Mount Gamalama on Ternate island in North Maluku province shot thick gray smoke up to 2,000 meters (6,560 feet) into the sky just before midnight Thursday, said Sutopo Purwo Nugroho, the spokesman for Indonesia's Disaster Mitigation Agency.
He said nine panicked hikers fell while fleeing to safety. Four of them were hospitalized and rescuers were searching for the tenth.
Slow-moving red lava was visible at the peak of the eruption Friday and villages were blanketed with thick ash, but no evacuation orders have been issued, Nugroho said.
An airport in Ternate, the provincial capital about30 kilometers (20 miles) from the volcano, was forced to close Friday as did schools and offices.
Mount Gamalama is one of about 129 active volcanoes in Indonesia. Its last major eruption was in 2012. No deaths were reported then.
Indonesia, a vast archipelago of 240 million people, is prone to earthquakes and volcanoes because it sits along the Pacific Ring of Fire, a horseshoe-shaped string of faults that lines the Pacific Ocean.