880-foot Megatsunami Slammed an Island off West Africa 73,000 Years Ago

Around 73,000 years ago, a massive landslide in West Africa’s Cape Verde islands triggered a giant tsunami. The wave formed off the coast of the volcanic island of Fogo and traveled 34 miles to the nearby island of Santiago, where it slammed into the land, reached an elevation of about 880 feet or more, and crested over a tall cliff, hurtling boulders from the shore’s coastline onto its summit.

Scientists recently published evidence of the megatsunami’s existence in the journal Science Advances. They believe the giant wave was caused by a sudden collapse in Fogo’s volcano flank, which sent miles of rock crashing into the ocean. (Tsunamis, which arise from the sudden displacement of large water masses, are more typically formed when underwater earthquakes, landslides, or volcanic eruptions suddenly displace water.)

The research was led by geologist Ricardo Ramalho, a professor who currently teaches at the University of Bristol in the UK. In 2007, Ramalho, who was then with Columbia University’s Lamont-Doherty Earth Observatory, visited Santiago, which today serves as home to some 250,000 people. While on the island, he noticed that massive boulders stood at the top of a high plateau surrounded by cliffs. How did they get there, he wondered?

A giant wave could have carried the boulders, he thought. Meanwhile, the nearby Fogo volcano—which erupts about every 20 years and is known as one of the world's largest and most active island volcanoes—showed evidence of a partial collapse. This was just the type of disaster that could have caused a megatsunami, Ramalho thought.

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The very suggestion of a megatsunami can be contentious among geologists. Researchers argued over which factors might cause the massive waves. Meanwhile, Fogo’s collapse might have happened gradually, instead of all at once. In this instance, smaller tsunamis might have swelled in the surrounding ocean—but not a giant one.

Nevertheless, Ramalho and his colleagues examined the boulders, and found that they matched the rock limestone types that crop out of the island’s cliff and the plateau’s bottom slope. They also dated the rocks back to Fogo's collapse. Their consensus? A giant wave must have arrived from Fogo, dislodging the rocks from the cliff’s edge and face before washing them uphill and inland. Using computer models, the scientists were also able to estimate the wave's staggering size, which was 10 to 15 times larger than the earthquake-triggered tsunami that devastated Japan in 2011, Ramalho says in the video below. 

The ancient disaster is fascinating to imagine. But does it carry implications for modern-day life? Ramalho thinks so. "These finds provide another line of evidence that volcanic flank collapses really can happen suddenly and catastrophically and trigger giant tsunamis,” he told Live Science. “We should not underestimate the hazard potential of these events—that is, the threat they pose to our society.”

And they're not unheard of in modern times: In 1958, a 7.7 magnitude earthquake caused 40 million cubic yards of rock to fall into Alaska's remote Lituya Bay, which in turn generated a wave that surged 1720 feet up the mountainside—the effects of which can still be seen today.

Ramalho discusses his research in the video below.

RISING ISLANDS, MONSTER WAVE from Earth Institute on Vimeo

[h/t Washington Post, Christian Science Monitor]