NSF PR 00-38 - May 30, 2000
Cheryl Dybas, National
Shelley M. Lauzon, Woods Hole
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Seafloor Off Mid-Atlantic Coast Highly Charged with
Vigorous gas expulsion could weaken
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.