Karangasem, Indonesia – A rumbling volcano on the resort island of Bali could erupt at any moment, authorities warned Monday as they raised alert levels to maximum, accelerated a mass evacuation and closed the main airport, leaving tourists stranded.

Massive columns of thick grey smoke that have been belching from Mount Agung since last week hae now begun shooting more than three kilometres (two miles) into the sky, forcing flights to be grounded.

Some 40,000 frightened people have fled their homes around the volcano but as many as 100,000 will likely be forced to leave, disaster agency officials said after raising the alert to its highest level.

The exclusion zone around Agung, which is 75 kilometres (47 miles) from the beachside tourist hub of Kuta, has also been widened to 10 kilometres.

“Continuous ash puffs are sometimes accompanied by explosive eruptions and a weak booming sound,” the National Board for Disaster Management said.

“The rays of fire are increasingly observed at night. This indicates the potential for a larger eruption is imminent.” Agung rumbled back to life in September, forcing the evacuation of 140,000 people living nearby. Its activity decreased in late October and many returned to their homes.

However, on Saturday the mountain sent smoke up into the air for the second time in a week in what volcanologists call a phreatic eruption – caused by the heating and expansion of groundwater.

Then on Monday so-called cold lava flows appeared – similar to mud flows and often a prelude to the blazing orange lava seen in many volcanic eruptions.

“I’m very concerned because I left my house behind and I’m also worried about family,” said 36-year-old farmer Putu Suyasa, who fled with some of his relatives from a village eight kilometres away from the volcano.

“The mountain is spewing thicker smoke than before.” 

We have to co-operate

Mt. Agung last erupted in 1963, killing about 1,600 people in one of the deadliest eruptions in a country that has nearly 130 active volcanoes.

The airport in Bali’s capital Denpasar, a top holiday destination that attracts millions of foreign tourists every year, has been closed, a move expected to affect tens of thousands of passengers.

“I have to make sure that the runway has no ash,” said Bali airport’s general manager Yanus Suorayogi.

While there was dismay from some tourists who were unable to return to their homes and jobs, others took events in their stride.

“What can I say? We have to co-operate because this is a natural disaster,” said Indian visitor Krisna Mustafa.

Many were told that even in the best scenario it would be several days before they could leave.

“My 7:00 am flight this morning got cancelled, just when we were about to board,” said 23-year-old Indonesian tourist Merry Handayani Tumanggor.

“Now we have to stay in Bali again – the earliest we can go is on Friday, they say.” The airport on nearby Lombok island – also a popular tourist destination east of Bali – closed on Sunday as ash from Mount Agung headed in that direction, but reopened early Monday.

The Australian government put out a travel advisory on Sunday instructing travellers to exercise a high degree of caution.

“Volcanic activity may escalate with little or no notice,” it said.”Past eruptions of Mount Agung have shown this volcano’s potential to cause significant impacts… including the potential for widespread ash fall outside the declared danger area.” 

Prayers

Dozens of Balinese Hindus took part in ceremonies near the volcano on Sunday, offering prayers in the hope of preventing an eruption.

Officials have said the activity could be a magmatic eruption – one which involves the decompression of gas and results in the spewing of ash – and advised people near the mountain to wear masks.

Indonesia is the world’s most active volcanic region. The archipelago nation with over 17,000 islands lies on the Pacific “Ring of Fire” where tectonic plates collide, causing frequent volcanic and seismic activities.

Last year, seven were killed after Mt. Sinabung on the western island of Sumatra erupted, while 16 were left dead by a Sinabung eruption in 2014.