A Projectile Shock wave Target rock
has been structurally uplifted by rebound (left). The surrounding annular depression contains breccias and impact-melt rocks. Postimpact modi- fication is severe in large impacts; collapse of the outer rim creates a final crater that is much wider than the transient cavity.
B Ejecta
Transient crater Vapor
Melt
Rarefaction Shock wave
C
Uplift of crater floor
Impact Facts and Figures The study of craters began in 1609 when Galileo Galilei observed and sketched circular features while training his telescope on the moon. The first suggestion of an impact origin to the moon’s craters was put forth by Robert Hooke in 1665. Hooke himself dismissed the idea because at the time, interplanetary space was thought to be empty, and he could not imagine where the pro- jectiles would come from; it was not until 1801 that asteroids were discovered. Through the 1800s most observers supported a volcanic origin for the moon’s craters. In 1893 the geologist G.K. Gilbert studied lunar craters and concluded they could be explained only by impact, but he could not understand why the craters were almost all circular even though many of the impacts were undoubtedly oblique. It is now known that craters produced by high-velocity impact are circular even at low incidence angles.9 On Earth, most craters currently recognized
D Ejecta
Gravitational collapse of crater walls
as impact related were once thought to be of volcanic origin. Scientists now realize that a vari- ety of mechanisms can cause terrestrial crater- like features, including volcanism, domal collapse, tectonic activity, subsurface dissolution and glaciation.
9. Melosh, reference 3.
10. Melosh, reference 3. 11. Koeberl (2002), reference 3.
E Final crater diameter Original plane
Structural uplift
F Apparent crater diameter Original plane
, Complex impact structure. In the formation of complex impact structures, the first two stages— contact and compression (A) and excavation (B)—are similar to the same stages in simple crater formation. The modification stage, however, is more complicated. The middle of the crater begins to rebound (C), creating an uplifted central area. Far from the center, the crater walls begin to collapse (D). The final diameter of the crater is much greater than that of the transient crater (E). After erosion of the crater (F), the diameter may appear to be even greater.
Oilfield Review Autumn 09 Impact Fig. 4
18
ORAUT09-Impact Fig. 4 Oilfield Review
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