The ferocity of volcanic eruptions – their penchant for either effusive or explosive behaviour – is to a large extent a matter of the ease with which volatiles are able to escape the volcanic system. Of particular importance are the mechanisms by which permeable networks within magma are fabricated and how they permit gas escape, thereby diffusing possibly calamitous explosions. Here, we present a series of experiments that confirms sample-scale fracture propagation and permeability development during shearing viscous flow of initially impermeable, bubble-bearing (<0.20 bubble fraction) magmas under conditions pertinent to volcanic conduits. These samples are deformed in torsion at constant shear strain rates until an applied differential pore fluid pressure across the sample equilibrates, confirming permeability development in situ . Permeability develops at moderate to high shear strain rates (γ˙>2×10−4 s−1). At moderate shear strain rates (2×10−4 s−1<γ˙<4.5×10−4 s−1), permeability initiates at high strain (γ>3) via en échelon Mode I fractures produced by repeated fracture events. At high shear strain rates (γ˙>4.5×10−4 s−1), permeability develops shortly after the onset of inelastic deformation and is, again, established through a series of en échelon Mode I fractures. Critically, strain is not immediately localized on Mode I fractures, making them long-lived and efficient outgassing channels that are ideally oriented for directing volatiles from the central conduit upward and outward toward the conduit rim. Indeed, Mode I fracture arrays may prove necessary for dissipating gas overpressures in the central regions of the magma column, which are considered difficult to outgas. These experiments highlight mechanisms that are likely active along conduit margins and constrain previously postulated processes under truly applicable conditions.