hydrothermal vents are springs of superheated water that are found scattered along the rift valleys that occur at the crests of the mid-ocean ridges. These valleys are the axes along which new ocean floor is being produced through eruptions of basalts. As the basalts erupt into the ocean water they tend to crack, creating channels through which water can percolate. The hot rocks generate convection currents in the fluids that penetrate deep within the ocean crust. The water is superheated, induces considerable changes in the mineralization of the rocks, and becomes saturated with metallic sulphides. The hottest water emitted from the vents is at a temperature of 350 °C (660 °F) and is kept from boiling by the high hydrostatic pressure. As it gushes out, it is rapidly cooled by mixing with the ambient sea water to temperatures of 2–5 °C (36–41 °F). The metal sulphides precipitate out of solution in black clouds, hence the colloquial name ‘black smokers’. In the immediate vicinity of the vents lives an extraordinary assemblage of clams, mussels, giant tube worms, and shrimps. These animals are characterized by having bacteria in their tissues that can synthesize organic molecules chemically using the energy derived from the oxidation of sulphide ions to sulphate. It is thought that these communities provide an analogue for the first living things that evolved on earth.

Because they occur where there are many earthquakes, many vents are destroyed by seismic events. Those that are destroyed gradually get furred up with metal deposits. The vent waters being emitted become cooler and no longer precipitate sulphides, and are then known as ‘white smokers’.

Fossil vents are found in a few places on land such as the Troodos mountains in Cyprus, and these were important sources of copper in the Bronze Age. The geochemistry of the vents is an important part of the long-term cycling of metals in both the oceans and continental rocks. It is estimated that all ocean water circulates through a vent system every 40,000 years. They are also remarkable in that their existence was predicted from theoretical considerations of seafloor spreading a decade before the first vent system was discovered near the Galapagos in 1977 by Dr Ballard.

See also chemical oceanography.http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/oceanography_recently_revealed1.html

M. V. Angel

Hydrothermal vents

A hydrothermal vent is a geyser that is located on the floor of the sea. The first such vent was discovered in 1977 on the floor of the Pacific Ocean. Since then, vents have been discovered at a variety of locations in the Pacific and Atlantic Oceans.

The vents tend to be located deep in the ocean. For example, in the Atlantic ocean, some 7000 feet beneath the surface, hydrothermal vents are associated with underwater mountain chain called the Mid-Ocean Ridge. This ridge is geologically active with an upwelling of hot magma and volcanic activity. The tectonic plate movements cause faulting and seawater that then enters the cracks is superheated by the molten magma. The superheated water and steam and spews out through hydrothermal vents.

Some vents, known as "black smokers," spew out a black-colored mixture of iron and sulfide. "White smokers" spew out a whitish mix of barium, calcium, and silicon.

This eruption through the hydrothermal vents is continuous, in contrast with the sporadic eruptions of surface geysers. The material that emerges from hydrothermal vents is extremely hot (up to 750° F [398.89° C]) and is very rich in minerals such as sulfur. The minerals can precipitate out of solution to form chimneys. The construction of a chimney can occur quickly. Growth of 30 feet in 18 months is not unusual. The tallest of these chimneys that has been measured was the height of a 15 story building.

A vibrant community of bacteria , tubeworms that are unique to this environment, and other creatures exists around hydrothermal vents. The entire ecosystem is possible because of the activity of the bacteria. These bacteria have been shown, principally through the efforts of the Holger Jannasch (1927–1998) of Woods Hole Oceanographic Institution, to accomplish the conversion of sulfur to energy in a process that does not utilize sunlight called chemosynthesis. The energy is then available for use by the other life forms, which directly utilize the energy, consume the bacteria, or consume the organisms that rely directly on the bacteria for nourishment. For example, the tubeworms have no means with which to take in or process nutrients. Their existence relies entirely on the bacteria that live in their tissues.

See also Chemoautotrophic and chemolithotrophic bacteria; Extremophiles; Sulfur cycle in microorganisms

hydrothermal vent crack along a rift or ridge in the deep ocean floor that spews out water heated to high temperatures by the magma under the earth's crust. Some vents are in areas of seafloor spreading , and in some locations water temperatures above 350°C (660°F) have been recorded. The vents' hot springs leach out valuable subsurface minerals and deposit them on the ocean floor. The disolved minerals precipitate when they hit the cold ocean water, in some cases creating dark, billowing clouds (hence the name "black smokers" for some of the springs) and settling to build large chimneylike structures.

Giant tube worms, bristle worms, yellow mussels, clams, and pink sea urchins are among the animals found in the unique ecological systems that surround the vents. All of these animals live—without sunlight—in conditions of high pressure, steep temperature gradients, and levels of minerals that would be toxic to animals on land. The primary producers of these ecosystems are bacteria that use chemosynthesis to produce energy from dissolved hydrogen sulfide. Some scientists believe such vents may have been the source of life on earth.

Hydrothermal vents were first discovered near the Galápagos Islands in 1977 by scientists in the research submersible Alvin. Vents have since been discovered in the Atlantic and Indian oceans as well. Although a number of species found around the vents in each ocean are also found in other oceans, many of the species are unique to the particular region in which they are found.

hydrothermal vent A place on the ocean floor, on or adjacent to a mid-ocean ridge, from which there issues water that has been heated by contact with molten rock, commonly to about 300°C. The vent water often contains dissolved sulphides. These are oxidized by chemosynthetic bacteria, which fix carbon dioxide and synthesize organic compounds. Near the vents, at temperatures up to 40°C, there are highly productive communities comprising animals that utilize the organic compounds or live symbiotically (see symbiosis) with the chemosynthetic bacteria; these organisms support carnivores and detritivores. Vent fluids containing high concentrations of iron, manganese, and copper tend to be hot (about 350°C) and black. They are known as ‘black smokers’. ‘White smokers’ flow more slowly, are cooler, and contain high concentrations of arsenic and zinc.