The role of magma composition and water content in explosive eruptions: 2. Pyroclastic dispersion dynamics

Abstract The net effect of magma composition and water content on the behavior of explosive volcanic eruptions was investigated by the use of thermofluid-dynamic models and numerical simulation. The two-phase flow models employed allowed the description of the eruptive process from the conduit entrance at the top of the magma chamber up to the pyroclastic dispersion process in the atmosphere. In this paper, vent conditions computed in the companion paper by Papale et al. [Papale, P., Neri, A., Macedonio, G., this issue. The role of magma composition and water content in explosive eruptions: I. Conduit ascent dynamics. J. Volcanol. Geotherm. Res.], as a function of magma composition and water content, are employed as boundary conditions at the crater base of the pyroclastic dispersion model. The eruptive mixture was described as a two-phase gas–particle fluid. Pyroclasts are represented as solid particles of one size whereas the gas phase consists of superheated water vapor leaving the vent and atmospheric air. Simulations were performed on physical domains extending several kilometers in both radial and vertical directions and for several minutes of real time. The results clearly show the net effects of the anhydrous magma composition, crystals, and water content on the spatial and temporal distribution of pyroclastic material during the first minutes of eruption. In detail, the results indicate a strong influence of magma composition on eruption intensity, whereas the eruptive style, i.e., the generation of Plinian, transitional, or collapsing columns, appears to be mainly controlled by the total water content of the magma, i.e., the weight fraction of water with respect to the entire magmatic mixture. The main effect of the crystals is to reduce the total water content of the mixture and lead to a less buoyant behavior of the column. As a summary, for the investigated ranges of variations, simulation results indicate a progressive change to a more collapsing behavior of the column whenever water-poorer or crystal-richer magma is erupted from the vent with the anhydrous composition of magma affecting just the intensity of the eruption. For most conditions, such a conclusion does not appear to be strongly dependent on a relatively large variation of vent diameter and, therefore, of the scale of the event. Simulation results appear to be consistent with field studies even though more complete data on specific eruptions are recommended for a more thorough comparison.