Volcanic rock and gas ride on a carpet of air

Often with little warning, pyroclastic flows—volcanic eruptions composed of gas, rock, and ash—obliterate everything in their path. The eruption of Mount Vesuvius in 79 CE buried the city of Pompeii and its inhabitants under several meters of ash. Gert Lube (Massey University), Eric Breard (University of Oregon), and their colleagues created analogues of pyroclastic flows in a lab to figure out how they travel meters per second for tens of kilometers without slowing down. The researchers learned that the flows redirect air downward to minimize both internal and ground-surface friction.

Measuring the dynamical properties of pyroclastic flows is dangerous, so Lube and several collaborators developed a facility in New Zealand known as PELE—for pyroclastic flow eruption large-scale experiment—that can safely simulate them. PELE operates by combining air heated up to 130 °C with 1000–1300 kg of rock and ash from the last eruption of New Zealand’s Taupo volcano, more than 1800 years ago. The mixture then falls freely down a vertical chute to an inclined channel where 10-m-high pyroclastic flows are created. A denser particle layer is overridden by a dilute, turbulent layer.

Lube, Breard, and their colleagues found that the kinetic friction acting on the particle layer varied with depth. The friction coefficient ranged from 0.2 to 0.3, in agreement with natural volcanic deposits. The researchers observed that a basal layer developed in the lower third of the particle layer that had even smaller values ranging from 0.05 to 0.21. That’s possible because of the low particle concentration in the basal layer. In the series of images shown here, the bottom meter of an experimental flow passes an observation point; the arrows point to the boundary between the basal layer, with its low particle concentration, and the overriding turbulent ash cloud.

A carpet of air at the bottom of the pyroclastic flow acts as a lubricant. In particle–gas flows, the gas responds more quickly to pressure gradients than particles do. Lube and his colleagues found that the high shear rates in pyroclastic flows generate a pressure maximum above the base of the flow. As the air gets squeezed toward the lower-pressure basal layer, it effectively decreases that layer’s particle concentration. The gas moves faster than the particles, so the pyroclastic flows can ride the air for long distances without slowing down. (G. Lube et al., Nat. Geosci., 2019, doi:10.1038/s41561-019-0338-2 ; thumbnail photo credit: Qfl247, CC BY-SA 3.0 .)