Strange craters discovered off U.S. mid-Atlantic coast - Brief Article

Scientists made a puzzling discovery while investigating tsunami-causing mechanisms along the U.S. mid-Atlantic outer continental shelf. The area was thought to be filled with large, en echelon-aligned cracks along a 25-mi section of the OCS from north of Cape Hatteras to southern Virginia. The shelf edge is located in 300- to 600-ft waters between the Norfolk Canyon and the Albemarle-Currituck submarine slide, which occurred 16,000 to 18,000 years ago.

However, a recent Woods Hole Oceanographic Institution cruise gathered more than 1,250 mi of geophysical data using a combination subbottom profiler and sidescan sonar towfish, as well as sediment core samples from several locations taken with a gravity core tool. The data showed that the cracks were actually craters.

Based on preliminary results, the scientists said that the craters are a system of large depressions along the shelf edge which appear to have been excavated by gas erupting through the seafloor. Some of the craters are as large as 4,900 ft across, 165 ft deep and up to 16,400 ft long. It suggests that there is some geological mechanism at work underneath the features, or their trend and shapes might be due to proximity to the shelf edge.

"We don't know the source of the gas," team leader Neal Driscoll said. "But it is clear that gas played an important role in forming these features." Seismic data in the form of bright spots show that gas is pervasive in and around the blowouts. Much more work is needed before definitive statements can be made, but the speculation is that layers of relatively impermeable sediment draped over the edge of the shelf to form a gas trap. Pressure builds until the seafloor explodes, presumably throwing sediment into the water column, where it is carried away by bottom currents.

Jeffrey Weissel of Columbia University's Lamont-Doherty Earth Observatory said, "The apparent size and violence of the gas-release episodes does concern us, as they could pose hazards in and of themselves. Shallow gas blowouts have damaged or destroyed oil drilling rigs in the Gulf of Mexico and the North Sea--something we should keep in mind when future work is conducted here."

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