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) exhibit a wide range in eruptive style and have produced some of the most catastrophic eruptions in human history, such as the 79 AD Plinian eruption of Vesuvius (Italy). However, eruptive dynamics are driven by complex, non-linear conduit processes during magma ascent, requiring a holistic approach to investigate their influence on explo- sivity. This study integrates synchrotron radiation X-ray computed microtomography (SRμCT) with a 1D steady- state conduit model, to investigate how crystal-bubble interactions, pre-eruptive conditions, outgassing, and magma composition affect eruptive style at alkaline volcanic systems, using Vesuvius as a case study. We analyse pyroclasts from the 79 AD Plinian and 1944 lava-fountaining eruptions using SRμCT. Our SRμCT results reveal that heterogeneous bubble nucleation can be promoted further by leucite crystals, contributing to the high bubble number densities (>104 mm−3) observed in Plinian products. Despite high bubble connectivity, low throat-pore size ratios (the ratio between the radii of the throat and connected vesicles) and elevated tor- tuosity restrict gas–melt separation during fast magma ascent, promoting fragmentation. Numerical simulations reveal tephri-phonolitic and phonolitic magmas are prone to fragmentation across diverse conditions, producing highly explosive eruptions. Only relatively high temperatures (>1050 ◦C) and low bubble number densities (102 to 103 mm-3) can promote lava flow and fountaining activity. Instead, phono-tephritic magmas exhibit highly explosive eruptions at considerably lower temperatures (<950 ◦C). Temperature controls magma viscosity, influencing the ascent rate and the outgassing efficiency, which, in turn, affects conduit dynamics and the eruptive behaviour. Our findings highlight that for alkaline systems, the parameter space which is conducive to highly explosive eruptions expands as the magma composition evolves and its viscosity increases. These insights enhance our understanding of eruption mechanisms, providing critical insights for assessing volcanic hazard and emergency planning at alkaline volcanic systems.