|Getting to an elite group’s core
CHERITON — Amid the constant drone of an 80-foot drill rig and the noisy scientists and drillers bustling about day and night, 10 squirming puppies go to sleep inside an adjacent cargo box.
Neither the puppies nor their mother, Gunner, a stray adopted by one of the drillers, pay attention to the discovery unfolding next to them. Once every hour, another core, several feet long and just 1? inches in diameter, is pulled from more than 4,000 feet below ground.
The litter and the rig sit alongside harvested soybean fields on the Eyreville Plantation. The site is just off-center from where geologists say a fiery space rock, probably an asteroid, exploded more than 35 million years ago, carving a crater that quickly filled with tons of water, rubble and debris.
Under a gloriously blue sky in early November, there’s no outward sign of the destructive force that shaped the 53-mile-wide crater now buried below the Chesapeake Bay. The only hint comes from the drill, which uses diamond-encrusted bits to bring to the surface fragments of rock that are millions — sometimes hundreds of millions — years old.
The Chesapeake Bay Impact Crater is the largest in the United States and the seventh-largest in the world. Twice the size of Rhode Island and as deep as the Grand Canyon, it sits some 1,000 feet beneath the lower part of the bay, its surrounding peninsulas and the inner-continental shelf of the Atlantic Ocean.
“It was one of the greatest catastrophes ever to hit Earth,” said Bevan French, a planetary geologist with the Smithsonian Institution who was observing the Eyreville drill on the Eastern Shore that November day.
“Here you’ve got the thing perfectly preserved,” he said, “a tremendous record for the history of the Earth.”
The $1.3 million drill project, teaming the U.S. Geological Survey with the International Continental Scientific Drilling Program, aims to dig 7,200 feet to the bottom of the crater. The drill was about 2,000 feet shy of the goal shortly before Thanksgiving.
About four dozen scientists are working the site through early December. Drillers constantly tend the rig to ensure that their bits don’t fall apart and that the coring goes smoothly. At night, they work under bright lights like a VDOT crew.
The scientists, graduate students and technicians stay much of the time in one of two trailers parked about 30 feet from the rig. Other team members sleep in a rental home nearby until their 12-hour shift begins at midnight. Most of the real science will begin next year when scientists divvy up the cores to take back to their labs for study.
“Most of the time, nothing happens,” said impact expert Christian Koeberl of the University of Vienna, Austria, one of the project’s principal investigators.
When a core rises to the surface, the scientists go to work. Leaping to their feet, they grab their hard hats and race to wash the cores clean of the drill mud. They measure and assemble them quickly into labeled boxes, photograph them and enter a brief description into the computers at a nearby trailer.
“Then we sit around another hour,” said Koeberl, who uses his down time at the site to write several scientific reviews and papers. “We just have to wait.”
When the drilling began in September, it brought up cores of ancient clay from atop the crater. Within a couple of weeks, excited scientists hit the top of the crater at about 1,450 feet, with cores of jumbled rock called breccia. Twenty years ago, this breccia gave two Virginia researchers the first clue to the prehistoric cataclysm.
Around 1,684 feet, the cores pulled up granite containing 612 million-year-old magma that had been blasted by the impact and then cooled slowly into crystals.
At about 3,650 feet deep, the cores contained water twice as salty as normal seawater. That suggests it was ocean water trapped and concentrated by the boiling caused by the intense heat of the impact, said marine ecologist James Murray of the USGS.
In late October, the cores surprised the geologists with long, uninterrupted sections of pink- and black-flecked gray granite. Geologists wondered if they had reached the bottom, meaning the crater was much shallower than they had thought.
Oleg Abramov, a Russian-born graduate student from the University of Arizona, lamented the lack of variety in the cores. “All I’ve seen so far is granite. I could have done that at home.”
After a couple of weeks of tension and some 900 feet of granite, the drill came out of granite and was back into the “resurge,” the rubble of melted sediments and rock as well as sand, said David Powars, one of the crater’s co-discoverers.
“The best guess at present is that [a] 900-foot-thick granite block slid toward the center of the collapsing crater from the crater rim,” said Greg Gohn, the USGS scientist leading the drill project.
Most importantly, the cores taken beginning about 4,600 feet now contain suevite, a rock mix of melted clumps considered a signature of impact craters.
“It’s some of the most beautiful rock I’ve ever seen,” Powars crowed.
Geologists expect the drilling to tell them more about the effect the prehistoric impact had on the seabed and to better estimate the space rock’s speed, size and energy as it slammed into the sea floor.
A computer modeling study to be published in next month’s issue of Geology suggests that the crater was formed by a 2-mile-wide space rock traveling about 40,320 mph. Had it smacked into land rather than water, it would have created a crater just half its size, the researchers say.
The team hopes to wrap up the drill by mid-December and have a “sampling party” early next year in which scientists take back to their home institutions cores or slices of core for their research.
They hope to capture clues about the Earth’s primeval climate, why the impact’s effects were relatively limited and didn’t cause wide extinctions of life forms, and where thirsty Tidewater residents can find drinkable water instead of the salty water they often find when drilling for wells.
They also hope to better calculate what will happen the next time one crosses paths with Earth.
Contact A. J. Hostetler at (804) 649-6355 or [email protected]