also loci of heat sources that may drive subseafloor hydrothermal 

 circulation. 



Significance 



Subseafloor hydrothermal circulation is important for 

 several reasons. The Earth is a water-cooled planet and some 20 

 percent of its internal heat is dissipated by hydrothermal 

 circulation at oceanic ridges (Williams and Von Herzen, 1974; 

 Wolery and Sleep, 1976). The two-way chemical exchange between 

 seawater and oceanic crust in hydrothermal circulation systems at 

 oceanic ridges is a significant influence on the composition of 

 seawater supplying certain metals in amounts comparable to the 

 input by rivers of material weathered from the continents (Table 

 1; Fig. 1; Edmond et al., 1980). This finding changes the role 

 of the ocean basins from passive sinks to active sources of 

 certain chemical elements and compounds which affect the entire 

 ocean environment. A byproduct of this chemical exchange is 

 mineralization in which metals present in trace quantities in 

 oceanic crust are concentrated into mineral deposits beneath and 

 on the seafloor by precipitation from the hydrothermal solutions 

 triggered by chemical and physical gradients (Rona, 1984). The 

 mineral deposits are analogs of certain ancient polymetallic 

 sulfides and related types of deposits uplifted from the seafloor 

 onto land where they are mined. Accordingly, the seafloor sites 

 are both natural laboratories for the observation of 

 mineralization processes as well as resources for the future. 

 The adaptation of animal communities at the seafloor vents to a 

 food net based on utilization by bacteria of certain dissolved 

 compounds in the hot springs entirely dependent on subseafloor 

 chemical exchanges is the subject of intensive biologic study, as 

 well as the evolution of the vent organisms and their possible 

 relation to the origin of life (Grassle, 1983). 



HYDROTHERMAL ACTIVITY AT THE MID-ATLANTIC RIDGE 



Characteristics 



The Mid-Atlantic Ridge extends about 15,000 km along the 

 center of the North Atlantic and South Atlantic Oceans between 

 latitudes 65°N and 55°S and is the prototype slow-spreading 

 oceanic ridge with full-rates up to about 4 cm/y (full-rate is 

 the sum of spreading rates to either side of spreading axis). It 

 exhibits features common to all oceanic ridges (Fig. 3) as well 

 as features specific to slow-spreading ridges. A common feature 

 of oceanic ridges parallel to their axes is a segmented 

 structure comprised of linear segments of the order of 10 km 

 long which are the loci of seafloor spreading, alternating with 

 various types of discontinuities including transform faults that 

 may offset the linear segments by distances up to hundreds of 

 kilometers (Fig. 3). A feature perpendicular to the axis common 

 to all oceanic ridges is a zone of volcanic extrusion of the 



