A total of 32 holes at 5 sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November–December 1979) provide unique data on the origin of the hydrothermal mounds on the south flank of the Galapagos spreading center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (⩽100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with sea water and pelagic sediments above locally permeable zones of ocean crust. Mounds as much as 25 m in height form in less than a few × 105 yrs, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

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