Thrown up by the sea? (Image: Ricardo Ramalho)
WITH waves taller than 100 metres, it’s no wonder they are called megatsunamis. After part of Cape Verde’s Fogo volcano fell into the sea around 70,000 years ago, a wave 170 metres tall slammed into nearby Santiago island, now home to over a quarter of a million people.
That’s the conclusion of work by Ricardo Ramalho, now at the University of Bristol, UK, and his team at Columbia University in New York. They say that the volcano has since grown back to its pre-disaster proportions and may be in for another catastrophic collapse. What’s more, we may be underestimating the risk of similar events on other volcanic islands.
Megatsunamis are defined as being at least 100 metres tall at their source. In contrast, the tsunami after Japan’s 2011 Tohoku earthquake topped out at 40 metres.
In theory, giant waves can be triggered by huge amounts of material crashing into the ocean: impacts from space, for example, or the collapsing flanks of volcanic islands. But clear evidence for a large collapse happening fast enough to cause one has so far been lacking. Given the abundance of volcanic islands, it would be good to know if it is a possibility.
While working on a separate project, Ramalho noticed huge boulders scattered across Santiago’s landscape, as high as 220 metres above sea level and 650 metres from the coastline. Some weigh more than 700 tonnes and are made of rock similar to that at the shore – suggesting they were deposited by a large wave.
By combining data on the boulders with the pattern of rock sediments at lower altitudes, his team estimated the height, path and date of the wave. That date matched with Fogo’s partial collapse, some 73,000 years ago.
Ramalho’s team says the event triggered a wave up to 170 metres tall, that travelled 55 kilometres to the shores of Santiago (Science Advances, doi.org/74r).
When the side of the Fogo volcano cracked off, it triggered a wave up to 170 metres tall
“That’s a huge tsunami. Even if their modelling overestimated the size, that’s still a huge tsunami,” says Bruce Jaffe of the US Geological Survey in California. He says work like this is crucial to understanding the risk megatsunamis pose in the present.
We still don’t know how often crumbling volcanoes lead to giant waves, but Ramalho’s team suspects these events may be more frequent than thought. There are numerous at-risk ocean volcanoes in the Caribbean and elsewhere, says James Goff of the University of New South Wales in Sydney, Australia. “We have been expressing increasing concern about flank collapse events in Pacific islands,” he says.
If and when they do happen, they are mostly a risk for nearby areas. This is because unlike the waves made by earthquakes, in which a fault movement pushes the wave like a paddle, megatsunamis tend to lose power more quickly over distance – like the spreading ripples from a pebble dropped into a pond.
This article appeared in print under the headline “100-metre waves are a threat today”