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NYTimes.com Article: Barrage of Meteors May Have Doomed the Dinosaurs



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Barrage of Meteors May Have Doomed the Dinosaurs

March 11, 2003
By KENNETH CHANG 




 

Scientists are arguing again over the idea that the
combination of cataclysms that doomed the dinosaurs 65
million years ago - titanic volcanic eruptions in India and
a meteor impact off the coast of Mexico - may not have been
a coincidence after all. 

For decades, some geologists have theorized that the force
of an extraterrestrial rock crashing into Earth could have
cracked its crust thousands of miles away and allowed
molten lava to spill out from the interior. But no one has
yet found any solid evidence. 

Now, though, researchers at University College London are
suggesting that the Indian lava flows are the impact site
of an earlier, larger meteor, and that evidence of the
impact was submerged by upwelling lava. In this view, the
mass extinction of dinosaurs and other creatures was caused
not by a single meteor, but by a barrage of them. 

The new work is provoking another burst of theories and
debate over the demise of the dinosaurs, which has never
been explained to everyone's agreement. 

The new theory, which the researchers described in a
scientific journal recently, holds that a meteor at least
12 miles wide - at least twice as wide as the one that
struck Mexico - would melt some rock, but not nearly the
amount seen in the lava flows, known as the Deccan Traps,
which cover hundreds of thousands of square miles of what
is now India. 

Rather, the researchers said, the impact would cause
"decompression melting" of already hot rocks deep within
the earth. Tens of miles below the surface, temperatures
reach more than 2,000 degrees Fahrenheit but rocks remain
solid because of the high pressure exerted by the rocks
weighing down above them. 

Computer simulations indicate that once the meteor impact
blew away the overlying rocks, the ones below, relieved of
pressure, could then have turned to lava. 

"The whole story is what happens underneath the crater,"
said Dr. Adrian P. Jones, a geologist at University College
London and lead author of an article that appeared in Earth
and Planetary Science Letters last year. 

"It's rather like having a hot-air balloon and a pin.
People have calculated the energy of the pin very
accurately, but they've forgotten the balloon is going
bang." 

This sequence may have played out several times in Earth's
history. Notably, the largest of all mass extinctions 250
million years ago, at the Permian geological period and the
beginning of the Triassic, coincided with the creation of
lava flows known as the Siberian Traps, the largest of all
of the volcanic eruptions. 

There are also intriguing but ambiguous hints of a meteor
impact at the Permian-Triassic boundary. Two years ago, a
group of scientists reported finding buckyballs - durable,
soccer-ball-shaped carbon molecules - that contained helium
and argon gases with un-Earthlike chemical signatures. 

The scientists said the buckyballs were molecular remnants
of the meteor, but other researchers have been unable to
verify the claim. Scientists have also found slightly
elevated levels of iridium - an element common in meteors -
in sediment layers dating to the Permian-Triassic boundary.


While the evidence for a connection any single event is
sparse, Dr. Dallas H. Abbott of Columbia University's
Lamont-Doherty Earth Observatory and Dr. Ann E. Isley of
the State University of New York at Oswego say a compelling
picture emerges when looked at over a longer view. They
compiled evidence of meteor impacts and massive volcanic
eruptions over most of Earth's history, dating back four
billion years. 

Dr. Abbott and Dr. Isley, writing in Earth and Planetary
Science Letters, report that their statistical analysis
shows, with 97 percent confidence, that 9 of 10 periods of
heavy meteor bombardment corresponded to periods of massive
volcanism. 


Skeptics like Dr. H. Jay Melosh, a professor of planetary
sciences at the University of Arizona, are utterly
unconvinced. "I know it's a fun idea," he said. "I think
that's why so many people have been advocating it. It makes
a good discussion after beer. But if you start looking at
the details and the real evidence for this, it really falls
apart." 

The dates of the ancient meteor impacts and eruptions in
Dr. Abbott's and Dr. Isley's analysis can only be roughly
estimated, within tens of millions of years, and the
results depend on how the statistical analysis is
performed. "Some people get correlations, and some people
don't," Dr. Melosh said. 

Dr. Melosh also said that decompression melting cannot
explain the Deccan Traps. While the meteor will punch deep
into the Earth, the Earth will almost immediately rebound.
"There's a certain amount of willful misunderstanding
here," he said. 

Further, he said, there is no evidence anywhere on Earth
that meteor impact has ever caused a volcanic eruption. And
scientists still do not have a convincing model of how an
impact could set off an eruption. 

But Dr. Jones of University College London said there was
evidence that decompression melting was a viable
explanation. He cited Iceland, where, he said, the melting
of glaciers has relieved enough pressure to accelerate
eruptions there. 

That is the latest in a multitude of theories that have
tried to connect meteors and volcanoes. 

An idea that caught scientists' fancy a decade ago was that
a meteor would not cause volcanism at the impact site, but
rather seismic waves from the impact would pass through the
Earth and then focus on the spot opposite the impact - the
antipode - rupturing the crust there. 

That would not work as a tidy explanation for the Mexican
impact and the Deccan Trap eruptions. While India is on the
opposite side of the world from Mexico today, it was in a
different position 65 million years ago, when the meteor
struck. Also, eruptions began at the Deccan traps a couple
of million of years before the meteor impact in Mexico. 

Dr. Jonathan T. Hagstrum, a geophysicist with the United
States Geological Survey who was among the first to propose
the idea of antipodal eruptions, said he believed that a
meteor impact in the eastern Pacific Ocean caused the
Deccan Traps eruptions, but that the evidence for it
vanished as tectonic forces pushed that part of the sea
floor back into the Earth's interior. 

But Dr. Melosh said that regardless of where an impact took
place, the mathematics do not work. Only about one
ten-thousandth of the kinetic energy of an impact is
transferred into seismic waves, and the temperature rise at
the other side of the Earth would be about one
five-hundredth of a degree, he said. 

Dr. Mark B. Boslough of Sandia National Laboratory said the
idea was still worth investigating. He said his computer
simulations, run in the mid-1990's, predict that even with
only one ten-thousandth of the kinetic energy transferred
into seismic waves, the impact would still generate about
six cubic miles of melted rock in the upper mantle at the
antipode, although the melt would be dispersed through a
much larger volume. 

If the impact instead transferred 5 percent of its energy
into seismic waves, 3,000 cubic miles of melt would be
produced. If that melt occurred beneath a weak portion of
the crust, that could perhaps still cause the volcanism. "I
look at this as a possible trigger," he said. "It's
worthwhile revisiting." 

Dr. Richard A. Muller, a professor of physics at the
University of California at Berkeley, has proposed an even
more novel mechanism of how a meteor could set off
volcanoes: avalanches deep inside the planet. "Everything
else I've seen has struck me as being wrong," he said. 

The Earth's inner core is solid, mostly iron, and is
growing in size. As iron in the outer core hardens, pockets
of lighter elements like sulfur and silicon remain in the
liquid outer core and start floating upward. As the
droplets rise, temperatures drop by more than 1,000
degrees, and the droplets condense into flakes "falling
like snow" that accumulate in piles at the boundary between
the outer core and the lower mantle, Dr. Muller said. "Or
if you turn it around, rising like foam," he said. 

The shock of a meteor impact could cause these piles of
flakes to collapse, exposing part of the mantle to the hot
outer core. That hot spot, in turn, could cause a stream of
magma to rise through the mantle to the surface, where it
erupts. 

Dr. Muller said the idea, published last year in
Geophysical Research Letters, was fanciful and added that
he was offering it more as an alternative theory to explain
why the Earth's magnetic field periodically flips. (The
avalanches would also disrupt the convection currents in
the core.) 

It may still turn out that the dinosaurs were merely very
unlucky.

http://www.nytimes.com/2003/03/11/science/space/11EXTI.html?ex=1048401619&ei=1&en=8d78a68462bdb3a8



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