The PDC flow units problem: deposit heterogeneity from varying cohesive behaviour and sediment flux

Pyroclastic Density Currents (PDCs) are rapidly moving, high-temperature currents of heterogeneous volcanic material and gas that can surmount topographic barriers and can form extensive deposits (ignimbrites) far away from source. They are a highly destructive, and a deadly hazard (~100 million people live at risk from PDCs1 ). Flow units are interpreted as deposits of individual PDCs and are defined by markers of hiatus in activity (such as ash fallout, reworking or paleosols). However, it has been shown that the arrangement of flow units can vary spatially within a deposit (from proximal to distal exposures, and laterally across drainages), recording a contradictory picture of PDC activity during a single eruption at different locations2 . The stratigraphic record of flow units within an ignimbrite may have been influenced by a number of factors, such as current unsteadiness or syn-depositional processes. Formation of ash within a PDC can be from magma fragmentation and/or by comminution processes as the current propagates. Entrainment from both internal and external environments can decrease temperatures and introduce water vapor (e.g. exsolving juvenile magma, external hydrological factors, combusting plant matter, water-laden sediment). These factors will likely affect cohesive and frictional behaviors within the flow causing internal variations affecting both the current dynamics and resulting deposits. This project will investigate how cohesive and frictional behaviors within a PDC may impact its ability to transport, deposit and erode material. Thus, impacting the flow unit record and determining the extent to which single pulsatory currents can be misinterpreted as separate flow events during major eruptions. Flume experiments3 will explore the significance of cohesion in influencing flow dynamics and resulting deposit behaviors, by exploring the role of fines and water vapour. Fieldwork will be undertaken to consider bedform and stratigraphic relationships of flow unit marker beds to ground-truth the experiments. This research will improve our understanding of the dynamics of PDCs, how they react to variations in internal and external conditions and factors that control the depositional record of PDCs. Better interpretation of ignimbrite successions is pivotal in improved volcanic hazard assessment.