Geoscience Reference
In-Depth Information
FIGURE 6
The formation of a simple
impact crater. [After Don Gault.
16
]
LOCATIONS AND AGES
As the photographs of other bodies taken from space began to be
returned to the earth, the full extent of cratering in the solar system
began to become apparent, at least to the more attentive, who con-
cluded that there must be many undiscovered craters on earth, and
set out to find them. They had their work cut out for them. A recent
impact, like that at Meteor Crater some 50,000 years ago, leaves an
obvious crater. But the earth is 4,500 million years old; most craters
will have been so eroded that they no longer have any surface man-
ifestation at all and may be detectable only through geological and
geophysical methods. These techniques work because the shock of
impact distorts the rocks at ground zero, raising central peaks and
causing terraces to slump, as at Tycho. But these surface features,
which on the earth are obscured by time and erosion, are underlain
by structural ones—rock beds bent and twisted into concentric
rings. Imagine, for example, that the moon had wind and water and
that Tycho had been eroded for millions of years. Then the central
peak, the terraces, indeed the crater itself would be gone, leaving no
surficial hint that a crater had once been present at that spot. Buried
in the rocks below the lunar soil, however, would be the bull's-eye
imprint of the now vanished impact crater, detectable by geophysical
