Two-phase dynamics of volcanic eruptions: Particle size distribution and the conditions for choking

Viktoriya M. Yarushina , David Bercovici , Chloé Michaut (1)

1 IPGP - Institut de Physique du Globe de Paris

Explosive volcanic eruptions are studied using a two-phase model of polydisperse suspensions of solid particles in gas. Eruption velocities depend on choking conditions in the volcanic conduit, which depend on acoustic wave propagation that is, in turn, influenced by the particle size distribution in the two-phase mixture. The acoustic wave spectrum is divided into three regions of superfast short waves moving at the pure gas sound speed, purely attenuated domain at intermediate wavelengths, and slower long waves for a dusty pseudogas. The addition of solid phases with differing particle sizes qualitatively preserves the features of two-phase acoustic wave dispersion, although it narrows the regions of short-fast and intermediate-blocked waves. Choking conditions, however, strongly depend on the number and size distribution of solid phases. Changes in particle sizes lead to variations in the choking conditions, which determine the eruption velocities and the resulting height of the erupting column. Smaller particles always exit the choking point faster than big particles, as expected. Even though particle-particle interaction is neglected, the particle distributions influence each other by momentum exchange through the gas. Therefore, the structure of the dispersion relation as well as the eruption or choking velocities and subsequent column height and particle deposition bear information on how eruption dynamics are controlled by size distribution and relative volume fractions of small and big particles. We suggest that unimodal distributions, with one dominant small particle size, favor development of vertical plinian eruptions, while bimodal distributions, with a comparable mean size, lead to pyroclastic lateral flows.

Mots clés: acoustic waves; dispersion relation; volcano acoustics; critical flow; fine ash; diversity of eruption styles

Domaines: Planète et Univers [physics]

Viktoriya M. Yarushina, David Bercovici, Chloé Michaut. Two-phase dynamics of volcanic eruptions: Particle size distribution and the conditions for choking. Journal of Geophysical Research : Solid Earth, 2015, 120, pp.1503-1522. ⟨10.1002/2014JB011195⟩. ⟨insu-03580011⟩