Megatsunami: Evidence of 800-Foot Wave Worries Experts

Around 73,000 years ago, the towering predecessor of the Fogo volcano - one of the most active in the world - collapsed. As a result, a unbelievably massive tsunami rippled across the Atlantic Ocean, washing its destructive force over islands that now boast over 250,000 human residents. Experts now wonder if such a disaster is more common than we'd like.

Even today, post-collapse Fogo remains some 2,829 meters (9,300 feet) above sea level, and erupts about every 20 years -- most recently last fall. This would hint that ancient Fogo was massive, and its collapse resulted in crashing waves equally towering.

That, as it turns out, is exactly what scientists working off the west African coast in the Cape Verde Islands have found. Several years ago Ricardo Ramalho, a researcher with Columbia University's Lamont-Doherty Earth Observatory, spotted unusual boulders lying as far as 2,000 feet inland and nearly 650 feet above sea level. They were kind of hard to miss -- big as delivery vans and completely out of place, lying on volcanic ground where limestone and basalt boulders do not belong. (Scroll to read on...)

The controversy partially hangs on the physics of waves. In the case of the Fogo collapse, that 800-foot wave would have had to travel 34 miles (55km) to reach Cape Verde. It has been argued that in the open ocean, waves created by geological collapse quickly lose energy simply because of all the surface area their momentum has to cover.

Bill McGuire, a tsunami experts with the University College London who was not involved in the research, said the study "provides robust evidence of megatsunami formation [and] confirms that when volcanoes collapse, they can do so extremely rapidly."

Based on his own work, McGuire s says that such megatsunamis probably come only once every 10,000 years. "Nonetheless," he added, "the scale of such events, as the Fogo study testifies, and their potentially devastating impact, makes them a clear and serious hazard in ocean basins that host active volcanoes."

Ramalho himself cautions against jumping to conclusions, adding that the Fogo event "doesn't mean every collapse happens catastrophically."

"But," he added, "it's maybe not as rare as we thought."

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