SEAFLOOR HYDROTHERMAL ACTIVITY: BLACK SMOKER CHEMISTRY AND CHIMNEYS

Abstract

Venting of hydrothermal solutions on the seafloor, first discovered in 1977 at the Galapagos spreading center (GSC) (Figure 1), has now been shown to be a relatively common phenomenon on the world ridge-crest system. It is a consequence of the emplacement of hot rock at divergent plate boundaries and the accompanying circulation of seawater through the oceanic crust. This mechanism for convective cooling of the crust was first suggested based on the deficiency of conductive heat loss extending from the ridge axes (zero crustal age) to several thousand kilometers away on the ridge flank� (crustal ages of �everal million years) (Anderson et al 1977). Although relatively little of the 55,000-km length of the world ridge­ crest system has been explored, at least 10 hydrothermally active sites have been visited and their fluids sampled by submersible. Photographic evidence from camera tows and dredging of hydrothermally precipitated or altered minerals and rocks suggest past or present hydrothermal activity at numerous other sites. The modes of occurrence of these seafloor hot springs differ widely; they are found over a wide range of spreading rates, at sediment-covered and sediment-starved ridge axes, and at temperatures ranging from just a few degrees above that of the ambient seawater (24°C) to � 350°C. Table I summarizes some of the physical properties of the areas that have had vent fluid sampled by submersible. Where the measured exit temperature and the inferred temperature of reaction within the hydrothermal system differ significantly, both values are given. Although the fluids have certain common characteristics to their chemistry,