Worst Volcanoes Even More Dangerous Than Feared

Some of the world’s most dangerous volcanoes can erupt much more quickly than scientists had suspected, according to a new study of the massive 2008 eruption of Chile‘s Chaitén volcano. (See Chaitén eruption photos.) Normally scientists can track the seismic rumblings that precede most volcanic eruptions for weeks or even months, as magma in the volcano slowly rises to the surface.

But when townspeople at the base of the Chaitén volcano first felt earthquakes on April 30, 2008, they had only 30 hours to get out before the long-dormant volcano began to blow its top.

On May 3, 2008, magma rocketed up through Earth’s crust, moving 3.1 miles (5 kilometers) up to the Chaitén volcano’s surface in only about four hours. An enormous eruption column soared 12 miles (19 kilometers) into the sky.

Thousands of Chaitén villagers had enough time to evacuate. But future victims who live in the shadows of these so-called rhyolitic volcanoes may not be so lucky.

Rhyolitic volcanoes are largely fueled by a silica-based, very flow-resistant magma and they tend to build pressure over time before erupting violently.

Large rhyolitic volcanoes exist in the Yellowstone region of Wyoming, in Long Valley, California, and in Valles, New Mexico. The Japanese islands and New Zealands Taupo Volcanic Zone are also home to such volcanoes.

Rhyolitic volcanoes erupt so infrequently that scientists hadn’t had a chance to observe one until Chaitén. The eruptions may go 10,000 years between episodes.

“The largest eruptions on the planet have been rhyolitic,” said study co-author Jonathan Castro of the Institut des Sciences de la Terre in Orléans, France.

“So you might have fewer of these volcanoes, but they pack a way bigger punch.”

“Bury You Alive”

Castro and colleagues studied crystalline evidence of Chaitén’s pre-eruption magma temperature, pressure, and water content to discover how the volcano had erupted so quickly. (See volcano photos.)

(Continued) – http://news.nationalgeographic.com/news/2009/10/091007-volcano-eruption-chaiten.html