Geology Reference
In-Depth Information
Steady-State Topography with a
Perennially Frozen Zone
process
zones
enhanced
erosion over
rockfall zone
rockfall
ava-
lanching
erosion deicit
cold-based
glacier
frozen
zone
over frozen
peak
&
glacial
erosion
T
1
T
1
T
1
warm-
based
glacier
future rockfall
collapse
T
2
land-
sliding
T
2
T
2
peak after
collapse
river
incision
river
incision
bedrock uplift
Fig. 10.42
Oscillating steady state in the presence of cold-based glaciers.
Process zones (left side) of river incision, landsliding, and glacier erosion are the same for ranges with and without
glaciers that are frozen to their beds. Where cold-based glaciers exist, glacial erosion drops to zero. Bedrock erosion
in the frozen zone occurs primarily due to episodic large rockfalls.
they grow upwards at the same rate as rock
uplift occurs beneath them. Farther down the
slopes of the mountains, the climate may be
sufficiently warm to permit warm-based glaciers
to exist. As described above, such glaciers are
likely to be capable of eroding as rapidly as the
rocks are uplifting. The topographic consequence
of the dichotomy between rapid erosion that is
focused low on the slopes versus no erosion on
the summits is similar to extruding a steeple that
keeps growing taller. Eventually, the summit
steeple on a mountain will exceed the strength
of the rocks beneath it and collapse in a large
rockfall. The time between successive summit
collapses provides a rough time scale for the
fitful oscillations around steady state of a range
with frozen summits (Fig. 10.42).
for past erosion. One such example comes from
the central Himalaya, where a pronounced gra-
dient in monsoonal rainfall appears decoupled
from the long-term erosion rates (Burbank
et al
.,
2003). Whereas a 10-fold decrease in rainfall
occurs across the Himalaya, apatite fission-track
ages (Blythe
et al
., 2007) appear quite uniformly
young across the entire monsoon gradient
(Fig. 10.43). This long-term gradient also
contrasts with a four-fold northward decrease in
modern erosion rates as deduced from river
sediment loads (Gabet
et al
., 2008).
This apparent mismatch of long-term rates with
modern rainfall forces one to ask why erosion
would be just as fast in a rather dry area (<40 cm/
year of rain) as in a very wet area (>4 m/year). Let
us accept that the data were correctly measured,
so that these differences are real. Is it possible
that the weather data, which were measured over
six years, are not valid for the long term? Certainly,
the magnitude of rainfall changes from year to
year (see inset, Fig. 10.44), but the physics of the
way storms interact with topography is constant,
so the basic gradient in average rainfall should be
persistent through time.
Rapid erosion tends to be event driven,
especially in non-glaciated landscapes. Hence,
we might ask whether the gradient in mean
Mismatches between tectonics,
topography, and climate
Although many studies have shown spatial
correlations between zones of high modern
precipitation and zones of young cooling ages
and, hence, inferred rapid, long-term erosion
(e.g., Reiners
et al
., 2003; Thiede
et al
., 2004),
sometimes we can learn more by studying
mismatches between climate and various proxies
