The role of pre-eruptive bubble characteristics in modulating andesitic magma fragmentation

Brittle magmatic fragmentation is pivotal in volcanic eruptions, influencing both eruption dynamics and pyroclast generation. Rapid decompression of a volatile-rich, viscous magma can initiate an explosive phase, which is directly influenced by gas overpressure in bubbles. However, natural systems and their associated bubble textures are rarely homogeneous, meaning that pressure and shear conditions can vary on different scales. Analytical and experimental methods are combined in this study to investigate the influence of vesicle texture on andesitic magma fragmentation. We examined diverse pyroclasts with varying porosity (10–80%) and permeability (10⁻¹⁶–10⁻¹¹ m²) from the sub-Plinian 1655 CE Burrell eruption of Mt. Taranaki and conducted textural analysis on polished samples representative of the main lithologies. Subsequent fragmentation experiments by rapid decompression enabled evaluation of the fragmentation threshold, and a synchronized high-speed camera facilitated the quantification of fragmentation speed and fracture density. Additionally, particle size distributions of experimental fragments were analysed to evaluate fragmentation efficiency. Burrell pumices show high vesicle number densities with evidence of continuous nucleation and growth patterns and various degrees of coalescence that likely occurred during magma ascent. Correlating the textural investigation with rock fragmentation, we observe that vesicle diameter and coalescence can affect grain size distribution and fracture location, as textural heterogeneities can increase the local permeability, and high vesicle number densities can increase the production of fine ash. These findings contribute to a new conceptual model linking homogeneous bubble textures to increased fine ash production and fracture density, while vesicle coalescence promotes localized pressure loss and production of lapilli-sized fragments. The results of this study augment the established eruptive models of Mt. Taranaki and other similar systems.