A huge cloud of ash spewed from the Chaiten volcano, some 1,300 km south of Santiago when it erupted in 2008.

Credit: AFP

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PARIS: Magma from a Chilean volcano shot through Earth's crust at around a metre per second, a speed highlighting the perils from so-called rhyolitic volcanoes, says a new study.

Volcanoes in this category provide some of Earth's most explosive events. They are characterised by a dome of hardened magma which covers their central vent and can blow with catastrophic force, often with scant warning.

They include Vesuvius, Krakatoa and Mount St. Helens - names that have gone down in history for inflicting loss of life and massive damage.

Hazard mitigation

"This result has implications for hazard mitigation because the rapidity of ascending rhyolite means that future eruptions may provide little warning," the authors write in the British journal Nature.

In the case of the Chaiten volcano, in Chile's northern Patagonia, local residents at a town 10 kilometres from the cone felt earthquakes at about 8 pm on April 30, 2008 that were powerful enough to knock objects off shelves.

The following day, the volcano erupted. On May 2, the volcano's lid, a caldera, was ripped off in a mighty blast. Further eruptions continued for a week, distributing a blanket of ash in a wide swathe near the volcano.

Explosive fragmentation

Jonathan Castro of France's National Centre for Scientific Research (CNRS) and Donald Dingwell, of the Ludwig Maximilian University in Munich, Germany, collected ash samples and analysed crystals that are formed under specific conditions of pressure, temperature and water content.

They calculate that the magma took just four hours to ascend from a depth of more than five kilometres. The molten rock surged upwards in the range of one metre per second before dispersing in an "explosive fragmentation" when it reached the surface.

The pair call for rhyolitic volcanoes that have not erupted since the last Ice Age, around 10,000 years ago, to be monitored. "In more densely populated regions this would be essential to avoid a major volcanic disaster," they said.

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