Geology Reference
In-Depth Information
Table 4 . 3. A comparison of the volume-expansion (“hydrofacturing”) and segregation-ice
models of frost weathering.
Volumetric-Expansion (Hydro-fracturing) Model
Segregation-Ice Model
1. No frost weathering if pore fl uid contracts
1. Frost weathering does not depend on the
upon freezing
volumetric expansion of water during
freezing. Frost weathering results from
heaving pressures that are universal in
freezing porous solids, whether the pore
fl uid expands or contracts upon freezing
2. No frost weathering under conditions
2. Saturation level infl uences rate of water
common in nature: saturation level less
migration in hydraulically-connected pores
than about 91%, and pores not effectively
(open system). Low saturation does not
sealed off (hydraulically-closed system)
preclude water migration and crack growth.
3. Water may be expelled from freezing sites,
3. Water attraction to freezing sites, due to
but never drawn towards such sites
chemical potential gradients, is a key factor
in frost weathering. If crack growth cannot
accommodate water-to-ice expansion, water
is expelled from freezing sites
4. Crack growth should occur in bursts as water
4. Slow, steady crack growth should occur as
freezes and expands
water migrates towards ice bodies within
cracks. Predicted crack-growth rates are
compatible with values inferred from
experimental data
Source: Hallet et al. (1991). Reproduced by permission of John Wiley & Sons Ltd.
4.5.3. Insolation Weathering and Thermal Shock
Insolation weathering, or “spalling,” refers to the cracking in bedrock that is thought to
be caused by temperature-induced volume changes such as expansion and contraction
(Ollier, 1963, 1984). For many years, this mechanism was not considered viable for the
natural disintegration of rock in cold regions. This was because early studies by E. Black-
welder (1925) and D. T. Griggs (1936) considered the process in the context of hot deserts.
In laboratory experiments, it was found that no thermal shock failure occurred when rocks
were heated and subsequently quenched through several hundred degrees Celsius. Sub-
sequently, it was demonstrated that the larger the rock specimen and the less polished the
surface, the larger were the thermal stresses generated. Still later, A. R ice (19 76) presented
a plot of a spalling-tendency index versus the number of quenches required to spall com-
mercial bricks heated to
500 °C (Figure 4.8A) and concluded that insolation may be suf-
fi cient to break rocks.
Theoretical considerations suggest that heating is certainly capable of breaking rocks if
it is combined with the availability of small amounts of moisture. For example, Figure 4.8B
compares the differential expansion of granite with that of pure water. It appears that the
heating of granitic rock from 10 °C to 50 °C can develop 250 atm as tensile strength, a force
strong enough to disrupt the soundest rock with time. Accordingly, there is renewed inter-
est in this mechanism, especially as it relates to cold environments. Today, the term
“thermal-shock resistance” is used to describe the ability of a rock to withstand sudden and
severe temperature change without fracturing (Marovelli et al., 1966).
In general terms, the tendency of any rock to resist disintegration under thermal cycling
depends upon such factors as its thermal diffusivity, tensile strength, thermal-expansion
 
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