GEOLOGY OF DEEP-SEA HYDROTHERMAL VENTS AND SEA-FLOOR SPREADING CENTERS

Crustal and lithospheric formation in the ocean occurs along extensive systems of mid-ocean ridges and rifts termed seafloor spreading centers. Characterized by variable magmatic and thermal budgets, seafloor spreading centers may be grouped according to their spreading rate into slow (1 to 5 cm/year), intermediate (5 to 9 cm/year), and fast (> 9 cm/year) types. All new oceanic crust bears the imprint of a complex cycle of magmatic/volcanic, hydrothermal, and tectonic processes within the spreading center plate boundary zone, which varies significantly in space and time. While magmatic/volcanic processes are directly responsible for the emplacement and accretion of hot rock at divergent plate boundaries, tectonic processes transport the ocean crust away from the site of its genesis and, together with thermal contractive forces, produce an elaborate network of fractures and fissures that enable seawater to circulate through the crust, thereby cooling it. Convective hydrothermal circulation at mid-ocean ridges culminates in the release of heated fluids or hot springs on the seafloor through diffuse, white smoker, and black smoker vents. Hydrothermal convection not only plays an important role in heat transfer at oceanic ridge crests, but also in mass transfer, manifested in the metal-rich sulfide deposits concentrated in stockworks and mounds along upwelling zones, and on the oceanic crust at and adjacent to discharge zones. In addition to cooling oceanic crust and generating potential ore deposits, hydrothermal vents support lush and exotic animal communities and appear to be a chief factor modulating global ocean chemistry.