Geology Reference
In-Depth Information
Figure 2.1. Geologist G. K. Gilbert
performed experiments in the late
1880s to simulate impact processes,
shown in this photograph of small
cannon balls that were dropped into a
target of stiff mud (courtesy of the US
Geological Survey).
Building on the work of Gilbert, Don Gault at NASA-
Ames Research Center constructed in the early 1960s a
facility for conducting sophisticated impact experiments
using a hydrogen gas-gun (
Fig. 2.2
). This gun can
re
projectiles at velocities as high as 7.5 km/s into a target
contained in a vacuum chamber to simulate the Moon. In
some experiments, the target is placed on a platform that
can be dropped at the time of the impact to simulate
reduced gravity conditions (much like the
“
weightless
”
feeling when an elevator descends rapidly). Results from
this facility enabled the fundamental physics of impacts to
be derived and provided critical insight into the geologic
aspects of impact cratering (
Fig. 2.3
).
At about the same time as impact experiments were
being conducted, Gene Shoemaker of the US Geological
Survey (USGS) was synthesizing results of his field stud-
ies of Meteor Crater in Arizona (
Fig. 2.4
), which included
assessments of rock structure and deformation, including
the presence of overturned stratigraphy in the crater rim,
which was so well seen in the Gault experiments
(
Fig. 2.3
). Subsequently, field sampling at Meteor Crater
led to the discovery of coesite and stishovite, the high-
pressure forms of quartz that are formed by impact pro-
cesses. Concurrently, other geologists were scouring
remote sensing data to identify possible impact structures
on Earth (
Fig. 2.5
), followed by
field investigations.
The mid 1960s also saw investigations of nuclear
explosion craters at the Nevada test site and their study
by Hank Moore of the USGS for comparisons with
Figure 2.2. The Vertical Gun at NASA-Ames Research Center
consists of an
frame mounted with a gun barrel that uses
compressed gasses to launch small projectiles at velocities as high as
7.5 km/s into a vacuum chamber tank to simulate impact cratering
processes. The
“
A
”
frame can be rotated so that the gun can
re
projectiles at different impact angles from near-horizontal (seen
here) to vertical.
“
A
”
