|Airlines have been told to avoid Mount Sinabung as the volcano erupted for a second day in a row [AFP]
A volcano on the Indonesian island Sumatra that had been dormant for more than four centuries has erupted for the second day in a row.
Mount Sinabung was again spewing out towering clouds of ash on Monday, forcing the evacuation of more than 21,000 people. Some aircraft had to be diverted because of poor visibility.
Villagers living along the slopes of the Sinabung in North Sumatra province have packed up their belongings and headed to emergency shelters, mosques and churches, Andi Arief, a presidential adviser on disasters, said.
Their abandoned homes and crops were blanketed in heavy, grey soot, and the air was thick with the smell of sulphur.
Eruption pattern unknown
Sinabung last erupted in 1600, so observers do not know its eruption pattern and admitted over the weekend that they had not been monitoring it closely before it started rumbling days ago in the lead-up to Sunday's first, less-powerful blast.
The island of Sumatra lies on the fault line of the Eurasian and Pacific tectonic plates that have been pushing against each other for millions of years.
Like other volcanoes along the Sumatra fault line, the Sinabung has the potential to be very destructive, Erick Ridzky, a disaster researcher, said.
A larger blast has the potential to darken skies across the region, he said, affecting air traffic in nearby Singapore and Malaysia.
Already, several domestic flights heading to the provincial capital of Medan had to be diverted, according to Bambang Ervan, spokesman for the transportation ministry.
"The eruptions are very difficult to predict. We don’t have enough data on the volcano," Surono, a government volcanologist who uses only one name, told Al Jazeera.
"We don't know what set it off, how long it will continue or whether we should expect pyroclastic flows or more powerful eruptions."
So far, 21,000 people have been evacuated, Arief said, and food, emergency tents, and medicine were on the way to the scene.
The government also has set up public kitchens for refugees and handed out more than 17,000 respiratory masks.
"The government has done a very good job in evacuating the population around the foot of the volcano," Wayne Ulrich, a Red Cross disaster management co-ordinator, told Al Jazeera.
"These people are being housed in function halls and public buildings. When these people can go home depends on the ash cloud and the volcano’s activity."
Indonesia is spread across 17,500 islands and is prone to volcanic eruptions and earthquakes because of its location within the so-called Ring of Fire, a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.
It is also home to some of the largest eruptions in recorded history.
The 1815 explosion of Mount Tambora buried the inhabitants of Sumbawa island under searing ash, gas and rock, killing an estimated 88,000 people.