The New York Times May14 2004
A buried geological formation off the west coast
of Australia, long thought to be remnants of an old volcano, is actually a
200-kilometer-wide crater formed by a devastating meteor strike
251 million years ago, scientists asserted.
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It was that meteor, they said on Thursday, that caused the largest mass
extinction in the Earth's history.
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Researchers led by Luann Becker, a geologist at the University of
California at Santa Barbara, said seismic and gravitational measurements
showed that the structure, called Bedout, had the general shape of a
crater. More significant, they said, jumbled and melted minerals in the
rocks could have formed only in the violent upheaval of a meteor hitting
the Earth, not from cooling lava.
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"This is not a volcano," Becker said. "It's an impact crater." She and her
colleagues presented their findings in a telephone news conference on
Thursday and in an article that was to appear Friday in the journal
Science.
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At the end of the
Permian geological period and the
beginning of the Triassic, 90 percent to 95 percent of the species in
oceans died out, as did at least half of the backboned species on land.
The extinctions appear to have happened quickly, at least on a geological
time scale, in less than 160,000 years and perhaps in a much shorter time.
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The new data is the latest evidence offered by this team of scientists
arguing that a meteor strike caused the Permian-Triassic extinction.
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Most scientists believe a meteor striking the Earth near
the Yucatan Peninsula in Mexico caused the best-known mass extinction, the
one that led to the disappearance of dinosaurs 65
million years ago.
The causes of
about half a dozen other mass extinctions remain unclear, however. In
2001, Becker, Robert Poreda, a professor of earth and environmental
sciences at the University of Rochester in New York State, and other
scientists published a paper in Science that said they found
soccer-ball-shaped molecules known as buckyballs in sediments dating to
the Permian-Triassic boundary.
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They said the buckyballs contained helium and argon gases that had an
extraterrestrial chemical signature - that is, they had come from a
meteor.
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Other scientists who have studied impact craters remain unconvinced that a
meteor hit Bedout. The "gold standard" of proof, mineral crystals within
the crater fractured and transformed by the shock of impact, is lacking.
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Michael Rampino, a professor of earth and environmental sciences at New
York University, said of the Science paper: "It's another piece of
evidence, but it's still equivocal."
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In the new study, Becker and Poreda examined Bedout core samples taken by
oil companies under about 60 meters, or 200 feet, of water and about three
kilometers, or two miles, of sediments. Becker said that even at first
glance the material looked like the tumultuous jumble that would be found
in an impact crater.
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Poreda said closer examination backed up the first impression. Crystals of
almost pure silica in the rock samples could not have formed from lava,
Poreda said. Crystals of the mineral feldspar show signs of shock.
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"There are things that are never seen in volcanic rock," he said. "This is
as close to a smoking gun as you get."
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