https://www.scientificamerican.com/article/deadly-volcanic-flows-glide-on-their-own-cushion-of-air/

When Guatemala’s Volcán de Fuego erupted in June 2018, it sent a billowing hot cloud of gas, ash and rock careening down the slope of the mountain. In the many smartphone videos of the eruption, the rush of debris often looks reassuringly distant—until suddenly it’s not.

How such pyroclastic flows, as they are called, can travel so quickly has long baffled volcanologists and disaster planners alike, leaving communities in volcanically active areas at risk. Now, though, a new study offers an explanation: forces within the flows create a cushion of air, allowing massive amounts of hot rock to slide like a puck on an air hockey table. “It’s a bit like the flow is surfing on something extremely low-friction,” says Gert Lube, a volcanologist at Massey University in New Zealand who led the research.