Unearthing Clues to a Cataclysm

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, a Washington native and 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. 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 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." 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."