Liquid jet breakup regimes in lava fountains

Primary magma fragmentation in "fluid-dominated" (as opposed to "ash-dominated") lava fountains involves the hydrodynamic breakup of a jet of magma. Lava fountains partly resemble industrial liquid jets issued from a nozzle into a quiescent atmosphere, on which there is a vast literature. Depending on the internal liquid properties, nozzle diameter and ejection velocity, liquid jet breakup in industrial applications occurs in four regimes: (I) coarse laminar breakup (Rayleigh regime); (II) transition region between laminar and turbulent breakup (first wind-induced regime); (III) turbulent breakup at the jet surface and unstable but intact liquid core (second wind-induced regime); (IV) fully turbulent fine spray (atomization regime). Ductile magma breakup associated with regimes II, III and IV have been reproduced during the initial expansion of experimental magma fragmentation pulses as part of this study. In each experiment, volcanic rocks were re-melted at 1200 degrees C, then fragmented through the injection of compressed argon gas within a few tens of milliseconds. Three compositions were used: olivine-melilitite, alkali basalt, and basaltic trachy-andesite. Each composition was ejected at 3 and 10 MPa gas driving pressure, yielding exit velocities between 11-13 and 33-44 m/s, respectively. The ultramafic magma ejected at high speed developed quickly into a fully developed spray (regime IV), whereas the basaltic trachy-andesite ejected at low-speed initially expanded as a coherent magma mass before breaking into coarse domains (regime II). The observed variability among the experiments is linked to the relative balance among surface tension, viscosity, density, jet diameter and ejection velocity of the magma versus external aerodynamic effects acting on the jet surface. These factors, particularly viscosity and exit velocity, are also likely to control jet breakup regimes in natural lava fountains and some Strombolian pulses.