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FUTURE DIRECTIONS IN OCEAN SCIENCES 81 



represent a coherent high-resolution continuous record of envi- 

 ronmental changes on time scales from years to millions of years. 

 Given the mobility of crustal plates, the geometry and location of 

 ocean basins and continents have changed through time. This 

 changing plate mosaic has had a profound effect on global sea 

 level and on ocean and atmospheric circulation. 



In essence, plate tectonics is the surface expression of a solid 

 Earth geochemical cycle. An understanding of how this cycle has 

 operated in time and space is a fundamental starting point for 

 Earth systems research, and it unifies the field of marine geology 

 and geophysics. The four principal elements of this research are 

 (1) the oceanic ridge and lithosphere, (2) off-ridge processes, (3) 

 ocean margins, and (4) ocean basin sediments. 



Oceanic Ridge and Lithosphere 



The global mid-ocean ridge is perhaps the most striking fea- 

 ture on the solid surface of our planet. Sections of the ridge 

 extend along the floor of the world's ocean to a length in excess of 

 50,000 kilometers. The mid-ocean ridge dominates Earth's volca- 

 nic flux and creates an average of 20 cubic kilometers of new 

 oceanic crust every year. Two-thirds of the annual heat loss from 

 Earth's interior occurs through the generation and cooling of the 

 oceanic lithosphere, partially by the circulation of seawater through 

 fractures in the hot oceanic crust. This hydrothermal circulation 

 facilitates a major chemical exchange between seawater and oce- 

 anic crustal rocks that acts as an important regulator of the chemistry 

 of the ocean and of the volatile content of Earth's interior. The 

 most stunning manifestations of this circulation are the high- 

 temperature hydrothermal vents along the ridge axis. 



Many discoveries of ridge phenomena have been made over 

 the past decade, and a number of sophisticated technological tools 

 have been developed for detailed investigation of the seafloor and 

 the subsurface crust. High-temperature hydrothermal vents, for 

 example, were discussed only as theoretical possibilities before 

 their discovery in the Pacific in the late 1970s. High-resolution 

 swath mapping and side-scan sonar imaging systems have only 

 recently begun to provide information on the detailed morphology 

 and structure of ridge systems. Multichannel seismic imaging 

 techniques have advanced and thus have enabled marine geolo- 

 gists to begin imaging the magma chambers that lie below the 

 ridge axis (Detrick et al., 1987). Much of the promise of this new 

 technology remains to be realized. Detailed sampling and map- 



