many years. During 1974, USGS issued a series of maps that 

 summarize present knowledge for the offshore portions. The maps 

 provide a picture of much greater geologic complexity for the region 

 than previously suspected, and help to demonstrate the wide 

 distribution of hazards about which detailed information is required 

 for planning offshore operations, such as siting of power plants, oil 

 platforms, and pipelines. 



Information on the hydrothermal circulation of seawater is being 

 provided by a number of programs, including DSDP. For example, 

 the basalt throughout the section of DSDP site 332, on the Mid- 

 Atlantic Ridge, shows evidence of alteration, possibly by hydrother- 

 mal circulation in the upper portion of the crust. This has been 

 postulated to explain large variations in measured heat flow near 

 spreading centers and measured heat-flow values significantly less 

 than those predicted by sea-floor spreading theories. Cold seawater 

 apparently flows down into the crust, is heated, and rises to the sea 

 floor where it discharges into the overlying bottom water, resulting 

 in a high percentage of heat transfer by convection that is not 

 recorded by normal heat-flow measurements. In programs spon- 

 sored by NSF on the Galapagos Rift Zone, and NOAA on the Mid- 

 Atlantic Ridge, temperature anomalies in seawater near the ocean 

 floor have been interpreted to be the result of sea-floor discharge of 

 hydrothermal waters. 



Mineral enriched sea-floor sediments may be products of 

 hydrothermal circulation. These metalliferous sediments were 

 initially found close to sea-floor spreading centers in all oceans. In 

 the DSDP they were discovered to be ubiquitous throughout the 

 ocean, lying just above basalt at the base of the sedimentary section. 

 As seawater circulates through the crust, it apparently leaches these 

 metals from the basalt. Upon discharge into the cold bottom waters, 

 these metals precipitate and settle to the sea floor. 



NSF currently sponsors a program to study extensive deposits of 

 these sediments on the Nazca Plate. The findings of this program 

 indicate that the formation of metalliferous sediments may be more 

 complicated than originally suggested. Apparently the mineralogy 

 and chemistry of these deposits is produced by a combination of 

 factors: hydrothermally produced elements transported from the 

 East Pacific Rise, the authigenic addition of elements from sea water, 

 and diagenetic remobilization of elements at the sediment/water 

 interface or within the sediment section. 



In addition to adding elements to sea water, hydrothermal 

 circulation may be responsible for removing certain elements; 

 laboratory studies have shown that significant amounts of Mg and 

 So" are removed from sea water in contact with basalt. Thus, 

 hydrothermal circulation through the 2 square kilometers of new sea 



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