Experimental investigation of the effect of bulk composition on the H2O bubble formation in hydrous silicate melts

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Large explosive volcanic eruptions of hydrous magma volumes exceeding 100 km3 are significant natural hazards, which may occur with a recurrence interval of over 100 years. The emission of ash and sulfur gases are suggested to significantly decrease the surface temperature on a global scale. The occurrence of such explosive volcanic eruptions is attributed to the rapid formation and growth of numerous H2O bubbles in the magma during pressure release. These eruptions are characterized by the fragmentation of magma into pumice and large amounts of fine ash particles, with a bimodal distribution of bubble sizes. Hydrous magmas with various compositions ranging from rhyolite, phonolite, phono-tephrite, andesite, and dacite tend to erupt explosively. Based on decompression experiments, two H2O-bubble formation mechanisms of hydrous silicate melt have been identified. The number density of bubbles with a broad size distribution formed in hydrous rhyolitic melt increases with decompression rate, suggesting nucleation of bubbles. Conversely, the number density of nearly equally sized H2O bubbles formed in phonolitic melt remains independent of the decompression rate and is consistent with the model of spontaneous phase separation. The dependence of H2O bubble formation mechanisms on bulk melt composition is not well understood. Part of the unresolved mechanisms of bubble formation in bulk compositions other than rhyolite and phonolite may be due to the inconsistency of experimental results from previous studies. This may be related to rare experimental data and different experimental protocols. To characterise the bubble formation mechanisms, hydrous phono-tephritic, andesitic, and dacitic melts will be decompressed experimentally. The following parameters will be determined (1) the bubble number density and size distribution, which are indicative of nucleation or spontaneous phase separation, (2) the pressure release required for bubble formation, and (3) in case of nucleation, the surface tension σ, as well as (4) the residual H2O contents of the vesiculated glasses. The collected data will help to answer fundamental questions on the dynamics of H2O bubble formation, which is expected to depend on silicate melt composition. These data, together with previously published data for rhyolitic and phonolitic melts, will improve the modelling of bubble formation. The expected results will help to quantify the required pressure release and the decompression rate ranges for the formation of bimodal bubble size populations with numerous small bubbles between few larger bubbles. This texture is often preserved in explosively ejected volcanic pumice and ashes with a variety of bulk compositions. The bubble formation model based on these data is an important part of the modelling of the eruption dynamics, which is essential for the risk assessment of explosive volcanic systems.