Government volcanologist Surono, who uses only one name, said Mount Sinabung in North Sumatra province started rumbling a few days ago and the minor morning eruption has mostly stopped.
It sent sand and ash up to a mile high but lava only moved near its crater. It caused no major damage and "only dust covered plants and trees," he said.
He said Mount Sinabung last erupted in 1600, so observers don't know the volcano's eruption pattern and are monitoring it closely for more activity.
Evacuations on the volcano's slopes started Friday at the first signs of activity. Up to 10,000 people who fled are staying in government buildings, houses of worship and other evacuation centers in two nearby towns.
The government has distributed 7,000 masks to refugees and set up public kitchens so people can cook food, said Priyadi Kardono, spokesman for the National Disaster Management Agency.
Indonesia, the world's largest archipelago, is on the so-called "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin.