Scientists: Ancient mega-tsunami hurled boulders nearly as high as the Eiffel Tower

A tsunami generated by a volcanic flank collapse is believed to have swept huge boulders like this one into the highlands of Santiago, a Cape Verde island. Photo / Washington Post

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 in a 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 90m in height that traveled about 50km 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 180m-high cliff, ultimately attaining water levels nearly 275m 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, of 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. 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 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.

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 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 tonnes - had been sitting exposed on the plateau.

The technique found that the dating corresponded with the time when Fogo's collapse occurred.

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.