The role of bubble-crystal interactions in the eruption dynamics of alkaline magmas: implications for Vesuvius volcano

Eruption dynamics and eruptive style are controlled by the interplay and feedback of non-linear conduit processes during magma ascent, such as gas exsolution, bubble expansion, outgassing and crystallisation. These processes control the evolution of magma viscosity and how easily the gas and melt phase decouple during ascent. Volcanism associated with intermediate and evolved alkaline magmas (from phonotephritic to phonolitic) is characterised by a wide range of eruptive styles, from lava flow to Plinian eruptions. This diversity in eruptive behaviour makes the eruption dynamics of alkaline volcanic systems challenging to predict. The volcanic history of Vesuvius (Italy) is characterised by complex transitions in eruptive behaviour, producing eruptions that are amongst the most catastrophic volcanic eruptions in human history. Here, we combine synchrotron X-ray microtomography with a numerical conduit model to investigate the processes and the pre-eruptive conditions that control the style of activity of alkaline magmas, using Vesuvius as a case study. We quantify crystallinity, vesicularity and connectivity of pore networks in pyroclasts from deposits of the 79 AD Plinian eruption and of the 1944 lava fountaining eruption of Vesuvius using 3D textural analysis. Our results reveal that heterogeneous bubble nucleation, driven by leucite crystals, contributes to the formation of large bubble populations during Plinian eruptions, with vesicle number densities exceeding 10⁴ mm⁻¹. The numerical results, obtained using a 1D steady-state model, indicate that phonolitic magmas are prone to fragmentation considering a wide range of pre-eruptive conditions, including temperatures from between 830 and 970 ºC and crystal fractions up to 0.40.