Geology Reference
In-Depth Information
In our view, the usefulness of numerical models is
not in creating a reproduction of some actual
landscape. Instead, models provide a means to
explore potential interactions within a landscape,
so that we have a firmer basis for understanding
how variables as diverse as crustal rigidity, sus-
ceptibility of bedrock to landslides, and the distri-
bution of precipitation may interact with each
other, perhaps in unexpected ways. Such models
are meant to develop our insight into the complex
interactions among the processes, and they can
serve to point toward measurements one might
make in the field that would most efficiently
constrain the process rates.
has been expended (or converted, really, to
potential energy) in elevating the crust of the
Himalaya and Tibet far above the geoid and
thereby creating the world's largest topographic
anomaly, averaging about 5 km above sea level.
The energy for driving surface processes
derives from a combination of gravitational poten-
tial energy and solar energy. All rocks that have
been elevated above the geoid have a potential
energy equivalent to the product of their mass,
gravitational acceleration, and their height above
the geoid (PE =
mgh
). At the top of the atmosphere,
the Earth receives solar energy equivalent to
about 1300 W/m
2
. The solar energy received at
the Earth's surface is several orders of magnitude
greater than the energy that leaks out from the
Earth's internal heat engine (
∼
40 mW/m
2
). Solar
energy evaporates water and heats the air that
carries the moisture with it as the air rises,
expands, and cools. The water attains its maxi-
mum potential energy at the top of its atmos-
pheric trajectory. When the vapor condenses
and falls as precipitation, it converts some of its
potential energy to kinetic energy that it delivers
to the surface of the Earth with the force of
its impact. The potential energy that is repre-
sented by the mass of water at some elevation
above the geoid is then available to be expended
doing geomorphic work or to be lost through heat
dissipation or frictional processes. Solar energy
is also an important factor in chemical weather-
ing processes, especially in any temperature-
dependent reactions, as well as in mechanical
weathering processes, such as freeze-thaw cycles.
Energetics
Energy drives the interactions between tectonics
and surface processes. In order to build topog-
raphy, work must be done against gravity. The
energy needed to accomplish this work comes
ultimately from the conversion of a small frac-
tion of the energy involved in the horizontal
motions of the lithospheric plates that constitute
the more rigid exterior of the planet. The energy
driving plate tectonics comes from primordial
heat associated with building of the planet, from
the decay of radioisotopes, and from phase
changes in the interior of the Earth.
It is perhaps surprising to cast the energy
expenditure represented by plate motion in
everyday terms. Consider, for example, a simple
calculation of the kinetic energy of that part
of the Indian Plate lying south of where it is
colliding with mainland Asia and building the
Himalaya. We may specify its approximate
dimensions as 3000 km wide by 7000 km long
by 50 km thick. If we assume a mean density of
3000 kg/m
3
, this yields a mass of approximately
3 × 10
21
kg. Taking a mean plate velocity of 5 cm/yr
yields a kinetic energy (½
mv
2
) for the plate of
about 4 × 10
3
J (joules), equivalent to about 1/200
of a “Snickers” bar (10
6
J)! On the other hand, the
rate of energy expenditure required to move the
Indian Plate (force × distance/second) is equiva-
lent to approximately 10
13
J/s or 10
7
“Snickers”
bars per second! Over the past 50 million years,
some tiny fraction (
∼
1/100 of 1%) of that energy
Active tectonics and models
of landscape development
Sharp contrasts in the appearances of land-
scapes in a given climatic or tectonic regime
inspired geologists in the past to devise schemes
to explain those contrasts. One of the most
prominent such geologists was William Morris
Davis, who in the late 1800s and early 1900s
developed the well-known geomorphic models
(Davis, 1899) showing a progression from
“youth” to “maturity” to “old age” (Fig. 1.2).
Living in the wake of the revolutionary ideas of
