Geology Reference
In-Depth Information
Figure
4.6.
Frost heave and ground temperature data recorded at a site in Adventdalen,
Svalbard,between August 1990 and December 1991. (A) Frost heave; (B) surface temperature;
(C) subsurface isotherms. The zero-curtain lies between 0 °C and
2 °C and is indicted by diagonal
shade. The thawed active layer is indicated by stipple. From Matsuoka and Hirakawa (2000). Repro-
duced by permission of the National Institute of Polar Research, Japan.
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increase in the rate of freezing at depth is the result of two factors. First, soil moisture
decreases with depth because it has already been drawn upwards towards the freezing
plane to form segregated ice lenses. Therefore, the amount of latent heat released as a
result of the water-ice phase change at depth is reduced and no longer offsets the
temperature fall. Second, up-freezing from the perennially-frozen ground combines
with down-freezing from the surface to quicken the overall movement of the freezing
plane at depth.
The implications of these observations with respect to weathering processes are uncer-
tain. It is clear that much of the soil profi le remains within a transitional freezing boundary
(
2 °C to 0 °C) for a long and continuous period. The restriction of short-term temperature
fl uctuations to the surface layers implies that they are incapable of inducing mechanical
weathering at depth. It follows that freezing boundary conditions, which are related to the
annual cycle, are probably more relevant when considering weathering at depth. However,
it is an open question whether mechanical or chemical weathering is more important
within a zone of near-zero constant temperatures combined with high moisture content.
There are certain areas for which the annual cycle, as described above, is not entirely
representative. Alpine environments of low latitudes are the obvious example since they
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