Lorenzo Cappelli, Paul A Wallace, Evelyne Mbede, Shimba Kwelwa, Edista Abdallah, Gerald G.J. Ernst, Karen Fontijn

The style and explosivity of volcanic eruptions are primarily governed by the interplay between conduit dynamics (e.g., magma ascent rate and degassing efficiency) and pre-eruptive magmatic conditions, both of which influence magma rheology. In highly alkaline magmas (i.e., agpaitic index > 1), the depolymerising effect of alkaline elements lowers melt viscosity at a given temperature, theoretically favouring less explosive eruptions—even in silica-rich systems. However, several well-documented eruptions demonstrate that silica-undersaturated peralkaline magmas (e.g., trachytes and phonolites) can produce highly explosive Plinian events, suggesting that additional factors control eruption style. In the East African Rift, volcanoes with peralkaline compositions have exhibited both explosive and effusive behaviour. To better understand the controls on explosivity, we investigated the pre-eruptive magmatic system of the Plinian eruption that produced the Rungwe Pumice (RP) deposit in southern Tanzania. The late-stage evolution of the plumbing system was reconstructed through haüyne-hosted melt inclusions (MIs), revealing that an evolved magma body was stored at relatively shallow depths shortly before the eruption. Water concentrations in MIs, measured via transmitted Fourier-transform infrared spectroscopy, indicate relatively water-poor conditions (~4.8 wt.%) and shallow water saturation depths (~3.5 km). Our findings suggest that volatile concentration alone does not fully explain the explosive behaviour; rather, the degree of water undersaturation and conduit dynamics must also be considered as key factors influencing eruption style.