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The microstructure of bubbles in magma controls the dynamics of magma flow in a volcanic conduit. To investigate the shear‐induced evolution of bubble microstructures, we performed a series of deformation experiments on vesiculated rhyolitic melts by twisting columnar obsidians at a temperature of 975°C and at rotation rates of 0.3–0.5 rpm for up to 10 rotations. Three‐dimensional shapes of the run products were analyzed using X ray–computed tomography. The experimental results demonstrate that shear strain and strain rate control the degree of bubble deformation and coalescence. The bubble connectivity, defined as the ratio of the volume of the largest bubble to the total bubble volume, starts to increase for a vesicularity of 20–30 vol%. A connectivity of >80% was achieved for a vesicularity of approximately 40 vol% and 10 rotations at 0.5 rpm, corresponding to a maximum strain of 30 at a strain rate of 0 …