A hydrothermal vent spews out hot water, that has been heated by a volcanic heat source.
Seawater enters the ocean crust through cracks which are commonly found around ocean spreading centers. The water is then heated to a high temperature typically between 300 C and 400 C. The seawater reacts with the rock and leaches out minerals. The hot chemically rich sea water is then expelled on to the ocean floor where it deposits it’s minerals. The water does not boil due to the high pressures been exerted on it by the ocean above.
Hydrothermal vents are typically located around mid ocean spreading ridges and back-arc basins. Spreading Ridges are located world wide.
Sources: V. Tunnicliffe, BRIDGE