen water
content, are especially suited to frozen-creep deformation.
Early attempts to investigate frozen creep involved in situ experimental studies (Ladanyi
and Johnston, 1973; Thompson and Sayles, 1972). Subsequently, a number of fi eld studies
have determined movement rates in several permafrost environments (see Table 6.2).
These data illustrate that the magnitude refl ects ice content, permafrost temperature, and
slope angle. At one locality in the Mackenzie Delta, Canada (Dallimore et al., 1996b), a
signifi cant upslope movement was recorded during late winter, thought to be due to
thermal contraction.
On slopes of moderate angle in discontinuous permafrost terrain, such as the Macken-
zie Valley, Canada, and on the Tibet Plateau, China, rates of deformation of 0.1-0.4 cm/
year appear typical. In colder permafrost terrain of the higher latitudes, movement rates
are approximately one order of magnitude less, in the vicinity of 0.03-0.05cm/year.
Finally, in alpine environments where slope gradient is high, permafrost is marginal
(warm), and ice content is high (as in rock glaciers), movement rates may be as great as
5.0-6.0 cm/year (Wagner, 1992).
Permafrost creep must be considered in any assessment of permafrost landscape evolu-
tion. Based on published data (Morgenstern, 1981), A. G. Lewkowicz (1988b) computes
that downslope volumetric transport rates of 800-1000 cm
3
cm
−1
year
−1
are possible. More
specifi c examples are that permafrost creep may result in the downslope curvature of ice
wedges (Bozhinskiy and Konishchev, 1982) and it may complicate thermal-contracting
cracking (Mackay, 1993b).
