Geoscience Reference
In-Depth Information
presence of stishovite at the surface means that the pressure at that
point once reached 16 gPa. As far as we know, only meteorite im-
pact produces such pressures. Coesite and stishovite were first dis-
covered in nature at Meteor Crater. At pressures above 60 gPa min-
erals melt entirely. When these melts cool and freeze, they do not
re-form the original minerals but instead harden into glasses that
resemble ordinary igneous rocks, which explains how those at Sud-
bury, for example, could have been mistakenly identified.
The final impact marker is less direct. Scattered around the globe
from Australia, through southeastern Asia, eastern Europe, the west-
ern coast of Africa, to Georgia and Texas, are large swaths of ground
strewn with small glassy globules called
tektites,
after the Greek word
for melted. Tektites usually have no relationship to the rocks with
which they occur, leaving their origin a mystery. Their rounded,
streamlined shapes and wide distribution suggest that they have trav-
eled through the atmosphere while molten. For decades a debate
raged over whether tektites had been splashed by impact off the
earth or off the moon, with Nobelist Urey arguing for a terrestrial
origin and Dietz and Shoemaker for a lunar one. Recently some tek-
tites have been linked to particular terrestrial impact craters, showing
that at least these tektites come from impacts on the earth.
CRATER TYPES
Gilbert, and other early observers of lunar craters through tele-
scopes, could see that they were of two types: smaller, rounded,
bowl-shaped depressions, and larger, more complex structures with
central peaks and collapsed rim terraces. Shoemaker, in his study of
Meteor Crater, discovered why. When an asteroid or comet traveling
at interstellar speeds strikes the earth, two powerful shock waves are
created. The first, the explosive wave, travels downward through the
target rocks, pushing them down and out. The second, the release
wave, moves in the opposite direction. The shock and release waves
interact in a complex manner, melting, vaporizing, and ejecting the
rocks at ground zero. Fractured rock and crater walls fall back into
the crater and mix with melt to form a breccia.
If the impactor is less than a few hundred meters in diameter, a
simple crater (Figure 6) like Meteor Crater is formed. Such craters
range up to about 4 km in diameter. Larger craters are not just bigger
versions of small ones, as Tycho (Figure 7) illustrates. Such large
craters start out in the same way as simple ones, but the greater
energy released by the larger (or faster) impactor causes the rocks at
ground zero to rebound to form the central peak. The crater rim can-
not hold and falls in on itself to form terraces.
