Mount Marapi on the island of Sumatra, with a peak of 2,891 metres, is on the third-highest alert level of Indonesia’s four-step system and authorities imposed an exclusion zone around its crater.
There were no immediate reports of damage or casualties after the eruption at 2:54 pm local time (0754 GMT), but 42 hikers were unaccounted for.
A total of 70 hikers had checked in at two entry points to Marapi since Saturday via an online booking system, according to the West Sumatra natural resources conservation agency.
“The… agency team are conducting evacuations. Twenty-eight people have been evacuated and efforts are still being made (to evacuate) the rest. Hopefully there are no fatalities and everyone is safe,” Dian Indriati, the interim head of the agency, said in a video statement.
Some hikers may not notify authorities about their presence when entering or leaving such areas, so the true number of unaccounted hikers remains unclear.
Hendra Gunawan, head of Indonesia’s Centre of Volcanology and Geological Hazard Mitigation, in a statement said ash was observed 3,000 metres (9,842 feet) above Marapi’s peak.
“The column of ash was observed in grey colour with thick intensity leaning toward the east,” Gunawan said.
“The eruption is still ongoing.”
Communities around the Marapi volcano and tourists are not allowed to hike the Marapi volcano within a three kilometre radius from its crater.
Ahmad Rifandi, an official at the Mount Marapi monitoring station, told AFP that ash rain was observed after the eruption.
“It has reached to Bukittinggi city,” he said, referring to the third-largest city in West Sumatra that has a population of more than 100,000.
“It is hoped for people who have activities around Marapi mountain to wear hats, glasses and masks.”
Mount Marapi, which means ‘Mountain of Fire’, is the most active volcano on Sumatra island.
The Indonesian archipelago sits on the Pacific Ring of Fire, where the meeting of continental plates causes high volcanic and seismic activity.
The Southeast Asian country has nearly 130 active volcanoes.