[meteorite-list] Chesapeake Bay Crater Yields Clues to a Prehistoric Cataclysm

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Nov 4 14:35:38 2005
Message-ID: <200511041930.jA4JUri26870_at_zagami.jpl.nasa.gov>

http://seattletimes.nwsource.com/html/nationworld/2002603058_crater04.html

Crater yields clues to a prehistoric cataclysm
By Michael E. Ruane
The Washington Post
November 4, 2005

EASTVILLE, Va. - A white fireball 2 miles across thunders from the sky
at 30,000 mph and crashes into the ocean off the Virginia coast. The
impact vaporizes billions of tons of water, rips a hole in the sea floor
6 miles deep and fractures the bedrock far into the Earth.

The splash is 30 miles high. Debris is lofted over the horizon and rains
down on an area of 3 million square miles, as distant as the Antarctic.
Towering tsunamis surge toward the Blue Ridge Mountains.

Nearby life - ferocious-looking sea creatures and dog-sized proto-horses
along the tropical shoreline - is blasted and then swept into the abyss
by the boiling ocean. A calamity of unimaginable scale, it is probably
the most stupendous geological event ever on the East Coast.

For more than a decade, geologists have believed that a gigantic object,
an asteroid or a comet, struck the Earth north of Norfolk about 35
million years ago in a cataclysmic occurrence that left behind a
53-mile-wide, long-buried crater.

An international team of scientists, seeking clues to the origins of the
planets, has assembled in a windblown bean field near the crater's
center to try to determine, among other things, exactly what happened
when the object struck.

Since early last month, the team has been working with a large drilling
rig that uses diamond-tipped bits and brings up core samples while
boring through eons of sediment toward the floor of the crater and the
place where the impactor hit, believed to be about 7,000 feet below the
surface.

It's hard to imagine the scale of what geologists believe happened there.

David Powars, a U.S. Geological Survey geologist, said he first
suspected the presence of an impact crater in the 1980s. "You could take
the whole nuclear arsenal in its heyday: Russia, China, U.S. ... That's
a firecracker compared to what this explosion would be."

In the course of a five-year project, the USGS has drilled six holes
probing the crater's landscape. This hole will be the program's deepest,
and the last, officials say.

Since the formal announcement in 1995 of what is now called the
Chesapeake Bay Impact Crater, studies have detailed its dimensions and
outline, experts say. Last year scientists for the first time found rock
that had been melted by the impact and fossils of microorganisms that
had been smashed in the event.

There are scores of known impact sites around the world and millions
more on planets and moons across the solar system. The one near Norfolk
is Earth's seventh-largest site and the biggest in the United States.

On Earth, such impacts can dramatically alter the landscape in seconds,
geologists say. And some scientists believe that understanding the
moment of impact might be a key to understanding the formation of the
solar system.

"If you think about how the Earth was formed," said geologist Henning
Dypvik of the University of Oslo, "The Earth was formed by a meteorite
that came from here, an asteroid that came from there and [a] comet that
came from here."

He moved his hands as if making a snowball. "This is the base process
for the formation of the Earth and the universe," he said. "By studying
[impacts], by understanding the mechanisms, then we can know much more
about the Earth and the formation of the planetary system."

And then there is the question: What if such an object struck today?
Even one a fraction of the size of the Chesapeake's would cause a
disaster, said Powars, one of the people who discovered the crater. An
impact by something a half-mile in size, and "the East Coast is in
trouble," he said. "Lights out."

Impact science is fairly young, the geologists said. The study of Earth
impacts by "rocks ... from heaven," as Dypvik put it, was long
considered crazy. The Earth's visible craters were thought to be
remnants of volcanoes, he said.

Gradually, the scientific community realized that the Earth, like other
planets, had been peppered over billions of years by renegade objects
streaking through space. There are now more than 170 impact "structures"
identified around the globe, more than 50 in North America.

The Earth's biggest, 186 miles across, is at Vredefort, South Africa.

The third-largest, the 100-mile-wide Chicxulub crater on Mexico's
Yucatan Peninsula, is believed to be the result of an impact 65 million
years ago that blew so much debris into the atmosphere that it darkened
the Earth for months and led to the extinction of the dinosaurs.

Geologists don't believe that kind of thing happened after the
Chesapeake impact. It "would have killed off the local population" for
hundreds of miles up and down the coast, said Jean Self-Trail, a
Geological Survey micropaleontologist. "But we don't really have any
evidence that there was a massive die-off."

Small impacts happen almost all the time on Earth, said Jens Ormo, a
Swedish crater expert working at Chesapeake site for the Spanish space
agency. The big, so-called hypervelocity impacts are quite rare. He said
one of the most recent occurred about 50,000 years ago and formed
Arizona's Barringer crater.

The Chesapeake crater is the result of what geologists say was a marine
impact. The object struck in several hundred feet of water far off the
coastline of the time.

"It basically vaporized billions of tons of seawater," Powars said.
"Billions of tons! And that's not exaggerating." There was a momentary
hole in the water down to the sea bottom. "Then you had the water coming
back in on this hot mixture of stuff, basically melted rock and
sediments that fell back in ... (creating) incredible steam explosions."

Another big marine impact crater in the Barents Sea off the northern
coast of Norway is named Mjolnir, for the legendary hammer of the
Scandinavian god Thor. There, geologists believe, the impact temporarily
ignited sediments on the bottom of the ocean.

The Chesapeake crater, which geologists describe as shaped like an
upside-down, broad-brimmed hat, is centered near Cape Charles and
extends north along the Delmarva Peninsula to about Wachapreague, Va. It
goes west across the bay into Gloucester County, south to Norfolk and
east into the ocean.

Powars said he first suspected the crater after routine geological
drilling found disorganized sediments underground. It was revealed after
oil explorations showed the outline of a crater. The impact object,
probably an asteroid, disintegrated, the geologists said, but traces
might be found if the drilling reaches deep enough.

The crater has been extremely well preserved because it is buried under
land and sea sediment, the geologists said. But it is accessible via
land and could become among the most-studied in the world.

"I often say how much I would love to see (an impact like) this happen,"
Powars said, "as long as I was on something that keeps me alive."
Received on Fri 04 Nov 2005 02:30:52 PM PST