By John Kerr 06/09/2016


New research has offered a glimpse into the inner workings of one of the most dangerous results of volcanic eruptions – pyroclastic flows.

“They are basically mixtures of hot volcanic particles and gas that race down the flank of a volcano to destroy everything its path,” says Dr Gert Lube from Massey University, describing these volcanic juggernauts.

He and his colleagues have just published a new study in Nature Geoscience offering insights into the mysterious internal structure of these pyroclastic flows.

While there is no doubt about the destructive nature of pyroclastic flows, what is happening inside the blistering streams of rock and gas has remained something of a mystery.

Getting up close to study a flow just isn’t an option. The flows created by some volcanic eruptions can reach temperatures of up to 1,000 °C and travel at up to 700 km/h. According to Dr Lube they are “amongst the most destructive phenomena on Earth.”

Looking inside

Given the difficulty of undertaking research on an active pyroclastic flow as blitzes its way down the side of a mountain, Dr Lube and his colleagues did the next best thing. They built an immense volcano simulation indoors. “We decided that if they could not see inside one of these flows, then maybe we could replicate one,” he says.

In the simulation, one and half tonnes of (room temperature- thanks goodness) volcanic material was dumped down a see-through channel and the resulting wave of dust and debris tracked with high speed cameras.

Rethinking the pyroclastic sandwich

The flows were thought to consist of an upper and lower layers, with the gas and particles in in the cloud-like upper layer moving turbulently as they interact with the air above the flow. The lower layer consists of denser material that travels down a slope in a more fluid-like fashion.

However, Dr Lube’s research found that pyroclastic flows actually consists of not two, but three different sections behaving in very different ways.

As suggested by previous research, there was a turbulent upper zone and a denser bottom zone, but sandwiched between the two was an intermediate layer where small swirling eddies of gas and solids – called mesoscale turbulence clusters – were observed.

“Intriguingly, these mesoscale turbulence clusters control how the internal structure and the damage potential of pyroclastic flows evolves during volcanic event,” said Dr Lube.

Read more about the research on Scimex.org.

Saving lives

“Pyroclastic flows are the most common and lethal volcanic threat, and by analysing the internal structure we are laying the foundations to understand how they will behave in an eruption,” says Dr Lube.

“This research opens a new path towards reliable predictions of their motion, and will be particularly topical for hazard scientists and decision makers, because they will lead to major revisions of volcanic hazard forecasts and ultimately more effective measures for keeping people safe.”

You can hear more about the volcanic eruption generator in an earlier interview with Dr Lube on RNZ’s Our Changing World.

Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 1984.
Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 1984.

Featured image: A pickup truck flees from the pyroclastic flows spewing from the Mt.Pinatubo volcano in the Philippines, on June 17, 1991. Credit: Alberto Garcia.