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Leg 118

January 16, 1988
COLLEGE STATION, TX
-- A team of oceanographers and engineers from the Ocean Drilling Program (ODP) finally got what earth scientists have wanted for two decades--more than a quarter of a mile of rock recovered from the deepest layer of the ocean's crust.

Scientists on board JOIDES Resolution, drill ship for the ODP, spent November and December drilling into one of Earth's great geologic features, the Atlantis II Fracture Zone located in the remote waters of the Indian Ocean. The Grand Canyon could easily nestle inside this giant gash in the seafloor that cuts north to south for more than 400 miles, plunges to depths of more than four miles and is as wide as 25 miles.

Previous surveys have indicated that layers of the ocean crust and the underlying mantle lie exposed on the walls of the world's great fracture zones. No team to this date, however, has been able to recover long, continuous sections of the deep ocean crust and mantle material at these sites.

The crew on board JOIDES Resolution first surveyed the fracture zone with an underwater television camera. They found a suitable site on the top of a giant submarine mountain that shoaled up more than three miles--as high as Mont Blanc in the Alps. The site was ideal; it lay only half a mile beneath the sea surface, its top sheared off by wave action, forming a broad mesa four miles long and two miles wide.

The crew first deployed a special guide base. Traditional drilling methods depend on thick layers of sediment to provide stability for the drill hit before it penetrates hard rock, but the lack of sediment at this site would have caused the drill bit to spin off the stony surface. ODP engineers manufactured a special structure, a 40,000 pound, 17 foot by 17 foot metal box supported by an additional 100,000 of cement, to give the drill bit the ballast that sediment usually provides.

The scientists and engineers recovered a little more than 500 meters (1,650 feet) of rock from an ancient magma chamber formed more than 1 million years ago. As the liquid cooled along the walls of the magma chamber, it formed the coarsely crystalline rock called gabbro which makes up the ocean crust's deepest layer. The recovered rocks contain a record of how molten magma rises from deep within the earth, cools and then solidifies.

The recovered material also reveal to scientists how the circulation of cold sea water dramatically alters the hot rocks deep within Earth's crust. For the first time, scientists have the material in hand in which to study how water is heated beneath the seafloor and driven to the surface.

The scientific and engineering success of Leg 118 will change the thrust of future deep-sea drilling. By being able to penetrate the deepest layers of the ocean crust, scientists now have excellent opportunities to study Earth's interior secrets in all the world's oceans.

Co-chief scientists for the cruise were Dr. Paul T. Robinson, Dalhousie University, Halifax, Nova Scotia, Canada, and Dr. Richard P. von Herzen, Woods Hole Oceanographic Institution, Woods Hole, Mass. Staff scientist was Dr. Andrew C. Adamson of Texas A&M University, College Station.

The drill ship departed Port Louis, Mauritius, on Oct. 22, 1987, and returned Dec. 14. Twenty-five scientists from Belgium, Canada, Japan, Federal Republic of Germany, France, Sweden, the United Kingdom and the United States sailed on the 18th cruise of the ODP.



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