Editing Abyssal plain (section)

===Hydrothermal vents===
[[File:Phase-diag2.svg|thumb|In this [[phase diagram]], the green dotted line illustrates the [[Water (molecule)#Density of water and ice|anomalous behavior of water]]. The solid green line marks the [[melting point]] and the blue line the [[boiling point]], showing how they vary with pressure.]]
{{Main|Hydrothermal vent}}

A rare but important terrain feature found in the bathyal, abyssal and hadal zones is the hydrothermal vent. In contrast to the approximately 2&nbsp;°C ambient water temperature at these depths, water emerges from these vents at temperatures ranging from 60&nbsp;°C up to as high as 464&nbsp;°C.<ref name=Haas2007/><ref name=Andrea2008/><ref name=Kosch2008/><ref name=Hill2008/><ref name=Karst2009/> Due to the high [[Atmospheric pressure|barometric pressure]] at these depths, water may exist in either its liquid form or as a [[supercritical fluid]] at such temperatures.

At a barometric pressure of 218 [[Atmosphere (unit)|atmospheres]], the [[Critical point (thermodynamics)|critical point]] of water is 375&nbsp;°C. At a depth of 3,000&nbsp;meters, the barometric pressure of sea water is more than 300&nbsp;atmospheres (as salt water is [[Density|denser]] than fresh water). At this depth and pressure, seawater becomes supercritical at a temperature of 407&nbsp;°C (''see image''). However the increase in salinity at this depth pushes the water closer to its critical point. Thus, water emerging from the hottest parts of some hydrothermal vents, ''[[black smoker]]s'' and [[submarine volcano]]es can be a [[Supercritical fluid#Submarine volcanoes|supercritical fluid]], possessing physical properties between those of a [[gas]] and those of a [[liquid]].<ref name=Haas2007/><ref name=Andrea2008/><ref name=Kosch2008/><ref name=Hill2008/><ref name=Karst2009/>

''Sister Peak'' (Comfortless Cove Hydrothermal Field, {{Coord|4|48|S|12|22|W|}}, elevation −2996&nbsp;m), ''Shrimp Farm'' and ''Mephisto'' (Red Lion Hydrothermal Field, {{Coord|4|48|S|12|23|W|}}, elevation −3047&nbsp;m), are three hydrothermal vents of the black smoker category, on the Mid-Atlantic Ridge near [[Ascension Island]]. They are presumed to have been active since an earthquake shook the region in 2002.<ref name=Haas2007/><ref name=Andrea2008/><ref name=Kosch2008/><ref name=Hill2008/><ref name=Karst2009/> These vents have been observed to vent [[Phase transition|phase-separated]], vapor-type fluids. In 2008, sustained exit temperatures of up to 407&nbsp;°C were recorded at one of these vents, with a peak recorded temperature of up to 464&nbsp;°C. These [[thermodynamics|thermodynamic]] conditions exceed the critical point of seawater, and are the highest temperatures recorded to date from the seafloor. This is the first reported evidence for direct [[magma]]tic-[[Hydrothermal circulation|hydrothermal]] interaction on a slow-spreading mid-ocean ridge.<ref name=Haas2007/><ref name=Andrea2008/><ref name=Kosch2008/><ref name=Hill2008/><ref name=Karst2009/> 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&nbsp;cm (1&nbsp;ft) per day have been recorded.[11] 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 (see iron cycle).

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.