Fluidization in pyroclastic flows T.H. Druitt (Université Blaise Pascal & CNRS, Clermont-Ferrand, France) G. Bruni, P. Lettieri, J. Yates (University College, London) Small volume pyroclastic flows are highly concentrated granular avalanches of hot (200-700 °C) volcanic debris fluidized, or partly fluidized, by escaping gases. They form by gravitational collapse of lava domes, by fallback of vertical eruption columns, or by rapid sedimentation from turbulent pyroclastic suspensions. Flows of different origins exhibit different degrees of friction reduction by fluidization. When fluidized at high temperature, and while being agitated mechanically (probably analagous to shear in the natural system) some pyroclastic flow materials pass through three distinct states as the vertical gas velocity is increased: (1) aerated, (2) homogeneously fluidized, and (3) bubbling states. Defluidization from high initial gas velocity, as might occur progressively in a propagating pyroclastic flow, takes place first by bubble evacuation, then by homogeneous hindered settling with progressive aggradation of a basal aerated layer. Friction in the aerated layer then increases from the base up by pressure diffusion in the loosely packed state until degassing is complete. Recent work has placed certain constraints on the origin of the gases that generate fluidization in pyroclastic flows. The development of physically realistic models of pyroclastic flows offers an important challenge for the future.