vendredi 9 octobre 2009

Nuevo en Chaiten........ News from Chaiten

Worst Volcanoes Even More Dangerous Than Feared
Brian Handwerk
for National Geographic News
October 7, 2009
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.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,
"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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