Transition from growth to collapse of the Plinian column during the 1783 eruption of Asama volcano, Japan, inferred from rock microtextures

The 1783 eruption of Asama volcano was the most recent high-magnitude explosive eruption (VEI 4) in central Japan. During the climactic phase, the eruption transitioned from the formation of Plinian columns to the generation of pyroclastic density currents (PDCs). This study identified key differences in pyroclastic deposits across eruption styles, showing that changes in density, porosity, bubble microstructure, and water content. The pyroclastic fall deposits were divided into 14 layers of either pumice or ash, five of which contained silty ash derived from earlier PDCs. The pyroclastic fall deposits were overlain by PDC deposits generated in the climactic phase. Compared to fallout pumice, the PDC pumice had lower vesicularity and bubble connectivity but contained finer bubbles and high bubble number densities, suggesting higher magma decompression rates. Furthermore, the temperature and initial water content of the magma calculated from the compositions of the groundmass glasses and the crystals remained almost unchanged during the 1783 eruption, whereas the groundmass glass of the PDC pumice had a higher water content, indicating that the amount of water exsolved from the rising magma decreased. Based on theoretically predicted changes in vesicularity caused by water exsolution, we estimated that the mass fraction 0.81–0.93 of the initial water content was supplied to the plume during the formation of the climactic Plinian columns, and the mass fraction 0.68 during the PDC phase. The reduction in the water supply might have contributed to destabilizing the eruption column and caused the generation of PDCs.