2,900 feet below the bay, piece of geological puzzle

Scientists drill into biggest impact crater in U.S. to study meteor's effects

May 23, 2004|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

CAPE CHARLES, Va. - David Powars takes a magnifying glass from his mud-splattered pants and peers at the 10-million-year-old pebbles that just gushed through a hose to the surface of the earth here.

The tiny gray stones are not particularly striking. But they are part of what covers the largest impact crater in the United States, a formation the size of Rhode Island created 35 million years ago when a massive meteor smashed into the planet. The meteor sent rocks flying as far as the Gulf of Mexico and carved out geological rifts that created the Chesapeake Bay.

What remains of the meteor, which was a mile in diameter, now lies under the bay and Virginia's Eastern Shore. But because it struck what was then ocean, the impact area hasn't seriously eroded or been buried.

"What we have here may be the best-preserved impact crater in the world," said Powars, a geologist with the U.S. Geological Survey.

Powars and other scientists are flushing out some of the rocks and sediment directly over the center of the crater this month in a $250,000 effort to piece together a geological puzzle that has intrigued them since they discovered the site in 1993.

Researchers began drilling down 2,900 feet this month at an industrial park not far from the water's edge, in this picturesque bayside community on Virginia's Eastern Shore. They hope to drill thousands of feet deeper next year in another location.

The meteor struck the water at 134,000 mph, creating a splash 30 miles high as it plunged 1.2 miles into the earth, shattering rocks 35,000 feet down as if they were glass. The fracturing of the earth created fissures and formed geological formations that hamper thirsty communities in the area - which makes studying the crater a practical concern.

Groundwater supplies are a major issue in the lower Chesapeake Bay. The growth of Newport News, Va., and nearby communities has forced officials to spend millions of dollars on desalinization plants because of the high salt content of the water.

Scientists want to understand why the freshwater aquifer goes down just 300 feet and how far they can drill for drinkable water without damaging wells.

"As the demand increases to pump water, the question becomes, is saltwater going to be flowing into these wells and into the pumping centers that are already operating?" said T. Scott Bruce, a groundwater hydrologist with the Virginia Department of Environmental Quality.

Early hypothesis

The saltwater mystery came close to being solved in 1943, when another USGS geologist named D. John Cederstrom described unusual patterns of freshwater and saltwater. He hypothesized that an underground structure was probably responsible.

But for years scientists discounted and criticized his hypothesis, which couldn't be validated without expensive drilling. When the current researchers found in the 1990s that the underground structure existed, as a result of the meteor's impact, Bruce contacted Cederstrom.

Cederstrom, by then living in a nursing home in Roanoke, Va., didn't react as expected.

"To tell you the truth, I don't think he believed me," Bruce said. "There was a real tone of skepticism in his voice."

Although the meteor's explosive impact evidently affected water supplies, it didn't alter the climate. A meteor strike that created a 125-mile-wide crater on Mexico's Yucatan Peninsula is blamed for causing climate changes that killed off the dinosaurs 65 million years ago.

The bay crater, shaped like an inverted sombrero, has a deep central hole about 23 miles in diameter and is surrounded by a larger, shallower depression about 56 miles wide. It's the largest of the 30 impact craters discovered in the United States and the sixth-largest of 170 in the world.

Powars believes that the crater may have been created by one of a shower of comets that rained down 35 million years ago because craters found elsewhere show signs of being about the same age.

"In the local area, this would have had a tremendous impact, flooding and killing off life for several hundred miles around it," said C. Wylie Poag, another USGS geologist working on the drilling project. "But while it wiped out populations, it didn't wipe out species. They came right back."

Drilling progress

Powars, Poag and other USGS scientists take turns supervising the drilling in 12-hour shifts, using a nearby tent as an office for their records and supplies. The USGS truck-mounted drill brought from Colorado uses a diamond-studded tip to create a hole about 8 inches in diameter. Water flushed into the hole keeps the drill bit cool and pumps the rock and debris to the surface for the scientists to examine.

Drilling is proceeding at a rate of about 200 feet every 12 hours and is expected to continue until the end of the month.

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