© 2006 Nature Publishing Group Long-term eruptive activity at a submarine arc volcano Robert W. Embley 1 , William W. Chadwick, Jr 1,2 , Edward T. Baker 3 , David A. Butterfield 3,4 , Joseph A. Resing 3,4 , Cornel E.J. de Ronde 5 , Verena Tunnicliffe 6 , John E. Lupton 1 , S. Kim Juniper 7 , Kenneth H. Rubin 8 , Robert J. Stern 9 , Geoffrey T. Lebon 3,4 , Ko-ichi Nakamura 10 , Susan G. Merle 1,2 , James R. Hein 11 , Douglas A. Wiens 12 & Yoshihiko Tamura 13 Three-quarters of the Earth’s volcanic activity is submarine, located mostly along the mid-ocean ridges, with the remainder along intraoceanic arcs and hotspots at depths varying from greater than 4,000 m to near the sea surface. Most observations and sampling of submarine eruptions have been indirect, made from surface vessels or made after the fact 1–6 . We describe here direct observations and sampling of an eruption at a submarine arc volcano named NW Rota-1, located 60 km northwest of the island of Rota (Commonwealth of the Northern Mariana Islands). We observed a pulsating plume permeated with droplets of molten sulphur disgorging volcanic ash and lapilli from a 15-m diameter pit in March 2004 and again in October 2005 near the summit of the volcano at a water depth of 555 m (depth in 2004). A turbid layer found on the flanks of the volcano (in 2004) at depths from 700 m to more than 1,400 m was probably formed by mass-wasting events related to the eruption. Long-term eruptive activity has produced an unusual chemical environment and a very unstable benthic habitat exploited by only a few mobile decapod species. Such conditions are perhaps distinctive of active arc and hotspot volcanoes. Direct observations of submarine volcanic eruptions have only been made when their vents or products reach the ocean surface, as happened during eruptions of Myojinsho 7 , Surtsey 8 , Macdonald seamount 2 and Kavachi 9 . Such shallow eruptions are usually too violent to study with existing technology, although some samples of fluids, microbes and volcanic products were obtained from the eruption of Macdonald seamount in the late 1980s 1,2,10 . Conversely, the more effusive deep-ocean basalt eruptions along the mid-ocean ridge have eluded direct observation. However, seafloor observations of fresh lavas, and sampling of fluids and microbes have been made within days to weeks following at least four deep submarine erup- tions 11 . These eruptions have been relatively small (#55 £ 10 6 m 3 ) 12 and short-lived (hours to days) 13,14 . Phenomena associated with these mid-ocean ridge eruptions include: (1) large increases in volatiles produced by magma degassing and subsurface phase separation of the hydrothermal fluids 6,15 , (2) blooms in microbial productivity driven by the increased volatile flux 4,16,17 , and (3) in some cases, extinction and subsequent rapid renewal of the macrofaunal vent communities 18,19 . Submarine volcanism associated with intraoceanic island arcs is quite different in character from mid-ocean ridge volcanism. Volatiles released during the subduction of an oceanic plate causes partial melting of the overlying mantle wedge, yielding lavas that are, on average, more siliceous and gas-rich than mid-ocean ridge basalt 20 . These volcanoes are fixed with respect to their magma source for longer periods than their mid-ocean ridge counterparts, allowing their growth into shallow water or emergence as islands. The shallower depth range and the more explosive nature of submarine arc volcanism results in the generation of a much larger proportion of volcaniclastic material than at the mid-ocean ridge. These volatile- rich habitats on shallow isolated peaks may support fauna capable of surviving on intermittent hydrothermal production 21 . A survey of more than 50 submarine volcanoes along the Mariana arc in February and March 2003 identified 12 sites with hydrothermal plumes 22 . The plume overlying the summit of NW Rota-1 (Fig. 1a) was distinguished by high rise height, elevated d 3 He values and low pH (ref. 22). Dives with the remotely operated vehicles (ROV) ROPOS and Hyper-Dolphin investigated NW Rota-1 in March/April 2004 and October 2005. NW Rota-1 is conical and about 16 km in diameter at its base at 2,700 m. The summit at 517 m lies along a ridge between a pair of southwest–northeast trending, inward-facing fault scarps that cut across the volcano. Rock samples collected during the dives at NW Rota-1 are vesicular, moderately fractionated, medium-K basalt and basalt andesites (51.8 ^ 0.5% SiO 2 , 0.62 ^ % K 2 O, 6.4 ^ 1.1% MgO) 23 . The south flank of the volcano between 700 and 884 m (Figs 1 and 2) is dominated by volcaniclastic debris and talus. Large-scale mass wasting is ubiquitous, with narrow spurs of lava forming headwalls of massive rock slides. The northwest–southeast trending summit ridge is composed primarily of volcaniclastic sand with some rock outcrop. Its western end has a sharp crest with a steep, unstable southern slope and a gentler northern slope (Fig. 3a). Deposits of black ash and lapilli (defined as volcanic particles ,2 mm and 2–64 mm in diameter, respectively) intermixed with milllimetre-sized sulphur globules were observed on flat surfaces or within small depressions on the outcrops along the summit ridge (Fig. 3b). ROPOS surveys discovered an active crater ,15 m in diameter and .20 m deep at a depth of 540 m on the south side of the summit. ‘Brimstone Pit’ (Fig. 2) was discharging a pulsating, opaque, yellowish smoky plume with characteristics unlike any known hydrothermal LETTERS 1 NOAA/PMEL, 2115 SE O.S.U. Drive, Newport, Oregon 97365-5258, USA. 2 CIMRS, Oregon State University, Oregon 97365-5258, USA. 3 NOAA/PMEL, 7600 Sand Pt Way, NE, Seattle, Washington 98115-6349, USA. 4 JISAO, University of Washington, Washington 98115-6349, USA. 5 Institute of Geological and Nuclear Sciences, 30 Gracefield Road, PO Box 31–312, Lower Hutt, New Zealand. 6 Department of Biology/School of Earth and Ocean Sciences, University of Victoria, PO Box 3020, Victoria, British Columbia V8W 3N5, Canada. 7 GEOTOP Research Centre; Universite´ du Que ´bec a ` Montre ´al, PO Box 8888 Montre ´al, Que ´bec H3C 3P8, Canada. 8 Department of Geology and Geophysics, University of Hawaii, 1680 East-West Road, Honolulu, Hawaii 96822, USA. 9 Geosciences Department, University of Texas at Dallas, 2601 N. Floyd Road, Richardson, Texas 75083-0688, USA. 10 National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7, 1-1, Higashi 1-Chome Tsukuba, Ibaraki 305-8567, Japan. 11 US Geological Survey, 345 Middlefield Road, MS 999, Menlo Park, California 94025-3591, USA. 12 Department of Earth and Planetary Sciences, Washington University, 1 Brookings Drive, St Louis, Missouri 63130-4899, USA. 13 (IFREE) Japan Agency for Marine-Earth Science and Technology, 237-0061 2-15 Natsushima-chou, Yokosuka-shi, Kanagawa-ken, Japan. Vol 441|25 May 2006|doi:10.1038/nature04762 494