Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 42

forms. Recent understanding of how erosion actually builds mountains recognizes the
intricate linkages and feedbacks between tectonics, isostasy, climate, and erosion (Hoff-
man and Grotzinger 1993; Pinter and Brandon 1997; Koons et al. 2002; Stewart et al.
2008). Orogeny leads to orography, the perturbation of the regional climate by topo-
graphy. An asymmetric pattern of precipitation results, wherein a dominant wind dir-
ection or storm track has moisture drawn out from the rising, cooling air mass on the
windward side of the range, thus producing a rain shadow to the leeward, where the air
is descending and warming. Erosion is enhanced on the windward side and reduced on
the leeward (Fig. 2.14). In ranges near oceans, where prevailing winds are in the same
direction as subduction, erosion denudes the coastal side of the range, which effectively
“pulls” or unloads the rocks so that the compressive forces of collision, coupled with
isostasy, move the buried rocks toward the surface. Where prevailing winds blow off-
shore and opposite to the direction of subduction, erosion is concentrated on the inland
side of the range, exposing the deepest and most deformed and metamorphosed rocks
in that area. The control of erosional agents—gravity, water, wind, and ice—acting upon
any particular mountain landscape depends largely upon the local climate, the steep-
ness of slopes, and local rock types (Pinter and Brandon 1997). In addition, high-alti-
tude/high-latitude mountain areas tend to be protected by cold-based glacier ice frozen
to its bed, whereas at lower altitudes and latitudes, warm-based ice with basal meltwa-
ter is highly erosive. Similarly, other factors that decrease erosion in mountain areas
are rock strength, greater aridity, and gentle slopes.
In recognition of the unusual juxtaposition of numerous massive Himalayan peaks
over 7-8 km high that are directly adjacent to major high-order rivers at low altitudes
at their base, Zeitler et al. (2001a, 2001b; Finnegan et al. 2008) discovered that some
actively uplifting metamorphic massifs are a direct consequence of erosion. Intense loc-
alized river erosion initiated as a result of capture of a major river by a steeper but
smaller one is capable of focusing deformation into small areas being rapidly eroded.
Intense metamorphism at depth results in a “tectonic aneurysm” that is accompanied
by a bowing up of the brittle-ductile transition zone, decompression melting, and intru-
sion (Fig. 2.15). New crustal mass is adverted in at depth to replace the continued loss
of surficial mass from the local high elevation and high relief that maintains the rap-
id surficial exhumation in a closed feedback loop. Passage of the rising mountain top
though high altitude causes formation of cold-based, protective ice that helps maintain
steep slopes against mass failure or further glacier erosion. Upward movement of rock
at depth into the tectonic aneurysm is thought to produce the distinctive petrology and
structure of mantled gneiss domes. These features are known from elsewhere in the
Himalaya where deep erosion is known or suspected (Finnegan et al. 2008; Shroder et
al. 2011), as well as in other ancient collision mountain zones, where these initially deep
internal dome structures are ultimately exposed at the surface by deep erosional un-
roofing (An Yin 2004; Fletcher and Hallet 2004; Maheo et al. 2004).
Erosional Relict or Residual Mountains
In many continental interior cratons, where tectonic stability has existed for millions of
years, long-continued, deep weathering to saprolites and their erosion has left behind
residual bedrock mountains. Examples occur on all continents, but those of Australia,