Quantifying degassing-driven crystal growth in basaltic lavas

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A primary aim of lava flow research is the development of accurate flow models that can be used to forecast areas of inundation, and to estimate how far lavas will advance before stopping. Lava flows are complex fluids comprising mixtures of crystals, liquid and gas bubbles and, as they flow, they cool and lose volatile species (mainly water and carbon dioxide) that were initially dissolved in the melt at high pressure beneath the surface. Both cooling and degassing lead to crystallisation of the liquid melt, and thus have significant influence on flow advance. Cooling is a major driver of crystallisation, but its effects are mainly restricted to the thermal boundary layers, where it is an integral process in the formation of surface crust and lateral levées. In contrast, degassing is not restricted to boundary layers and occurs throughout flows, with the potential to affect the entire bulk rheology. Although the effects of cooling-driven crystallisation are accounted for in the current generation of lava flow models, crystal growth due to degassing has not yet been sufficiently quantified to allow its incorporation into models. In recent laboratory experiments, we have been able to simultaneously measure degassing and crystallisation for the first time, and we propose to further this research by examining the growth of crystals directly using hot stage microscopy. This will provide the data on crystal sizes, growth rates and morphologies necessary to quantify the contribution of degassing to the overall crystallisation of lavas. Ultimately, these results will allow degassing-induced crystallisation to be accounted for in numerical lava flow models.