Geology Reference
In-Depth Information
karst. They are especially applicable to the areas of discontinuous permafrost in Canada,
but similar terrain that exists on Svalbard (Salvigsen and Elgersma, 1985) and in Siberia
(Popov et al., 1972) suggests these models are of wide applicability.
4.6.3. Salt Weathering
Frost weathering, via ice segregation, relies upon the crystallization pressures generated
by growing ice crystals that feed off migrating pore water. Commonly, the water is not
pure. This leads to solute effects, which range from the relatively simple pressure gener-
ated by salt crystallization itself to the complexities of depressed freezing points and
physico-chemical weathering.
Laboratory experiments have shown that some rock types normally resistant to frost
weathering become highly susceptible once they are immersed in salt solution. A recent
study indicates a range of salts are capable of this effect (Williams and Robinson, 2001).
As a result, it is diffi cult to predict the aggressiveness of salt weathering because different
combinations of salts may act in different ways. The damage caused by the combination
of salt and frost to buildings, bridges, highways, and other infrastructure is well known in
cold countries such as Canada, the United States, and Sweden. For example, a considera-
ble literature exists upon the durability of cement and concrete in frost-dominated cli-
mates (Rosli and Harnik, 1980).
Geomorphological studies into salt and frost weathering usually concentrate upon their
effects on limestone and dolomite terrains (Dredge, 1992; Goudie, 1974; St-Onge, 1959),
on coastal rock platforms (Trenhaile and Rudakas, 1981), or on how rock type is affected
by different freezing regimes and varying strengths of salt solutions (Fahey, 1985; Jerwood
et al., 1990a, b). In addition, a number of descriptive studies document honeycomb weath-
ering and granular disintegration phenomena that are partially or wholly attributed to a
combination of salt and frost weathering (André and Hall, 2005; Calkin and Cailleux,
1962; Cailleux and Calkin, 1963; Czeppe, 1964; French and Guglielmin, 2000, 2002a;
Selby, 1971a; Watts, 1983).
Various explanations have been suggested as to why salt accelerates frost weathering.
The more important are described here. First, salt can accumulate in the outer layers of
rocks as a result of surface evaporation. This tends to block the pores and seal the surface,
often leading to surface (“case”) hardening. As a result, water cannot escape by extrusion
through freezing, thus increasing the stresses within the rock. Second, salt may intensify
frost weathering because the rock is subject to both ordinary frost action (i.e. volume
expansion and/or ice crystallization) and salt crystallization. In theory, however, it is only
at temperatures below the depressed freezing point that weathering can be due to the
combined pressures. Therefore, this simple explanation does not account for the enhanced
frost weathering, in combination with salt, which is known to occur at temperatures above
this. Third, frost damage may result from the expansion and contraction of adsorbed water
on clay particles. This is sometimes referred to as “hydration shattering” (White, 1976).
If salts are present, freezing of adsorbed water is delayed and salts continue to expand as
freezing progresses, causing enhanced rock disintegration. This explanation does not
account for frost and salt weathering of non-sorption-sensitive rocks such as sandstones.
Fourth, the enhanced damage associated with salt and rock weathering may partly be the
result of the slower rates of freezing associated with dilute salt concentrations. Thus, the
resulting ice crystals will be larger than normal. In this case, experimental studies have
yet to demonstrate that slower rates of freezing cause more frost disintegration than faster
rates.
