Hydrothermal vents are the result of sea water percolating down through fissures in the ocean crust near spreading centers or subduction zones. The cold seawater is heated by hot magma, and re-emerges to form the vents. As the seawater is heated, it dissolves many minerals and metals from the crust. As super-heated, metal-infused hydrothermal fluids rise, they mix with near-freezing seawater. Rapid chemical reactions cause sulfur and other minerals to precipitate. They may form chimneys or towers as well as mineral-rich deposits on the seafloor. “Black smokers” are chimneys formed from deposits of iron sulfide, which is black. “White smokers” are chimneys formed from deposits of barium, calcium, and silicon, which are white.