Hydrothermal vent

Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspots. The dispersal of hydrothermal fluids throughout the global ocean at active vent sites creates hydrothermal plumes. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.
Hydrothermal vents exist because the Earth is both geologically active and has large amounts of water on its surface and within its crust. Under the sea, they may form features called black smokers or white smokers, which deliver a wide range of elements to the world's oceans, thus contributing to global marine biogeochemistry. Relative to the majority of the deep sea, the areas around hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic bacteria and archaea found around hydrothermal vents form the base of the food chain, supporting diverse organisms including giant tube worms, clams, limpets, and shrimp. Active hydrothermal vents are thought to exist on Jupiter's moon Europa and Saturn's moon Enceladus, and it is speculated that ancient hydrothermal vents once existed on Mars.
Hydrothermal vents have been hypothesized to have been a significant factor to starting abiogenesis and the survival of primitive life. The conditions of these vents have been shown to support the synthesis of molecules important to life. Some evidence suggests that certain vents such as alkaline hydrothermal vents or those containing supercritical CO₂ are more conducive to the formation of these organic molecules. However, the origin of life is a widely debated topic, and there are many conflicting viewpoints.

Physical properties

Hydrothermal vents in the deep ocean typically form along the mid-ocean ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. These are locations where two tectonic plates are diverging and new crust is being formed.
The water that issues from seafloor hydrothermal vents consists mostly of seawater 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. On land, the majority of water circulated within fumarole and geyser systems is meteoric water and ground water that has percolated down into the hydrothermal system from the surface, but also commonly contains some portion of metamorphic water, magmatic water, and sedimentary formational brine released by the magma. The proportion of each varies from location to location.
In contrast to the approximately ambient water temperature at these depths, water emerges from these vents at temperatures ranging from up to as high as. Due to the high hydrostatic pressure at these depths, water may exist in either its liquid form or as a supercritical fluid at such temperatures. The critical point of water is at a pressure of 218 atmospheres.
However, introducing salinity into the fluid raises the critical point to higher temperatures and pressures. The critical point of seawater is and 298.5 bars, corresponding to a depth of ~ below sea level. Accordingly, if a hydrothermal fluid with a salinity of 3.2 wt. % NaCl vents above and 298.5 bars, it is supercritical. Furthermore, the salinity of vent fluids have been shown to vary widely due to phase separation in the crust. The critical point for lower salinity fluids is at lower temperature and pressure conditions than that for seawater, but higher than that for pure water. For example, a vent fluid with a 2.24 wt. % NaCl salinity has the critical point at and 280.5 bars. Thus, water emerging from the hottest parts of some hydrothermal vents can be a supercritical fluid, possessing physical properties between those of a gas and those of a liquid.Image:Phase-diag2.svg|thumb|upright=1.2|In this phase diagram, the green dotted line illustrates the anomalous behavior of water. The dashed gray line from the triple point falls onto the melting point and the one from the critical point falls onto the boiling point on the Temperature axis, showing how they vary with pressure; the solid green line shows the typical melting point behavior for other substances.Examples of supercritical venting are found at several sites. Sister Peak vents low salinity phase-separated, vapor-type fluids. Sustained venting was not found to be supercritical but a brief injection of was well above supercritical conditions. A nearby site, Turtle Pits, was found to vent low salinity fluid at, which is above the critical point of the fluid at that salinity. A vent site in the Cayman Trough named Beebe, which is the world's deepest known hydrothermal site at ~ below sea level, has shown sustained supercritical venting at and 2.3 wt% NaCl.
Although supercritical conditions have been observed at several sites, it is not yet known what significance, if any, supercritical venting has in terms of hydrothermal circulation, mineral deposit formation, geochemical fluxes or biological activity.
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 per day have been recorded. An April 2007 exploration of the deep-sea vents off the coast of Fiji found those vents to be a significant source of dissolved iron.

Black smokers and white smokers

Some hydrothermal vents form roughly cylindrical chimney structures. These form from minerals that are dissolved in the vent fluid. When the superheated 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. An example of such a towering vent was "Godzilla", a structure on the Pacific Ocean deep seafloor near Oregon that rose to before it fell over in 1996.

Black smokers

A black smoker or deep-sea vent is a type of hydrothermal vent found on the seabed, typically in the bathyal zone, but also in lesser depths as well as deeper in the abyssal zone. They appear as black, chimney-like structures that emit a cloud of black material. Black smokers typically emit particles with high levels of sulfur-bearing minerals, or sulfides. Black smokers are formed in fields hundreds of meters wide when superheated water from below Earth's crust comes through the ocean floor. This water is rich in dissolved minerals from the crust, most notably sulfides. When it comes in contact with cold ocean water, many minerals precipitate, forming a black, chimney-like structure around each vent. Chimneys thicken due to heat conduction encouraging crystallization. The deposited metal sulfides can become massive sulfide ore deposits in time. Some black smokers along the Azores segment of the Mid-Atlantic Ridge are exceptionally metal-rich; for instance, hydrothermal fluids from the Rainbow Vent Field contain up to 24,000 μM of dissolved iron.
Black smokers were first discovered in 1979 on the East Pacific Rise by scientists from Scripps Institution of Oceanography during the RISE Project. They were observed using the deep submergence vehicle ALVIN from the Woods Hole Oceanographic Institution. Now, black smokers are known to exist in the Atlantic and Pacific Oceans, at an average depth of. The most northerly black smokers are a cluster of five named Loki's Castle, discovered in 2008 by scientists from the University of Bergen at 73°N, on the Mid-Atlantic Ridge between Greenland and Norway. These black smokers are of interest as they are in a more stable area of the Earth's crust, where tectonic forces are less and consequently fields of hydrothermal vents are less common. The world's deepest known black smokers are located in the Cayman Trough, 5,000 m below the ocean's surface.

White smokers

White smoker vents emit lighter-hued minerals, such as those containing barium, calcium and silicon. These vents also tend to have lower-temperature plumes probably because they are generally distant from their heat source.
Black and white smokers may coexist in the same hydrothermal field, but they generally represent proximal and distal vents to the main upflow zone, respectively. However, white smokers correspond mostly to waning stages of such hydrothermal fields, as magmatic heat sources become progressively more distant from the source and hydrothermal fluids become dominated by seawater instead of magmatic water. Mineralizing fluids from this type of vent are rich in calcium and they form dominantly sulfate-rich and carbonate deposits.

Hydrothermal plumes

Hydrothermal plumes are fluid entities that manifest where hydrothermal fluids are expelled into the overlying water column at active hydrothermal vent sites. As hydrothermal fluids typically harbor physical and chemical properties distinct from seawater, hydrothermal plumes embody physical and chemical gradients that promote several types of chemical reactions, including oxidation-reduction reactions and precipitation reactions.
Hydrothermal vent fluids harbor temperatures well above that of ocean floor seawater, meaning that hydrothermal fluid is less dense than the surrounding seawater and will rise through the water column due to buoyancy, forming a hydrothermal plume; therefore, the phase during which hydrothermal plumes rise through the water column is known as the "buoyant plume" phase. During this phase, shear forces between the hydrothermal plume and surrounding seawater generate turbulent flow that facilitates mixing between the two types of fluids, which progressively dilutes the hydrothermal plume with seawater. Eventually, the coupled effects of dilution and rising into progressively warmer overlying seawater will cause the hydrothermal plume to become neutrally buoyant at some height above the seafloor; therefore, this stage of hydrothermal plume evolution is known as the "nonbuoyant plume" phase. Once the plume is neutrally buoyant, it can no longer continue to rise through the water column and instead begins to spread laterally throughout the ocean, potentially over several thousands of kilometers.
Chemical reactions occur concurrently with the physical evolution of hydrothermal plumes. While seawater is a relatively oxidizing fluid, hydrothermal vent fluids are typically reducing in nature. Consequently, reduced chemicals such as hydrogen gas, hydrogen sulfide, methane, Fe²⁺, and Mn²⁺ that are common in many vent fluids will react upon mixing with seawater. In fluids with high concentrations of H₂S, dissolved metal ions such as Fe²⁺ and Mn²⁺ readily precipitate as dark-colored metal sulfide minerals. Furthermore, Fe²⁺ and Mn²⁺ entrained within the hydrothermal plume will eventually oxidize to form insoluble Fe and Mn hydroxide minerals. For this reason, the hydrothermal "near field" has been proposed to refer to the hydrothermal plume region undergoing active oxidation of metals while the term "far field" refers to the plume region within which complete metal oxidation has occurred.