Gregory Smith, Pete Rowley , Rebecca Williams , Guido Giordano, Matteo Trolese, Aurora Silleni, Dan Parsons, Samuel Capon

Pyroclastic density currents are a life-threatening volcanic hazard. Our understanding and hazard assessments of these flows primarily rely on interpretations of their deposits. The occurrence of stratified layers, cross-stratification, and dune bedforms in these deposits has been assumed as indicative of dilute, turbulent, flows causing traction-dominated deposition. Here we show, through analogue experiments, that a variety of dune bedforms can be produced by denser, aerated, granular currents, including regressive dune bedforms that are formed in waning flows by an upstream-propagating granular bore. We are able to, for the first time, define phase fields for the formation of dune bedforms in PDC deposits. We examine how our findings impact the understanding of bedform features in outcrop, using the example of the Pozzolane Rosse ignimbrite of the Colli Albani volcano, Italy, and thus highlight that interpretations of the formative mechanisms of these features observed in the field must be reconsidered.