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Shear‐induced brittle fractures in vesicular magmas are thought to be possible pathways for open‐system degassing that controls the explosivity of volcanic eruptions. To investigate the detailed processes involved in the shear‐induced degassing, we performed torsional deformation experiments on columnar rhyolites with water contents and averaged vesicularities of 0.4–0.5 wt % and 20–41 vol %, respectively, at temperatures of 780–930°C and strain rates of <0.03 s⁻¹. The experimental conditions, such as viscosity and vesicularity of the sample, strain rate, and total strain, simulated those of typical natural dehydrated rhyolites in shallow volcanic conduits. At relatively high temperatures (≥855°C), the columnar samples were homogeneously twisted and deformed. On the other hand, at temperatures of ≤830°C, the deformation was localized and finally resulted in brittle failure into a column and a disk, followed …