Imagine this premise for a Hollywood disaster movie: A giant volcano on an island collapses during an eruption, generating a massive landslide that in turn causes an absolute Godzilla of a tsunami, an 840-foot-tall wave that travels for 30 miles and engulfs anything in its path.
Sound far-fetched? Well, it's already happened at least once and some scientists say it could actually happen again.
Evidence that a 9,300-foot-tall volcano on the island of Fogo, one of the world's largest and most active volcanoes, collapsed during an eruption 73,000 years ago and generated a wave so big that it swept over nearby Santiago Island. The findings, recently published in the journal Science Advances, raise the possibility that Fogo or another volcano could cause a similarly scary event today.
"Our observations therefore further demonstrate that flank collapses may indeed catastrophically happen and are capable of triggering tsunamis of enormous height and energy, adding to their hazard potential," the scientists noted in the article.
The study's lead author, Ricardo Ramalho, is an adjunct scientist at Columbia University's Lamont-Doherty Earth Observatory. He said in a press release that while such massive volcano collapses don't happen very often, "we need to take this into account when we think about the hazard potential of these kinds of volcanic features."
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Ramalho and fellow researchers were working on the island of Santiago several years ago when they spotted a strange phenomenon at 650 feet above sea level - giant boulders, as big as delivery vans, that turned out to match the marine-type rocks that line the island's shoreline.
The only explanation that the scientists could find was that a giant wave must have picked up the 770-ton rocks and swept them uphill. They came up with the size of the wave by calculating the amount of energy it would have taken to move the massive rocks.
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When Ramalho and Lamont-Doherty geochemist Gisela Winckler measured isotopes of the element helium embedded near the boulders' surfaces, they found that the rocks had been lying in the open for about 73,000 years.
That conclusion is bound to provoke some scientific controversy, since not all scientists believe that volcanoes would collapse suddenly and generate huge tsunamis. But others have theorized about much larger prehistoric collapses and resulting megatsunamis - in the Hawaiian islands, at Italy's Mt. Etna, and the Indian Ocean's Reunion Island.
The latest study, however, seems to provide the best evidence that such an event already has occurred. The researchers note that other volcanic islands, including some in the northeastern Atlantic Ocean, are close to heavily-populated areas.
"It doesn't mean every collapse happens catastrophically," Ramalho said. "But it's maybe not as rare as we thought."