Fragmentation efficiency of low viscosity magmas: Which mechanisms control ash formation?

Laura Pioli

First

;Andrew Harris;Matthew Edwards

2017-01-01

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Abstract

Explosive fragmentation of basaltic magmas display peculiar features with respect to their silicic counterpart. It is generally believed that low viscosity magma fragmentation is less efficient and generate larger clasts. However, even basaltic explosions can generate large proportions of fine material (ash), suggesting that the mechanisms controlling mafic explosions are poorly known. The fragmentation process depends on the effective physical properties of the bulk magmatic mixture: brittle magmatic fragmentation is expected when the magma characteristic decompression time is smaller than its relaxation time; when these conditions are not met, fragmentation is inertia-driven (i.e. ductile). Inertia-driven fragmentation is typical of low viscosity magmas. In basaltic explosive eruptions, the most likely processes determining magmatic fragmentation are gas shearing in annular flow in the conduit, jet acceleration in fountaining activity, gas slug rupture in Strombolian explosions, and bubble expansion phenomena in larger explosions. In these cases the liquid rupture is related to the deformation and instability of the liquid/gas interface and its consequent disruption into several droplets which are then entrained in the gas jet. These instabilities form in a range of conditions, in both laminar and turbulent jets, and with or without gas shearing effects. Further instabilities can also affect the magma particles formed after primary fragmentation leading to further (secondary) disruption. Understanding the role of fragmentation in controlling the mean particle size and particle size distribution is of fundamental importance for pyroclastic dispersal modeling, hazard quantification and the reconstruction of the eruptive dynamics based on tephra deposit characteristics. We have investigated the role of primary and secondary magma fragmentation processes and their typical length scales by coupling theoretical and experimental observations with detailed observations of small scale Strombolian explosions occurred in 2012 at Stromboli volcano (Italy). Based on our results, we discuss the role of fragmentation regimes on the mean size and size distribution of scoria particles formed in explosive mafic eruptions.