Deep inside a rhyolitic volcano, the mostly silica-based magma is thinned by water and other fluids, according to the findings published tomorrow in the journal Nature.
Pressure from above keeps the water in place. But as the magma rises toward the surface, the pressure eases until nearly all the water is removed—making the material one of the most viscous liquids known in nature.
Meanwhile, the extracted water forms a layer of bubbles, increasing pressure on the surrounding sticky magma.
Eventually, that built-up pressure breaks to the surface in an explosive eruption, like the uncapping of a well-shaken bottle of soda.
Ongoing eruptions are continuing to rebuild Chaitén's dome, as seen in this September 2009 picture.
"Fine ash and larger blocks, up to the size of a house, come rumbling out," said study co-author Donald Dingwell, of the University of Munich in Germany.
"They can go straight up or tumble down the side of the mountain. Those pyroclastic flows are very hot and very fast. They can asphyxiate you, burn you, bury you alive, or crush you under debris."
Sounding the Alarm
Castro and Dingwell both stressed the need for improved monitoring of large rhyolitic volcanoes, even those that have not shown activity in the past 10,000 years.
John Pallister, of the U.S. Geological Survey's Volcano Hazards Program, agreed, adding that sensitive monitoring equipment should be placed on the volcanoes themselves to pick up microearthquakes or other advance warnings of eruptions.
That's because distant seismometers miss the early warning signs of volcanic activity—before the 2008 eruption, the nearest seismometers to Chaitén were 125 miles (200 kilometers) away, said Pallister, who was not involved in the new study.
"When you have to rely on people feeling the ground shake," he said, "you're already at the point where something is about to happen."
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