Sedimentary peels from pyroclastic density current deposits: revealing basal flow-boundary processes.

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Dilute pyroclastic density currents (PDCs) represent some of the most mobile and dangerous events occurring during explosive volcanic eruptions. They are a transport process of gas and particle mixtures at low concentration that belong to the class of particulate density currents. They can be directly emitted from a vent or be produced kilometres away from the eruptive centre by flow stripping from dense pyroclastic flows. Their low density is responsible for their high mobility, allowing them to overcome topographic obstacles and to date, little is known of their internal dynamics.In a dilute PDC, particles play the role of both driving momentum and make up the resultant deposit. Sedimentological studies on the cross-bedded deposits left by dilute PDCs thus represent the richest and most revealing source of natural data for these flows. A unique high-resolution dataset has now been acquired by impregnating the deposits of the 2006 eruption of Tungurahua volcano (Ecuador) with a solidifying resin during extensive field campaigns. This technique permits to produce sedimentary peels that preserve both the original grain organization and stratification. The resulting data-set is a rare tool for fine-scale investigation and educational purposes. As a complement to the sedimentary peels, two surveys of ground penetrating radar and terrestrial laser scanner mapping provide additional information on the three dimensional geometry of the deposits, both internal and surficial. We plan a detailed investigation of these datasets through laboratory characterisation in combination with specially designed, targeted, analogue experiments. Our goal is to gain unique insights into the dynamics of dilute PDCs and the morphodynamics of grain transport at the flow-bed interface. The peel dataset will also serve as the central piece of an educational dissemination tool for students, researchers, and the public.