Schematic diagram illustrating hydrothermal circulation

Cross-section of ocean crust, with volcanic ridge axis at left, and progressively older crust to the right. Large arrows represent the flow of heat from the mantle. Small arrows represent the flow of seawater and hydrothermal fluids through the crust. Primary sites of fluid exchange in older, sedimented crust may be exposed at rocky outcrops (1). There is exchange between sediment pore fluids (2) and circulating fluids (3) within the upper, permeable volcanic layer. The deeper section of vertical dikes is much less permeable and fluid flow is more restricted. Hydrothermal circulation continues as the crust ages, but the temperature and composition of the fluids that vent to the ocean in old crust are very poorly known. (After Butterfield et al., 2001)

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