We live on a dynamic, sometimes violent planet. It’s just that we also live on it for a relatively short period of geologic time, and so we miss most of the action.

Scientists drove the point home today in a new study published in Science Advances, which suggests the occurrence of a stupendously powerful megatsunami in the Cape Verde Islands off the coast of Africa some 73,000 years ago. Around that time, they believe, a large flank of the volcanic island of Fogo collapsed into the ocean, unleashing a gigantic wave more than 300 feet in height that traveled about 30 miles to the island of Santiago — where it would have done simply incredible things.

When the wave hit, the theory goes, it was so powerful that it surged all the way over the top of a more than 600 foot high cliff, ultimately attaining water levels nearly 900 feet above sea level — nearly as high as the Eiffel Tower. It also scoured large boulders from below — or perhaps directly tore them from the rock itself — and carried them to the top of the plateau where modern scientists would later identify them.

“You’re displacing a huge mass, which must generate movement of water,” says Ricardo Ramalho, the lead researcher behind the study who conducted the work while at the Lamont-Doherty Earth Observatory at Columbia University, and is currently based at the University of Bristol in the UK. “And in the case of volcanic flank collapses they can be very acute, because you have all this mass collapsing basically into the oceans.” Ramalho published the work along with a team of researchers from Columbia University as well as several universities in Portugal and Japan.

The new study originates with a simple mystery — Ramalho was on Santiago in 2007 and saw large boulders on top of the high plateau, which ends in a steep cliff face. “I was puzzled by their origin. I didn’t know what they meant,” he says.

But a few years later, other researchers published evidence suggesting a tsunami had hit Santiago long ago. They only documented tsunami impacts at low elevations, however, not atop the high plateau. This inspired Ramalho and colleagues to take a closer look at the boulders and other associated geological evidence at much higher elevations.

Adding to the picture was strong evidence that Fogo, a nearby island that consists of a large and still active volcano rising four miles from the seafloor, had undergone a partial collapse — the seafloor nearby shows evidence of a huge rock avalanche. That’s precisely the kind of event scientists had long thought could create a megatsunami.

Yet the entire concept of megatsunamis has been “highly controversial,” in the researchers’ words, featuring big scientific debates and lots of ambiguous evidence. And it was certainly possible that Fogo’s collapse had happened in stages rather than all at once — in which case it might have created several smaller tsunamis, rather than one gigantic one. This has long been one of the arguments against megatsunamis – that volcanic island flanks might collapse, but surely not all at once.

But after examining the boulders and other associated geologic evidence at high elevations on Santiago — an area that is across the sea from where Fogo’s collapse would have occurred — Ramalho and his colleagues now assert that they must come from far below, up the side of a sheer vertical cliff. And they say only a megatsunami could do that.

The evidence hinges on the nature of the boulders, which are composed of rock types that “exclusively crop out on the cliff faces and lower slopes of the plateau, implying a source at considerably lower elevations,” the researchers write.

The scientists also used cosmogenic techniques, based on how cosmic rays that bombard the Earth create unique isotopes on rock surfaces, to date how long the large rocks — maxing out at 700 tons — had been sitting exposed on the plateau. The technique found that the dating corresponded with the time when Fogo’s collapse occurred.

The researchers therefore concluded that the rocks were “quarried from the cliff edge and face” by the gigantic wave and then “transported uphill and inland onto the surface of the plateau.”

Here’s a diagram, by Ramalho, that documents what the researchers think happened, and its incredible scale:

“You can only explain the existence of those deposits from the impact of a giant tsunami approaching from the western side of the island, and of course, that’s where Fogo is,” says Ramalho.

Flank collapses of this type, says Ramalho, are mainly possible with volcanic islands, because these kinds of islands thrust upward so dramatically from the seafloor.  “They are some of the tallest features on Earth,” he says. “The big island of Hawaii, if you take into account from the base of the seafloor up to the summit, it’s even higher than Mount Everest.”

Indeed, there is also published research suggesting that a megatsunami happened in the Hawaiian islands, over 100,000 years ago. And there is the longstanding suggestion that a collapse of the Cumbre Vieja volcano on the Canary Islands’ LaPalma island could create a tsunami that travels across the entire Atlantic and impacts the United States.

The researchers say they don’t want to scare people, but they do think that certain volcanic islands are theoretically capable of producing similar events. There should be more study of volcanic islands and their potential for flank collapse so as to “realistically assess the full hazard potential of such low-probability but high-impact events,” the study concludes.

“I’m not saying that this is going to happen on Fogo or elsewhere, tomorrow,” says Ramalho. “I’m just saying, this happened in the past, so we need to be vigilant.”

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