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

Various processes involving nucleation, growth, and interaction of bubbles occur as water-rich magma —e.g., of andesitic composition— ascends through a volcanic conduit towards shallower depths. These mechanisms significantly affect the properties of the magma upon fragmentation, including porosity, permeability, and magma strength. Such properties, in turn, can influence the dynamics of fragmentation —such as its speed and efficiency— thus impacting eruptive behaviour. Precisely measuring and understanding how the pre-eruptive bubble size distribution and texture affect fragmentation dynamics poses a scientific challenge that lacks comprehensive quantification methods to date. In this study, we employed a combination of analytical and experimental methods to explore the processes governing bubble formation and to quantify the impact of their size, distribution, and texture on andesitic magma fragmentation. Our investigation utilised a series of andesitic products originating from the AD 1655 Burrell eruption of Mt. Taranaki, New Zealand. The pyroclasts from this eruption exhibited varying porosity (10 – 80 %) and permeability (10-16 - 10-11 m²). Using scanning electron microscope imagery, we employed FOAMS, an image analysis software, to compute bubble size distributions. Subsequently, we performed rapid decompression experiments on selected samples, capturing the process of fragmentation and clast ejection using two synchronized high-speed cameras. This approach allowed us to track fracture location and evolution over time, enabling the assessment of fragmentation speed and fracture density. Further analyses involved examining the grain size distribution of generated clasts to evaluate the efficiency of fragmentation. Our texture analysis unveiled that bubble nucleation and growth occurred concurrently during various eruption stages, as evidenced by changes in bubble number density, porosity, and polydispersivity. Correlating these results with the high-speed camera analysis, we observed a significant influence of bubble number density and average diameter on fragmentation speed. Moreover, large bubble clusters appeared to intensify fragmentation speed by weakening local rock strength. Both the number and diameter of bubbles exhibited additional relationships with particle size, efficiency of fragmentation, and fracture density in our samples. Drawing from the correlation between bubble texture and petrophysical properties, we proposed a conceptual model elucidating the impact of bubble textures on fragmentation dynamics. Furthermore, we evaluated and discussed our findings in the context of previously developed eruptive models of Mt. Taranaki. Our findings, together with variation in the density of the expelled particle-gas mixture, align well with the model depicting a transition in eruptive behaviour and plume stability of Mt. Taranaki during the deposition of the Burrell formation.