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Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard. Our understanding and hazard assessments of these flows rely on interpretations of their deposits. The occurrence of stratified layers, cross-stratification, and bedforms in these deposits has been assumed as indicative of dilute, turbulent, supercritical flows causing traction-dominated deposition. Here we show, through analogue experiments, that a variety of bedforms can be produced by denser, aerated, granular currents, including backset 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 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.

The gravitational collapse of eruption columns generates ground-hugging pyroclastic density currents (PDCs) with highly variable temperatures, high enough to be a threat for communities surrounding volcanoes. The reasons for such great temperature variability are debated in terms of eruptive versus transport and emplacement processes. Here, using a three-dimensional multiphase model, we show that the initial temperature of PDCs linearly correlates to the percentage of collapsing mass, with a maximum temperature decrease of 45% in the case of low percentages of collapse (10%), owing to an efficient entrainment of air into the jet structure. Analyses also demonstrate that column collapse limits the dispersal capabilities of volcanic plumes, reducing their maximum height by up to 45%. Our findings provide quantitative insights into the mechanism of turbulent mixing, and suggest that temperatures of PDC deposits may serve as a marker for determining column collapse conditions, which are of primarily importance in hazard studies.

We studied the temperature of emplacement of pyroclastic density currents deposits that destroyed and buried the Villa dei Papiri, an aristocratic Roman edifice located just outside the Herculaneum city, during the 79AD plinian eruption of Mt Vesuvius (Italy). We used the Thermal Remanent Magnetization of lithic clasts embedded in the PDC deposits to retrieve accurate measures of the deposit temperature. The deposit shows substantial internal thermal disequilibrium. In areas affected by convective mixing with surface water or with collapsed walls, temperatures average at around 265°C (min 190°C, max 300°C). Where the deposits show no evidence of mixing with external material, the temperature is much higher, averaging at 350°C (min 300°C; max 440°C).Numerical simulations and comparison with temperatures retrieved at the very same sites from the reflectance of charcoal fragments indicate that such thermal disequilibrium can be maintained for time-scales well over 24-48 hours, i.e. the acquisition time for common *Manuscript Click here to view linked References proxies of emplacement temperatures. We therefore reconstructed in detail the history of the progressive destruction and burial of Villa dei Papiri and infer that the deposit temperature is virtually the same as that of the incoming PDCs. This conclusion is very important as it solves a long standing debate on the actual relationships between the PDC deposit temperatures and those of the parent flows. Here we suggest that PDC deposit temperatures are excellent proxies for the temperatures of basal parts of PDCs close to their depositional boundary layer and therefore that mapping of deposit temperatures gives essential insights for thermal processes within PDCs and during their interaction with the affected environment.