New York, Oct 5: Scientists working off west Africa in Cape Verde islands have found evidence that the sudden collapse of a volcano tens of thousands of years ago generated a tsunami that dwarfed anything ever seen by humans.
The researchers said an 800-foot wave engulfed an island more than 50 km away.
The study could revive a simmering controversy over whether sudden giant collapses present a realistic hazard today around volcanic islands or even along more distant continental coasts.
"Our point is that flank collapses can happen extremely fast and catastrophically and therefore are capable of triggering giant tsunamis," said lead author Ricardo Ramalho from Columbia University.
"They probably don't happen very often. But we need to take this into account when we think about the hazard potential of these kinds of volcanic features," he said.
The apparent collapse occurred some 73,000 years ago at the Fogo volcano, one of the world's largest and most active island volcanoes.
Nowadays, it towers 2,829 metres (9,300 feet) above sea level, and erupts about every 20 years, most recently last year.
Santiago island, where the wave apparently hit, is now home to some 250,000 people.
There is no dispute that volcanic flanks present a hazard. At least eight smaller collapses have occurred in Alaska, Japan and elsewhere in the last several hundred years, and some have generated deadly tsunamis.
But many scientists doubt whether big volcanoes can collapse with the suddenness the new study suggests.
Rather, they envision landslides coming in gradual stages, generating multiple, smaller tsunamis.
A handful of previous other studies have proposed much larger prehistoric collapses and resulting megatsunamis, in the Hawaiian islands, at Italy's Mount Etna and the Indian Ocean's Reunion island.
But critics have said these examples are too few and the evidence too thin.
The new study adds a new possible example - it says the estimated 160 cubic km of rock that Fogo lost during the collapse was dropped all at once, resulting in the mega wave.
By comparison, the biggest known recent tsunamis, which devastated the Indian Ocean's coasts in 2004 and eastern Japan in 2011, reached only about 100 feet.
And like most other well documented tsunamis, it was generated by movements of undersea earthquake faults and not volcanic collapses.
Tsunami expert Bill McGuire, a professor emeritus at University College London 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".
Ramalho cautions that the study should not be taken as a red flag that another big collapse is imminent here or elsewhere. "It doesn't mean every collapse happens catastrophically," he said.
The study appeared in the journal Science Advances.