Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 223

CHAPTER FIVE
Mountain Landforms and Geomorphic Processes
JASON R. JANKE and LARRY W. PRICE
The mountain landscape is the product of both constructive and destructive processes.
Mountains are created by forces originating from within the Earth, but they are soon
modified and are eventually destroyed by external forces. Many mountain ranges have
been created and destroyed throughout geologic time. The Alaska Range, Alps, Andes,
Cascades, Himalaya, Rockies, and Sierra Nevada are all very young mountains and are
still growing. Plate movement, which produces earthquakes and volcanic eruptions, may
have destructive effects, but these processes are also fundamental to mountain construc-
tion. Present rates of orogeny in western North America exceed rates of erosion by about
7.5 m per 1,000 years (Schumm 1963). The Appalachian Mountains in eastern North
America, created through collisions forming the super continent Pangaea, are considered
old mountains. At one time, they may have been as high as the Alps today, but erosion
has diminished their relief and elevation. In extreme cases, all evidence of relief and el-
evation may be erased, but the roots of the former mountain range give evidence of a
more glorious past. The expansive, thinly soiled region of eastern Canada (the Canadian
Shield) is an example of this.
While mountain ranges display different ages and degrees of development, all are
relatively young compared to the vastness of geologic time. One reason for this may
be the increased effectiveness of erosion at higher altitudes because of greater relief,
steep slopes, unconsolidated material, and uplift. This provides much potential energy
for erosion and redistribution of the eroded material, and greater precipitation totals in-
crease the possibility of rock weathering. Mountains are “rapidly” worn away over the
course of millions of years, and consequently, can never be very old in a geologic sense.
Relief has been shown to be an important control on the rate of denudation or removal
of material through erosion and weathering that result in the lowering of the land (Hin-
derer 2001). Estimates from some of the world's largest river systems indicate that
mechanical denudation ranges from 4 to more than 500 mm ka −1 , whereas rates of
chemical denudation are lower, ranging from 1 to 30 mm ka −1 (Caine 2004). In the Alps,
mean denudation rates have been about 6.2 cm per 100 years over the past 17,000 years
(620 mm ka −1 ). The exact rate of denudation varies in basins of different size, climate,
vegetation regime, and rock type. For example, in the Wind River Range, denudation
rates of 0.115 mm ka −1 have been reported, whereas in the nearby, yet geologically dif-
ferent, Front Range, rates of 0.1 mm ka −1 have been estimated (Ahnert 1970). Mean de-
nudation in the Wasatch Mountains of Utah is consistent with longer-term exhumation,
which suggests that denudation has been steady or decreasing over the past 5 million