Bali volcano: What is pyroclastic flow? Mount Agung to spew out hot rock, ash and gas

The people of Bali are scrambling to get out of range of Mount Agung, which officials fear could erupt in a “matter of hours”.

About 130,000 Balinese locals have already been evacuated, many from within the 7.5 mile danger zone that surrounds the .

Mount Agung has not erupted since 1963, when a series of eruptions killed more than 1,100 people, injuring about 300 more. 

Most of the victims were killed by powerful pyroclastic flows.

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What is a pyroclastic flow?

A pyroclastic flow is a deadly cocktail of superheated ash, lava blocks and volcanic gas that can travel at hundreds of miles per hour.

According to the United States Geological Survey (USGS), pyroclastic flows are made up of two main parts. 

A lower layer consists of a flow of fragments and rocks that tear across the ground, while the upper section is a billowing cloud of scolding ash.

Professor Ray Cas told Sky News Australia that a pyroclastic flow poses a “greatest volcanic hazard” to the people of Bali.

He said: “The worst case scenario is if that column of ash and gas actually becomes overloaded with rock debris and collapses down to the ground surface, acquiring enough momentum to produce very dramatic turbulent flows of gas, ash and rock debris over the landscape that we call a pyroclastic flow.

“They’re incredibly dynamic – they flow irrespective of topography and they flow at temperature of hundreds of degrees centigrade.

“So they are the greatest volcanic hazard, they represent the greatest threat to life.” 

Pyroclastic flows have the power to decimate anything it their path.

Debris the size of boulders can be propelled along the ground at speeds greater than 50mph and rocks caught in the pyroclastic flow can reach 700C.

The vast amount of debris caused by pyroclastic flows can lead to widespread flooding if streams or rivers are blocked. 

Hundreds of evacuation camps have been set up across Bali, but the Indonesian government insists that it is still safe to travel to the island.

Ages Santoso, director of air transport, said: “Bali tourism is safe. Do not spread misleading news that Bali is not safe because Mount Agung is on the highest alert status. Please come and visit Bali.”