Megatsunamis Can Develop, Drown Coasts in an Instant, New Study Shows

Evidence of prehistoric monster wave 250 meters high off the African coast indicates that the theory of slow collapses and 'small' tsunamis doesn't hold water.

Aldo Bien, Wikimedia Commons

Are today's coastal populations in danger of drowning from a mega-tsunami caused by sudden collapse of a volcano, say, or a coastal shelf? Yes, according to a new study that found evidence of a mega-tsunami 250 meters in height caused by the collapse of a volcanic flank 73,000 years ago, scientists say.

To put things into proportion, the wave they found would have reached more than halfway up New York's Empire State Building.

It isn't that any danger is imminent. But we may have precious little warning if it happens.

Although it happened in prehistory, the latest data has reignited the scientific squabble over whether collapses, whether due to volcanic explosions or continental shelf disruptions, can trigger gigantic tsunamis. The study appears today in the journal Science Advances.

The Cape Verde collapse is not the only known example of a mega-tsunami, though most occurred in prehistory, so science is guided almost exclusively by geological smoking guns.

The catastrophic waves that struck Indonesia in 2004, killing around 230,000, and Japan in 2011, causing the Fukushima meltdown, were not mega-tsunamis, with all due respect to media hyperbole. They were about 30 meters in height. In any case, both were caused by seabed earthquakes, not collapsing volcanic slopes. Even the all-but-isolated Mediterranean Sea has suffered major tsunamis: There is evidence of a 50-meter high wave triggered by Mt Etna losing six square miles of rock some 8,000 years ago.

The 2011 tsunami hitting the Japan coastline

On the other hand, the swell caused in Lituya Bay, Alaska in 1958, which also followed an earthquake, rose over 900 meters in height and definitely does qualify as a mega-tsunami. Only five people died in that event, by the way, due to the low population density in the area.

The point is that the Lituya incident - and now the evidence from Cape Verde - provide more evidence that collapses of mountain flanks can happen extremely fast and catastrophically and trigger giant tsunamis, though that such events seem to be rare.

Tossing a 770-ton rock like a feather

Scientists have been mixed on the suddenness of major volcanic collapses, with some insisting that landslides come in stages, causing multiple, smaller tsunamis.

The Cape Verde wave was apparently caused by a flank collapse 73,000 years ago at Fogo, one of the world’s largest and friskiest island volcanoes, explains the paper's lead author Ricardo Ramalho, who did the research as a postdoctoral associate at Columbia University. Today Fogo is 2,830 meters above sea level, and erupts about every 20 years, most recently in the autumn of 2014.

A 2011 French study suggested that the Fogo collapse, for instance, which it says took place somewhere between 124,000-65,000 years ago, involved more than one landslide. But their calculations would have resulted in waves no more than 45 feet high, not that those wouldn't hurt when they hit. Not so, it seems.

The new study postulates that a vast amount, around 160 cubic kilometers of rock, fell off that Fogo in one fell swoop, creating a monster wave.

One sign of the sheer power of the Fogo wave is giant boulders weighing as much as 770 tons, which are as far as 2,000 feet inland and nearly 650 feet above sea level. They are atypical of the geology where they lie – but they match seabed rocks around the shoreline. The only explanation the scientists could come up with is that a gigantic wave must have ripped them from the shoreline and lofted them up, tossing them like feathers.

Tsunami expert Bill McGuire, a professor emeritus at University College London who was not involved in the research, estimates that mega-tsunamis of this type probably come only once every 10,000 years.

The Lituya mega-tsunami happened in a confined space. In the open sea, landslide-generated waves are generally thought to lose energy quickly, and thus to pose mainly a regional hazard.  But that is based on modeling, not real-world experience. So, will it happen again? Yes. When? We don't know. What will happen when it does? We don't know that either.

Cape Verde map
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