抄録
Kikai Caldera, 17 km wide and 20 km long, is a Quaternary volcano located, nearly submerged, in the East China Sea, southern Kyushu. Two volcanoes, Iwo-dake (a rhyolitic volcano) and Inamura-dake (a basaltic volcano) on Satsuma Iwo-jima Island at the marginal part of this caldera, were formed after the great caldera-forming eruption of 6.5 ka known as the Akahoya Eruption. We made a geological study to understand the eruptive history of Satsuma Iwo-jima Island after the caldera formation. The volcanic activity in the island after the Akahoya Eruption is divided into three main stages and ten sub-stages based on volcanic edifice development and tephra deposits; the old Iwo-dake stage (stage OIo-I-II), the Inamura-dake stage (stage In-I-IV), and the young Iwo-dake stage (stage YIo-I-IV). The old Iwo-dake stage was initiated by phreatomagmatic eruptions and pumice fallout (stage OIo-I), followed by the effusion of rhyolitic lava with continuous ejection of ash and lithic fragments, resulting in building up an old volcanic edifice (stage OIo-II). In the stage OIo-II, intermittent explosive eruptions also occurred. The Inamura-dake stage is characterized by the effusion of basaltic lava flows and scoria-cone building (stage In-I-II). After that, at the western foot of the cone, phreatomagmatic explosions occurred (stage In-III) and andesitic lava flow effused (stage In-IV). In the young Iwo-dake stage, the explosive eruption occurred at the beginning (stage YIo-I), followed by the effusion of multiple rhyolitic lava flows from the summit crater and the formation of hyaloclastite in shallow sea that made up the marine terrace (stage YIo-II). During the past 1000 years, the volcanic activity changed to intermittent ones with pumice and bomb fallout (stage YIo-III-IV). Total phenocryst contents of the Iwo-dake lavas increased up to 8 vol.% through the young stages. The magma of the young Iwo-dake stage is different from that of the Akahoya Eruption and the old Iwo-dake stage, bordered by the Inamura-dake stage.
