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Extensional Rheology of Crystalline Magma

Angela Tatsch , Pranabendu Moitra

Abstract

During magma ascent, bubble growth can be significantly hindered by the surrounding melt+crystal matrix, affecting volcanic eruption styles. The growth of bubbles imposes an extensional stress on the surrounding matrix. Thus, while bubble growth dynamics is significantly affected by the rheology of the crystalline melt under extension, it remains poorly constrained. In this study, we use dynamically similar analog experiments to investigate crystalline magma rheology under extension. The suspensions up to 0.64 volume fractions of particles were prepared using 10 Pas silicone oil and hollow glass spheres (7--13 µm diameter). Using a linear drive in a rotational rheometer (Anton Paar MCR 702e), extensional tests were performed where samples were stretched following an exponential velocity profile to produce a purely uniaxial extension. Suspensions with ≤0.50 and 0.64 particle volume fractions (near maximum packing) were stretched following the capillary breakup extensional rheomtery (CaBER) and filament-stretching rheometry (FiSER), respectively. The change in mid-diameter of the sample filaments was determined by analyzing the experimental videos in ImageJ. Additionally, shear rheology experiments of the same particulate suspensions were performed using parallel plate geometry. The stress-strain rate data were fitted to the Herschel--Bulkley model. The shear rheology data shows that increasing particle fractions increases viscosity and yield stress while decreasing the flow index of the suspensions. Our preliminary results show that the shear and extensional rheology of crystalline magma are broadly comparable. Future experiments will investigate the effect of crystal shape on the extensional rheology to further inform the magma ascent dynamics during volcanic eruptions.