Geography Reference
In-Depth Information
firmer ground upon which to base their concepts branched off into detailed process
studies, especially as the quantitative revolution unfolded.
During the mid-twentieth century, geomorphology became less concerned with evol-
ution and more concerned with process, morphometry, and systems (Chorley 1965). In
order to accomplish this and define the discipline, Hack (1960) and Chorley (1962) re-
jected the time dimension, thus examining the landscape within narrow temporal limits
and justifying short-term process studies. Change was viewed as dynamic, moving about
an average condition or causing a shift in the condition to a new or preexisting state.
By viewing process at different temporal scales, one can pass from short-term static
equilibrium states, to graded time over hundreds of years, to a dynamic equilibrium or
metastable state in which thresholds are crossed, and finally to cyclic time over millions
of years (Schumm 1977). These studies pursued a reductionist path. Unfortunately, re-
searchers were unable to extend from a small spatial scale to a regional scale (Thorn
1992). Process could not always explain form at different spatial scales. This same di-
lemma exists today when trying to understand the formation of mountains. As a result,
some feel that Davis's ideas will again become mainstream; geomorphology may be on
the threshold of another golden era where cyclical approaches become the custom once
again (Bishop 2007).
Another school of thought suggests that late Cenozoic uplift of mountains is a con-
sequence of climate change (Molnar and England 1990). Based on the principle of
isostasy, mean elevation should decrease by ΔT (amount of material eroded) divided by
6; thus the underlying rock (the crust and the Moho) should rise by 5ΔT/6. In other
words, enhanced relief production by erosion can uplift mountains, while at the same
time the mountains will experience a decrease in mean elevation, since material is being
removed from between the peaks and high ridges. For example, glaciation will increase
the depth of incision of the terrain and, concurrently, streams will increase erosion and
deposition in the forelands, creating greater relief. Large sediment deposits in the At-
lantic, Pacific, and Indian Oceans provide evidence of recent erosion and concurrent
deposition. In the Gulf of Mexico, deposition rates were greater during Quaternary gla-
ciation than during the 60 million years before glaciation. This suggests that glaciation
was responsible for the late Cenozoic uplift of the Rocky Mountains. In the northern
and central Apennines, Italy, hillslope erosion and river incision have balanced uplift
for 1 My, but rates of exhumation have slowed since the emergence of the mountain
chain, suggesting that some other process is operating (Cry and Granger 2008). In the
southern Alps, both tectonics and uplift as a result of incision are present. Adams (1980)
found that uplift and erosion rates are roughly equal. Isostatic forces must be active
because if 1 km of material is removed, the landscape should lower by 1/6 km; thus tec-
tonics are thickening the crust at a rate proportional to this. Plate convergence in this
region has occurred for 10-15 million years, but deposits in the oceans are only 2.5 mil-
lion years old, correlating well with recent glaciation, not tectonic events. Late Ceno-
zoic uplift has been inferred for mountain regions across the world, yet the correlation
with plate motions is small; only global climate change is regionally extensive enough
to explain uplift. The combination of climate change, weathering, erosion, and isostatic
rebound might even have created a system of positive feedbacks that allows glaciers to
continue to grow, causing more erosion and therefore uplift.
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