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Scientists discover megatsunami 73,000 years ago. Could it happen again?

Scientists discover megatsunami 73,000 years ago. Could it happen again?

By Story Hinckley for CSMonitor

Waves the size of the Chrysler building may seem like they belong in a movie trailer, but scientists have recently found that megatsunamis are all too real.

Scientists say that 73,000 years ago, a large flank (or slope) from the volcanic island Fogo in the Cape Verde islands off the coast of Africa fell into the ocean and triggered a tsunami that could – quite literally – move mountains.

“You’re displacing a huge mass, which must generate movement of water,” Ricardo Ramalho, the lead researcher behind the study, told The Washington Post. “And in the case of volcanic flank collapses they can be very acute, because you have all the mass collapsing basically into the oceans.”

Scientists knew Fogo experienced a collapse thousands of years ago because a nearby seafloor shows evidence of a huge rock avalanche. They also already expected that this avalanche caused a tsunami 30 miles away on the island of Santiago, but there has been disagreement over just how big it was. Until now, scientists assumed the collapse happened in stages instead of all at once, only triggering several smaller tsunamis. A recent study of the Fogo collapse in 2011 by French scientists suggests that there were multiple waves, registering only 45 feet in height.

So what was Dr. Ramalho’s proof for a megatsunami? Rocks. Really, really big rocks. 

When Ramalho was on Santiago in 2007, he found large boulders on top of a high plateau, near a sheer, vertical cliff. Ramalho and his colleagues were able to trace the boulders’ origin to the cliff below because the rock types “exclusively crop out on the cliff faces and lower slopes of the plateau, implying a source at considerably lower elevations.” As The Christian Science Monitor's Joseph Dussault reports, the scientists found that the boulders were composed of marine rock, whereas the surrounding terrain was made of young volcanic rock.

There seemed to be only one force powerful enough to move these rocks 800 feet above their original location: a megatsunami.

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