Conceptually, we break the process into three phases. In the recharge zone, cold seawater penetrates into the ocean crust and is gradually heated along its flow path. Seawater sulfate is removed from solution by precipitation as anhydrite and by reduction to hydrogen sulfide.
Fluids reach their maximum temperature in the high-temperature reaction zone near the heat source.
Reactions are fast at high temperature, and fluids may approach equilibrium with a set of alteration minerals. Hot hydrothermal fluids are buoyant and rise toward the ocean floor in the upflow zone. The residence time in this zone is probably very short, but fluids may continue to react with the rock through which they flow. Hydrothermal vents are the interface between the hot, anoxic upflow zone and cold, oxidized seawater. Some of these minerals are formed as part of the chimney structures that build up on the sea floor, while others are formed within the plume and dispersed through the water.