Vulcanologists have expected a big eruption of Mount Awu on Sangihe Island since last week, and the nearly 12,000 people living around the mountain had been evacuated to a nearby town. There were no reports of injuries in Thursday's blast.
``We cannot predict when it will end since many smaller blasts and aftershocks continue to occur,'' said Syamsul Rizal, the vulcanologist.
It was the second volcanic eruption in Indonesia this week. On Tuesday, two hikers were killed by an eruption of Mount Bromo, a volcano popular with tourists on the eastern end of Java island.
Thursday's explosion at Mount Awu, 1,350 miles northeast of Jakarta, threw up rocks near the volcano crater and spewed smoke 9,900 feet into the air, but there was no lava flow, Rizal said.
It was followed by several smaller blasts and aftershocks, and thick black smoke billowed from the crater for much of the day, witnesses said.
The island's airport was closed and authorities blocked roads leading to the mountain.
Authorities in recent days evacuated people living in a radius of four miles around Mount Awu. Evacuees were housed in the nearby town of Tahuna, in government offices and other public buildings.
A light coat of ash from the volcano, which lies just south of the Philippines' Mindanao Island, has covered local beaches and villages.
In August 1966, the 4,330-foot volcano erupted, killing 40 people. It last erupted in October 1992.
Tuesday's eruption on Mount Bromo also injured seven people who were hit by a shower of hot rocks expelled by the volcano, which also sent a plume of smoke 9,900 feet into the air, officials said.
By Suzanne Plunkett