A massive chunk of La Palma, the most volcanically active island in the Canaries archipelago, is unstable, says Simon Day, of the Benfield Grieg Hazard Research Centre at University College London.

He calculates that its flank could collapse the next time the volcano, Cumbre Vieja, erupts.

If so, that would send a dome-shaped wall of water up to 100 metresfeet) high racing across the Atlantic at 800 kilometers (500 miles) per hour, hitting the western coast of Africa and southern coast of England within a few hours.

Some eight hours after the collapse, the US East Coast and Caribbean would bear the brunt.

Cities from Miami to New York would get swamped by waves up to 50 metres (160 feet) high, capable of surging up to 20 kilometers (12 miles) inland, according to Day's research.

Tsunamis, mistakenly called tidal waves, are commonly caused by earthquakes that occur under or beneath the sea.

But around three decades ago, the theory was born that these gigantic waves can also be caused by collapsing islands. The evidence for this came from debris in the mid-Pacific believed to have been strewn from a titanic landslide in Hawaii.

Day published his findings on Cumbre Vieja in 1999 after a two-year study into the volcano, which occupies the southern half of La Palma.

He identified dozens of volcanic vents formed by successive eruptions over the past 100,000 years and collected samples of lava to built up a detailed geological picture.

He found that the volcano's vents are laid out in the shape of a three-pointed "Mercedes star", whose western flank -- a mass comprising some 500 billion tonnes of rock -- is gradually becoming detached as volcanic activity forces magma to the surface.

The flank is very slowly falling into the sea, but a major eruption by Cumbre Vieja could cause it to fall with catastrophic effect.

The big question is when this might occur. Some geologists say the threat cannot be assessed accurately because of the way in which volcanic pressures build up in the volcano's porous rock.

"Eruptions of Cumbre Vieja occur at intervals of decades to a century or so, and there may be a number of eruptions before its collapse," Day said, in a followup study in 2001 that estimated the astonishing speed at which the tsunami could travel.

La Palma reaches 6,500 metres (21,320 feet) from the surrounding ocean floor and to a height of 2,426 metres (7,957 feet) above sea level.

The island has had seven known eruptions, the last of which was in 1971, at a location on the island's southern tip and well away from an unstable ridge at the summit which is Day's biggest fear.

In August this year, one of Day's colleagues, Bill McGuire, told a conference on global geophysical disasters that Cumbre Vieja could blow "any time" and warned that there was insufficient watch on the volcano.

Only two or three seismographs were deployed there and this number falls way short of giving the weeks-long warning needed to evacuate coastal cities, McGuire said.

"Eventually, the whole rock will collapse into the water, and the collapse will devastate the Atlantic margin," McGuire said. "We need to be out there now looking at when an eruption is likely to happen... otherwise there will be no time to evacuate major cities."

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