Geoscience Reference
In-Depth Information
The first markers were discovered just after the turn of the cen-
tury in a hill at the center of the Steinheim structure. The striated
and broken cones of rock found there, known as shatter cones, had
clearly formed from shock pressure, though its source was un-
known. Some 40 years later, Robert Dietz, an early proponent of
impact, studied the cryptoexplosion structure at Kentland, Illinois,
located in the middle of the sedimentary rocks and cornfields of the
American Midwest. In a large limestone quarry, he found shatter
cones 6 feet long.
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The narrow ends of shatter cones tend to point back toward the
center of their structure, showing that the fracturing pressure had
come from there. Dietz believed that shatter cones would only be
found at impact craters. Experiments (with Shoemaker participat-
ing) in which a gas gun fired pellets into limestone at 18,000 mph
produced tiny but perfect shatter cones. Eventually they turned up
in scores of other structures, including Meteor Crater, and came to
be regarded, as Dietz had proposed, as an indicator of impact.
As noted earlier, the Sudbury structure in Ontario is one of the
world's largest nickel ore bodies and one of the most thoroughly
studied geologic features in the world. Decades of traditional geo-
logical approaches, however, had by the early 1960s produced no
satisfactory theory to explain its origin. In a way analogous to the
proposal of the Alvarez team that something completely outside
normal experience had destroyed the dinosaurs, Dietz came up with
the notion that Sudbury was created by a process so rare that no one
had even thought to invoke it. In 1964 he proposed that Sudbury
was a giant impact structure and, in his first visit, found the pre-
dicted shatter cones (Figure 5).
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Dietz went even further by endorsing the suggestion made in
1946 by Harvard's Daly that impact had also created the ancient
South African structural dome known as the Vredefort Ring. (An
impact structure that is very old and highly eroded would have
ceased to exist as a topographic feature. All that would be left would
be the concentrically warped rocks that were present at ground
zero, hence the name "ring.") Dietz predicted the presence of shatter
cones and again, in his first visit to Vredefort, found them. But shat-
ter cones notwithstanding, most geologists thought that by proposing
that impact had created the classic and intensely studied Sudbury
and Vredefort structures, Dietz had crossed the line into heresy. At
Meteor Crater, little was at stake and the misguided pro-impactors
could muse as they liked. Sudbury and Vredefort were another mat-
ter; at these famous sites, decades of study and reams of publications
placed reputations and geological orthodoxy on the line.
