Pyroclastic density currents (PDCs) pose substantial risk to populations living on and around active volcanoes, but their structure and internal dynamics are poorly understood. Much of this understanding is derived from interpretation of their widespread deposits. Scaled experiments are able to probe different conditions, to explore how changing flow dynamics relate to the wide variety of depositional styles observed in nature. Here we present two suites of work, first exploring the generation of spontaneous unsteadiness, and how it can impact the partitioning of sediment between dense granular under-currents, and over-riding dilute particle clouds. Second, we introduce grainsize variation to the dense granular regime and explore the formation of grading patterns. We demonstrate that unsteadiness in flow can be important in capturing different grading structures in deposits, and that granular sorting mechanisms are highly effective in thin fluidized grainflow. We conclude that this may raise challenges for the interpretation of common poorly sorted lithofacies (massive lapilli tuff) in natural deposits, as it must require substantial vertical mixing within these grainflows.

Stationary antidunes are a product of critical flow in open channel systems, but with poor preservation potential. They are related to the existence of stationary surface waves in the overriding current, but their existence in the dense pyroclastic density current regime has been unrecognized to date. Experiments presented here demonstrate that surface waves in simulated dense pyroclastic density currents show both supercritical downstream-migrating and critical stationary wave behaviour. Deposits from the Pozzolane Rosse ignimbrite (Italy) demonstrate the presence of stationary wave antidunes in deposits from dense pyroclastic currents which imply progressive aggradation from long-lived quasi-stable critical flow conditions during their emplacement. The narrow stability fields for the formation of these deposits reinforces that they are unlikely to be widely preserved in the geological record, but highlights that dense pyroclastic density currents cannot be assumed to be simply supercritical flows, and they may be substantially slower than over-riding dilute currents.