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Hydrothermal vents in the deep ocean typically form along the Mid-ocean ridges, such as the Mid-Atlantic Ridge. These are locations where upwellings of ascending mantle plumes occur between two tectonic plates at a divergent boundary.
The water that issues from seafloor hydrothermal vents consists mostly of sea water drawn into the hydrothermal system close to the volcanic edifice through faults and porous sediments or volcanic strata, plus some magmatic water released by the upwelling magma.
In terrestrial hydrothermal systems the majority of water circulated within the fumarole and geyser systems is meteoric water plus ground water that has percolated down into the thermal system from the surface, but it also commonly contains some portion of metamorphic waters, sedimentary formational brines and magmatic water that is released by the magma. The proportion varies from location to location.
The water emerges from a hydrothermal vent at temperatures ranging up to 400°C, compared to a typical 2°C for the surrounding deep ocean water. The high pressure at these depths significantly expands the thermal range at which water remains liquid, and so the water doesn't boil. Water at a depth of 3,000 m and a temperature of 407°C becomes supercritical. However the increase in salinity pushes the water closer to its critical point.
Some hydrothermal vents form roughly cylindrical chimney structures. These form from minerals that are dissolved in the vent fluid. When the super-heated water contacts the near-freezing sea water, the minerals precipitate out to form particles which add to the height of the stacks. Some of these chimney structures can reach heights of 60 m. An example of such a towering vent is "Godzilla", a structure in the Pacific Ocean near Oregon that rose to 40 m before it fell over.
The initial stages of a vent chimney begin with the deposition of the mineral anhydrite. Sulfides of copper, iron and zinc then precipitate in the chimney gaps, making it less porous over the course of time. Vent growths on the order of 30 cm per day have been recorded.
Chimney structures that emit a cloud of black material are called "black smokers", named for the dark hue of the particles they emit. The black smokers typically emit particles with high levels of metal sulfides. Vents that emit lighter-hued minerals have also been discovered, and these are named "white smokers". They are typically lower in temperature than black smokers, and are deficient in copper, iron and hydrogen sulfide, while being rich in zinc.
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