La Palma eruption – Volcanoes, tsunamis and risk

Don’t bother your pretty head about it” is the prevailing media take on the risk of the volcanic eruption on La Palma in the Canary Islands turning into a mega-tsunami disaster. The media definitely over-hyped that risk when it was first suggested 20 years ago, so now they have to work the other side of the street.

But the original story still has legs. The eruption is now more than two weeks old, but the explosions and lava flows are still increasing. Part of the main cone of Cumbre Vieja (“old summit”) collapsed last weekend.

La Palma and its neighbour, El Hierro, are so volcanic that similar cone collapses have removed about half of their above-water mass during the past million years. The biggest single landslide, about half a million years ago, dumped an estimated 200km3 into the Atlantic. That’s an entire mountain’s worth.

Also Read: Watch: Streams of lava engulf houses, plantations in La Palma, Spain

The volcanoes constantly rebuild the islands, too, so massive landslides are a normal part of their geology. There have been at least ten in the past million years. That’s why volcanologist Joan Martí, when asked if the flank of Cumbre Vieja might slide into the sea and cause a huge tsunami, replied that “it is possible, but it is not likely”.

But there will be another collapse on La Palma and then a tsunami, maybe tomorrow, maybe in 100 000 years or so.

What we don’t know is the size of the resulting tsunami.

The original scientific article warning of a possible mega-tsunami from La Palma was written by Steven Ward and Simon Day in 2001. They estimated that the giant waves generated by a flank collapse would hit the Moroccan and Spanish coasts in two to three hours, and make it all the way across the Atlantic to strike the Brazilian, US and Canadian coasts in nine hours.

Hundreds of metres high at first, the tsunamis waves would be down to 100m by the time they reached Spain, and perhaps only 25m when they struck the North American coast from Florida to Newfoundland. But that’s still a lot.

Or so Ward and Day estimated – whereupon other scientists immediately piled in to insist that they had got the geology wrong.

They did knock this one down in the public mind and the media. But all the basic facts were correct: the relatively frequent massive slides (one every 100 000 years in the Canaries), the tsunamis that follow, and the immense damage they do.

We can do nothing about flank collapses on volcanic islands except have a good early warning system, but they are only common in the Hawaiian Islands, the Canary Islands, and the Indonesian archipelago.

Just east of the Rocky Mountains in the US, Yellowstone has staged three long-lasting “super-eruptions” at 2.1 million, 1.3 million and 631 000 years ago. Each time it covered the surrounding states with volcanic ash a metre thick, coated the entire continent with enough ash to kill most green plants, and boosted pulverised rock and gas into the atmosphere.

That blocked incoming sunlight for the next six to 10 years and caused 4°C lower average global temperature. If that happened today, it would cause global crop failures and mass starvation.

There have been at least 47 such “super-eruptions” in the world’s history.