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Abstract:
During volcanic eruptions, changes in explosivity may be attributed to the rheological response of the magma. The tendency for magmatic suspensions to localise strain ensures that magmas can shift from a regime controlled by viscous flow to that where they can fracture and fragment, owing to increasing viscosity of the degassing, crystallising magma and the elastic response of the melt when subjected to stress variations shorter than the structural relaxation timescale. If the conditions for magma failure are met over a protracted temporal or spatial scale, a shear zone may flank the ascending magma.Slip and traction along such marginal shear zones has been posited as the cause of geophysical signals observed at differing timescales during volcanic eruptions, yet the complexities of such structural features are still poorly understood. Melt viscosity, crystal cargo, vesicularity and ascent rate all impact the type and scale of shear textures developed, and as temperature, strain-rate and the presence of fluids fluctuate, fault products may repeatedly form and deconstruct in unison or in subsequent events. Here, we review some recent advances shaping our understanding of magma failure and the post-development influence of faulted magmas on ascent. We focus on magmatic pseudotachylytes formed by frictional melting, which can alter the physical and chemical properties of the magma: driving mineral reactions; melting crystalline phases; triggering devolatilisation and vesiculation; inducing fragmentation; lowering interstitial melt viscosity; altering magnetic properties; efficiently healing fractures and redistributing permeable pathways. Such processes may be vital to understand shallow controls on eruptive behaviour.
