If I was living in Miami or New York and I heard that the Cumbre Vieja was erupting, I would keep a very close eye on the news

Prof Bill McGuire

Swiss researchers who have modelled the landslide say half a trillion tonnes of rock falling into the water all at once would create a wave 650 metres high (2,130 feet) that would spread out and travel across the Atlantic at high speed.

The wall of water would weaken as it crossed the ocean, but would still be 40-50 metres (130-160 feet) high by the time it hit land. The surge would create havoc in North America as much as 20 kilometres (12 miles) inland.

Dr Day told BBC Science's Horizon programme: "This event would be so huge that it would affect not only the people on the island but people way over on the other side of the Atlantic Ocean - people who've never heard of La Palma."

Destructive power

His latest work on the subject has been published in the Journal of Volcanology and Geothermal Research.

On the back of this work, the Geological Society of London is to write to the UK science minister, Lord Sainsbury, to make him aware of the dangers posed by so-called mega-tsunami in the Atlantic.

The society hopes he will take the issue as seriously as he has the threat from asteroid strikes.

Scientists have known of the destructive power of tsunami - huge tidal waves - for many centuries. As recently as 1998, over 2,000 people were killed by a large wave hitting the coast of Papua New Guinea.

This was caused by an offshore earthquake. But researchers believe far bigger phenomena can be created by giant landslides.

The largest wave in recorded history, witnessed in Alaska in 1958, was caused by the collapse of a towering cliff at Letuya Bay. The resulting wave was higher than any skyscraper on Earth and gouged out soil and trees to a height of 500 metres (1,640) feet) above sea level.

Summit eruptions

Geological studies have found evidence of giant landslides elsewhere in the world such as Hawaii, the Cape Verde Islands and R�union in the Indian Ocean.

Dr Day has identified dozens of volcanic vents in the Cumbre Vieja volcano that have been formed by successive eruptions over the past 100,000 years.

He thinks water trapped between dykes of impermeable rock could create pressures that eventually lead to the western flank of the mountain falling away during some future eruption.

Dr Simon Day: This would be a huge event

Hermann Fritz, of the Swiss Federal Institute of Technology, which has equipment to model waves created by landslides, said: "If the Cumbre Vieja were to collapse as one single block, it would lead to a giant mega-tsunami with an initial wave height of 650 metres.

"It would have a wavelength of 30 to 40 kilometres (18 to 25 miles) travelling westwards across the Atlantic at speeds up to 720 km/h (450 mph) towards America."

But researchers caution that such a catastrophe may not occur for many decades.

"There could be five more summit eruptions of the Cumbre Vieja before the western flank collapses," said Professor Bill McGuire, of the Benfield Greig Hazard Research Centre.

"There could be 10 or there could be 20 - we simply don't know. But put it this way: if I was living in Miami or New York and I heard that the Cumbre Vieja was erupting, I would keep a very close eye on the news."

Dr Simon Day's work is featured in a Horizon programme to be broadcast on BBC Two on Thursday, 12 October.