Spontaneous Unsteadiness and Sorting in Pyroclastic Density Currents and their Deposits

Abstract

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.