Oomurodashi Volcano: Overview of an Active Shallow Submarine Silicic Volcano in the Northern Izu-Bonin Arc

Tuesday, 31 January 2017: 15:00
Sovereign Room (Hobart Function and Conference Centre)
Iona M McIntosh, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, Kenichiro Tani, National Museum of Nature and Science, Ibaraki, Japan, Alexander RL Nichols, University of Canterbury, Department of Geological Sciences, Christchurch, New Zealand, Fumihiko Ikegami, University of Tasmania, Earth Sciences, Hobart, Australia, Rebecca Carey, University of Tasmania, Earth Sciences, Hobart, TAS, Australia, Osamu Ishizuka, Geological Survey of Japan, Tsukuba, Ibaraki, Japan and Shipboard Scientific Groups of Cruises NT07-15, NT12-19, KS-13-3, and KS-16-6
Abstract:
Oomurodashi is a shallow submarine silicic volcano in the volcanic front of the northern Izu-Bonin Arc, located between the inhabited islands of Izu-Oshima and Miyakejima. Oomurodashi has a large (~20 km diameter) flat-topped summit located at 100 – 150 mbsl, with a small central crater, Oomuro Hole, located at ~200 mbsl that contains numerous active hydrothermal vents. We present here the findings of cruises NT07-15, NT12-19, KS-13-3 and KS-16-6 to Oomurodashi.

ROV surveys of the deeper edifice found basaltic dykes and lavas on the northern flank and dacitic volcaniclastics on the eastern flank, suggesting that Oomurodashi is a complex of smaller edifices with various magma compositions. Meanwhile, extensive deposits of fresh rhyolitic pumice and lava are exposed in the crater walls of Oomuro Hole and the surrounding area of the flat-topped summit, with minimal biogenetic or manganese cover. FTIR spectroscopy analyses of their dissolved water contents and speciation, i.e. the relative amounts of molecular H2O versus hydroxyl groups (OH), reveal that these rhyolites have undergone secondary hydration by seawater; however, we use the unaltered OH data to reconstruct their original total water contents, which are consistent with shallow quench (< 50 mbsl). Furthermore, Oomurodashi has been identified as the likely source of a ~10 ka pumiceous tephra layer on neighbouring Izu-Oshima, indicating that eruptions from this shallow summit are capable of generating subaerial plumes. Finally, single-channel seismic (SCS) surveys have revealed thick seismically-transparent deposits underneath the flat-topped summit of Oomurodashi that are potentially gas-rich sediments. This finding, in conjunction with ROV observations of a small, shallow crater in the summit deposits and domed sediments on the floor of Oomuro Hole, suggests that sudden gas release may be an important phenomenon and previously unrecognised hazard at Oomurodashi.

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