Mount Sinabung has been showing signs of life since September, after being dormant for three years.
It erupted more than eight times on Sunday and Indonesia's National Disaster Mitigation Agency (Badan Nasional Penanggulangan Bencana) has evacuated more than 15,000 people.
"People panicked as the eruption was accompanied by a loud thunderous sound and vibrations. Then it started raining down rocks," Robert Peranginangin, a local government official, told reporters.
Airlines have been advised not to fly near the area.
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