significantly increases the resource potential of seafloor 

 sulfides. 



Hydrothermal Activity 



The active mound at the TAG Hydrothermal Field can be 

 visualized as a feature the size and shape of the Houston 

 Astrodome (Fig. 5). It is surrounded by a field of predominantly 

 pillow lava flows, veneered with light tan sediment, indicating 

 that the flows are not fresh. Near the edges of the mound the 

 lava flows are fractured by faults in the marginal zone of active 

 extension that may create permeable pathways for upwelling 

 hydrothermal solutions. An abrupt boundary exists between the 

 lava flows and the outer edge of the mound, composed of talus of 

 massive sulfide fragments with surfaces oxidized to hues of red 

 and orange. Hot spring activity progressively increases in 

 temperature and intensity with changes in surface morphology from 

 edge to center of the mound. Warm springs with estimated 

 temperatures of tens of degrees Celsius above ambient and diffuse 

 discharge emanate as shimmering water from areas of talus and 

 relict chimney-like forms up to several meters high, composed of 

 massive sulfides in the outer one-third of the mound; white 

 anemones and crabs are the predominant animals present. 

 Proceeding toward the center, the next one-third of the mound 

 exhibits similar diffuse flow with the addition of organized flow 

 of white and blue-white smoke from bulbous-shaped chimneys 1 to 

 2 m high. Temperatures up to 300°C are inferred for the 

 solutions containing the smoke by analogy with white smokers at 

 Pacific sites and from the zinc sulfide-rich composition of the 

 recovered chimney (Thompson et al . , 1988). The central one-third 

 of the mound consists of an edifice constructed of massive 

 sulfides surmounted by a group of about 10 chimney-like forms up 

 to 20 m high vigorously discharging black smoke at flow rates up 

 to several meters per second. Black smoke also discharges at 

 slower flow rates from numerous cracks around the base of the 

 central edifice. Layers of calcium sulfate (anhydrite) up to 

 several meters thick interspersed with massive sulfides crop out 

 around the base of the central edifice. Eel-like grey fish 

 typically about 0.5 m long live in crevices of the irregular 

 surface of the mound. Swarms of grey shrimp, between 2 and 5 cm 

 long of a new genus (Williams and Rona, 1986), cover the sulfide 

 surfaces of all active black smoker cracks and chimneys where 

 they are inferred to be feeding on bacteria chemosynthetically 

 growing in the hot spring environment (Van Dover et al., 1987). 



Solution Properties 



A plume of hydrothermal solutions buoyantly rises from the 

 central area of the mound. The hydrothermal plume entrains 

 surrounding seawater as it rises until its density equals that of 

 surrounding seawater and attains equilibrium in a layer between 

 200 and 350 m above the venting zone extending about 6 km along 



15 



