[meteorite-list] Barrage of Meteors May Have Doomed the Dinosaurs

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:47 2004
Message-ID: <200303111539.HAA11607_at_zagami.jpl.nasa.gov>

http://www.nytimes.com/2003/03/11/science/space/11EXTI.html

Barrage of Meteors May Have Doomed the Dinosaurs
By KENNETH CHANG
New York Times
March 11, 2003

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.
Received on Tue 11 Mar 2003 10:39:10 AM PST