PREDICTING when the world鈥檚 most dangerous volcanoes are about to erupt just got easier. It seems that one type of eruption can be triggered by heavy rain, so simply watching the weather forecast could help save lives.
The type of eruption in question is a 鈥渄ome collapse鈥�. This is the most dangerous form of volcanic eruption, having caused more than 70 per cent of volcano-related deaths over the past century.
鈥淭hey tend to be big, violent eruptions,鈥� says John Murray, a volcanologist at the Open University in Milton Keynes. The Mount St Helens eruption on 18 May 1980, for example, sent a cloud of 520 million tonnes of ash 25 kilometres into the air. Rock and debris fell to Earth up to 30 kilometres from the volcano.
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The build-up to an eruption is often easy to see months or years ahead, as magma pushing from below produces a visible dome on the side of the mountain. Small eruptions of lava add to the bulge, making it more unstable until eventually it collapses, releasing the pressure in a massive explosion. But it鈥檚 extremely difficult to tell exactly when this will happen.
Now researchers studying the Soufri猫re Hills volcano on the Caribbean island of Montserrat have linked the last three eruptions to an unlikely culprit 鈥� the weather. The volcano became active on 18 July 1995 after a break of some 300 years.
On all three occasions, heavy rain destabilised a dome that was already primed to collapse, Adrian Matthews at the University of East Anglia told New Scientist. He says the eruption on 29 July last year was particularly striking, as it happened immediately after tropical storm Barry hit the island. 鈥淭hat was the first heavy rainfall event in seven months,鈥� he says, and within minutes the dome collapsed (Geophysical Research Letters, DOI: 10.1029/2002GL014863).
Matthews thinks rain may have played a part in triggering other volcanoes too. There鈥檚 a statistical link between the wettest times of the year and eruptions at Mount Etna and Mount Saint Helens, but the connection is not clear-cut.
Matthews says the next stage will be to work out how rainfall triggers a blowout. One possibility is that heavy rain washes away rocks at the base of the dome, destabilising it. But it鈥檚 more likely that large amounts of water seep into cracks in the dome and are vaporised by the molten rock (see Graphic). The massive increase in pressure as the water heats up and turns to steam then triggers a blast.
鈥淧eople on Montserrat will pay attention to this,鈥� says Rosalind Helz at the US Geological Survey in Virginia. Plymouth, the British colonial capital of Montserrat is only 4 kilometres from the summit of Soufri猫re Hills. After successive evacuations of the town as the volcano became more dangerous, the city was finally abandoned in 1996. Some two-thirds of the island is now off-limits to inhabitants.
Helz says factoring rainfall into eruption predictions could improve accuracy, though she cautions that rain won鈥檛 necessarily be the trigger in every case.
鈥淒omes will collapse eventually in the absence of rainfall,鈥� she says, 鈥渋t鈥檚 just an extra kick.鈥�
But being able to predict any dome collapse would be a major step forward. These events are particularly deadly because of the avalanche of searing toxic gas, hot ash and boulders they send down the mountain. These 鈥減yroclastic flows鈥� 鈥� which travel at up to 80 kilometres per hour 鈥� are much more hazardous than lava.
The massive amounts of rubble flow by a sort of 鈥渉overcraft effect鈥�. Huge boulders are suspended by hot gases that have built up under the dome, and the whole lot behaves like a raging torrent as it gushes down the mountain, destroying everything in its path.
So great is their momentum, they have been known to mount obstacles 1 kilometre high. 鈥淟ava you can get out of the way of, but no one knows of anyone surviving a pyroclastic flow,鈥� says Murray.