The role of crystal-bubble interactions, outgassing and magma composition in the ascent dynamics of alkaline magmas: Implications for eruptions at Vesuvius

Intermediate to evolved alkaline magmas (phono-tephritic, tephri-phonolitic, and phonolitic) display a broad spectrum of eruptive behaviours and have generated some of the most catastrophic eruptions recorded in human history, including the AD 79 Plinian eruption of Mount Vesuvius. Nevertheless, eruptive dynamics are governed by complex, non-linear processes occurring within volcanic conduits during magma ascent, which calls for an integrated approach to evaluate their role in controlling explosivity. In this study, synchrotron radiation X-ray computed microtomography (SRμCT) is combined with a one-dimensional steady-state conduit model to examine how crystal–bubble interactions, pre-eruptive conditions, outgassing processes, and magma composition influence eruptive style in alkaline volcanic systems, using Vesuvius as a case study. Pyroclasts from the AD 79 Plinian and 1944 lava-fountaining eruptions were analysed using SRμCT. The results show that heterogeneous bubble nucleation may be further promoted by leucite crystals, contributing to the high bubble number densities (>10⁴ mm⁻³) observed in Plinian products. Despite a high degree of bubble connectivity, low throat-to-pore size ratios (i.e., the ratio between the radii of the throats and their connected vesicles) and increased tortuosity limit gas–melt separation during rapid magma ascent, thereby favouring fragmentation. Numerical simulations indicate that tephri-phonolitic and phonolitic magmas are susceptible to fragmentation under a wide range of conditions, leading to highly explosive eruptions. Only relatively high temperatures (>1050 °C) and low bubble number densities (10²–10³ mm⁻³) favour effusive behaviour such as lava flows and lava fountaining. In contrast, phono-tephritic magmas can produce highly explosive eruptions at significantly lower temperatures (<950 °C). Temperature exerts a primary control on magma viscosity, which affects ascent rate and outgassing efficiency and, consequently, influences conduit dynamics and eruptive behaviour. Overall, our results indicate that in alkaline volcanic systems the range of conditions favourable to highly explosive eruptions broadens as magma composition evolves and viscosity increases. These findings improve our understanding of eruption mechanisms and provide important constraints for volcanic hazard assessment and emergency planning in alkaline volcanic settings.