Seamounts play a fundamental role in facilitating the exchange of fluids, heat, and solutes between the oceanic lithosphere and the overlying ocean. Global heat flow compilations indicate that much of the seafloor loses a significant fraction of lithospheric heat because of fluid flow from the crust, and most of this advective heat loss occurs on ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers. The driving forces available to move fluid between the crust and ocean are modest, and most of the seafloor is blanketed by low-permeability sediment that prevents vertical fluid flow at thermally significant rates. Thus, most of the thermally important fluid exchange between the crust and ocean must occur where volcanic rocks are exposed at the seafloor; little fluid exchange on ridge flanks occurs through seafloor sediments overlying volcanic crustal rocks. Seamounts and other basement outcrops focus ridge-flank hydrothermal exchange between the crust and the ocean. We describe the driving forces responsible for hydrothermal flows on ridge flanks, and the impacts that these systems have on crustal heat loss, fluid composition, and subseafloor microbiology. We show data collected from two ridge-flank areas that illustrate how the extent of fluid exchange, lithospheric heat loss, and chemical reaction and transport depend on the rate of fluid flow, fluid residence time, and temperature in crustal hydrologic systems. Seamounts are ideal places to sample crustal fluids as they exit the crust and enter the ocean, to determine their chemical and microbial characteristics, and to assess the importance of this global hydrogeologic system on the evolution of Earth’s lithosphere, ocean, and biosphere.

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