Scientists were caught by surprise when Indonesia's Mount Sinabung suddenly erupted over the weekend, and an even stronger eruption on Monday sprayed soot and debris more than a mile into the air. Plumes of white smoke continue to pour out.

Since the volcano hadn't erupted since 1600 A.D., many vulcanologists considered it a low priority — especially in Indonesia, which has 129 live volcanos to worry about, thanks to its location within the "Ring of Fire." But Lee Siebert, director of the Global Volcanism Program at the Smithsonian Institution, told NPR that it's that surprising for a volcano to erupt suddenly after 400-odd years — that's a very short time geologically, and any volcano that's been active in the past 10,000 years is still considered active.

Some experts are worried that the volcano may produce more than just ash and soot, given its position on the Sumatra fault line. Some of the 30,000 evacuated villagers are already returning home, but a state vulcanologist named Surono cautions:

I'm not going to send people back until the mountain shows me it's safe. It's too early. And in the end I'll be judged by how many lives are lost.

Photos by Binsar Bakkara/Associated Press [NPR]

Indonesian volcano erupts for the first time in over 400 yearsS

AP Photo/Binsar Bakkara

Indonesian volcano erupts for the first time in over 400 yearsS

AP Photo/Binsar Bakkara

Indonesian volcano erupts for the first time in over 400 yearsS

AP Photo/Binsar Bakkara

Indonesian volcano erupts for the first time in over 400 yearsS

AP Photo/Binsar Bakkara

Indonesian volcano erupts for the first time in over 400 yearsS

An Indonesian youth drags a goat as he briefly returns to his village to check on his livestock with rumbling Mount Sinabung in the background in Tanah Karo, North Sumatra, Indonesia, Tuesday, Aug. 31, 2010. Villagers briefly returned home Tuesday to check their farms along the fertile slopes of an Indonesian volcano that erupted after laying dormant for more than four centuries, catching many scientists off-guard. (AP Photo/Binsar Bakkara)