2020 Volume 126 Issue 9 Pages 473-491
Kusatsu-Shirane Volcano is one of the most active volcanoes in Japan. The summit of the volcano comprises three young pyroclastic cones: the Motoshirane Pyroclastic Cone Group (MPCG), which erupted on January 23, 2018, with almost no eruptive precursors; the Ainomine Pyroclastic Cone, of unknown age; and the Shirane Pyroclastic Cone Group (SPCG), which is the site of numerous historical phreatic eruptions. In this study, we present the results of an investigation of the stratigraphy and eruption history of the MPCG. The MPCG consists of overlapping pyroclastic cones including, from south to north, Motoshirane-nishi, the Older and Younger Motoshirane, Kagamiike, and Kagamiike-kita. The stratigraphic relationships and geochemical characteristics of the erupted material suggest that construction of most of these cones involved initial lava flow and subsequent cone-forming stages. Four lava flows, the Daimyozawa, Ishizu, Sessho, and Furikozawa lavas, form the bases of the Motoshirane-nishi, Older Motoshirane, Kagamiike, and Kagamiike-kita cones, respectively. The subsequent cone-forming stages included Vulcanian and phreatomagmatic eruptions. Products of the Kagamiike and Kagamiike-kita eruptions can be correlated with the ca. 4800 cal yr BP 12L Volcanic Sand (Hayakawa and Yui, 1989), which lies at the eastern foot of the volcano, and the juvenile jointed block-rich horizon (after ca. 1500 cal yr BP) on the southern flank of the Kagamiike cone, respectively. In summary, the MPCG eruption sites shifted from south (the Motoshirane-nishi cone) to north (the Kagamiike-kita cone), and magmatic eruptions continued until just after ca. 1500 cal yr BP. Petrological analyses of the erupted material suggest that the MPCG eruptions were fed by a mixture of dacitic magma from the long-lived crustal magma chamber, and repeatedly recharged mafic magmas with different compositions for each pyroclastic cone.
