Inherent duality of vesiculation kinematics revealed through 4D imaging

Magma vesiculation controls the ability of gas to escape from a volcano to the atmosphere and strongly influences volcanic eruption style. In silicic magmas, vesiculation in the conduit is often accompanied by elements of pure and simple shear deformation at different scales. To constrain the impact of shearing on outgassing in volcanic conduits, we performed synchrotron-based, time-resolved X-ray tomographic imaging during a thermally-induced vesiculation experiment on an obsidian from Lipari volcano, Italy. We tracked the real time, 4D evolution of vesicularity, bubble size distribution and bubble number density, bubble shape, and bubble orientation during vesiculation and shearing. We employ numerical simulations of heat (diffusion), diffusive bubble growth and pressure and velocity (linearized compressible Navier-Stokes) to map the temperature, pressure, water content and shear conditions throughout the sample. Vesiculation occurred in two main domains, (i) a shear zone that rapidly established at the boundary with the silicon carbide crucible during vesiculation and underwent pure and simple shear and (ii) the interior, in which initial pure shear was followed by isotropic vesiculation. Shear bands first promoted outgassing due to system-spanning bubble coalescence creating permeable pathways, followed by collapse, densification and creation of impermeable domains. This evolution leaves behind only faint textural traces that could be overlooked during analysis of natural samples, or ex-situ experiments. Our study reveals, in 4D, the role of shearing on transient permeability evolution at the microscale, which may drive dynamic shifts from effusive to explosive behavior and regulate hybrid volcanic activity commonly observed at silicic volcanoes. • Shear bands promote rapid shifts in permeability. • This behavior may regulate hybrid volcanic activity observed at silicic volcanoes. • Rapid feedback exist between local expansion and shearing-induced outgassing. • Permeability evolution leaves behind only faint textural traces.