TANAH KARO, Indonesia — An Indonesian volcano that had been dormant for more than four centuries erupted for the second day in a row Monday, spewing out towering clouds of ash and forcing the evacuation of more than 21,000 people.

Some airplanes had to be diverted because of poor visibility.

Villagers living along the slopes of Mount Sinabung in North Sumatra province have packed up their belongings and headed to emergency shelters, mosques and churches, said Andi Arief, a presidential adviser on disasters.

Their abandoned homes and crops were blanketed in heavy, gray soot, and the air was thick with the smell of sulfur.

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."

So far, 21,000 people have been evacuated, said Arief, and food, emergency tents, and medicine were on the way to the scene. The government also has set up public kitchens for refugees and handed out more than 17,000 respiratory masks.

Several domestic flights from the provincial capital of Medan had to be diverted, according to Bambang Ervan, spokesman for the Transportation Ministry.

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.

Associated Press Writers Irwan Firdaus and Ali Kotarumalos contributed to this report from Jakarta.