「しんかい2000」による海底軽石火山の観察 : 明神海丘潜航調査

抄録

Many caldera volcanoes occur in the northern part of the Izu-Ogasawara Arc. These submarine calderas produced a large amount of pumice and show high gravity anomalies in the central parts along with small amplitude of magnetic anomalies on the volcanic edifices. They have no equivalent in the usual classification of on-land calderas as far as known. Elsewhere I proposed that the submarine "calderas" associated with pumice in the arc were pumice cones which were difficult to form under subaerial conditions but easy under subaqueous conditions. The Myojin Knoll is one of the typical pumice cones with calderas in the arc. It is located between Aogashima Island and Myojinsho Reef. The size of the knoll is about 18 km in diameter at the base and 950 m in height. The knoll has caldera structure on its summit. The nearly circular caldera rim is 5-7 km in diameter. The caldera floor is about 1400 m deep and 5-6 km across. A central cone rises from the floor. Relief of the caldera wall is about 700-900 m with the inner slope being at about 20-30°. The submersible study by Shinkai 2000 reported here shows that the top of knoll is constructed mainly of stratified pumice deposits underlain by rhyolitic lava. These rocks have characteristic chemical compositions with high SiO₂ (more than 71 wt%) and low K₂O (less than 0.86 wt%), and show similar range to the rocks from Aogashima Rift volcanoes to the west of the knoll rather than those from adjacent Myojinsho Reef Volcano. The eruption under the deep water conditions caused the deposition of pumice near the crater. The repetition of such eruptions formed the submarine pumice volcano. Acid volcanism under subaqueous conditions in the northern part of the arc formed the volcanoes with pumice cones and calderas.