Geoscience Reference
In-Depth Information
fied by Tycho (see Figure 7), the moon has many complex craters,
with central peaks, collapsed rim terraces, and internal zones of melt
rock and impact breccia. Their ejecta can be seen to be scattered
over hundreds or thousands of kilometers. From studying smaller
craters on the earth, and larger ones on other bodies in the solar sys-
tem, geologists have built up an accurate and detailed picture of
what the K-T crater and its associated features would be like.
• If the impactor were 10 km in diameter, as the Alvarezes
calculated, then based on studies of nuclear explosions and
cratering on other bodies, the crater would have a diameter of
about 150 km. However, if some of the impact material had been
blasted completely out of the earth's gravity field, then the
boundary clay would not contain all the ejecta, and the resulting
calculation would give too small a size for the meteorite and the
resulting crater.
• Magnetic, gravity, and seismic anomalies will reveal a circular,
bull's-eye pattern and a buried central peak.
• If the crater is not too deeply buried, it might have an obscure
but recognizable surface topography, perhaps represented by
concentric, arc-shaped ridges and valleys.
• The structure will contain impact breccia and once-molten
rock. (However, volcanic rocks were also once molten and can be
mistaken for impact melts.)
• The melt rock will be enriched in iridium, display reversed
magnetism, and have a radiometric age indistinguishable from 65
million years.
• Glass spherules resembling tektites and dating exactly to the
K-T boundary will be found in the vicinity.
• Ejecta layers located farther away will contain shocked minerals
and iridium. If they can be dated, they too will be 65 million years
old.
• If the impact occurred in the oceans near a continent, say on a
continental shelf, it might have given rise to giant waves that would
leave unusual sedimentary rocks behind.
This is a long list. Were any putative K-T impact structure to
meet even most of these predictions, the search would be over. A
site that met them all would close the case for impact.
