Geology Reference
In-Depth Information
(latitude 69° N), approximately 200 km east of Tuktoyaktuk, when there was bright sun-
shine and the air temperature was approximately
43 °C. A sound like a rifl e shot was
heard to emanate from a rock about 60 cm in diameter. Upon examination, the surround-
ing snow was seen to be covered in rock fragments. Mackay cautions against interpreting
this anecdotal evidence as proof of either thermal shock or freeze-thaw activity. He
merely emphasizes the sub-zero temperature at the time, the low angle of the sun (6°),
and limited daylight at that time of the year. These are all factors that might affect, in
various ways, the disintegration mechanism, whatever that might be.
In summary, it is diffi cult to differentiate between the ice segregation, hydro-fracturing,
and thermal-shock mechanisms. It seems wise to conclude that all are valid, depending
upon specifi c climatic conditions, moisture availability, and physical properties of the rock
concerned.
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4.6. CHEMICAL WEATHERING
4.6.1. General
There is a tendency to underestimate the role of chemical weathering in periglacial envi-
ronments. This is because attention usually focuses upon the more dramatic effects of
mechanical weathering, as described above. However, limited evidence suggests that
chemical weathering processes in cold climates are just as effective, if not more so, than
in non-periglacial environments. If one focuses attention upon soil climate (Pope et al.,
1995), geographical variations in chemical weathering appear less obvious than previously
thought.
There are several reasons why the importance of so-called “normal” chemical weather-
ing processes in cold environments is often underestimated. First, the presence of perma-
frost in many periglacial environments imparts a distinct character to soils and soil-forming
processes. Most pedologists are more interested in soils where horizon differentiation is
more marked. Cold-climate soils, now recognized as cryosols, are discussed later in this
chapter. Second, a number of unusual weathering phenomena, such as tafoni and honey-
comb weathering, divert attention from more traditional weathering studies. These phe-
nomena refl ect an intimate, but largely unknown, interaction between saline conditions,
the freezing process, and sub-zero (cryogenic) temperatures. These are also discussed
later in this chapter. Third, the few detailed studies upon cold-climate weathering that are
available are not necessarily of widespread applicability. For example, the classic study of
A. Rapp (1960a) was undertaken in a glacially-overdeepened and recently-deglaciated
valley (Kärkevagge) in northern Sweden that is currently experiencing paraglacial transi-
tion. As seen in Chapter 2, this is not truly representative of the periglacial domain.
Moreover, the importance of solution processes that was so convincingly demonstrated in
that study (see Table 9.3) is now seen to refl ect the presence of pyrite-rich rock that, upon
weathering, produces sulfuric acid, which enhances the weathering of muscovite and
calcite (Darmody et al., 2000, 2001; Thorn et al., 2001). Furthermore, clay mineral evi-
dence suggests that pedogenesis is more advanced on the upland surfaces that surround
the valley than in the warmer and wetter valley-bottom itself (Allen et al., 2001). This
raises the possibility that the parent materials on the ridges are not residual but are pre-
weathered materials that have been transported from elsewhere.
It is not unreasonable to suggest that sub-aerial weathering and soil formation in cold
environments is not very different to that in other environments. For example, when a
number of machine-polished disks were inserted into the ground at Kärkevagge and then
