Deep down in the ocean at tectonic plate boundaries, hot fluids rise from hydrothermal vents. The fluids are devoid of oxygen and contain large amounts of metals such as iron, manganese or copper. Some may also transport sulfides, methane and hydrogen. When the hot water mixes with the cold and oxygenated surrounding seawater, hydrothermal plumes develop containing smoke-like particles of metal sulfide.
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Source: Phys.org