Geology Reference
In-Depth Information
original height
of "summits"
change in mean
height
change in summit
height
eroded mass
ice cube
(
new mean height
new mean height
ρ
= 0.9)
before isostatic
compensation,
increased relief
after isostatic
compensation,
increased relief
initial conditions,
no relief
3
1
2
change in
depth of base
ρ
water (
= 1.0)
Erosionally Driven Summit Uplift
Fig. 1.6 Isostatic uplift of mountain summits due to enhanced erosion.
The density contrasts of ice and water are analogous to crust and mantle contrasts, respectively. Erosion of the top
of the ice cube decreases its mass and its mean height. More rapid erosion of valley bottoms than of summits leads
to increased relief and uplift of summits, at the same time that the mean elevation decreases. Note that stage 2 will
never occur unless some force restrains the ice cube from rebounding due to melting at the surface.
chemical processes involved in erosion of bed-
rock by both fluvial and glacial processes.
Can enhanced erosion drive uplift of mountain
summits? Yes, but only if there is an increase in
topographic relief associated with the erosion. In
essence, there has to be less erosion of the sum-
mits than there is of the valley bottoms. Think
about an ice cube floating in a drink (Fig. 1.6).
Its upper surface is at a mean elevation equiva-
lent to
of erosion tend to increase topographic relief and,
therefore, would promote the uplift of summits.
On the other hand, theoretical studies of river
profiles suggest that increased erosive power
causes river gradients to decrease (Whipple,
2004). If this is true, then, in order to generate an
increase in topographic relief in the landscape,
hillslopes would have to be concurrently length-
ened and steepened. Tectonic-geomorphic studies
that document the temporal evolution of both the
valley bottoms and the adjacent hillslopes are
needed to resolve these issues.
Another modern controversy in tectonic
geomorphology revolves around earthquake
prediction. Over the past two decades, a
substantial research effort has focused on answer-
ing such questions as: “Which fault is most
likely to rupture in the next large earthquake?”,
“When is that event likely to occur?”, and “How
large an earthquake can we expect to occur?”
Many scientists would maintain that the best
way to make such predictions is to understand
the past history of faulting. Such topics fall into
the realm of paleoseismology with its focus on the
reconstruction of past earthquakes  in terms of
their distribution in space and time,  coseismic
displacements, and interactions among faults.
1/10th of its total thickness, equivalent
to (1 − r ice / r water ), where r = density. If you were
to cut canyons into the upper surface of the
cube, the mean elevation of the cube's surface
would decrease and the base of the cube would
bob upward in the water in order to maintain the
isostatic balance (maintaining 1/10th of the total
mass above the water surface). If you could cut
the canyons without “eroding” other parts of the
upper surface of the cube, these remnants would
actually rise higher than their original height in
response to the lowering of the mean elevation.
Although melting of an ice cube demonstrates
that peak uplift can occur due to enhanced ero-
sion, has this commonly occurred in the past, is it
related to a more “erosive” climate, and what are
the magnitudes of the uplift of peaks involved?
The common view has been that increased rates
 
Search WWH ::




Custom Search