Deadly danger beneath Bali’s tropical paradise

Towering 3,000 metres above the island of Bali, Mount Agung has for decades served as a dramatic backdrop for holiday snaps. But now it is showing its true nature – a volcano bursting into life after more than 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.

Seething hot rock and lava are welling up from depths of 10 kilometres or more, and pouring up into the volcano’s body. 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 eruption in 1963, which rumbled for almost a year and killed almost 2,000.

Indonesia’s disaster management authority moved 100,000 people out of a 10km exclusion zone, only for the tremors to subside.

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

The result was a phreatic eruption of the kind seismologists believe destroyed the 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 on the type of magma.

If it is relatively thick and sticky, it can contain high-pressure gas that 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.

But it can also trigger the most feared outcome: pyroclastic flows. Reaching temperatures of 1,000°C and exploding out from a volcano at up to 700kph, they can kill thousands in seconds.

Pyroclastic flows from Mt Vesuvius wiped out the Roman cities of Herculaneum and Pompeii in 79AD.

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

Evacuations are reported to have been well planned and orderly, although many people are said to have refused to leave their homes.

This may come as small comfort to about 59,000 tourists now trapped on Bali after the closure of the international airport at Denpasar.

But with ash clouds already soaring kilometres into the sky, the airlines have little alternative.

If 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 this way, with nine cases of engine failure.

With the eruption more than 50km from Denpasar, by far most of those on Bali have little to fear over the coming days. But in the longer term, its effects are likely to be much more widely felt.

After the 1963-64 eruptions, farmland around Mt Agung was ruined for years. The ash and gases released reflected the sun’s light back into space, cooling the planet by about 0.25°C for a year.