Vigorous gas expulsion could weaken shelf edge
A team of scientists investigating whether possible cracks along the outer continental shelf off the mid-Atlantic coast might lead to a tsunami-causing landslide has discovered that the entire area is charged with gas.
Based on preliminary results from a just-completed two-week cruise to the area funded by the National Science Foundation (NSF), the scientists say the suspected cracks are a system of large depressions along the shelf edge that appear to have been excavated by gas erupting through the seafloor.
"We don't know the source of the gas," team leader Neal Driscoll of the Woods Hole Oceanographic Institution (WHOI) said. "But it is clear that gas has played an important role in the formation of these features. The gas is trapped under layers of sediment on the shelf edge until some circumstance causes it to escape, blowing holes in the seafloor to form these large pockmark features we thought were cracks."
In a paper published in the journal Geology days before their May 7 departure for the Mid-Atlantic coast, Driscoll and colleagues Jeffrey Weissel of the Lamont-Doherty Earth Observatory of Columbia University and John Goff of the University of Texas Institute for Geophysics, speculated that rising gas might play a part in triggering shelf-edge collapse. Even so, they were surprised at the quantity of gas and the apparent vigor of the "blowout" process.
"Our seismic data show that gas is pervasive in and around the blow-outs," Goff said. "Gas has a characteristic signal, which commonly shows up as a bright, high-amplitude reflection that obscures any deeper signals."
"These are exciting findings," said Mike Purdy, director of NSF's division of ocean sciences. "We now know that biological, chemical and physical processes, like those that created the gas reservoirs and gas expulsions described by these investigators, are going on every day in the depths of the sea. Driscoll, Goff and Weissel have reaffirmed that exciting and significant discoveries are waiting to be made on the ocean floor. The importance of exploring our own planet is again made very clear."
"We were a bit taken aback, to be sure," Weissel said. "A great deal more work is needed before any definitive statements can be made. Regardless of the source, it is apparent that gas charging plays a critical role in the area, and that the ongoing process of blowouts could conceivably weaken the shelf edge and contribute to a submarine landslide/tsunami scenario."
The scientists say the features, some as large as 2,000 meters (6,500 feet) across, 50 meters (165 feet deep) and up to 5,000 meters (16,400 feet) long, occur in a line along the shelf edge. That suggests there is some geological mechanism at work underneath the features, or their trend and shapes might be due to their proximity to the shelf edge. A submarine landslide and resulting tsunami along the shelf, although a very low risk when compared to the chance of hurricanes and severe storms striking the area, remains possible.
The May 7-20 research cruise was funded under a grant from NSF’s Marine Geology & Geophysics program and was conducted aboard the Research Vessel Cape Hatteras, operated out of Beaufort, North Carolina, by Duke University and the University of North Carolina Consortium. The NSF grant will also cover the next year of data synthesis and analysis, and the eventual publication of the detailed results.
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