Geology Reference
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described as being both physico-chemical and cryogenic in nature. Unfortunately, this
explanation is not entirely satisfactory because it is inapplicable to the hot deserts of the
world and therefore, by implication, is not necessarily the answer for cold environments
either.
A second example is provided by the interpretation of tafoni that has formed upon
Mesozoic-age sandstone blocks on Alexander Island, Antarctic Peninsula (André and
Hall, 2005). In some ways, a similar conclusion involving both physical and chemical
weathering is reached. In order to accommodate apparently confl icting evidence that sug-
gests both current activity and inactivity of tafoni, the authors attribute its initiation to
coastal spray weathering by halite during the last 6500 years. Current activity, in the form
of fl aking and granular disintegration, is then explained in terms of thermal shock. The
latter is invoked on the basis of rock temperature measurements made on the back wall
of a hollow that indicated a number of thermal events in which the rate of temperature
change exceeded the assumed critical threshold of
2 °C/min for thermal-shock effective-
ness (see pp. 64-66). While we may assume that thermal shock is a viable mechanism,
this explanation leaves unanswered the more fundamental questions concerning salt
weathering and the initiation of cavernous weathering.
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4.7. CRYOGENIC WEATHERING
Cryogenic weathering refers to the combination of mechanico-chemical processes which
cause the in-situ disintegration of rock under cold-climate conditions. Salt weathering is
one such example (see above) but here, discussion is more wide-ranging.
A number of fi eld and experimental studies in the former Soviet Union show that one
of the main effects of cold-climate weathering is the production of silty particles with grain
sizes of between 0.05 mm and 0.01 mm in diameter. Analysis of this material indicates that
it consists mainly of primary minerals such as quartz, feldspar, amphibolite, and pyroxene
(Konishchev, 1982 ; Konishchev and Rogov, 1993). This evidence suggests that frost weath-
ering occurs within the layer of unfrozen water that is adsorbed on the surfaces of these
particles. The susceptibility of these particles to weathering depends not so much on their
mechanical strength but more on the thickness and properties of this unfrozen water fi lm.
According to V. N. Konishchev (1982), the protective role of this stable fi lm of unfrozen
water is highest with silicates, such as biotite and muscovite, and lowest with quartz. Cryo-
genic disintegration occurs when the thickness of the protective unfrozen water fi lm
becomes less than the dimensions of the various micro-fractures and defects that charac-
terize the surface of mineral particles.
In a series of laboratory experiments in which minerals were subject to repeated
freeze-thaw, it was established that 0.05-0.01 mm grain sizes are the limit for the cryo-
genic disintegration of quartz, amphibole, and pyroxene, 0.1-0.5 mm for feldspar, and
0.25-0.1 mm for biotite. Quartz grains, in all the size fractions investigated, proved less
resistant when compared to corresponding grain sizes of unchanged feldspar (Figure 4.12).
These relationships are opposite to the normal weathering behavior of these minerals
under temperate or warm climates. The yield of heavy minerals is also unusual in that the
0.05-0.01 mm fraction is lower than that in the 0.05-0.1 mm fraction.
The relatively high degree of instability of quartz under cold-climate conditions has
been confi rmed in several other Russian studies (Minervin, 1982; Rogov, 1987; see also
Table 4.7). The mechanism of cryogenic disintegration is thought to be based on the
wedging effect when ice forms in micro-cracks and produces volume widening following
repeated freeze-thaw. The freezing of gas-liquid inclusions, commonly containing salts,
is the specifi c cause of the widening. Konishchev and Rogov (1993) have also been able
