Enhanced Mobility in Concentrated Pyroclastic Density Currents: An Examination of a Self‐Fluidization Mechanism

Pyroclastic density currents (PDCs) are a significant volcanic hazard. However, their dominant transport mechanisms remain poorly understood, in part because of the large variability of PDC types and deposits. Here we combine field data with experimental and numerical simulations to illuminate the twofold fate of particles settling from an ash cloud to form the dense PDC basal flow. At solid fractions >1 vol %, heterogeneous drag leads to formation of mesoscale particle clusters that favor rapid particle settling and result in a mobile dense layer with significant bed weight support. Conversely, at lower concentrations the absence of particle clusters typically leads to formation of poorly mobile dense beds that deposit massive layers. Based on this transport dichotomy, we present a numerical dense‐dilute parameter that allows a PDC's dominant transport mechanism to be determined directly from the deposit geometry and grainsize characteristics. Key Points The first numerical modeling of self‐fluidization of mesoscale clusters is presented We illuminate the role of normal stress reduction on the frictional properties of dense PDCs New mechanism to explain the dichotomy of dense dilute regimes of volcanic flows at all scales is presented