Volcano threatens giant tsunami

The possibility of a collapsing volcano unleashing a kilometre high tsunami wave has captured the public imagination following the release of new research. But don't bother watching the horizon or moving to high ground, at least for the present.

The research, to be published this Saturday in Geophysical Research Letters, predicts the frightening outcome of a collapse of the Cumbre Vieja volcano in La Palma in the Canary islands. Much of the volcano's western flank sits poised to drop into the sea, a mass of rock equal to twice the volume of the Isle of Man.

The researchers point out, however, that "there seems no significant risk that the volcano will collapse spontaneously". It would need fresh eruptions from the sleeping giant to spark a collapse, and these occur only rarely.

But it will happen eventually, according to Prof Bill McGuire of the Benfield Greig Hazard Research Centre at University College London, who was involved in the research. "These collapses occur every several thousand years," he said.

Tsunami means "harbour" wave in Japanese; in English such large waves are commonly called tidal waves, although they are not caused by tides.

When the volcano decides to collapse computer models predict it will release the biggest tsunami wave in recorded history. It would wipe out the eastern seaboard of Brazil and North America and smash Ireland's south and west coasts with waves higher than a two storey house.

The tremendous volume of rock breaking off the mountainside would push out a massive blob of water up to 900 m high that would then rush away from the island at speeds approaching 800 km an hour, Prof McGuire explained.

The volcano has been building over hundreds of centuries and now rises 6 km above the ocean floor with only the top 2 km reaching above sea level. "The volcano we are talking about started to collapse after an eruption in 1949," Prof McGuire said.

Much of the western face of the mountain began to slide towards the sea but stopped after displacing four metres. Since then, the loose block has been moving a few centimetres a year. Prof McGuire has identified a fault line associated with this movement which indicates that between 200 and 500 cubic km of rock could break free.

"The really big waves are going to go west," he said, because of the likely direction of the collapse. Brazil could expect waves up to 50 m high, according to a computer model, and even higher waves could reach North America.

Reflected waves would be directed towards Ireland and Britain which could stand seven to 15 metres high when they reached the coastline, he added. These would swamp coastal towns and cities such as Cork.