Geology Reference
In-Depth Information
Figure 3.15. Collapse depressions are craters
typically less than 15m in diameter that can form
on basalt
flows as a result of lava draining from
beneath a solid or partly solidi
ed crust. This
oblique aerial view shows collapse depressions
formed on the Amboy lava
flow in southern
California. Similar-appearing depressions can
form by in
ation of active lavas surrounding a
zone not encroached upon by the lava.
40 km/s. Using the simple expression for kinetic energy of
KE = 0.5 mv
2
, in which m is the mass of the bolide and v is
its velocity, an average nickel
-
iron meteoroid 30 m in
diameter traveling at 15 km/s could transfer about
1.7 × 10
16
J of energy onto a planetary surface, the equiv-
alent of exploding about four million tons of TNT! Such
an impact is considered to have been responsible for the
formation of Meteor Crater in northern Arizona (
Fig. 2.4
),
in which more than 175 million tons of rock were exca-
vated to leave a crater more than 1 km across and 200 m
deep. Unlike most geologic processes, impact events are
of very short duration. For example, Meteor Crater prob-
ably formed in about one minute.
3.4.1 Impact cratering mechanics
Much of our understanding of how impact cratering oper-
ates has come from laboratory experiments in which the
process could be studied under controlled conditions.
From analyses of high-speed motion pictures and detailed
analyses of cratered targets, Don Gault and his colleagues
at the NASA-Ames Research Center derived the general
sequence of the impact process (
Fig. 3.22
). First is the
compression stage, in which the projectile contacts the
target and penetrates the surface (
Fig. 3.23
), resulting in
high-speed jetting of material outward from the zone of
contact. At the same time, intense shock waves (
Fig. 3.24
)
pass through both the target and the projectile. In this stage
of the impact, shock pressures of several megabars are
common, exceeding by three to four orders of magnitude
the effective strength of common rocks. It is this high
shock pressure that sets impact cratering apart from
other geologic processes. Impacts result in intensely
crushed and broken target material, some of which is so
Figure 3.16. An aerial photograph of part of Glass Mountain (see
Fig. 3.9
), showing small (5
10m) explosion craters formed by the
rapid release of volatiles from the viscous obsidian
ows.
-
Wyoming, a dome of sedimentary rocks pushed upward
by an intrusion of magma.
Figure 3.21
shows several
intersecting ridges composed of cooled magma that
intruded as vertical sheets termed dikes.
3.4 Impact cratering
Impact cratering involves the nearly instantaneous trans-
fer of energy from an impacting object, called the bolide,
to a target surface. Bolides can include meteoroids, aster-
oids, and comets, which impact Earth at velocities of 5
-
