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Location Narrowed For Permian/Triassic Impact

has a couple pics of rock sections


SN FRANCISCO, Dec. 12  Scientists who support a controversial theory that
a meteor crash coincided with the largest mass extinction in earth's
history now assert that they have narrowed the location of the impact
somewhere on land in the tropics.

The extinction occurred 250 million years ago at the boundary between the
Permian and Triassic geological periods, and killed 90 percent of the
living species. 
Evidence at the Permian-Triassic extinction is sketchier, but most
scientists theorize that the most likely cause was a series of giant
volcanic eruptions in Siberia that spewed 600,000 cubic miles of lava and
might have induced catastrophic ecological changes.

Last month, a team of scientists headed by Dr. Asish R. Basu, a professor
of earth and environmental sciences at the University of Rochester,
reported in the journal Science that it had found shards of the meteorite
in rocks from Antarctica. Other researchers agreed that the fragments
looked extraterrestrial, but questioned whether they were as old as

On Friday, at a meeting of the American Geophysical Union here, the same
group of scientists reported finding tiny glass spheres about
one-thousandth of an inch in diameter in the same layer of rock in

The spheres, the scientists said, are pieces of the earth's crust melted
by the meteor impact and then cooled into spheres.

Such spheres can also form in volcanic eruptions. But the composition of
glass does not resemble lava. Instead, the spheres contain a mix of
elements typical of old continental soil in the tropics, low in sodium,
calcium and magnesium, which are washed away by heavy rains, and
relatively high in metals like titanium, aluminum, silicon and iron, which
are left behind.

"This seems to really convince people," said Dr. Stein B. Jacobsen, a
professor of geochemistry at Harvard and a member of the research team.
"There are no volcanic rocks of that composition."

No crater corresponding to the extinction has been located. But, Dr.
Jacobsen said, "this gives us a clue where we should look."

If scientists become convinced of the meteor evidence, that would reopen
the debate on whether meteors can not only cause extinctions, but also set
off huge volcanic eruptions thousands of miles away, because the Siberian
eruptions coincide exactly with the Permian-Triassic extinction. The
dinosaur extinction 65 million years ago coincided with similarly
extensive eruptions in India. Each type of event is rare, and two
coincidences strike some scientists as highly unlikely.