High bubble number densities in Plinian pyroclasts are a necessary condition for magma fragmentation

Plinian eruptions involve magma fragmentation that breaks magma into gas and pyroclasts. These pyroclasts contain unique vesicle textures that preserve the only direct record of magma degassing during ascent. Yet the origin of these vesicle textures has remained unresolved. To address this, we develop a conduit flow model that incorporates disequilibrium degassing, including bubble nucleation and growth, without imposing constraints on conduit geometry. Focusing on crystal-free to crystal-poor silicic Plinian eruptions, we demonstrate that the vesicle textures preserved in their pyroclasts, including vesicle number densities and size distributions, are the natural outcome of repeated bubble nucleation during magma ascent that culminates in magma fragmentation. Bubble nucleation builds bubble number density up to a critical threshold ( ∼ 10 14 bubbles m − 3 ), at which rapid diffusion of H 2 O into bubbles sharply increases melt viscosity and pushes the melt across the glass transition. This critical BND threshold persists across the wide range of discharge rates examined for silicic Plinian eruptions.