University-U. S . Geological Survey research team in 1985 and 1986. 

 The southern one-third of the Gorda Ridge is sediment-filled. 

 Polymetallic massive sulfides hosted in the sediment of the 

 Escanaba Trough of the southern Gorda Ridge were discovered by 

 USGS scientists in 1985. By analogy with the Mid-Atlantic Ridge, 

 the discovery of black smokers, additional massive sulfide and 

 related types of hydrothermal deposits, and vent animals is 

 anticipated at various sites along the Gorda Ridge. The recent 

 discoveries described have opened the Gorda Ridge and the Mid- 

 Atlantic Ridge as new frontiers for investigation of hydrothermal 

 mineralization, chemical and thermal effects of venting on the 

 ocean environment, and the adaptation of vent organisms. 



INTRODUCTION 



The Gorda Ridge and the Mid-Atlantic Ridge were largely 

 bypassed by the scientific community in the past and have only 

 recently become frontiers for undersea research (Fig. 1). The 

 Mid-Atlantic Ridge is the classic example of a slow-spreading 

 oceanic ridge. It was the site of the earliest systematic 

 investigations of an oceanic ridge by several projects including 

 the Canadian Hudson Geotraverse at latitude 45°N between 1960 and 

 1975 (Loncarevic et al . , 1966; Loncarevic, 1976), the French 

 American Mid-Ocean Undersea Study (FAMOUS) near latitude 37°N 

 between 1971 and 1974 (Heirtzler and van Andel, 1977) and the 

 Trans-Atlantic Geotraverse (TAG) Project near latitude 26°N 

 between 1970 and 1975 (Rona, 1970, 1976). The first active 

 hydrothermal field at an oceanic ridge in an open ocean basin was 

 discovered at the Mid-Atlantic Ridge by the TAG Project in 1972 

 (Rona, 1973; Rona and Scott, 1974; M. R. Scott et al . , 1974; R. 

 B. Scott et al . , 1974). However, the evidence at that time was 

 for the presence of warm springs and low-temperature mineral 

 deposits. When massive sulfides deposited from high-temperature 

 solutions were discovered at latitude 21°N at the fast-spreading 

 East Pacific Rise in 1978 (CYAMEX, 1979), followed by the 

 discovery of hot black smoker-type vents (350°C) at the same site 

 in 1979 (RISE Project Group, 1980), the oceanic ridge research 

 "bandwagon" shifted from the Atlantic Ocean to the Pacific Ocean. 



With the shift in emphasis of oceanic ridge research to the 

 Pacific in 1979, most attention was devoted to the faster 

 spreading East Pacific Rise, Galapagos spreading center, and Juan 

 de Fuca Ridge in the belief that faster spreading thermal 

 regimes were necessary to sustain high-temperature hydrothermal 

 activity. The Gorda Ridge with slow-spreading characteristics 

 received relatively little attention (e.g., Raff and Mason, 1961; 

 McManus 1966; Moore, 1970; Atwater and Mudie, 1973; Riddihough, 

 1980; Malahoff, 1985). The proclamation of the U.S. Exclusive 

 Economic Zone in 1983 drew attention to the Gorda Ridge as the 

 only oceanic ridge off the conterminous United States within that 

 zone. The discovery of polymetallic massive sulfide deposits, 



