Experimental Study of the Generation of Pore Gas Pressure in Pyroclastic Density Currents Resulting From Eruptive Fountain Collapse

Pyroclastic density currents formed through collapse of eruptive fountains commonly have runout distances of the order of tens of kilometers. A possible cause of this high flow mobility is elevated interstitial pore gas pressure, which may have various origins. We investigated experimentally the generation of pore pressure at the impact zone of an eruptive fountain, where concentrated pyroclastic density currents emerge from compaction of a free falling gas-particle mixture. We simulated pyroclastic fountain collapse by releasing glass beads of mean sizes of 29-269 mu m from a hopper at height of 3.27 m above a 5 m-long horizontal channel, and we measured pore air pressure in the impact zone. During free fall, the granular mixtures accelerated and expanded to reach particle concentrations of 1.6-4.4 vol.% before they impacted the base of the channel. Upon impact, the particles accumulated to form concentrated granular flows with particle concentrations of 45-48 vol.% and pore air pressures that indicated almost full weight support for particle sizes <= 76 mu m. Both the amount of pore pressure in the impact zone and the flow runout distance increased as we decreased the particle size and hence the hydraulic permeability of the concentrated granular mixtures. Our results suggest that pore gas pressure in concentrated pyroclastic density currents can be generated at the impact zone of collapsing fountains and that small particle size conferring low permeability and long pore pressure diffusion timescale is one of the main causes of long flow runout distances. Collapse of volcanic eruptive fountains leads to the formation of pyroclastic density currents that can propagate over long distances. This mobility may be due to the high pore gas pressure in the pyroclastic mixture, which reduces intergranular friction. The origin of the pore gas pressure inside these mixtures is not well understood. We performed laboratory experiments to study the free fall from a hopper, and the subsequent impact on a rigid channel base, of dilute granular mixtures with particle concentrations of a few volume percent and different mean grain sizes. High-speed videos and pressure measurements showed that the granular mixtures compacted upon impact to form dense flows with high air pore pressure. As particle size decreased, both the pore pressure generated at the impact zone and the flow runout distance in the channel increased. For particle equal or smaller than 76 mu m, the generated pore pressure was enough to counterbalance particle weight. Simple scaling arguments imply that full weight support would occur in nature for larger particle sizes. These results suggest that pyroclastic fountains can lead to the formation of concentrated ground-hugging currents with high interstitial pore gas pressure, and that particle size is key to controlling their travel distance. We present experiments on pyroclastic fountains by considering the impact of free-fall dilute granular mixtures on a horizontal channel baseAccumulation of particles generates dense flows with pore gas pressurePore gas pressure and flow runout distance increase as particle size decreases