Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 234

cracks (Tricart 1969). Coarse and angular debris is the typical product of frost wedging.
The exact size and shape of the by-products depend upon the type of rock and the in-
tensity of the frost action. Rocks with stratification planes, such as slate or schist, typic-
ally break into flat slabs, whereas massive rocks, such as limestone or granite, shatter
more randomly.
Once the bedrock has been broken, the effectiveness of frost weathering increases,
regardless of the size of rock fragmentation. More surface area is exposed, the material
diminishes in size, and its water-holding ability increases. The ultimate size to which
material can be reduced by frost wedging is thought to be that of silt, although a small
amount of clay may result from additional chemical weathering (Washburn 1980). Al-
though the absence of clays in most mountain environments may be interpreted as the
dominance of physical weathering over chemical weathering, it is likely that high winds
remove most clay-sized particles.
Physical forces have been thought to account for the greater share of rock disin-
tegration compared to chemical weathering. Rocks are subjected to rapid and unequal
heating and cooling as well as to pressures exerted by ice-crystal growth. Insolation,
or radiation from the sun, is intense at high altitudes, creating rapid and extreme tem-
perature differences. In theory, each heating and cooling cycle causes unequal expan-
sion between the surface and the interior of the rock, so that it may eventually become
weakened and break apart. Other lines of evidence, however, suggest that heating and
cooling alone is not sufficient to break rocks apart. Temperature fluctuations often do
not penetrate deeper than 5 cm and would likely not break rock apart (Caine 1974).
Laboratory experiments involving rapid heating and cooling cycles on rocks in dry air
have also failed to produce any noticeable weakening of rocks, but when water is added,
a definite weakening is observed. It was originally thought that number of freeze-thaw
cycles were important for rock weathering, when in fact, the duration, intensity, and
fluctuation in temperatures below 0°C are more important parameters of rock break-
up (Barsch 1993). Fahey (1973) observed 238 diurnal freeze-thaws over 22 months at
2,600 m, but only 89 cycles at 3,750 m. On Baffin Island, there is little relationship
between the air temperature outside and at the bottom of the bergschr-und (30 m), the
highest crevasse on a glacier. The sheer coldness of a frigid alpine environment cre-
ates stress which may help weather rock (Gerrard 1990). Therefore, it is likely that both
physical and chemical processes are involved, each complementing and reinforcing the
other to maximize weathering effects.
One result of these combined processes is granular disintegration: Grains are
loosened around the periphery of a rock and crumble away. It is possible to brush the
surface of such rocks and have the particles crumble into your hands! Granular disin-
tegration is most effective in rocks with large crystals, particularly in rocks composed
of light and dark minerals, which enhances differential heating and cooling (Washburn
1980). A related process is exfoliation: The rocks spall, or peel off, in concentric scales
or layers, much like the skin of an onion, creating rounded features. Exfoliation may res-
ult from the chemical decay of minerals as well as from the volumetric expansion and
contraction of the rock surface through heating and cooling. A similar process called
unloading is an important weathering factor in mountains that have been heavily glaci-
ated or are tectonically active and have had their overburden removed (Gerrard 1990).