Large-scale volcanic deposit fluidization by dilute pyroclastic density currents

There is increasing evidence that fine-grained deposits of pyroclastic density currents can be remobilized on a large scale, resulting in concentrated flows. These flows can be a major hazard; for example, at Soufrière Hills Volcano (Montserrat) in 1997, some travelled beyond the designated danger zone to inhabited areas. Despite their hazard potential, the scale and generation mechanism of these flows are poorly understood. Here we demonstrate using laboratory experiments and numerical modelling that decompression following the passage of dilute pyroclastic density currents can cause rapid deposit fluidization over areas of several square kilometres and to depths of tens of centimetres. The fluidized volume can be substantially greater than that deposited by the triggering pyroclastic density current because of remobilization of previously emplaced deposits, and the fluidized volume can flow even on slopes of a few degrees. The capacity for remobilization of a deposit is limited by its particle cohesion, which rapidly increases with atmospheric humidity. This mechanism of fluidization should alert us to an under-appreciated volcanic hazard of long-run-out pyroclastic flows that can be generated by remobilization very rapidly and with little warning.