Shear deformation experiments on vesicular rhyolite: Implications for brittle fracturing, degassing, and compaction of magmas in volcanic conduits

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 by a slip at the fractured interface. This slip prevented further brittle failure and shear-induced bubble coalescence in the remaining parts of the sample. A permeable fragmented zone was formed near the interface, while in the rest of the sample, the bubbles remained isolated and permeability did not increase. From this, we infer that a single event of magma fracturing may enhance open-system degassing locally near a fracture, but repeated fracturing and healing processes are necessary for the effective degassing of the entire magma that leads to nonexplosive eruptions.