Geology Reference
In-Depth Information
Table 3.3. Criteria for the recognition of impact craters (modi
ed from Dence
1972
)
Criterion
Characteristics
Reliability
Remote sensing
Plan view
Distinctly circular; may be modi
ed by slumping, tectonic
Fair, but can be attributed to
other processes
patterns, or erosion
Rim structure
Inverted stratigraphy
De
nitive
Central zone
Floor lower than surrounding plain; may contain central uplift Fair, but can be attributed to
other processes
Geophysical
observations
Gravity
anomaly
Generally negative
Supportive, but not
conclusive
Magnetic
eld
Variable; may be distinct anomaly over melt rock
Supportive, but not
conclusive
Seismic
velocities
Generally lower in brecciated zones
Supportive, but not
conclusive
Ground
observations
Presence of
meteorites
Rare except in very young craters
De
nitive
Shock
metamorphism
Features such as high-pressure minerals, impact melt, planar
shock features, and shatter cones
De
nitive
Brecciation
Observed in ejecta, rim, and
floor of craters
May be attributed to other
processes
The 24 km Ries Kessel impact structure in southern
Germany provides insight into the mechanics of ejecta
emplacement. Extensive mapping and drilling to obtain
subsurface core samples have revealed the extent of the
ejecta deposit and its properties. The ejecta is composed
mostly of fragmented rock, or breccia, some of which was
mixed with melted rock and volatiles as it was thrown
from the transient cavity. This material rained down on the
surrounding terrain, where it churned up and mixed with
additional local rock. Then the entire mixture continued to
slide outward a short distance and settled into its
nal
resting place.
Many other structures on Earth have yielded important
data on the morphology of impact craters and the cratering
process. For example, the Clearwater Lakes in Canada
appear to be a double impact, while the Henbury Craters
in Australia consist of at least 13 craters, probably re
ect-
ing multiple impacts from the breakup of an incoming
bolide.
The largest recognized impact structures on Earth
include Sudbury in Ontario, Canada, and Vredefort in
South Africa; both are about 140 km in diameter, and
they formed at 1.85 Ga and 1.97 Ga, respectively. The
Popigai structure in Siberia is 100 km in diameter and
involved an impact
Figure 3.26. Shatter cones formed in limestone at the Haughton
impact structure, Canada, showing typical striations on the
flanks of
the cones, the apexes of which are oriented toward the point of
impact (courtesy of R. A. F. Grieve).
into limestones that show tiny
