To make matters worse, ashes may come to Buenos Aires
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Explosions and loud groaning noises resounded from the crater of the 3,280-foot (1,000-meter) volcano, which had been dormant for thousands of years.
No lava flow has yet been detected down Chaiten's sides, but Chile's National Emergency Office said the volcano was spitting bits of molten rock and that remaining civilians and troops were being evacuated across a fjord.
"The situation has changed suddenly," national emergency official Rodrigo Rojas said in an interview. "Today the volcano is erupting with pyroclastic material on a different scale."
The towering ash cloud was clearly visible from the southern town of Puerto Montt, where many desperate evacuees were being sheltered.
"I am very worried to have left my house, my pet, my animals behind. All I want is for this to be over," said Carola Perez, a 22-year-old housewife evacuated to the town.
The government ordered the evacuation of a 30-mile (50 km) radius around the volcano -- which lies some 760 miles (1,220 km) south of the capital, Santiago -- including two dozen people who had refused to leave their homes and animals.
It appealed to anyone still on remote farms in the area to leave.
Military personnel, police and journalists were being ferried to join dozens of civilians already aboard warships waiting in the fjord off Chaiten. Around 4,200 people, nearly the whole population of Chaiten, have already been evacuated.
Sparsely populated Patagonia is the southernmost swathe of Latin America that cuts across Chile and Argentina and is home to towering snow-capped peaks, some of them volcanoes, glaciers and log cabins, and is a gold mine for dinosaur fossil hunters.
DENSE CLOUD OF ASH
Luis Lara, a government geologist, said he did not expect a catastrophic collapse of the Chaiten volcano, but that a cloud of dense, very hot material could coat the surrounding area.
"This produces a more complicated scenario," Lara said. "A dense cloud of pyroclastic material could move down its slopes, and that causes much more damage (than a spray of lava)."
"The entire volcano will not (collapse), but the eruptive column could, and that is sufficient material to be displaced down its sides and into areas nearby," he added. "Lava flow would not reach Chaiten, but hot fragments, ash and gas could."
A second town, Futaleufu, has also been coated with ash and is being evacuated. The area is some distance from Chile's vital mining industry farther north.
Some of Futaleufu's 1,000 or so residents have already crossed into neighboring Argentina, where some areas have also been showered with thick ash and where flights and schools were suspended.
Argentina is not evacuating residents from the worst-affected zones, instead advising them to stay indoors.
"It's a horrible situation. Sometimes it goes all dark and it doesn't stop raining ash," said Cecilia Rimoldi, a resident of the southern Argentine tourist town of El Bolson.
The ash is more than 6 inches (15 cm) thick in some places near Chaiten, contaminating water supplies and coating houses, vehicles and trees. Thousands of head of cattle are being moved out of the area.
Chile has the world's second most active string of volcanoes behind Indonesia. It is home to 2,000 volcanoes, 500 of which experts say are potentially active. Around 60 have erupted over the past 450 years.