Sumisu volcano,Izu-Bonin arc,Japan: site of a silicic caldera-forming eruption from a small open-ocean island |
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| Authors: | Kenichiro Tani Richard S Fiske Yoshihiko Tamura Yukari Kido Jiro Naka Hiroshi Shukuno Rika Takeuchi |
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| Institution: | (1) Institute for Frontier Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan;(2) Smithsonian Institution MRC-119, Washington, DC 20013-7012, USA;(3) Department of Earth and Planetary Science, University of Tokyo, Tokyo 113, Japan |
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| Abstract: | Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km3 of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km
in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research
cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study
the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam
seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and
basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began
on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed
the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly
settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea
floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been
deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer
of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera
pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera
edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically
conspicuous wavy slump structures that disturb much of the volcano’s surface. |
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| Keywords: | Submarine caldera Silicic volcanism Caldera-forming eruption Tephra dispersal Late Pleistocene eruption Edifice instability Izu-Bonin arc |
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