An ancient landslide on an island volcano is providing a worrisome lesson about tsunamis, thanks to some geologic sleuthing. According to a new study in the Cape Verde archipelago, a landslide triggered a tsunami powerful enough to push massive boulders on a neighboring island onto a high plateau. The scientists warn that although such events are extremely rare, they could also be devastating if they hit a populated coastal area.

The catastrophe took place about 73,000 years ago off the western coast of Africa. Researchers already knew that a wedge of an ancient volcano on Fogo, one of the archipelago’s southern islands, had collapsed and slid into the sea. Telltale scars on the rim of the volcanic caldera, or basin, indicate the edges of the section that collapsed. It was enormous. The 160 cubic kilometers of rock, more or less, that once made up that slope are piled on the sea floor. And on Santiago Island, 55 kilometers away, researchers have found sand layers likely deposited by the one or more tsunamis caused by the collapse. But the size of the tsunami and whether there was just one massive or a succession of minor waves has been unclear.

A team of researchers took another look around Santiago and found numerous boulders anomalously strewn across two plateaus. The 49 boulders they studied ranged from refrigerator-sized to as large as a truck and weighing more than 700 tons. Analyses showed that the boulders must have come from rock exposed on the cliffs and had been shoved up onto the plateaus at about the time the volcano on Fogo collapsed. The international team, led by researchers from Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, conclude that only a massive tsunami could have done that.



Ramalho A tsunami apparently heaved boulders like this on top of an island plateau.

Taking into account the sizes of the boulders, the height of the plateaus, and the local topography, they estimate that a single tsunami resulting from the sudden, total collapse of the volcano’s flank must have run up on Santiago to elevations of more than 270 meters above sea level. Writing online today in Science Advances, the researchers warn that this evidence "stands as a warning that such hazard should not be underestimated."

The claim of a 270-meter run up, which is a measure of the elevation above sea level a wave reaches on shore, is “certainly possible," says James Goff, a paleotsunami expert at University of New South Wales, Sydney, in Australia. He notes that in 1958 an earthquake in Alaska triggered a landslide into Lituya Bay that generated a wave that climbed 525 meters up the walls of the surrounding valley, the highest ever documented.

For comparison, the tsunami that devastated Japan's northeast coast in 2011 had a maximum run up of about 40 meters. But because it was triggered by the jolt of a long fault, waves inundated hundreds of kilometers of Japan’s shoreline. (Across the ocean, a 2.4-meter wave struck America’s Pacific Northwest, damaging docks and harbors. Smaller waves hitting the coast of South America flooded several hundred seaside homes.) Tsunamis triggered by point-source events such as volcanic collapses or submarine landslides are not likely to have as wide an impact because the waves appear to dissipate quickly with distance. Goff cautions that the dynamics of these waves are not well understood.

But he emphatically agrees with the authors of the Science Advances paper that the threat posed by collapsing volcanic slope and submarine landslides has been overlooked in the tsunami hazard field. Waves from such events could be devastating if they were to hit a city. And Goff thinks the threat is highest in the Pacific, where there are thousands of volcanic islands. He hopes that awareness of the threat will lead to preparation for a possible disaster and steps to mitigate the hazard.