How dangerous is Bali's Mount Agung and what action has been taken?

What’s happening?

Mount Agung, a volcano on the Indonesian island of Bali, has been in a state of unrest for several weeks and is now emitting smoke and ash. Mudflows, known as lahars, are running down the mountainside and loud booms have been heard. Experts say a large eruption could happen imminently.

What action has been taken?

The volcano alert status has been set at the highest level and 100,000 people have been told to evacuate from a six-mile (10km) radius around the volcano, about 40,000 of whom have left the area. The airports on Bali and Lombok have been closed.

David Rothery, professor of planetary geosciences at the Open University, said the move to evacuate was wise. “It is very hard to have people in emergency shelters for such a length of time – there are all sorts of social issues that can arise – but they have done it and people have largely complied. And those few people still dangerously close to the volcano I hope will now be getting away because there could be something catastrophic about to happen.”

Why might Mount Agung be about to erupt?

It’s all down to pressure.Geological processes at the ocean floor mean molten rock is rising upwards with dissolved gases, including water vapour and sulphur dioxide. As the mixture – known as magma – rises, the gases form bubbles. As the pressure builds, an eruption becomes more likely.

Has Agung erupted before?

Yes, the last eruption was in 1963 and killed more than 1,000 people. “Far more would die this time in a similar eruption [without evacuation] because population density has gone up,” says Rothery.

David Pyle, professor in earth sciences at the University of Oxford, said: “The evacuated area could be totally devastated by this. It is a young active volcano, and it has a long history of repeated explosive eruptions. It is the sort of volcano that probably erupts once every hundred years or so.”

How hard is it to predict an eruption?

Telltale signals picked up by those monitoring the volcano, including changing patterns in the frequency and intensity of earthquakes beneath it, have suggested since September that an eruption is brewing, and evacuations were also carried out then. “It wouldn’t have been a surprise if it had erupted straight away, but in fact the earthquake activity died away,” said Pyle.

The last eruption, which began in February 1963, involved about a month of small explosions, ash emissions, lava flows and mudflows. The big explosions, including pyroclastic flows (superheated and fast-flowing currents of gas and rock), came later, in March and May, with activity lasting nearly a year.

Predicting what happens next is largely based on what a particular volcano has done before. In this case, scientists have data from only one recent eruption, so for now they are assuming the activity is likely to follow a similar pattern.

What threat does the volcano pose?

Although mudflows and falling ash are hazardous, the biggest threat to humans would come from pyroclastic flows which can travel at 70 miles per hour. The likelihood of pyroclastic flows occurring depends on how the column of ash develops.

“If the ash just settles out of the sky, depending on how thick the ashfall is, you can kill crops or collapse roofs of buildings and so on,” said Rothery. “But if the rising column gets too heavy to support itself and it collapses, that will produce a pyroclastic flow, rushing down the sides of the volcano. Pyroclastic flows are absolutely deadly. They are hot … and they are very, very destructive.”

Will it affect tourists?

The area around Mount Agung is not a major tourist hotspot. However, Bali airport has closed, with about 59,000 people thought to have been stranded on the island.

How long would an eruption last?

It’s hard to tell. Based on the previous eruption, the activity seems unlikely to subside any time soon. “Our starting point would be that we’d expect the activity to play out over the next few weeks to months,” said Pyle, adding that the principle hazards at present were the mudflows and ash – which can cause disruptions both on the ground and for air transport, since ash can affect jet engines.

“Monitoring agencies will be running weather simulations to predict where the ash cloud will go,” he said, noting that whether flights were grounded could change from day to day.