三宅島火山西斜面のカルデラと溶岩流制御について

Two calderas are developed in the Miyakejima Volcano, one is on the summit area (ISSHIKI, 1960), and the other is on the western slope. The latter was first recognized and reported by the present authors (SHINDO, 1980 ; CHIHARA et al., 1973). The topographic and geological features of the caldera of western slope are summarized as follows : (1) On the western slope of the volcano, a contrasting drainage pattern (radial valley systems) is seen between the upper and lower slopes, the boundary line running along the contour line of about 300 to 350 m above the sea level (Fig. 1). All radial valleys cut deeply the deposits of older somma stratovolcano. On the other hand, the upper valley system is shallower than the lower valley system and both are independent and discontinuous. (2) Wide and gentle slopes are developed at the altitude of 300 to 450 m above the sea level inside of caldera. The southern flat area is called Kuwanokidaira. (3) Along the western side of the gentle slope, a caldera rim about 4.5 km long, is clearly traced, the height of caldera rim being 0 to 15 m. The amount of collapse of the inside is presumed to be more than 50 m. (4) The inside of the caldera is covered by the aphyric basalt lavas and reddish scoria bed of the younger parastic volcanoes. (5) The caldera of the western slope was formed presumably after the older main stratovolcano and prior to the formation of younger somma and the younger parastic volcanoes. (6) The 1983 basalt lavas flowed down along the caldera rim west of Kuwanokidaira. The lavas partly overflowed the lower parts of the caldera rim and followed just the same course as the 1643? basalt lavas. Therefore, construction of man-made bank in addition to using the natural rim might make it possible to protect the lava disaster for the villages, and at least to change the lava flow course into another valley. These lava flow controls should be planned in the future in the Miyakejima Volcano.