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 ascending 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). Curiously, 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 structure remains, allowing them to be interpreted as either of cryptoexplosion 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.

The first markers were discovered just after the turn of the century 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 unknown. 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.14

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 participating) 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 geological 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 predicted shatter cones (Figure 5).15

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 shatter 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 matter; at these famous sites, decades of study and reams of publications placed reputations and geological orthodoxy on the line.

FIGURE 5 Shatter cones from Sudbury, Ontario. [Photo courtesy of R. Grieve and Geological Survey of Canada.]

Today we know from experiments that shatter cones mark the lowest pressures of impact, in the range of 5 gigapascals to 10 giga-pascals. (Named in honor of a seventeenth-century mathematician and physicist, Blaise Pascal, a gigapascal [gPa] equals 10,000 times the pressure of the earth's atmosphere at the surface.) At slightly higher pressures—10 gPa to 20 gPa—quartz and feldspar, the two most common minerals in the earth's crust, begin to fracture in the characteristic crisscrossing planes, a few millionths of an inch apart, that I had originally seen in K-T zircon on the cover of Nature.

When a mineral with a certain crystal structure is subjected to sufficient heat and pressure, its atoms rearrange themselves into a structure that better accommodates the new conditions. For example, at low temperatures and pressures, pure carbon exists in the sheetlike structures that we call the mineral graphite. At higher temperatures and pressures, and under certain other conditions, carbon changes into the interlocking, three-dimensional structure that we call diamond.

Laboratory experiments show that quartz has two mineral phases that appear at high pressure but low temperature: The first to form is coesite, followed at about 16 gPa by stishovite. Thus the presence of stishovite at the surface means that the pressure at that point once reached 16 gPa. As far as we know, only meteorite impact produces such pressures. Coesite and stishovite were first discovered in nature at Meteor Crater. At pressures above 60 gPa minerals 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 western 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 traveled 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.

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