Abstract

Explosive volcanic eruptions eject dense, hot, multi-phase jets which can exhibit several behaviours. The evolution of these flows in the atmosphere crucially depends on the rate of entrainment of surrounding air into the jet, on the dynamics of the particles within the jet and on the thermal exchanges between the particles and gas. If the entrainment and the heating of atmospheric air within the jet is important, the density of the volcanic mixture can become smaller than the one of the atmosphere and the jet forms a buoyant eruption column. Otherwise, the jet naturally collapses producing pyroclastic density currents. The study of the eruptive deposits of the Montagne Pelée volcano (Martinique, F.W.I) shows that these different dynamic behaviours often follow one another during a single eruption. However, the theoretical predictions for a column collapse are in disagreement with field data. A new model of turbulent entrainment of surrounding fluid within the flow is presented to reconcile both. Observations of historical eruptions also reveal that a third dynamical regime exists in which the volcanic jet separates into a buoyant rising part and a denser collapsing part. Two experimental studies are presented to investigate this particular regime and the stability of the buoyant rising part.