Paleozoic Pockmarks

Monday, July 01, 1996
Splayed across southern Illinois, Missouri, and eastern Kansas are eight large, gently sloping depressions, 2 to 10 miles wide and an average of 60 miles apart. Although the structures have been reliably dated--they are some 310 million to 330 million years old--no one has been quite sure how they formed. The alignment and similarity of the pockmarks has led some geologists to conclude that a series of subterranean volcanic explosions created them. No volcanic rock, though, has been found at any of the sites. Michael Rampino, a geologist at New York University, believes he has a better explanation. He says the features are the eroded traces of a string of craters formed when pieces of a comet or asteroid broke up and slammed into our planet.

Rampino was inspired by the collision of 21 fragments of Comet Shoemaker-Levy 9 with Jupiter in 1994. Strings of impact structures have been identified on our moon and Jupiter’s moons, but never on Earth. Still, icy comets are constantly breaking up as they pass into the inner solar system. According to Rampino’s calculations, if a comet fragmented within a few million miles of Earth, its pieces would have had time to drift apart enough to strike the Midwest at 60-mile intervals--and at 50,000 miles per hour.

An impact would explain a number of features of the sites, says Rampino. The rock in the depressions is folded along circular fractures radiating from a center like a bull’s-eye, suggesting an intense disruption. Also, researchers have found crystals of shocked quartz at two sites in Missouri. Such fractures form only when quartz is subjected to extreme pressures. What’s more, Rampino noted that cone-shaped, overlapping splinters of rock, also known to form only at high pressures, had been found at two sites. A volcanic explosion would not have had the force necessary to form those features; a hypervelocity impact would have.

Rampino says the craters date from a time when a lot of marine life mysteriously disappeared. Although each of the comet fragments would have been less than one-fifteenth the estimated diameter of the body that may have wiped out the dinosaurs, the impacts might still be linked to the marine extinctions. This Shoemaker-Levy occurrence on Jupiter shows people that they should be looking for chains of impact structures--that these chains might actually be relatively common, says Rampino. They’re on the moon, they’re on the moons of Jupiter, and here we think we see one on Earth. Are there more? There may be.
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