Geologists delve into giant crater off Virginia
- From: alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (Alan)
- Date: Mon, 7 Nov 2005 18:28 +0000 (GMT Standard Time)
http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2005/11/07/MNGTOFK2H01.DTL
Geologists delve into giant crater off Virginia
Huge object blasted 6 miles into seabed 35 million years ago
Michael E. Ruane, Washington Post
Monday, November 7, 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 seafloor 6 miles deep
and fractures the bedrock far into the Earth.
The splash is 30 miles high. Debris are lofted over the horizon and rain 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-size 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, Va., 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 September, the team has been working with a large drilling rig that uses
diamond-tipped bits and brings up core samples to bore 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.
As a farmer harvested his soybean crop just north of Cape Charles on Virginia's
Eastern Shore and the wind off the Chesapeake Bay blew dust and grasshoppers
across the drilling site, it was hard to imagine the scale of what geologists
believe happened there.
"This is so big that we can't really picture it," said David Powars, a U.S.
Geological Survey geologist, who 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."
The men and women of the small but intense crater community who gather at the
spot attempt to picture it every day. "I dream this all the time," Powars said.
"People say, 'Did you sleep?' I say, 'I worked all night dreaming it.' I try,
but I'll be honest: I can't imagine the event."
Their work is the culmination of a five-year project in which 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, "the soul ... the spirit" of the collision, as one said, might be a key
to understanding the formation of the solar system.
"If you think about how the Earth was formed," geologist Henning Dypvik of the
University of Oslo said Wednesday at the drilling site, "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 a
mile in size, and "the East Coast is in trouble," he said. "Lights out."
Impact science is fairly young, the geologists said. As recently as 20 years
ago, the study of Earth impacts by "rocks ... from heaven," as Dypvik put it,
was 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 Meteorite 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, which
was west of Richmond during the period of high global sea levels.
"It basically vaporized billions of tons of seawater," Powars said. "Billions of
tons! And that's not exaggerating."
Alan
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