Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 237

Rock types show considerably different rates of chemical weathering. In the Wind
River Range, Wyoming, the solution removal from granitic rocks on the southwest side
is about 7 t km −2 yr −1 , while sedimentary rocks on the northeast side dissolve at a rate
of 19 t km −2 yr −1 (Hembree and Rainwater 1961). Similarly, in the Sangre de Cristo
Range, New Mexico, average solute contents of stream waters draining quartzite, gran-
ite, and sandstone are in the proportion of 2:5:20, an index of the relative solubility of
each of these rocks in this area (Miller 1961). Similarly, Bluth and Kump (1994) sug-
gested that granitic rocks tend to have lower rates of chemical weathering compared to
sedimentary rocks.
Other forms of chemical weathering also deserve mention. Dissolution is a process
whereby rock material turns directly into solution, like salt in water. It occurs because
water is one of the most effective and universal solvents. Because of the polarity of the
water molecule, practically all minerals are soluble to some extent in water. Oxidation
is the combination of oxygen with a mineral to form a new mineral with a higher oxid-
ation state (ionic charge). Of the elements that have variable charges, iron is the most
important on Earth. After oxidation, rocks show rusted or reddish colored surfaces.
Frost-related Features and Processes
Frost also plays an important role in the unconsolidated deposits that result from weath-
ering. Frost heaving, frost thrusting, and needle-ice growth give rise to distinctive geo-
morphic features. Since frost processes often increase as vegetation is destroyed, a
landscape can become more vulnerable to disturbances such as overgrazing or shrub
removal, so that it may take a very long time for disturbed surfaces to be colonized or
stabilized by vegetation. The effectiveness of frost action is determined by its intensity
and duration. Both of these factors are reflected in the presence of frozen ground.
Seasonally Frozen Ground
Seasonally frozen ground freezes and thaws every year. Winter freezing penetrates the
surface most deeply in the subarctic and much less in lower latitudes. The zone of freez-
ing and thawing in mountains migrates seasonally. The vertical mobility and extent of
this belt are largely determined by latitude and altitude. The zone of freeze and thaw
on a tropical mountain occupies a relatively narrow vertical extent and remains station-
ary. If the mountain is high enough, there may be an area near the top where thawing
seldom occurs. At middle elevations, a transitional zone occurs where seasonal freezing
and thawing take place. Freezing seldom takes place at lower elevations. In the middle
latitudes, however, there may be a zone of continually low temperature at the top, but
freezing and thawing extend to the lowlands in winter. The zone of freeze and thaw is
narrowest in summer and widest in winter. It is compressed as the zone moves up the
mountains in the spring, and is extended as it moves down in the fall; the width of this
belt may also vary significantly with aspect.
Although the intensity and duration of low temperatures are the major factors con-
trolling the depth of frost penetration, site conditions are also very important. Soil com-
position, moisture presence, vegetation, snowcover, and snow depth all greatly affect