HR: 1340h
AN: T53A-1393    [Abstracts]
TI: Sumisu Volcano, Izu-Bonin Arc, Japan: Site of a Rhyolitic Caldera-Forming Eruption From a Small Open-Ocean Island
AU: * Fiske, R S
EM: rfiske@volcano.si.edu
AF: Smithsonian Institution, NMNH 10th St. and Constitution AVE. NW, Washington, DC 20560 United States
AU: Tani, K
EM: kentani@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Tamura, Y
EM: tamuray@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Naka, J
EM: nakaj@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Shukuno, H
EM: shukuno@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kido, Y
EM: kidoy@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: Sumisu submarine caldera, an 8 x 10 km Crater Lake type structure along the front of the Izu-Bonin arc, was the site of a 30-60 ka eruption that introduced 50-70 km3 of rhyolite tephra into open-ocean environment. A manned submersible, two ROVs, a DeepTow camera system, and dredge samples (augmented by newly acquired single-channel seismic profiles and SeaBeam mapping) were used to study the caldera and surrounding areas. Caldera-wall studies show that pre-caldera eruptions built a complex of overlapping submarine dacitic and basaltic cones that formed an edifice 40 x 25 km in diameter; its summit grew above sea level to form an island about 200 m high. The caldera-forming eruption likely began on this island and sent a Plinian column high into the air. We interpret that prodigious rates of tephra fallback overwhelmed the Sumisu area, forming huge rafts of floating pumice and forcing large volumes of hot, proximal fallback debris beneath sea level-- generating gravity flows of quenched pumice that traveled downslope along the sea floor. The caldera rim, currently 100-400 m below sea level, is mantled by thin and discontinuous deposits of syn-caldera pumice showing no evidence of heat retention. Ocean currents apparently eroded most proximal pumice away (especially during periods of late Pleistocene sea-level lowering), and the caldera rim is locally mantled by lag lithics. ODP Leg 126 cores in the Sumisu Rift, atop the Rift's east margin, and on the forearc slope, show the sea floor south and southeast to be underlain by non-welded pumice deposits 10-20 m thick. Geochemical data point to Sumisu as the source of these distal deposits, rather than the nearby arc-front centers of Minami Sumisu or Torishima-indicating that the Sumisu pumice gravity flows traveled for distances of at least 70 km. Post-caldera edifice instability resulted in the collapse of a 15 degree sector of the east caldera rim and the formation of spectacular thin-skin slumps that ring much of the edifice.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
DE: 8440 Calderas
SC: Tectonophysics [T]
MN: Fall Meeting 2005