Mount Agung: the deadly danger beneath Bali's picture postcard mountain

Half a century since its last eruption, Mount Agung serves as a reminder of how the tropical paradise sits on the most volcanic land on Earth

epaselect epa06353121 A long expossure photograph shows the Mount Agung volcano spewing hot volcanic ash as seen from Amed, Karangasem regency, Bali, Indonesia, 26 November 2017 (issued 27 November 2017). According to media reports, the Indonesian national board for disaster management raised the alert for the Mount Agung volcano to the highest status and closed the Ngurah Rai International Airport in Bali due to the ash cloud rising from the volcano.  EPA/RONI BINTANG

Towering 3,000 metres above the island of Bali, Mt Agung has for decades served as a dramatic backdrop for holiday snaps. But now it is showing its true nature as a volcano bursting into life after over half a century of quiescence.

Deep below Mt Agung a colossal slab of the Earth’s crust carrying Australia and India is being forced below another bearing Indonesia.

In the process, seething hot rock and lava are welling up from depths of 10km or more, and pouring up into the body of Mt Agung.

The symptoms of these primordial events have been felt for weeks.

First came the wisps of steam and tremors, rekindling memories of Mt Agung’s last eruption in 1963, which rumbled on for almost a year and left almost 2,000 dead.

The Indonesian Disaster Management Authority began an evacuation, moving over 100,000 out of a 10km exclusion zone – only for the tremors to subside.

But just as people began to think about returning to their homes, Mt Agung growled a warning. The searing-hot magma within it had reach a layer of water and turned it into high-pressure steam with explosive speed.


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The result was a so-called phreatic eruption of the kind seismologists believe destroyed the entire Indonesian island of Krakatoa in 1883, killing 40,000.

This initial outburst has now given way to an eruption driven by magma. What happens next depends critically on what that magma is like.

If it’s relatively thick and sticky, it can contain high-pressure gas which bursts out of the volcano with devastating violence.

The gas shreds the magma so finely it cools and turns into ash, huge clouds of which are already billowing over Mt Agung.

But it can also trigger the most feared outcome of an eruption: pyroclastic flows. Reaching temperatures of around 1,000C and exploding out from a volcano at up to 700km per hour, they can bring death to thousands in seconds.

It was pyroclastic flows from Mt Vesuvius which wiped out the Roman cities of Herculaneum and Pompeii in 79 AD.

While seismologists fear that Mt Agung may release pyroclastic flows, they have also been quick to praise the actions of the Indonesian authorities.

The evacuations are reported to have been well-planned and orderly, though many people are said to have refused to leave their homes.

The IDMA is seen as highly professional in dealing with such emergencies – a reputation made necessary by the 130 active volcanoes in the republic.

This may come as small comfort to the estimated 59,000 tourists now trapped on Bali following the closure of the international airport at Denpasar, the island’s capital.

But with ash clouds already soaring thousands of metres into the sky, the airlines have little alternative.

Sucked into jet engines, the ash damages turbine blades and chokes filters, leading to engine failure. Since the 1950s, at least 26 aircraft have been severely affected after running into ash clouds, with at least nine cases of engine failure.

With the eruption over 50km from the capital, the vast majority of those on Bali have little to fear over the coming days.

In the longer term, however, its effects are likely to be much more widely felt. Following the 1963-4 eruptions, the farmland surrounding Mt Agung was ruined for years.

The ash and gases released into the atmosphere even reflected the sun’s light back into space, cooling the entire planet by around 0.25C for a year.

Whether Mt Agung’s latest outburst will have even greater consequences remains unclear.

Either way, it serves as another reminder of the tropical paradise and primordial hell that co-exist in Indonesia, the most volcanic land on Earth.

Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK