Cross-section of a typical submarine volcano with an active hydrothermal system. As magma ascends, water and gases contained in magma exsolve and enter the deep hydrothermal system as pressure is released. Cold seawater (blue arrows) permeates the volcano and becomes heated by the hot rock near the magma body. Hybrid hydrothermal fluids (red arrows), derived from seawater and magmatic gas and fluid, buoyantly rise to the summit while altering the volcanic rock (white=alteration). Fluids discharged at the seafloor rise 10s to 100s of meters before they spread laterally in the hydrothermal plume. The plume is dispersed by the local currents, carrying various dissolved and particulate chemical species derived from the magma and the volcanic rock. Image courtesy of Institute of Geological and Nuclear Sciences.