Pyroclastic flows, fast-moving currents of hot gas and volcanic matter, achieve great speeds and distance by traveling on a self-generated layer of air with little friction according to a study published in Nature Geoscience this week.

Volcanic eruptions can produce pyroclastic flows when part of the eruptive column collapses or as segments of a lava dome collapse. Flows can reach temperatures of up to 1000 °C and can travel many kilometres from their source. They present a deadly hazard and are responsible for an estimated 50% of volcanic fatalities globally. However, little is known about how they move so far and so fast.

Gert Lube and colleagues conducted large-scale eruption simulation experiments. They released over one tonne of hot volcanic material down a 35 m long instrumented runout section, and recorded the results on high-speed videos. The authors observed that an air-rich layer develops at the base of the pyroclastic flow. The current self-organizes when the rate of change of velocity is high as it flows over the ground. An area of high-pressure densely packed volcanic material forms close to the base of the currents. Then, gas moves towards the ground and creates a layer of air between the ground and high-density region above. The air layer allows the current to flow over the ground with little friction.

The authors suggest that these findings may help to enable more-accurate hazard assessments of future pyroclastic flow runout distances and speeds.