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Influence of bubbles on the extensional rheology and fragmentation of magma

Ceri Allgood , Thomas Jones

Abstract

Fluid fragmentation dynamics govern the formation of pyroclasts in eruptions of low-viscosity magma such as basalt. This is in contrast to the relatively well-studied brittle fragmentation dynamics of more viscous eruptions. Understanding the fluidal or ductile fragmentation of magma is necessary for modelling potential tephra production during eruptions, and for interpreting past eruption characteristics from tephra deposits. The fragmentation of low-viscosity magma depends on its extensional rheology (i.e., its response to being stretched), which is fundamentally different to its rheological behaviour in simple shear. When magma is stretched, it forms a narrow filament, and a balance between surface tension and viscosity determines the maximum filament length and lifetime, influencing pyroclast production. The extensional rheology of Newtonian fluids is well-established, but erupting magmas are typically non-Newtonian due to the inclusion of gas bubbles, with basaltic tephra often containing up to 80% bubbles by volume. Here, we explore the effect of bubbles on extensional rheology and fluid fragmentation using analogue experiments. We create a range of bubble suspensions in well-characterised Newtonian fluids (e.g., silicone oils), and investigate the extensional rheology by rapidly separating two small plates, stretching the bubbly fluid into a narrow filament between them. Using a high-speed camera, we observe the dynamic response of bubble shapes and their spatial organisation, measure the time at which the filament breaks, and extract an apparent extensional viscosity based on the change in filament width. Our results demonstrate that bubbles play a crucial role in the fragmentation of low-viscosity magmas.