Conduit processes during the climactic phase of the Shinmoe-dake 2011 eruption (Japan): Insights into intermittent explosive activity and transition in eruption style of andesitic magma

摘要

The climactic phase of the Shinmoe-dake 2011 eruption was characterized by sub-Plinian events (January 26 p.m., 27 a.m., and 27 p.m.) and lava accumulation in the crater (late on January 27 to January 29), all of which were accompanied by Vulcanian events. The magma discharge rate for lava accumulation was close to that of the sub-Plinian events. We discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression-induced crystallization of groundmass microlites, in order to reveal how the final eruption style was determined at the explosive-effusive transition. We examined pumice and lava-like pyroclasts from three sub-Plinian events, the January 28 Vulcanian event, and a lava block ballistically ejected during the February 1 Vulcanian event. The samples exhibit variable groundmass textures due to variable syneruptive ascent conditions, not due to pre-eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching.