Shear-induced bubble coalescence in rhyolitic melts with low vesicularity

We have experimentally demonstrated for the first time, shear-induced development of bubble size and shape in a rhyolitic melt. The deformation experiments have been performed by using an externally heated, piston-cylinder type apparatus with a rotational piston. At 975°C, the vesiculated rhyolitic melts having cylindrical shape were twisted at rotational speeds of 0.3, 0.5 and 1.0 rpm. The number, size and shape of bubbles were then measured by using the X-ray computed tomography. The experimental results show that coalescence of bubbles occur even at low vesicularity (20 vol.%) and the degree of coalescence is enhanced with the shear rate. Because the shear-induced deformation seems to be produced for magmas ascending in a volcanic conduit, we propose the possibility of the vesiculated magma undergoing bubble coalescence at a significant depth (low vesicularity), resulting in the formation of permeable magmas, at least near the conduit wall.