A unifying model for pyroclastic surge genesis and pyroclastic flow fluidization

International audiencePyroclastic density currents are hot and fast ground−hugging mixtures of volcanic fragments and gases, which represent a major threat to people living near to explosive volcanoes. Mechanisms causing the separation into the concentrated (the pyroclastic flow) and dilute (the pyroclastic surge) layers, as well as the mechanism causing their remarkably high mobility are still unclear. Here we present a conceptual model based on field observations of lava dome collapses, laboratory experiments, and numerical modelling that unifies these mechanisms. Our model shows that they are caused by the fall of fine volcanic particles onto steep, irregular topography. The ambient air entrapped during the fall both creates the pyroclastic surge through elutriation and induces high fluidity in the pyroclastic flow by increasing its pore pressure. Our conclusion reveals the importance of topography in the destructive capacity of pyroclastic density currents