Geology Reference
In-Depth Information
Figure
4.5.
Annual ground temperature regime recorded at Fenghuo Shan, Tibet Plateau, 1991-
1993. The thawed active layer is indicated. The elevation of the station is 4850 m a.s.l. From Wang
and French (1995c). Reprinted with permission from Elsevier.
the year. Second, the deeper sections of the seasonally-thawed layer are in a transitional
freezing zone, between
4 °C, for over seven months of the year. Third, the
length of time during which ground temperature remains within the transitional freezing
boundary progressively decreases towards the ground surface. Fourth, the near-surface
experiences an increasing number of short, shallow temperature fl uctuations related to
diurnal, cyclonic, or other infl uences.
From a geomorphic viewpoint, the spring thaw is of special interest because it infl u-
ences the nature of spring runoff. Thaw occurs quickly, and approximately 75% of the soil
thaws within the fi rst fi ve weeks of air temperature rising above 0 °C. The rapidity of thaw
is the result of meltwater percolating through the soil, thereby transferring heat to frozen
material beneath. Coarse sediments are particularly suited to rapid thaw since percolation
is easy and thermal conductivity of such material is high.
The fall freeze-back is more complex and may extend over 8-10 weeks. This is illus-
trated with data from Adventdalen, Svalbard (Figure 4.6). Freezing occurs mostly from
the surface downwards but also, to a lesser extent, from the permafrost surface upwards.
This so-called “two-sided” freezing is an important differentiating criterion between areas
experiencing perennial frost (permafrost) and those experiencing seasonal frost. In
Adventdalen, freezing begins in early September and ends in early November. During the
majority of this time, ground temperatures remain remarkably constant in the range of
−
+
2 °C and
−
2 °C to 0 °C. This transitional temperature zone is the “zero curtain” (see earlier) and
occurs because the onset of freezing during autumn releases latent heat to the soil, which
temporarily compensates for the upward heat loss associated with the drop in air tempera-
tures. It is analogous to the inferred “exotherm” phenomenon reported by Hall (2004)
(see above) but is a large-scale effect.
A second feature of interest in the freeze-up period is the initially slow rate of freezing
from the surface downwards and then the dramatic speed-up in freezing at depth. This
