Geology Reference
In-Depth Information
principal process involved in the formation of limestone
caves.
Space weathering occurs from the bombardment of
planetary surfaces by energy from the Sun and deep
space, such as cosmic rays. Being only
“
skin-deep,
”
this
process is more important on airless bodies that are poorly
shielded from bombardment. For example, the Apollo 11
astronauts conducted an experiment to measure the solar
wind (charged particles streaming from the Sun) impact-
ing the Moon by exposing a sheet of gold foil on the
surface, which was returned to Earth at the end of
the mission and analyzed. Results enabled the
flux of the
charged particles from the Sun to be quanti
ed.
Although space weathering is a relatively shallow surface
process, it can have a big in
uence on the exposed rocks and
minerals, altering their chemical and physical properties.
For example, the molecular structure of ices (as on the
outer Solar System satellites) and glasses can be altered,
which can in
uence their signatures in remote sensing data.
Cosmic rays, in addition to
“
zapping
”
minerals, can also
be used for age-dating rocks, much like counting craters.
“
Zap
”
pits and traces of cosmic rays can be counted in
mineral grains; from knowledge of the
flux of cosmic rays,
it is then possible to calculate the length of time that the
specimen has been exposed on the surface. Of course, one
must assume that the specimen has not been overturned in
its history and that the
flux has been constant through time.
Despite these uncertainties, age determinations based on
cosmic-ray abundances have been used to date some lunar
samples, as well as some rocks on Earth, as on the rim of
Meteor Crater, to help determine the age of the impact.
Once material has been weathered, it is subject to ero-
sion through various agents, such as wind and water. The
driving force of these agents is primarily gravity. Through
gravity, material is moved by mass wasting (such as
landslides),
flowing liquid water, ice (glaciers), or wind.
Figure 3.33. A rock glacier on McCarthy creek, Copper River region,
Alaska, showing
flow-lobes into the valley. Rock glaciers consist of
poorly sorted rocks and
fine debris held together by ice (US
Geological Survey photograph by F. H. Mof
t).
lubricants and can destroy the cohesion between par-
ticles; (b) in many materials, particularly the clay min-
erals, water enters the crystal structure, causing swelling
and disrupting of the strength of the material; (c) water
adds weight to the potential landslide and thus helps
to
“
push
”
the mass of rock and soil down hill; and
(d)
fluid pore pressure can reduce the amount of energy
necessary to initiate movement.
Figure 3.33
shows a
rock glacier, representing one form of mass wasting on
Earth.
3.5.2 Mass wasting
Mass wasting is the downslope movement of rock and
debris and is a universal geologic process. Even very
small bodies, such as the asteroid Gaspra (
Fig. 1.5
),
where gravity is only a tiny fraction that of the Earth,
display downslope movement of surface material.
Mass wasting is categorized on the basis of the rate of
movement (
“
slow,
”
or imperceptible to an observer, and
“
3.5.3 Processes associated with the hydrologic
cycle
The hydrologic cycle defines the movement of water
among the atmosphere, surface reservoirs, such as oceans,
glaciers, streams, and groundwater systems (the term
used for water beneath the surface). On Earth, water is a
dominant geologic agent, and the hydrologic cycle
gures
), the types of material that are involved (rock,
soil, etc.), and the water content. Water acts in several
ways to enhance mass wasting: (a)
films of water act as
fast
”
