Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 233

Physical Weathering
Frost action has typically been thought of as the main physical weathering process in
mountains and cold climates. Other than frost action, little attention has been paid to
other forms of weathering. The fundamental process of rock breakdown under the gen-
eral category of frost action is frost wedging, the mechanical prying apart or shattering
of rocks upon freezing. When water freezes, it expands by about 9 percent, producing
pressure (Washburn 1980). For example, when liquid inside a capped bottle freezes,
the expansive force can cause the bottle to break or the cap to be forced out! When
water contained within rock freezes, it expands and breaks the rock apart. However,
the force needed to break apart rock is much greater than that needed to break apart
a bottle! The pressure exerted by freezing water is 2,100 kg/cm 2 at −22°C. Pressure
also decreases at lower temperatures as the ice begins to crack. Under ideal condi-
tions, ice must be in a closed system; if air bubbles were present, pressure would be re-
duced (French 1996). Unfortunately, rock does not meet these conditions; therefore, the
simple volumetric expansion of water to ice cannot sufficiently explain frost shattering
of rock. There may be other processes that enhance weathering by frost action. Taber
(1929, 1930) found that ice crystals grow as additional water is attracted to the freez-
ing plane through molecular cohesion. In this respect, water is attracted to the freezing
surface from the surrounding area and accumulates (Taber 1929). As this occurs, the
additional volume may help wedge rocks apart. The most important of several factors
controlling the efficiency of these processes are the availability of water (height of the
water table and degree of saturation), the composition of the rock (tensile strength, sur-
face area, porosity, and hydraulic conductivity), and temperature (the rate, extent, and
intensity of cooling) (Matsuoka 1991).
Some question the role of frost action in rock breakdown, and instead support other
processes such as hydration shattering (White 1976). In this process, molecular water
is adsorbed on the surface of silicate minerals, so that pressure is exerted on surround-
ing rock surfaces. According to White (1976), a force of over a t cm −2 can be gener-
ated, greatly enhancing weathering. Numerous other types of weathering such as wet-
ting and drying, rock fatigue, thermal shock, and salt crystal growth, as well as chemic-
al and biological processes, could likely enhance frost wedging (Williams and Robinson
1991; French 1996; Hall 1998). For this reason, some prefer the term cryogenic weath-
ering, which refers to mechanical-chemical process of rock breakdown (French 1996).
Frost action remains difficult to corroborate in the field; what mountain geomorpholo-
gists need more than anything else is a method to determine the processes responsible
for the weathering of rock fragments (Thorn 1992).
Frost weathering in mountains can be observed in the moist zone immediately below
late-lying snow patches (Thorn 1978). The effects can also be seen at the moist bases of
notched cliffs and rock walls (Gardner 1973). Porous and multijointed rocks that allow
water to infiltrate usually experience more weathering than impervious, dense rocks.
Sedimentary rocks are generally more susceptible to frost shattering than hard crystal-
line rocks, although much depends on their individual characteristics. Crystalline rocks
with deep cracks may be more susceptible to frost action than compact sedimentary
rocks. Frost wedging is most effective on rocks with reticulate hairline fractures, where
moisture is allowed to penetrate but does not drain away, as is often the case with large