They're one of the most fearsome natural phenomena known to man: avalanches of super-hot gas and rock that tear down the sides of erupting mountains at speeds of up to 700km and obliterate landscapes for tens of kilometres around.
One of the most violent eruptions recorded in New Zealand - the Hatepe or Taupo eruption that would have created effects visible in China and Rome around 1800 years ago - produced a devastating 1.5km-high pyroclastic flow that covered the landscape with ash and pumice for 80km.
The danger of pyroclastic flows, or pyroclastic density currents, have long been known: the buried city of Pompeii is just one example of a killer pyroclastic flow.
Now scientists at Massey University have made the first observations of the inner-workings of these deadly torrents, which are responsible for about half of fatalities from volcanic eruptions, and threaten half a billion people worldwide, including many communities in New Zealand.
For years, the mechanics of the flows have been a hotbed of debate between earth scientists, geophysicists and applied mathematicians, each offering their own explanations of what may be happening inside.
"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," said Dr Gert Lube, who has co-authored a major paper published today in the journal Nature Geoscience.
The research sought to create a quantitative view inside the flows to define how two separate transport regimes - non-turbulent underflow and fully turbulent ash cloud-regions - were able to harmonise and control the severity of the flow.
However, measuring the inside of an avalanche of several tonnes of rock, gas and ash has proven impossible because of the heat and destructive force of the flows.
"We decided that if they could not see inside one of these flows, then maybe we could replicate one," Lube said.
This involved using the university's one-of-a-kind eruption simulator, which works by dropping ash and pumice down a narrow channel while high-speed cameras and sensors capture the data, to synthesise the natural behaviour of the flows in unique large-scale experiments.
The results showed that the currents met in a previously unrecognised turbulent middle zone, meaning there were not two currents but three.
"Inside this middle zone, the gas-particle mixture behaved fundamentally different from the turbulent suspension cloud above and the particle-rich avalanche of pumice below," Lube said.
Instead, the volcanic particle spontaneously associated in a pattern of particle clusters called "mesoscale clusters".
"Intriguingly, these mesoscale turbulence clusters control how the internal structure and the damage potential of pyroclastic flows evolves during volcanic event," Lube said.
"This 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.
"This research replaces the existing theories and long-standing paradigms that have underpinned how we understand and ultimately protect people against the flows."
The research represents a cross-campus and international collaboration effort funded through the Marsden Fund and the Natural Hazards Research Platform.
The study was co-authored by Dr Eric Breard, Professor Jim Jones and Dr Anja Moebis, and Massey researchers also received support from scientists from the University of Auckland, Georgia Institute of Technology in Atlanta and the State University of New York.
• If a large volume of volcanic debris is thrown quickly from a volcano, the eruption column can collapse, like pointing a garden hose directly up in the sky. As the eruption column collapses it can transform into an outwardly expanding flood of hot, solid ejecta in a fluidising gas cloud. This is known as a pyroclastic flow, or surge if the flow is dilute and turbulent.
• The flow direction may be topographically controlled. Flows and surges often travel at speeds up to 200km/h, and totally destroy the areas they cover.
• Flows and surges may be up to several hundred degrees celsius, and can start fires. Some pyroclastic surges are cooler, usually less than 300C, and often deposit sticky wet mud.
• Pyroclastic flows and surges represent the most destructive manifestations of volcanic activity. People caught in the direct path of a flow or surge are most unlikely to survive. Those who are lucky enough to survive will be badly injured. Buildings offer some protection at marginal areas of flows and surges but they are often destroyed. The best protection is to leave before it happens.
Source: GNS Science