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Calendar Friday April 13 MARINE BIOLOGY SEMINAR - Rob Knight, UC Irvine, will present a title to be announced in 4500 Hubbs Hall at 12 noon. Monday, April 16 GEOSCIENCES MARINE CHEMISTRY & GEOCHEMISTRY SEMINAR - Dan McKenzie, Cambridge University, will present "Rheology of the Continental Lithosphere" in 4500 Hubbs Hall at 4 p.m. (Julie Bowles, jbowles@ucsd.edu) Tuesday, April 17 MSPPC MEETING - (Note revised date) A special joint meeting of the MSPPC and the SIO Machine Shop Building Advisory Committee will be held in 114 Scripps Building, from 11 a.m. - 1 p.m. Wednesday, April 18 ECOLOGY LUNCHEON SEMINAR - Bruce Menge, Oregon State University, will present "Integrating Bottom-up with Top-down Perspectives in Rocky Intertidal Communities" in 4500 Hubbs Hall at 12:15 p.m. (Chris Janousek, cjanouse@ucsd.edu) SPECIAL SEMINAR - Richard Norris, WHOI, will present "Climate Transients During the Cretaceous and Early Cenozoic" in 4500 Hubbs Hall at 1:30 p.m. (Susan Jorgensen, sjorgensen@ucsd.edu) Friday, April 20 MARINE BIOLOGY SEMINAR - Bob Shadwick, SIO, will present "How to Tune a Fish: Muscle Dynamics in Swimming Skipjack and Yellowfin Tuna" in 4500 Hubbs Hall at 12 noon. (Bonnie Becker, bjbecker@uscd.edu) SEMINAR - Ida Broker, head of the Coastal Engineering Department at the Danish Hydraulics Institute, will discuss DHI's coastal modeling capabilities in 4500 Hubbs Hall at 2:30 p.m. Notices THANKS FROM RON - Ron McConnaughey writes, "Thank you all very much for the wonderful retirement party and for your outpouring of friendship and generosity! " RESEARCH ACTIVITIES - Do you value having an annual hard copy edition
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staff. Ship News Weekly Science Report, R/V Melville - We are steaming west at 14d20'N to complete our sampling of the southern Mariana Trough spreading center. We have just completed mapping and sampling a large volcano at 14o20' N, 145o20'E. We recovered large volumes of pumice and evidence of extensive silicic eruptions, presumably related to collapse and formation of a substantial (5 km x 3 km) caldera. As at some of the other large calderas, there quite mafic lavas recovered from the flanks of the edifice. We will come back east tomorrow afternoon, towards Guam, and sample the two southernmost large seamounts in the Mariana arc before entering Apra Harbor on Thursday morning. Our last dredge should be completed an hour from the buoy. Our return to the southern reaches of the arc has greatly improved the fishing, and grilled mahi mahi and sashimi are on the evening menu. The weather has remained lovely and our work has gone very smoothly. The Melville has proved, as usual, to be a very able vessel for marine geological sampling. Her crew has been superb, providing very professional seamanship, good company, and a remarkable willingness to improvise, innovate, and explore. It has been a real pleasure to work with them. A summary of our principal scientific results to date follows. We've come away with enough survey data and rock samples to keep ourselves and many of our colleagues busy for some time. We have studied the southern Mariana Arc system intensively, with a focus on volcanoes along the magmatic arc and associated cross-chains. Sonar backscatter imagery of about 15,000 square miles or 50,000 square kilometers was obtained with the towed HMR-1 sonar towfish. This allowed us to image 28 submarine volcanoes of the Mariana arc which had not been previously studied. We also imaged the spreading axis of the Mariana Trough from 13d45' to 17d30', about half the length of this slow-spreading ridge. In addition, we sampled the back-arc basin extension axis from 13d20'N to almost 15d N, which completes a first-order along-axis sampling of this archetypal example of a back-arc basin spreading axis. Our survey of 450 km along the arc magmatic axis, from 13d30'N to 17d20'N, indicates that these volcanoes vary widely in volume, from a third to 1000 cubic kilometers. They are constructed on a base that ranges in depth from 1000 to 3000 meters, are from 200 to 2700m in height, and are spaced at distances of 7 to 45 km. The average volcano along the Mariana magmatic front is spaced 20km from its neighbor, is built on a platform that lies at 1800m below sealevel, rises 1000m above this platform, and occupies a volume of about 200 cubic kilometers. Lavas collected from 24 edifices along the magmatic front include abundant basalt (many with olivine) and dacite; we also collected several cumulate gabbroic xenoliths. Phenocryst phases are dominated by plagioclase, olivine, pyroxene and hornblende. Five of the volcanoes in the study area - the islands of Guguan, Sarigan, and Anatahan, and the submarine edifices of S. Ruby and Esmeralda -are active. The other volcanoes are extinct or dormant, and our study suggests that, to a first approximation, the smaller the edifice, the less likely it is to be active. Some extinct edifices have flat summits at depths that range from 40 to 300 m that are capped with coral, or, farther north in the study area, shelly material or carbonate sand. These are guyots, although smaller and much younger than the drowned carbonate platforms that formed in the Western Pacific during the Cretaceous period. Those we have found in the Marianas probably drowned during Pleistocene time and provide opportunities for others to study how guyots form. Volcanoes extending westward from the magmatic front were an important target of our survey. A major cross-chain of small volcanoes along 14d40'N was studied and sampled for the first time. These volcanoes yielded a range of lavas, from basalt to dacite. Volcanoes from another cross-chain, extending along 17d20'N latitude west of Guguan, yielded only basalt. Shorter cross-chains were also studied in the Diamantes and near Sarigan. Comparisons of lava compositions will provide the basis for us to test the hypothesis that convecting mantle is sequentially melted first beneath the back-arc basin spreading axis and then under cross-chains and finally under the volcanoes along the magmatic axis. This provided the philosophical basis for our work, sponsored as part of the Izu-Mariana-Bonin "Subduction Factory" experiment of NSF's MARGINS initiative. Recovery of basalts which have not experienced a lot of fractionation was an important objective of our sampling, and HMR-1 imagery was very useful for identifying basaltic parasitic cones and associated flows as dredge targets. Using this imagery allowed us to recovered a large number of aphyric and sparsel phyric olivine basalts. We recovered abundant pumice from 6 volcanoes in the northernmost 100km of the arc. Each volcano yielded a homogeneous and compositionally distinctive suite. This fact, coupled with the large size and angular shape of pumice blocks, indicates that these pumice hauls are in situ. These dredges were made at depths of 800 to 1700m, indicating that vesiculation to form pumice can happen when felsic lavas erupt at relatively high pressures. These hauls also reinforce the idea that felsic magmas are a much more important part of the Mariana arc system than previously appreciated. Another interesting feature related to igneous activity in the region is unusually thick crusts of Mn- and other oxides. This probably manifests hydrothermal activity in the vicinity of these deposits. Our surveys traversed active areas of sediment deposition between the active arc and the back arc basin. Here, the sediment-covered seafloor is the upper surface of a volcaniclastic 'apron' that slopes westward at 2-4 degrees and buries the igneous infrastructure of the arc and the join between arc and backarc basin crust. The HMR-1 imagery revealed two features on the seafloor that provide important --
R/V ROGER REVELLE
R/V MELVILLE
R/V NEW HORIZON
R/V ROBERT GORDON SPROUL
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