hot black smokers and vent animal communities at the Mid-Atlantic 

 Ridge in 1985 ( Rona et al., 1986; Ocean Drilling Program Leg 106 

 Scientific Party, 1986) dramatically changed perception of the 

 role of hydrothermal activity at slow-spreading oceanic ridges. 

 This change initiated a redress of the imbalance in research 

 emphasis between the Pacific Ocean and the Atlantic Ocean and, 

 at the same time, positioned the Gorda Ridge and the Mid-Atlantic 

 Ridge at the leading edge of undersea research. 



SEAFLOOR HYDROTHERMAL ACTIVITY 



Process 



A major factor driving research at oceanic ridges is 

 interest in seafloor hydrothermal activity, because such activity 

 has wide ramifications on basic and applied levels. The ocean 

 basins are leaky as containers of the oceans. Cold, dense 

 seawater penetrates kilometers downward through permeable 

 fractures in oceanic crust. In most places in the ocean basin 

 where this penetration occurs it is a one-way trip downward and 

 the water is assimilated into the crust which is about 5 km 

 thick, and into the underlying upper mantle. In localized areas 

 where heat sources exist within the oceanic crust in the form of 

 magma chambers and related offshoots of hot volcanic rock the 

 down-welling seawater is heated, thermally expands, and buoyantly 

 upwells as a less dense hot solution forming a subseafloor 

 convective circulation system which discharges as hot springs at 

 the seafloor (Fig. 2). Subseafloor hydrothermal convection 

 systems are highly efficient in exchanging chemicals and heat 

 between the ocean and the oceanic crust. A two-way chemical 

 exchange occurs between the circulating seawater and the rocks. 

 During circulation certain elements and compounds are removed 

 from the seawater and other elements, notably metals, are 

 dissolved from mineral phases in the fractured volcanic rocks 

 (Table 1; Fig. 2). 



The major locus of heat sources that drive hydrothermal 

 circulation in oceanic crust is an intermittent line of magma 

 chambers and hot volcanic rock generated in the process of 

 seafloor spreading at the submerged volcanic mountain ranges of 

 the oceanic ridge system. The submerged volcanic mountain ranges 

 are divergent plate boundaries that extend some 55,000 km through 

 all the ocean basins of the world (Fig. 1). Other volcanic heat 

 sources that drive hydrothermal circulation in oceanic crust are 

 associated with volcanic island arcs such as those that lie 

 along the western margin of the Pacific Ocean and comprise 

 convergent plate boundaries where oceanic crust and upper mantle 

 is reassimilated into the Earth's interior (Fig. 1). Certain 

 volcanic islands like Hawaii, isolated from plate boundaries, are 



