Mount Sinabung, on the Indonesian island of Sumatra, has blasted volcanic ash as high as 4.2 kilometres, one of its biggest eruptions in the past several months of high activity.
Isya Nurrahmat Dana, an official at a volcano monitoring post in North Sumatra province, said at least 19 eruptions followed by lava flows were recorded by midday on Wednesday.
Hot ash tumbled down the mountain's slopes as far as 4.5km east and south-east into a river.
Students from a school in Kuta Rakyat were sent home because volcanic ash covering the village was considered dangerous, according to media reports.
There were no reports of casualties.
Thousands of people in 10 villages were directly affected by ash fall from the latest eruption, National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho said.
He warned villagers to stay out of a danger zone that extends as far as 7km from the peak.
Mr Nugroho said villagers urgently need face masks and clean water.
He called on people to be vigilant about the possibility of flash floods and lava flows because Wednesday's eruption created a natural dam on the upper reaches of the river.
The 2,460-metre high Sinabung burst back to life in 2010 after a long period of dormancy.
The volcano has been erupting sporadically ever since, killing 17 people and forcing more than 30,000 to evacuate their villages.