Modeling magma viscosity and ascent dynamics of the 472 CE sub-Plinian eruption of Somma-Vesuvius (Italy)

• First viscosity model for Pollena phonotephritic melt accounting for water content. • Melt fragility increases with water, contradicting empirical models. • Adding 5.18 wt. % of H 2 O reduces glass transition temperature by 285 °C. • Our model predicts a 23,500-fold viscosity increase with dehydration at eruptive temperatures. • Our model predicts shallower fragmentation depth (∼1800 m) compared to empirical models (∼2700 m). The 472 CE sub-Plinian eruption of Somma-Vesuvius represents a critical reference scenario for volcanic hazard assessment, yet the physico-chemical melt controls on magma dynamics remain poorly constrained. We present an experimental and numerical investigation of magma ascent, incorporating a newly developed temperature- and water-dependent viscosity model for the phonotephritic melt involved in the climactic phase of the eruption. Hydrous glasses with water contents up to 5.18 wt. % were synthesized and characterized using electron microprobe analysis, FTIR spectroscopy, Mössbauer spectroscopy, Raman spectroscopy, and small/wide-angle X-ray scattering. Glass transition temperatures and melt fragility were determined through differential scanning calorimetry and Brillouin spectroscopy, respectively, avoiding nanocrystallization and dehydration artifacts inherent to traditional viscometry. Our results reveal that empirical viscosity models systematically overestimate melt viscosity at eruptive conditions. The newly parameterized model shows that melt fragility increases with water content, leading to dramatically enhanced viscosity sensitivity during dehydration. At an estimated eruptive temperature of 950 °C, viscosity increases by approximately 23,500-fold from hydrous (5 wt. % H 2 O) to anhydrous conditions, representing a much larger variation than predicted by existing models. The integration of this improved viscosity parameterization into a one-dimensional conduit model provides new insights into the syn-eruptive processes controlling the 472 CE eruption, highlighting the role of rheological models in providing important constraints for understanding the dynamics of explosive volcanic activity.