Petrological Constraints on Magma Ascent Processes During the 1977 Eruption of Usu Volcano, Japan

Sub-Plinian eruptions often repeat at short intervals, and the magma discharge rate during these eruptions changes. The mechanism that controls the variation in discharge rate remains unclear; however, its understanding is essential for predicting the evolution of explosive eruptions and mitigating disasters. Here, we used petrological methods to investigate the magma ascent processes during the 1977 eruption of Usu volcano, which included four sub-Plinian eruptions and two relatively small pumice-rich eruptions. The water content of the melt inclusions in the first sub-Plinian eruption indicated a pressure of 100–125 MPa under the assumption of water saturation. From the second to fourth sub-Plinian eruptions, the minimum water content gradually decreased. The crystal size distributions of plagioclase microlites in the groundmass show large variations in small size (<1 μm), and these small crystals are inferred to crystallize at low pressure (<40 MPa), confirmed by decompression-induced crystallization experiments. Based on these observations, we inferred the following scenario: the magma in the first sub-Plinian eruption ascended from the magma reservoir at a depth of ∼4 km; however, the magma stagnated in a shallow conduit and then ascended to the surface during the second to fourth sub-Plinian eruptions. Moreover, the position of the magma head in the conduit gradually became shallower. The results of this study indicate that the evolution of explosive eruptions during the 1977 eruption of Usu volcano originated from the processes and conditions of both the magma reservoir and the shallow parts of the conduit.