FLUIDIZATION IN NATURE: PYROCLASTIC FLOWS

Pyroclastic flows are mixtures of gas and particles generated by gravitational collapse of lava domes or fall-back of eruption columns during explosive volcanic eruptions. They represent major natural hazards because they can propagate far away from the source at speeds that may largely exceed those of most natural or anthropogenic granular flows. Hazard mitigation requires the development of reliable models based on mechanistic understanding of their complex dynamics to predict flow path, velocity, momentum, and run-out distance.

It has long been hypothesized that fluidization of pyroclasts - by either endogenous gases or entrainment of ambient air - may represent one key to the emplacement of dense pyroclastic flows, and a key factor for their extraordinary mobility. Although early speculations on the role of fluidization have received substantive support, the debate is still open as to whether the apparently inviscid nature – hence large mobility – of pyroclastic flows arises mostly from fluidization or from inherent rheological patterns of fast dry granular flows. Broad uncertainties characterize fundamental aspects like the prevailing nature and source of the fluidizing gas, the extent of particle-phase stresses in rapid aerated/fluidized dense granular flows, the perturbation to classical fluidization regimes due by turbulence and shear, the influence of solids polydispersity, the dynamics of the frontal zone of the flow and its relevance to gas entrainment. The current lack of fundamental understanding, despite the extensive published literature on the subject, is largely due to limitations of fundamental research, mostly based on scaled-down physical models of granular currents.

The lecture will survey and critically discuss the literature relevant to the subject, with the aim of establishing an up-to-date mechanistic framework of pyroclastic flows embodying consideration of pyroclast fluidization.