Geology and previous eruptions of Miyakejima Geological Outline of Miyakejima
Miyakejima Volcano ((N: 34.08, E: 139.53, 814 m above sea level before the 2000 eruption) is an active basaltic andesite stratovolcano about 200 km south of Tokyo, developing on the volcanic front of the Izu-Mariana arc. In the northern part of the Izu-Mariana arc, some active volcanoes Fuji, Hakone, Izu-Oshima, Hachijo-jima etc. are located on the volcanic front.
Tectonic setting of Miyakejima is complex. Magmatic activity of Miyakejima relates the subdiction of the Pacific Plate at Izu-Bonin trench running the east side of the volcanic chain. The edifice of Miyakejima volcano is building on the Philippine Sea Plate, which is moving to NNW direction and subducts to Eurasian Plate and North-American Plate at Suruga trough and Sagami trough at the southern off of the Japan Islands. Northeren boundary of the Philippine Sea Plate is locally bending northward by the collision of Izu-Peninsula about 100 km north of Miyakejima and forms compressional tectonic setting around the peninsula.
Miyakejima volcano has a sub-circular outline with a diameter of about 13 km and a height of about 1,000 m above the sea floor. Above the sea part, the volcano has a flat-conical shape with about 8 km in a diameter and 800m height. The summit area is characterized by concentric double calderas: the Kuwakidaira Caldera (3.5 km across) and the Hatchodaira Caldera (1.5 km across). A central stratocone of Mt. Oyama developed in the Hatchodaira caldera.
Topographic image of the Miyakejima before the 2000 eruption
The main part of the volcanic edifice of Miyakejima was built more than 10,000 years ago. Kuwakidaira Caldera is considered to have formed during the building of the main part of the volcanic edifice, although the associated eruptive materials are not recognized. The main part of the volcanic edifice has been covered with lavas and pyroclastics within the last 10,000 years. In the last 10,000 years, largest eruption called Hatchodaira eruption was occurred at 2,500 years B.P. and about 0.4 km3 (DRE) of eruptive materials (scoria, explosive breccia, and accretionary lapilli) were produced during the eruption. Hatchodaira caldera was formed with this eruption (Tsukui and Suzuki 1998; Tsukui et al. 2001). After the formation of Hatchodaira Caldera, activity of basaltic andesitic magma formed a stratocone of Mt. Oyama within the caldera. No summit collapse event has been recognized since the formation of the Hatchodaira Caldera. he volcanic activity in the last 600 years has been characterized by flank fissure eruptions that have produced scoria and lava flows at 21- to 69-year intervals (Miyazaki 1984; Tsukui and Suzuki 1998). The volume of eruptive material at each eruption was ~1010 kg (~107 m3). Phreatomagmatic eruptions often occurred when the fissure reached coastal areas to generate explosive craters and tuff rings.
Recent eruptions
TIn the 20th century, eruptions occurred in 1940, 1962, and 1983, before the 2000 eruption (Miyazaki 1984). The 1940 eruption was characterized by fissure eruption on the northeastern flank with a minor summit eruption. About 4*1010kg of lava and scoriae were produced (Tsuya 1941). The 1962 eruption was characterized by eruption from a 2.5-km-long fissure on the northeastern flank, producing ~2*1010 kg of lava and scoria (Suwa 1963). The 1983 eruption was also characterized by eruption from a 4.5-km fissure on the southwestern flank, which produced ~2*1010 kg of scoriae and lava flows (Aramaki and Hayakawa 1984). Phreatomagmatic eruption formed explosive craters and tuff cones around the southern coast during the 1983 eruption.
