Scientists have found evidence that the sudden collapse of a volcano off the west coast of Africa 73,000 years ago generated an 800-feet tsunami which engulfed an island 55 kilometres away.
The collapse occurred at the Fogo volcano in Cape Verde, one of the world's largest and most active island volcanoes.
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, researchers said.
"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 the Columbia University's Lamont-Doherty Earth Observatory.
"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," said Ramalho.
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 that the new study suggests.
A 2011 French study also looked at the Fogo collapse, suggesting that it took place somewhere between 124,000-65,000 years ago; but that study said it involved more than one landslide.
According to the new study, the estimated 160 cubic kilometres of rock that Fogo lost during the collapse was dropped all at once, resulting in the 800-feet wave.
The new study focused on Santiago, the largest of the Cape Verde Islands off the west coast of Africa. Santiago is home to about 250,000 people and lies about 55 kilometres from Fogo.
Ramalho and his colleagues were conducting research on Santiago when they discovered unusual boulders lying as far as 2,000 feet inland and nearly 650 feet above sea level.
Some boulders were up to roughly 25 feet wide and weighed up to 770 tons and utterly unlike the young volcanic terrain on which they lie.
Rather, they match marine-type rocks that ring the island's shoreline - limestones, conglomerates and submarine basalts.
The only realistic explanation the scientists could come up with was that a gigantic wave must have ripped them from the shoreline and lofted them up.
They derived the size of the wave by calculating the energy it would have taken to accomplish this feat.
Ramalho cautioned that the study should not be taken as a red flag that another big collapse is imminent.
"It doesn't mean every collapse happens catastrophically. But it's maybe not as rare as we thought," he said.
The study was published in the journal Science Advances.
