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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.