Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, we report the first high-resolution (20 μm) neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of our findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. We conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.