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 (