Geography Reference
In-Depth Information
spent much time above the timberline in mountains, you know that a very common
sound is that of falling rocks. Large-scale features such as mudflows, landslides, and
avalanches can reach catastrophic dimensions and do more geomorphic work in a mat-
ter of minutes than day-to-day processes accomplish in centuries. Such spectacular phe-
nomena epitomize the inherent instability of the mountain system (Caine 1974). These
catastrophic events begin, however, with the weakening and breakdown of bedrock.
FIGURE 5.3
Components of an alpine hillslope in the Rocky Mountains. Other extensively glaciated
mountain systems would have additional horns, cirques, and greater relief at high elevations. (Ad-
apted from Caine 1974.)
Weathering
Weathering is the alteration and reduction of rock into finer particles, whereas erosion
involves transporting the weathered particles. The processes of rock breakdown are
normally divided into chemical and physical (mechanical) weathering. Recently, other
biological weathering mechanisms, including biofilms such as fungi, algae, cyanobac-
teria, and heterotrophic bacteria, have been recognized for their potential to chemically
and physically change rock (Gorbushina 2007). Some have suggested that weathering
should be viewed from a critical zone framework in which unweathered rock and water
are delivered to a reactor, enhancing the potential for physical, chemical, and biological
weathering (Anderson et al. 2007).
Physical weathering
refers to the mechanical forces that pry rocks apart or make
their surfaces disintegrate. Minerals remain unchanged through physical weathering
processes such as frost action, unloading, and salt crystal growth.
Chemical weathering
is the decomposition of rock by chemical alteration of the original minerals. The internal
structure of minerals is destroyed, and new minerals are created. Chemical reactions
tend to round the edges of rocks, producing pitted and grooved surfaces.
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