Geology Reference
In-Depth Information
Figure 3.27. Cross-section through the eroded Sierra Madera impact structure, Texas, showing reconstruction of the structure based on drill-
holes (indicated by the vertical lines) and evidence for a prominent central uplift, or peak (reprinted from Wilshire, H. G. and Howard, K. A.,
1968
, Structural patterns in central uplifts of crypto explosive structures, as typi
ed by Sierra Madera, Science, 162, 258
-
261).
Figure 3.28. A diagram showing the trajectories
of ejecta as a function of gravity on Earth, the
Moon, Mercury, Venus, and Mars for an impact of
equal magnitude, all other factors being equal
(courtesy of Peter Schultz).
diamonds, apparently resulting from the high shock
pressures. The largest impact found in the United
States is the Chesapeake Bay structure at 85 km in
diameter and containing a central uplift some 38 km
across. Knowledge of this structure comes primarily
from drilling and recovered core samples, along with
geophysical data. This impact occurred at about 35.5 Ma
in a shallow marine environment (Ma = mega-annum or
10
6
years).
3.4.3 Impact craters and planetary environments
The primary factors governing the size and shape of
impact craters are the impact energy, gravity, and proper-
ties of the target, as described by planetary scientist Jay
Melosh (
1989
) of Purdue University. In general, the
greater the energy (i.e., larger bolide mass, higher impact
velocity), the larger the crater. Gravity affects the cratering
process by in
uencing the dimensions of the excavation
