開聞岳火山の噴火史

抄録

Kaimondake Volcano, situated in the Ibusuki Volcanic Region of southern Kyushu, is an undissected volcano which consists of a basal stratovolcano and a small central volcano. We established the eruptive history of this volcano by tephrochronology. Kaimondake Volcano started its eruption ca. 4 ka, and the latest eruption occurred in A. D. 885 (ca. 1.1ka). For about 2,900 years during this period, the volcano had been active, and 12 major eruption deposits (Km 1-Km 12) were recognized. The repose periods between these eruptions were estimated to range from 100 to 400 years. The mode of eruption of this volcano was mainly scoriaceous sub-plinian type, and was frequently associated with phreatomagmatic eruptions because the volcano originated from the shallow sea or near-shore environment. Lava flows were often associated with the scoria eruptions. Submarine lava flows which flowed southeastward are topographically divided into three; among them the lowest one is the most voluminous and is thought to have flowed out in the early stage, probably before Km6 eruption period. Among the 12 major eruption deposits, Km1, Km9 (ca. 2 ka), Km11 (ca. 1.5 ka), and Km12 (ca. 1.1 ka) were voluminous, and largely contributed to the formation of the volcanic edifice. During the latest eruption (Km 12), a central volcano was formed in the summit crater. This central volcano is not a simple lava dome, but a mound of complex volcanic materials with a composite structure. It consists of a basal scoria cone associated with fluid lava flows, which is later capped by viscous lava dome, and then subsequently penetrated by volcanic plug around the summit. The summit crater, which is named Hachikubo, had been thought to be a collapse crater, but it was geologically proved to be a large explosion crater which was successively enlarged during the eruption of Km12a. The total amount of volcanic products was calculated to be 3.1 km³ and 2.3 km³ for tephra and lava flows, respectively. Although there are no systematic relations between eruption volumes and the preceding repose period, the eruption materials containing tephra were more voluminous in the later stage than in the early stage, while those of lava flows were exceptionally large in the early stage of volcanic history.