Geology Reference
In-Depth Information
bowl, the extent of the ejecta, and various post-impact
crater modi
cations. For equal-size impacts, blocks of
ejecta can be lifted and excavated more easily on low-
gravity planets, leading to larger craters in comparison
with high-gravity environments. Furthermore, in low-
gravity environments, ejecta is thrown a greater distance,
as shown in
Fig. 3.28
. Thus, we would expect to see a
wider (but thinner) zone of ejecta surrounding impact
craters on the Moon than on higher-gravity planets such
as Mars and Mercury. In the modi
cation stages of impact
cratering, gravity also plays a role by in
uencing the
degree of slumping, perhaps governing the size of poten-
tial central uplifts.
Target properties in
uence crater morphology in all
stages of the impact process. As noted for Meteor Crater,
the structure of the rocks (such as joints) can control
the planimetric form, or the outline of the crater. Impacts
into soft sediments tend to be larger because less energy is
needed to break up the rocks and more energy is available
for excavation. Target rocks containing water tend to be
fluidized in the impact process, leading to slurry-like
ejecta deposits, as seen at the Ries Kessel in Germany
and as proposed for many martian craters (
Fig. 3.29
).
Impact craters show a distinctive progression in mor-
phology with increasing size (
Fig. 3.30
). Small craters,
such as Meteor Crater, are simple bowl-shaped depres-
sions. Larger craters display central peaks and terraces on
their inner walls, and at still larger sizes, clusters of central
peaks. The largest impacts formmulti-ringed basins. The
size ranges for these morphologies are different among the
planets, being controlled primarily as a function of gravity
(
Fig. 3.31
).
The shape of impact craters in plan view is partly
controlled by the angle of the incoming projectile.
Because impacts involve essentially point-source trans-
fers of energy, both the crater and the distribution of ejecta
for most impacts are concentrically symmetric about the
point of impact. Although intuition might suggest that
oblique angles of impact would cause elongate craters,
experiments have shown that only for very low angles
(<15°) above the surface do impact craters and ejecta
become noticeably non-circular (
Fig. 3.32
).
3.5 Gradation
Gradation is a complex process that begins with weath-
ering and erosion, continues with transport of the weath-
ered debris, and ends with deposition of the material.
Think of a road
on a dirt track that cuts off the
tops of bumps and
fills in the ruts with the debris. Thus,
gradation is a
“
leveling off
”
process in which topograph-
ically high areas are worn down by erosion and low areas
are
filled by deposition.
“
grader
”
3.5.1 Weathering
Weathering is the
first step in gradation; it is the process
that
“
softens up
”
rocks, making them amenable to erosion,
either through physical events or by chemical reactions.
For example, rocks can be fragmented by impacts, broken
to smaller pieces in a stream, or ground into dust by
glaciers. On a much smaller scale, grains can be reduced
in size by a variety of processes, including salt weathering,
in which salt-laden water seeps into tiny cracks, and, as
the salt crystallizes and expands, the grains are split into
smaller fragments.
Chemical weathering on Earth typically occurs through
reactions with water and the atmosphere and includes
oxidation (the combination of materials with oxygen,
forming, for example, rust), hydration (the combination
of minerals with water molecules), solution (in which
materials are dissolved, commonly in water), and carbon-
ation (a more complicated reaction involving carbon
dioxide from the atmosphere and the formation of a
weak acid that reacts with minerals). Carbonation is the
Figure 3.29. Many craters on Mars show distinctive
flow-lobes that
are generally considered to re
ect impacts into water- or ice-
saturated targets that formed slurry-like ejecta masses (NASA
THEMIS mosaic).
