Eruptive History of Miyakejima Volcano during the Last 7000 Years

Abstract

Miyakejima volcano is located in the Pacific, 200 km south of Tokyo, as a member of lzu-Mariana Arc active volcanoes. The main cone of the volcano has nested two 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. The volcano-stratigraphy of Miyakejima during the last 7000 years was revealed and described in detail, mainly by tephrochronological method and historic records. 18 isopach maps and distribution of lavas were presented together with volume estimation of 42 erupted units. Based on the eruption-recurrence period, production rate and eruption style, the developing history was divided into four stages; 1) inactive stage of 7000-4000 yBP, 2) the Caldera-forming stage of 4000-2500 yBP, 3) the Oyama stage, from 2500 yBP to the 15th century, and 4) the Shinmio stage, 1469 A.D. to the present. The inactive stage of 7000-4000 yBP is characterized by small scale eruption with a long interval of quiescence. In the Caldera-forming stage, voluminous eruption of two accretionary lapilli and a scoria resulted in the formation of the Hatchodaira caldera. In the Oyama stage, caldera had been filled by products from central and lateral eruptions. It is noteworthy that phreato-magmatic eruptions from central vent prevail over dry magmatic eruptions. In the Shinmio stage, most eruptions took place from lateral fissures. The typical volume of materials ejected in a single eruption ranges from the order of l0^-3 km³ to 10^-1 km³ (DRE), most cases are about 10^-2 km³, and a few exceed 10^-1 km³ during the last 7000 years.