Conduit dynamics of the Rungwe Pumice eruption (Tanzania): From storage to fragmentation of phonolitic-trachytic magmas

The eruptive style and explosivity of a rising magma are initially shaped by the conditions established in the magmatic plumbing system. However, processes in the conduit during ascent exert a large influence on the final eruption style. Peralkaline magmas—bearing an agpaitic index>1—typically have high water saturation levels, promoting explosive behaviour during ascent. At the same time, their relatively low viscosities—resulting from alkaline-induced Si-O bonds depolymerisation—suggest higher pressure thresholds for bubble bursting or the capacity to endure prolonged plastic deformation under intense stretching stresses. This implies an increased resistance to fragmentation compared to high-silica, calc-alkaline magmas. Yet, trachytic or phonolitic magmas can still produce highly explosive eruptions as confirmed by many documented events. The Rungwe Pumice eruption (Tanzania) serves as a striking example of unexpected eruptive behaviour. This Plinian, VEI 5 eruption was generated by a crystal-poor, microlite-free phonolitic/trachytic magma stored at high temperatures and relatively low water concentrations. Based on these characteristics, a milder eruption might have been expected. However, through detailed 2D and 3D textural analyses of pumiceous ash clasts, we identified a delayed homogeneous bubble nucleation event (ΔPsat50 MPa) occurring abruptly at shallow depths (Pn40 MPa). The rapid nucleation and growth of bubbles during fast magma ascent (6 MPa·s⁻¹) left insufficient time to form a highly vesicular foam (<75%), while low magma permeability hindered efficient outgassing. This maintained a strong coupling between magma and gases, and, combined with a sudden rheological shift likely triggered by volatile loss and a temperature drop, ultimately led to fragmentation and the explosive nature of the eruption. The Rungwe Pumice eruption highlights the critical role of conduit dynamics in shaping the behaviour of peralkaline magmas, which can unexpectedly deviate from predictions based solely on their composition and storage conditions.