The Tsunami That Engulfed An Island

A collapsing volcano generated a wave that engulfed an island over 30 miles away

About 73,000 years ago, a volcano collapsed into the sea.

Pico do Fogo is located on Fogo island, part of the Cape Verde archipelago off the north-west coast of Africa. As its rocks smashed into the ocean, the impact created a 240m (800ft) tsunami wave that engulfed Santiago island 34 miles away.

Fogo erupts around every 20 years (Credit: Ricardo Ramalho)

The impact would have been catastrophic for anyone living on Santiago. Fortunately, the Cape Verde islands were uninhabited until the mid-1400s.

Pico do Fogo is still active today, with eruptions roughly every 20 years. Collapses like the one 73,000 years ago are extremely rare, but researchers can look to the past to understand the impact of any potential future volcanic collapses.

Various sized boulders litter the island (Credit: Ricardo Ramalho)

Geologist Ricardo Ramalho of Bristol University in the UK started working on the Cape Verde islands in 2005. He soon noticed a series of strange boulders on Santiago island.

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The more he looked, the more he discovered. Eventually he found more than 40 boulders, some weighing over 700 tonnes.

The Fogo volcano last erupted in late 2014 (Credit: Ricardo Ramalho)

Strangely, these large rocks were quite different from any other volcanic deposits on the island. Ramalho discovered they were made of the same stuff as the rocks on the island's shoreline, which had been generated by tsunamis.

Since these deposits were situated at sea level, it had been assumed that the tsunami that created them was quite small.

Bolders like this were forged by a powerful tsunami (Credit: Ricardo Ramalho)

Ramalho's latest study indicates that the tsunami was a big one.

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The boulders he analysed are 220m above sea level. That could only mean one thing, he says: the tsunami generated by Pico do Fogo's collapse was powerful enough to push rock deposits all that way inland.

"Tsunamis are not waves that splash, they are like floods of immense water that can go uphill," says Ramalho.

Eruptions shape the landscape (Credit: Kim Martineau/Lamont-Doherty Earth Observatory)

By dating the boulders, Ramalho and his colleagues have figured out when the tsunami occurred, a finding that is nowpublished in the journal Science Advances.

These things are real and can potentially happen again

The team was also able to estimate the size of the wave by calculating the energy needed to drag the boulders over 600m inland, though Ramalho admits it is a "crude estimate".

More broadly, the work provides further evidence that large volcanic collapses can trigger mega-tsunamis. It was already clear that collapses could trigger smaller tsunamis, but nobody was sure if bigger waves could be generated.

"Flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis," says Ramalho.

Fogo is one of the world's largest and most active island volcanoes (Credit: Ricardo Ramalho)

"These things are real and can potentially happen again," he says.

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"Because they have a high impact we should be vigilant. We need to start thinking about what we can do to improve resilience to such an event."

Mega-tsunamis like this might happen every 10,000 years, says Bill McGuire from University College London in the UK, who was not involved with the study.

"The scale of such events… and their potentially devastating impact, makes them a clear and serious hazard in ocean basins that host active volcanoes," says McGuire.

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