Science: Lethal shockwave from an island in the sun

Volcanic activity on the Canary Isles could send a tidal wave to devastate Florida. Phillip Henry monitors the changing shape of La Palma

It reads like the plot from a disaster movie. Florida is devastated by a tidal wave tens of metres high. The destruction and loss of life is immeasurable. The wave which caused so much devastation crossed the Atlantic in just a few hours, unseen until it reached the American coast. Its source is an unstable geological fault on the Canary Isles, more usually thought of as an idyllic holiday destination of thousands of European tourists than as the cause of disaster.

To ensure such a scenario remains in the realms of Hollywood, a group of British scientists recently travelled to the Canaries. By monitoring the fault which threatens to create the tidal wave, they hope to predict any hazard long before it could happen.

In the middle of the ocean, these waves of mass destruction - called tsunami - are almost invisible. Only when they reach the shallow waters around coasts do they become huge breakers.

The ruin caused by even a relatively small one can be apocalyptic. When the Krakatoa volcano blew itself to pieces in the last century, a tsunami six metres high killed 30,000 people.

Tsunami can also be generated when a huge landslide falls into the sea. This has never been seen in historical times but scientists have now identified the island of La Palma as a potential hazard.

"There is a danger that the side of the volcano facing west may fall into the Atlantic," says Professor Bill McGuire of the Centre of Volcanic Research in Cheltenham, who was part of the recent expedition.

"It could literally happen during the next few weeks or months or years," he said. "Equally, it could happen 100 years or more into the future. The island is very unstable and this is something which could happen fairly soon."

La Palma is not only the steepest island in the world but has also been the most volcanically active of the Canary Isles in the past 500 years. There have been two eruptions on the island this century alone - the last one was in 1971.

The volcanoes themselves do not present much danger. La Palma lava moves so slowly that most people could easily outrun it, so there is no cause for anxiety to the many tourists who visit the island. The real danger lies in the possibility that an eruption might trigger the collapse of a volcanic ridge which is unsound.

The problems started when an eruption in 1949 caused several cubic kilometres of rock to slide a few metres toward the sea. This also opened a two- kilometre-long fracture which can quite easily be seen to this day.

There are not only fears that a future eruption would cause the rock to move again, but that next time, the landslide will not stop. If this happened, the resulting tsunami would be catastrophic.

"There have been three of these collapses in the history of the island," says Juan Carlos Carracedo of the Spanish National Research Council. Not only does the landscape bear the scars of these cataclysms but submarine photos show rock from the peaks of old volcanoes far out to sea. "Another collapse is impending. The only way to prevent this hazard is to study the island closely."

By monitoring the change in shape of the mountainside, the team hope not only to discover if the western flank is slipping due to gravity but to predict if the sleeping volcano is growing restless. Before eruptions, volcanoes always swell. This swelling may be imperceptible to the human eye. Only by surveying the shape of the ground with sensitive instruments can small changes be detected.

The team of scientists used a system called electronic distance measuring (EDM). By bouncing an infra-red beam off a mirror on another ridge of the volcano and timing how long the beam takes to return, the EDM can be used to measure distances to an accuracy of a few centimetres per mile.

In late 1994, scientists set up a network of stations on the mountainside and accurately measured the distances between them. After one and a half years, they returned to measure the network again. If the distances between the stations had become greater over that period, this would suggest that either the fault had slipped or the ground was bulging as molten rock inflated the volcano.

For the moment, results show there has been no movement. While the rest of us might breathe a sigh of relief, the measurements are highly valuable to the scientists because they give them a "baseline", illustrating the behaviour of the volcano under normal conditions.

Should future studies reveal that the volcano has deviated from this, the prospect of the east coast of America being flooded by a wave from the other side of the Atlantic may turn from fiction to horrifying reality.