Need another worry? Gas bubbles in seabed

Phenomenon could sink ships, scientists say

June 02, 2000|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Did we say tidal waves? Sorry, it's giant gas bubbles from the deep that should be troubling your dreams.

It turns out those "cracks" in the sea floor off the Virginia coast - the ones that geologists said last month could trigger an undersea landslide and send 20-foot tidal waves crashing ashore - aren't really cracks after all.

The same scientists now say the "cracks" are actually huge, elongated craters formed by natural gas "blowouts" - giant, flammable bubbles from the seabed that can stir the ocean and swallow ships.

"If you were in a boat, you would see roiling brown water, and it [the boat] may actually sink because of the bubbles. Ships would lose their buoyancy," and fall into the bubbles, said Jeffrey K. Weissel, senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University.

Oil rigs have collapsed and burned in such blowouts, he said. And in the 1960s, a Japanese research vessel was exploring a submarine volcano in the Pacific when "there was a gas eruption, and they lost everybody."

And if the bubbles don't sink you, the gas might snuff you out. "You have to extinguish all smoking materials," Weissel joked.

While there is a risk of tidal waves from a gas blowout, too, Weissel said, the amount of material dislodged in these blowouts is just 1 percent of that moved in a typical landslide on the continental shelf. "That wouldn't produce anything that would threaten the coastline."

The blowouts were so close to the shelf edge that they could have jostled the sea floor into a more precarious position, making it more vulnerable to a tumble into the abyss, and a big tidal wave.

All this fretting about the bottom of the Atlantic began May 7. That's when a paper by Weissel; Neal Driscoll, of the Woods Hole Oceanographic Institution; and John Goff, of the University of Texas Institute for Geophysics; appeared in the journal Geology, describing what appeared to be cracks in the sea floor 80 miles off Virginia.

Such cracks, they said, could indicate a dangerous instability on the continental shelf. The likelihood of a tsunami was "slight," however, and the potential damage no worse than a big hurricane storm surge.

But during a two-week return cruise that ended May 20, the team took a closer look. "What we saw was that the entire outer part of continental shelf is charged with natural gas," Weissel said.

The "cracks" were round-bottomed depressions -- three miles long, less than a mile wide and 165 feet deep -- formed by the violent eruption of gas, sediment and water from under the seabed into the water above.

"You could fit Central Park into each one of these things," he said.

The natural gas, formed from organic material buried in the ocean sediments, normally seeps out harmlessly. But in that region, the older ocean sediments are capped by a dense layer of river delta sediments deposited during the last ice age, 20,000 years ago.

"So now it [the gas] is trapped beneath a lid," Weissel said. "The pressure builds as more gas accumulates. Then something happens - an earthquake or maybe a shallow landslide. That allows these sediments to fracture, and you get a massive expulsion of the gas below them, forming these blowout craters."

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