Abstract

Abrupt transitions in eruptive style are common at mafic volcanoes, possibly leading to styles that potentially damage infrastructures and threaten the surrounding communities. During the 19th September–13th December 2021 Cumbre Vieja eruption (La Palma, Canary Islands), rapid changes in eruptive style were observed, from vigorous lava fountaining to sporadic intense ash columns, alongside continuous lava effusion, providing an ideal case study for investigation of the mechanisms underlying such transitions. We used a 1D steady-state numerical model for magma ascent to study the driving processes behind the eruptive style transitions. The model considers the complex and non-linear interdependence among the major processes occurring during magma ascents, such as temperature and viscosity evolution, non-ideal gas behavior, outgassing, and disequilibria in crystallization and volatile exsolution. Input parameters of the simulations (inlet pressure and temperature, water and CO2 content, initial phenocrystal content) are constrained by analytical studies and empirical data. Simulation outputs are compared with field observations and analytical data. Results suggest that a change in the degree of coupling between gas and melt might have caused the eruptive style change. We hypothesize that a change in the conduit geometry triggered this shift. One possible explanation could be a shallow branching of the conduit, which leads to variable magma and gas partitioning between ramifications. Another possible cause could be the temporary partial collapse of the conduit walls near the surface, which might have generated an acceleration of magma, reducing volatile outgassing, increasing magmatic strain, and consequently triggering the observed transition to explosive activity. © 2025. The Author(s).