An Indonesian volcano that had been dormant for more than four centuries erupted for the second day in a row Monday, spewing white clouds of smoke and ash more than 2,000 yards (meters) into the air, officials and witnesses said.

Thousands of people living along the slopes of Mount Sinabung in North Sumatra province have been evacuated to emergency shelters, mosques and churches, said Priyadi Kardono, a spokesman for the National Disaster Management Agency.

Their abandoned villages and crops are blanketed in heavy, gray soot.

Mount Sinabung last erupted in 1600, so observers don't know its eruption pattern and are monitoring it closely for more activity. They raised the volcano's alert to the highest level after its first blast Sunday, which followed days of rumbling.

"The problem is, we really have no idea what to expect," said Surono, a government volcanologist who uses only one name. "We don't know what set it off, how long it will continue or whether we should expect pyroclastic flows."

The number of people evacuated climbed to 12,000 on Monday, said Kardono, adding that food, emergency tents, and medicine were on their way to the scene. The government also has set up public kitchens for refugees and handed out more than 17,000 respiratory masks.

Indonesia, the world's largest archipelago, is on the so-called "Ring of Fire," an arc of volcanos and fault lines encircling the Pacific Basin.