Hydrothermal Vent - Physical Properties

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 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 water, magmatic water, and sedimentary formational brine that is released by the magma. The proportion of each varies from location to location.

In contrast to the approximately 2 °C ambient water temperature at these depths, water emerges from these vents at temperatures ranging from 60 °C up to as high as 464 °C. 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 (pure) water is 375 °C at a pressure of 218 atmospheres. At a depth of 3,000 meters, the hydrostatic pressure of sea water is more than 300 atmospheres (as salt water is denser than fresh water). At this depth and pressure, seawater becomes supercritical at a temperature of 407 °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 can be a supercritical fluid, possessing physical properties between those of a gas and those of a liquid. Besides being superheated, the water is also extremely acidic, often having a pH value as low as 2.8 – approximately that of vinegar.

Sister Peak (Comfortless Cove Hydrothermal Field, 4°48′S 12°22′W / 4.8°S 12.367°W / -4.8; -12.367, elevation -2996 m), Shrimp Farm and Mephisto (Red Lion Hydrothermal Field, 4°48′S 12°23′W / 4.8°S 12.383°W / -4.8; -12.383, elevation -3047 m), are three hydrothermal vents of the black smoker category, located on the Mid-Atlantic Ridge near Ascension Island. They are presumed to have been active since an earthquake shook the region in 2002. These vents have been observed to vent phase-separated, vapor-type fluids. In 2008, sustained exit temperatures of up to 407 °C were recorded at one of these vents, with a peak recorded temperature of up to 464 °C. These 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 magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

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. 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.