Geologists think that the eastern slope of Fogo volcano crashed into the sea some 65,000 to 124,000 years ago, leaving a giant scar where a new volcano can be seen growing in this satellite image. Image credit: NASA.

A team of geologists from Japan, the United States and Europe has found evidence that the sudden collapse of Fogo volcano – one of the tallest and most active oceanic volcanoes on our planet – approximately 73,000 years ago generated an ocean megatsunami that dwarfed anything ever seen by humans. The wave was up to 885 feet (270 m) high, tall enough to submerge Statue of Liberty and second level observation deck of the Eiffel Tower.

Several years ago, Dr Ricardo Ramalho of the University of Bristol and co-authors were working on Santiago Island, the largest of the Cape Verde archipelago, when they spotted unusual boulders lying as far as 2,000 feet (610 m) inland and nearly 650 feet (200 m) above sea level.

Some are as big as delivery vans, and they are utterly unlike the young volcanic terrain on which they lie. Rather, they match marine-type rocks that ring the island’s shoreline: limestones, conglomerates and submarine basalts. Some weigh up to 770 tons.

These observations imply a catastrophic turbulent surge of water, compatible only with a giant tsunami impact which approached the island from the west, i.e. incoming from Fogo.

The wave generated by the collapse of Fogo volcano apparently swept boulders like this one up from the shoreline into Santiago island’s highlands. Here, a scientist chisels out a sample to establish the date of the tsunami. Image credit: Ricardo Ramalho.

Using cutting-edge rock-dating techniques, the team estimated the tsunami happened 73,000 years ago, an age also consistent with collapse at nearby Fogo volcano.

Since sea level at the time was about 164 feet (50 m) lower than today, the scientists also estimate the tsunami hit Santiago with water levels rising to heights of 885 feet.

Nowadays, Fogo volcano towers 9,300 feet (2,829 m) above sea level, and erupts about every 20 years, most recently between November 2014 and February this year. Santiago Island lies 34 miles (55 km) from Fogo and is now home to some 250,000 people.

“The active volcano we see today grew on top of this collapse’s scar, being almost as tall and steep as the old volcano before the collapse,” said Dr Ramalho, lead author on a study published in the journal Science Advances on October.

“The potential energy for a new collapse therefore exists but what we don’t know if or when this is ever going to happen.”

The study’s findings provides a causal link between the field evidence of marine deposits at Santiago and the steep volcanic collapse at Fogo, indicating this collapse must have happened catastrophically, resulting in the megatsunami.

“We need to be vigilant, these findings stand as a warning that the hazard potential of volcanic island lateral collapses should not be underestimated, and consequently our society needs to do more to improve its resilience to such a threat,” Dr Ramalho said.

“To achieve this, we need to improve our understanding of what trigger volcanic collapses, how they operate and cause the generation of giant tsunamis, and how likely they are to reach distant coastlines. We also need to reinforce our volcano monitoring capabilities.”

Ricardo S. Ramalho et al. 2015. Hazard potential of volcanic flank collapses raised by new megatsunami evidence. Science Advances, vol. 1, no. 9, e1500456; doi: 10.1126/sciadv.1500456

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