Silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous (Plinian) explosive episodes and gentle effusion of lava. Whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate. We measured porosities and permeabilities in samples from the A.D. 1912 eruption of Novarupta volcano, Alaska, and analyzed them within the context of a well-constrained eruptive sequence that encompasses sustained explosive and effusive activity. For the explosive samples, we find that the degree of vesicle interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Permeabilities of explosive samples show a weak dependence on porosity. Dome samples are not significantly different in permeability, but are of lower porosity, which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing. Quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity. Rather, the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates.