Blast in Bali

It's normal for tiny earthquakes to happen near a volcano. But in late September, volcanologists took notice of Mount Agung. That's because hundreds of small rumbles were happening there every day. Then the mountain started spewing steam out of its vent, or top opening. All signs pointed to magma, or molten rock, building within Mount Agung.

“The earthquakes coming from the volcano meant the magma was moving from deep within Earth toward the volcano,” explains Janine Krippner. She’s a volcanologist at the University of Pittsburgh in Pennsylvania.

Scientists closely monitored Mount Agung in the weeks leading up to the eruption. The people living nearby had plenty of time to prepare. On September 25, residents were ordered to evacuate. Anyone living within 12 kilometers (7.5 miles) of Mount Agung left the area.

People left behind farms and livestock that might not be there when they return. But there was no question that evacuating was the right choice. The last time Mount Agung erupted, in 1963, 1,100 people died.

So far, the eruption hasn't caused much damage. But Krippner cautions that volcanic eruptions aren’t quick events. Mount Agung could become very dangerous very quickly. On November 27, mud created by the steam from the eruption began flowing down the volcano. These mudflows, called lahars, can quickly cover areas below the mountain.

But the biggest danger is a pyroclastic flow, says Krippner. A pyroclastic flow is a hot mix of volcanic material, such as ash, gas, lava, and broken rock. These dangerous flows move down the side of a volcano at up to 700 kilometers (435 miles) per hour. “This is the most deadly case you could have,” says Krippner.

Unfortunately, Mount Agung has a history of producing deadly pyroclastic flows. They were responsible for most of the deaths in the 1963 eruption. In addition, volcano scientists have used ancient rocks to study 5,000 years of Mount Agung’s activity. They’ve found evidence of these flows for each eruption.

“What a volcano has done in the past is a likely indicator of what it will do in the future,” says Krippner. “Knowing there was the potential for an explosive, dangerous volcano, [authorities] knew they had to get everyone out.”