The Wave of the Future?

A monster sea wave might have whacked North America’s East Coast in prehistoric times. Could it happen again?

You’ve just a heart-stopping ride on the Tidal Wave, the beachfront roller coaster in Ocean City, Md. You’re feeling washed out but elated. Suddenly, a 30-foot wall of water crashes over the boardwalk, toppling the roller coaster like toothpicks and sending people flying. What is this? Another coastal thrill ride?

No, it’s a tsunami (soo-NAH-mee), a monster wave caused by a violent geologic disturbance, such as an earthquake, underwater landslide, volcanic eruption, or even an asteroid impact.┬áTsunamis almost always happen in the Pacific Ocean. But new scientific evidence suggests that a killer wave could one day swamp the Atlantic coast of the United States.

WHAT A CREEP

A team of scientists investigating the seafloor off Virginia and North Carolina has found evidence that the continental shelf might be weakening there. A continental shelf is a gently sloping area of seabed between the edge of a continent and the deep ocean. Last summer, the scientists discovered several large blowout features, or craters, where gas from beneath the seafloor had escaped.

Some of the craters are “quite large,” said team leader Neal Driscoll of Woods Hole Oceanographic Institution in Massachusetts. The four largest craters are, on average, 50 meters (164 feet) deep, 4 kilometers (2.5 miles) long, and 1 kilometer (0.6 miles) wide. A crater that size could easily swallow New York City’s Central Park!

Driscoll’s team is still trying to find out where the gas came from and how it escaped through the seafloor. Team member Jeffrey Weissel, senior scientist at Columbia University’s Lamont-Doherty Earth Observatory, explained that gas is always moving up through the layers of sediment on the ocean bottom. Those layers of sediment usually block gas from reaching the seafloor. However, in several places along the mid-Atlantic coast, the gas somehow penetrated all the layers and blew through the seafloor.

Creeping might have caused that gas to blow. Creeping is the sliding of layered sediments down a continental slope, like a carpet sliding down stairs. As the layers creep down a slope, they stretch and deform, opening avenues for underground gas to escape. “That’s why these [blowout] features are so close to the shelf edge,” Weissel explained.

COASTAL TERRORS

Weissel worries that the gas blowouts could be weakening the edge of the continental shelf. If the shelf were to cave, it could set off an underwater landslide large enough to trigger a tsunami.

A boat cruising directly above such a landslide probably wouldn’t detect the wake of the resulting tsunami amid normal wind waves and swell. That’s because as a tsunami moves from its source to the closest coast, its length can be as much as 100 miles and its height just a few feet. Only when a tsunami nears land does it become monstrous. In shallow water, it bunches up and gains height. Coming ashore, it pummels unsuspecting coastal areas with devastating force.

Tsunamis often catch coast dwellers by surprise. Moving at speeds of up to 965 kilometers (600 miles) per hour, they frequently arrive before people know they’re coming.

ATLANTIC MONSTER

Most of the areas that have felt the punch of a tsunami are located in and along the Pacific Ocean. Surrounding the Pacific is a band of volcanoes, mountain chains, and earthquake zones. This Ring of Fire is the most geologically active area on the planet.

On average, two destructive tsunamis occur every year in the Pacific Ocean. Alaska, Hawaii, California, Oregon, and Washington are the states most vulnerable to the monster waves. However, other states might be vulnerable too. There are indications that a tsunami may have hit the East Coast of the United States at least once–and one might strike again.

Along the southern end of the region where Weissel, Driscoll, and their team found the seafloor dotted with craters, they also uncovered features of a past underwater landslide. That landslide likely happened 20,000 years ago, at the peak of the last Ice Age, and could have spawned a tsunami that walloped the East Coast of the United States.

“A big landslide, the size of the one that occurred [during] the last Ice Age, could affect a large amount of the East Coast from Cape Hatteras, N.C., to Long Island, N.Y.,” Weissel said.

The possibility of such a wave striking again is rare, but real, both Weissel and Driscoll stressed. Their team plans to continue mapping the seafloor and keeping a watchful eye on areas vulnerable to underwater landslides. If a tsunami ever does hit the eastern seaboard, the last place you’ll want to be is on the Tidal Wave coaster.

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