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Sediment waves and the gravitational stability of explosive eruption columns and ash clouds : towards a new classification of explosive eruptions Gilchrist, Johanand


It is increasingly recognized that the gravitational stability of volcanic jets is governed by complex ash-pumice-gas (multiphase) interactions and the mechanics of turbulent entrainment in the lower momentum-driven (fountain) and upper buoyancy-driven (plume) regions of these flows. We use analogue experiments on relatively dense particle-freshwater and particle- saltwater jets injected into a linearly-stratified saltwater layer to revisit, characterize and under- stand how transitions among Buoyant Plume (BP), Total Collapse (TC) and Partial Collapse (PC) multiphase jet regimes in a traditional source strength ( Ri0) - particle concentration ( phi0) parameter space are modified by particle inertial effects expressed through a Stokes number (St) and particle buoyancy effects expressed through a Sedimentation number (Sigma0). We show that “coarse particles” (0.1 ≤ St0 ≤ 10) modify significantly published conditions favouring BP and TC, causing the transition between these endmember regimes to occur smoothly over a PC regime that represents the majority of the Ri0 - phi0 parameter for eruptions. Large volume annular sedimentation waves excited periodically in PC and TC regimes produce terrace deposits and lead to “phoenix clouds” spreading at multiple altitudes. Consistent with most eruptions having fluctuating source strengths, we carry out additional experiments on “unsteady” jets. We identify an additional key source Pulsation number Pu0 and develop a new Ri0 - phi0 - Pu0 parameter space for jet regimes. Applied to volcanic jets generally, and to data we recover from Doppler radar monitoring of two erupting volcanoes, we develop a new set of conceptual models for steady jets in the BP, TC and PC regimes and for unsteady jets in newly defined “Connected Thermals” and Discrete Thermal” regimes, all of which make readily-tested links among source parameters, column heights, sedimentation processes, cloud structures and deposit architectures. The predictions for cloud structures and deposit architectures agree with field-based and remote-sensing observations made for well-studied historic and pre-historic eruptions and explain the origin of common but enigmatic near-field features of explosive eruption deposits. The predictive power of our new Ri0 - phi 0 - Pu0 classification for explosive eruptions provides exciting new pathways for future observational and modelling studies.

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