Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 229

Evidence for the link between erosion and climate also stems from a variety of other
sources. Avouac and Burov (1996) hypothesized that removal of eroded material from
mountains and its deposition in the foreland opposes the spreading of a crustal root,
thus driving material toward the orogeny (inward flow). This idea could explain the
formation of Colorado's Front Range during the Laramide orogeny. The Tien Shan and
Himalaya both experience similar tectonic forces, but the Himalayas are shortening
twice as fast (2 cm/yr compared to 1 cm/yr), which is attributed to accumulation of sed-
iment in the forelands. The Tien Shan exists in an arid climate, but the Himalayas are
in a monsoonal climate conductive to erosion, which suggests that climate may be the
driving force of uplift.
Small and Anderson (1998) examined relief production for the Wind River (Wyom-
ing), Beartooth (Montana), and Front Range (Colorado). Evidence suggests that erosion
rates of summit flats in the Laramide Ranges are approximately 10 mm ka −1 , but
erosion of valleys is about 100 mm ka −1 years. If summit erosion is slower than rock
uplift driven by isostasy, then summit height will increase. Using digital elevation mod-
els (DEMs) and geographic information system (GIS) techniques, erosionally driven up-
lift was estimated to be 50-100 m. This rate of uplift is similar to summit erosion; thus
summit erosion will be offset by uplift. Summit elevations have remained constant, but
several hundred meters of relief have been produced. The onset of valley growth began
some 2-3 million years ago, correlating well with the growth of ice sheets, suggesting
that relief must be climatically driven.
Other studies have questioned the link between erosion (the process) and the growth
of mountains (the form). While Small and Anderson (1995) showed that a significant
component of uplift in California's Sierra Nevada was linked to the lithosphere's re-
sponse to erosion coupled with deposition in the Central Valley, Brocklehurst and
Whipple (2002) questioned these results after examining glaciated and nonglaciated
basins in the Sierra Nevada. Relief was greatest in areas that experienced full gla-
ciation. In fact, peaks were reduced at a rate greater than the isostatic uplift they
would have induced. Relief was greatest where glaciers extended into other basins. This
suggests that the isostatic response to incision is exaggerated in the Sierra Nevada.
However, focused erosion at high elevations could contribute to a flexural response to
uplift. Using cosmogenic nuclides to measure erosion in the Sierra Nevada, Riebe et
al. (2001) showed that erosion rates vary by only a factor of 2.5 and are not correlated
with climate across an eight-fold range in average annual precipitation and mean an-
nual temperature conditions. These findings raise questions about the basic idea that
changes in climate can produce more erosion, thus enhancing uplift.
Whipple et al. (1999) suggested that, although climate change may cause an increase
in denudation, neither fluvial nor glacial erosion is significant enough to induce isostatic
uplift. In a fluvial environment, both tributary and trunk relief would be reduced in a
more erosive climate, which contradicts the idea that more erosion leads to greater re-
lief. The transition to more erosive conditions will not only increase incision, but also
increase landsliding concentrated in the upper part of the basin, thus leveling the land-
scape. Molnar and England (1990) thought that a transition to glacial erosion could cre-
ate relief. However, Whipple et al. (1999) stated that this was not demonstrated empir-
ically, and have challenged the claim that glacial erosion produces relief. In a glacial
environment, they argue that glaciers only produce relief through valley widening, ice