Geoscience Reference
In-Depth Information
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Sedimentary rocks of the type that rim Meteor Crater are de-
posited, naturally, with younger rocks above resting on older ones
below. Yet at Meteor Crater, Shoemaker found just the opposite: The
rocks on the crater rim were actually upside down geologically, with
younger underneath older. He concluded that they had been blasted
into the air, flipped over, and then had fallen to the earth again, but
still upside down, forming a kind of upside-down layer cake. To lift
huge masses of rocks and turn them over would have taken a great
deal of energy. He too found the crater floor filled with breccia. Com-
parison with craters produced by nuclear test explosions allowed him
to calculate that Meteor Crater had been formed by an iron meteorite
weighing 60,000 tons, measuring 25 m in diameter, and traveling at
15 km/sec. Shoemaker calculated that the explosion was equivalent to
the detonation of a 1.7 megaton nuclear device (85 times the magni-
tude of the bomb dropped on Hiroshima; recent estimates are higher)
and destroyed all of the impactor save a few fragments. In 1964, the
old generation and the new came together when Shoemaker guided
Walter Bucher on a field trip to Meteor Crater. The evidence appar-
ently convinced Bucher that the crater after all was due to impact, but
he died before he could make his change of heart known.
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CRYPTOEXPLOSION STRUCTURES AND IMPACT MARKERS
The Steinheim Basin in Germany was one of the first cryptovolcanic
structures to be described. Although it was initially put down to
meteorite impact, this idea quickly gave way. to the more orthodox
view that the basin and others like it had been formed by ascend-
ing volcanic gases that fractured the rocks but whose associated
lavas remained hidden, giving rise to the name
cryptovolcanic
for the
structures
{cryptoexplosion
later became the preferred term). Curi-
ously, however, the deeper the structures were probed, the less the
rocks are deformed. If produced by volcanic activity, it should have
been just the opposite. Some observant geologists wondered if the
cryptoexplosion structures had been hit, not from below, but from
above, and set out to find evidence.
Unfortunately, terrestrial craters, especially the older ones, are
often so heavily eroded that only the barest trace of a circular struc-
ture remains, allowing them to be interpreted as either of crypto-
explosion origin or of impact origin, if not formed by some entirely
different process. What was needed to resolve the issue was a marker,
or set of markers, produced only by impact. It seemed theoretically
possible that such markers exist, for the shock of impact is so intense
and sudden that it produces conditions radically different from the
low pressures typical at the earth's surface.
