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

Abstract

The eruptive style of magma is shaped by both storage conditions and ascent processes. Peralkaline melts, with relatively high water concentrations and low viscosity, are expected to better resist magmatic fragmentation compared to peraluminous melts. However, trachytic and phonolitic magmas can still generate highly explosive eruptions, as demonstrated by the Rungwe Pumice Plinian eruption (Tanzania). This VEI 5 event involved a crystal-poor, microlite-free phonolitic/trachytic magma stored at high temperatures and relatively low water concentrations. 2D and 3D textural analyses, coupled with embayment speedometry, reveal a delayed homogeneous bubble nucleation event (ΔP_sat ~50 MPa) at shallow depths. Rapid bubble nucleation and growth during fast ascent (~6 MPa·s⁻¹) prevented the formation of a highly vesicular foam and consequently, low permeability restricted outgassing. Strong melt-gas coupling, combined with a sudden rheological shift, ultimately led to fragmentation. This eruption underscores the critical role of conduit dynamics in peralkaline magma explosivity, beyond storage conditions alone.