Deep Drilling Reveals Ancient Crater
For more than two weeks, scientists drilled around the clock alongside a parking lot across the harbor from Cape Charles, Va. They stopped at 2,700 feet. From the depths came jumbled, mixed bits of crystalline and melted rock that can be dated, as well as marine deposits, brine and other evidence of an ancient comet or asteroid that slammed into once-shallow waters near the Delmarva Peninsula.
Cape Charles is considered ground zero for the resulting 56-mile-wide depression below what's now the Chesapeake Bay. The drilling project marks the first time the geologists explored the inner portion of the inverted-sombrero-shaped crater.
“We expected to see some pretty strange rocks because of the extreme pressure and temperatures that occurred'' approximately 35 million years ago, says geologist Greg Gohn, who led the $180,000 project for the U.S. Geological Survey (USGS).
Over the past decade, USGS and Virginia scientists have investigated indications that a 2-mile-wide brilliant ball traveling tens of thousands of miles per hour crashed off the Virginia coast, burrowing thousands of feet and depressing and fracturing the bedrock.
Billions of tons of ocean water vaporized. Millions of tons of debris spewed 30 miles high before collapsing back into the excavation. A train of giant waves inundated the land. The waves then dragged debris as they washed back into the crater, preserving it beneath a blanket of rock and sediment.
It probably took just a few minutes to create the largest crater in the United States and sixth-largest known on the planet, according to computer simulations.
The catastrophe squeezed fresh water from many of the aquifers of southeastern Virginia and filled others with briny water. Its legacy is well-known to residents who try to drill for drinkable ground water and encounter the saltwater “wedge,'' pockets of brine nestled in an arc from the lower Eastern Shore to the Hampton Roads-Newport News area.
Geological research off the coast of New Jersey and in Virginia, begun in 1983, led to the crater's discovery a decade later. Drilling and further study of seismic data narrowed the location in the Chesapeake Bay.
“We're getting evidence about how hot this thing (was) and what was the energy,'' says USGS hydrologist David Powars, one of those credited with the crater's discovery.
More clues to the space rock's identity will come from cores taken in the drill's final 280 feet.
A $1.2 million proposal to drill 7,000 feet not far from Cape Charles is before the International Continental Scientific Drilling Program, which then would assist the USGS with funding.