A team of scientists found fields of boulders, some as big as camper vans, strewn up to 650 meters inland on Santiago Island. Marine sediments were also found all over Santiago’s topography, an island that sits 34 miles away from the island of Fogo.

These discoveries suggest an incredible surge of water hit Santiago. And the only force strong enough to push large boulders that far inland is a mega-tsunami.

Using the latest rock-dating techniques, the scientists believe this tsunami happened around 73,000 years ago. It was also around this time that a collapse occurred at the nearby Fogo volcano. The collapse of the Fogo volcano is believed to have created an immense displacement of water. A mega-tsunami rushed towards Santiago Island with a wave estimated to be as high as 885 feet or more striking the island.

Fogo volcano and Santiago Island

The different ways a tsunami forms

We all usually associate tsunamis with earthquakes. The catastrophic tsunamis of 2004 (Indian Ocean) and 2011 (Japan) immediately come to mind. But a tsunami can form in other ways too. All that’s needed is a sudden and massive displacement of water. Meteor impacts are one example. We’ve seen it illustrated in movies like Deep Impact. Landslides (or rockfalls), such as what occurred in Fogo, are another example.

One of the most famous examples of a rockfall-generated tsunami happened in Lituya Bay, Alaska on July 9, 1958. An earthquake struck along the Fairweather Fault in the Alaska Panhandle and caused 40 million cubic yards of rock to fall 3000 feet into the waters of Lituya Bay. The sudden displacement of water destroyed trees up to 1,720 feet above the bay (seen in the image below).

alaska tsunami

Another image shows where the mass of rock and ice that was dislodged from the cliff.

lituya bay rockfall

You can see all the trees that were swept away in the image below. The lighter brown areas had just as many trees as areas surrounding it prior to the tsunami.

lituya bay tsunami

Credits: United States Geological Survey

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As for Fogo Island?

“Fogo Island presently towers 2,829 metres above sea level, and erupts about every 20 years, most recently between November 2014 and February this year,” said Dr Ricardo Ramalho, lead author of the study. “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. 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 main message from Ramalho is that we need to stay vigilant. Rockfalls have the capability to produce very large and very destructive tsunamis. “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,” Ramalho adds.

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