三宅島火山の形成史

Abstract

Miyakejima Volcano is located 200 km south of Tokyo, Japan, and is one of the active volcanoes situated on the Izu-Mariana Arc. The main cone of Miyakejima has two nested calderas : outer Kuwanokitaira Caldera, 4 km in diameter, and inner Hatchodaira Caldera, 1.8 km by 1.6 km across. The central cone, Oyama, grew in the Hatchodaira Caldera.A short recurrence time of about 22 year-period and continuous crustal inflation in the recent years suggested that the volcano was proceeding to the next eruption. Activity began on June, 26, 2000, resulting in subsidence to form and the forming of a new caldera 1.6 km in diameter. The new Hatchodaira Caldera almost overlaps the site and size of the former Hatchodaira Caldera. This paper presents the volcano-history of Miyakejima during the last 10000 years.Based on the eruption style, together with whole-rock bulk chemistry, the development history during the last 10000 years is divided into four stages. These are 1) the Ofunato Stage of 10000-4000 y.B.P., 2) the Tsubota Stage of 4000-2500 y.B.P., 3) the Oyama stage, from 2500y.B.P. to the early 15th century, and 4) the Shinmio stage, 1469 AD to the present. Since 1085 AD, at least 14 eruptions are documented. The last four eruptions occurred in 1874, 1940, 1962, and 1983.The Ofunato stage is characterized by porphyritic basalts. These lavas and pyroclastics contributed the growth of the main cone and filling up of the Kuwanokitaira Caldera. The Tsubota Stage, reopened after a 3000-year repose, is distinguished by andesitic products from lateral and central eruptions. The Oyama Stage began with the formation of the Hatchodaira Caldera, which resulted from the discharge of ca. 0.37km3 scoria, explosion breccia, and accretionary lapilli. Subsequent products from central and lateral eruptions filled the caldera. It is noteworthy that phreato-magmatic eruptions from the central vent prevail over dry magmatic eruptions. Overflows of lavas from the rim of the Hatchodaira Caldera occurred in 9th century. At the Shinmio stage, 12 documented eruptions, without exception, took place from lateral fissures, with some accompanied by central eruption.The variation of Mg# (=Mg/ (Mg+Fe) × 100) versus erupted age show an abrupt increase of the ratio 2500, 1300, and 500 y.B.P. with a mild decrease. This pattern suggests that relatively undifferentiated magma was supplied to the magma plumbling system underneath the Miyakejima Volcano, which was slowly differentiating.