GORDA RIDGE AND MID-ATLANTIC RIDGE: NEW FRONTIERS 

 FOR UNDERSEA RESEARCH 



Peter A. Rona 



National Oceanic and Atmospheric Administration 



Miami, Florida 33149 



ABSTRACT 



Investigations of the Gorda Ridge within the proclaimed U.S. 

 Exclusive Economic Zone off northern California and southern 

 Oregon, and the Mid-Atlantic Ridge extending along the center of 

 the Atlantic ocean, have received new impetus from the discovery 

 of hot black smokers, sizeable polymetallic massive sulfide 

 deposits, and vent animals at the Mid-Atlantic Ridge by the NOAA 

 VENTS Program in 1985. Prior to that time, the oceanographic 

 community focused investigations of seafloor venting at the 

 faster-spreading oceanic ridges of the Pacific Ocean including 

 the East Pacific Rise, the Galapagos spreading center and the 

 Juan de Fuca Ridge because a consensus held that the faster 

 spreading rates at these ridges were necessary to supply the heat 

 to drive high-temperature black smoker-type venting. As the only 

 oceanic ridge in the Pacific with slow-spreading characteristics, 

 the Gorda Ridge is an outlier of the slow-spreading ridge system 

 that extends through the Atlantic Ocean and western Indian Ocean 

 comprising more than half the 55,000 km length of the globe- 

 encircling oceanic ridge system. 



In response to recent discoveries at the Mid-Atlantic Ridge 

 the oceanographic community is shifting more of its activity to 

 investigation of hydrothermal venting and related processes at 

 slow-spreading oceanic ridges. The discovery cruise by a team of 

 NOAA and university scientists in 1985 followed by DSRV Alvin 

 dives by a NOAA-WHOI-MIT team in 1986, supported by NOAA ' s Office 

 of Undersea Research and the National Science Foundation, 

 revealed that the black smokers emanate from the center of a 

 mound the size and shape of the Houston Astrodome. The mound is 

 constructed primarily of polymetallic massive sulfides and 

 situated in the rift valley of the Mid-Atlantic Ridge near 26°N, 

 45°W. Massive sulfide samples from the mound exhibit supergene 

 enrichment of gold attaining the highest grade (16ppm; 0.5 oz per 

 ton) yet found in such seafloor deposits. The chemistry, heat 

 content, and dynamics of the venting solutions exhibit 

 similarities and differences with Pacific vents. The vent 

 animals differ from those of the Pacific. 



The northern two-thirds of the 300 km long Gorda Ridge is 

 sediment-starved like the Mid-Atlantic Ridge. Evidence for past 

 and present high-temperature black smoker-type venting was 

 detected at the northern Gorda Ridge by a NOAA-Oregon State 



