Chile's Chaiten volcano one of scores of active volcanoes in region, says CU-Boulder professor

Only a few dozen of the 200 to 300 active volcanoes in the Andean Arc are actively monitored, said Stern. More than 25,000 people were killed by the eruption of the Nevado del Ruiz volcano in Columbia in 1985, he said.

By Tuesday morning Chilean authorities had evacuated more than 4,000 people from the region, including the population in the town of Chaiten six miles from the volcano and the town of Futaleufo, roughly 70 miles to the east near the Argentine border. The five-day eruption of Chaiten has sent a thick column of ash and smoke into the stratosphere moving east across Patagonia to the Atlantic Ocean.

"The volcano went into a higher state of activity this morning," Stern. "What happens after today is anybody's guess."

In addition to covering towns and villages and polluting air and water, the ash fall will undoubtedly affect agricultural and ranching activities, Stern said. "Because there is relatively little precipitation in the region of Patagonia east of the volcano, it will take a long time to naturally wash the ash from the landscape."

Stern and Jose Naranjo of the National Service of Geology and Mining, who published a 2004 paper in the journal "Geology of Chile," used radiocarbon methods to date the last eruption of Chaiten and concluded the eruption generated layers of volcanic tephra on the surrounding landscape up to five feet thick. The same prehistoric eruption apparently created the two-mile-diameter crater where the current eruption is centered, the authors said.

Stern said the possibility of the Chaiten volcano affecting Earth's climate is probably fairly low. "In to order to significantly affect the climate, a volcano has to put out a lot of sulfur dioxide aerosols into the stratosphere for an extended period, which then reflects sunlight away from the Earth," he said. "Our data from Chaiten showed the last eruption was high in silica and low in sulfur."

In contrast, the massive eruption of Mount Pinatubo in the Philippines in 1991 cooled the global climate for about one year because of high sulfur dioxide emissions, he said. The eruption of Tambora in Indonesia in 1815 affected the world's climate for about three years and caused what is known as the Year Without a Summer in 1816 by cooling Europe and North America with huge atmospheric sulfur dioxide emissions.