Black Smokers

Black smokers are hydrothermal vents on the ocean floor that discharge plumes of very hot water laden with suspended, fine-grained, dark-coloured minerals, giving the rising fluid the appearance of black smoke. ^{[1] [1]} Where the discharge contains no suspended dark minerals, the vents are called white smokers instead. ^[1] Black smokers are among the most striking geological features on the seafloor, and they are geochemically important as agents of chemical exchange between the crust and the ocean.

Structure and Mineral Deposits

At the sites of many oceanic hot springs, vents composed of sulfide, sulfate, and oxide deposits up to 10 m or more tall discharge plumes of hot solutions. ^[1] These chimneys are constructional features built by the precipitation of minerals from the vent fluid as it makes contact with cold ambient seawater. When the hot hydrothermal solutions mix with the cold seawater, they precipitate various minerals - particularly pyrite (FeS₂) and chalcopyrite (CuFeS₂) - that build sulfide deposits around the vents. ^[1] Pyrite and chalcopyrite are economically important minerals - pyrite is a common iron sulfide, while chalcopyrite is a primary ore of copper. Active black smokers are therefore modern analogues of the ancient ore-forming systems that produced many of the world’s base-metal deposits.

Temperature and Plumes

The temperature of the water when it emerges from the vents may exceed 350°C. ^[1] This is remarkable: at the pressures found at the seafloor, water does not boil even at these temperatures, so the fluid remains liquid and rises buoyantly through the overlying seawater rather than flashing to steam. The heated water rises as hydrothermal plumes 100-300 m above the vent field, with exceptional plumes reported to rise as high as 1000 m. ^[1] As these plumes ascend, they dilute into the ambient seawater and spread laterally, distributing hydrothermal components - including iron, manganese, and silica - across wide areas of the deep ocean.

Geological Settings

Hydrothermal hot springs are found along mid-ocean ridges in both the Pacific and Atlantic oceans, as well as along convergent plate margins, in backarc basins, and even on midplate volcanoes such as those in the Hawaiian chain. ^[1] The diversity of settings reflects the fundamental requirement for hot volcanic rock in contact with seawater, which is not limited to the spreading centres at mid-ocean ridges but occurs wherever volcanic activity and seawater circulation overlap.

Fossil Hydrothermal Systems

Deposits of fossil hydrothermal systems have now been observed in ancient oceanic ophiolite complexes exposed on land. ^[1] Ophiolites are slabs of ancient oceanic crust that have been obducted - thrust up - onto continental margins during plate collisions. Their preservation on land allows geologists to study the full architecture of ancient hydrothermal systems, including the stockwork of veins and the massive sulfide ore bodies that formed at vent sites. These ancient analogues confirm that black smoker-style mineralization is a persistent feature of ocean-floor geology through geologic time.

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