hydrothermal vent

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hydrothermal vent

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℃ (660℉) have been recorded; temperatures at vents in the Cayman Trough in the Caribbean Sea have been measured at above 400℃ (750℉). The deepest known vents are those of the Beebe Vent Field in the Cayman Trough, some 16,273 ft (4,960 m) below the sea surface.

The hot springs found at hydrothermal vents leach out valuable subsurface minerals and deposit them on the ocean floor. The dissolved minerals precipitate when they hit the cold ocean water, in some cases creating dark, billowing clouds of sulfides (hence the name “black smokers” for some of the springs) and settling to build large chimneylike structures. “White smokers” tend to release lower temperature fliuds with lighter-colored minerals. Diffuse flows result when the hot springs mix with cold seawater before the exit from beneath the ocean floor; the fluids in these flows are typically cooler, are less toxic as a result of mixing with seawater, and spread over a larger area without creating “smoke” or 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, Indian, and Southern 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.

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Hydrothermal vent

A hot spring on the ocean floor, where heated fluids exit from cracks in the Earth's crust. Most hydrothermal vents occur along the central axes of mid-oceanic ridges, which are underwater mountain ranges that wind through all of the deep oceans. The best-studied vents are at tectonic spreading centers on the East Pacific Rise and at the Mid-Atlantic Ridge. However, vents are also found over hot spots such as the Hawaiian Islands and Iceland, in back-arc basins such as those in the western Pacific, in shallow geothermal systems such as those off the Kamchatka Peninsula, and on the flanks of some underwater volcanoes and seamounts. Hydrothermal vent sites, or closely grouped clusters of vent deposits and exit ports, may cover areas from hundreds to thousands of square feet (tens to hundreds of square meters). Individual vent sites may be separated along mid-ocean ridges by more than 1000 mi (1600 km).

All of the hydrothermal vent sites occur in areas where quantities of magma exist below the sea floor. Cold seawater is drawn down into the oceanic crust toward the heat source. As the seawater is heated and reacts with surrounding rock, its composition changes. Sulfate and magnesium are major components of seawater lost during the reactions; sulfide, metals, and gases such as helium and methane are major components gained. This modified seawater is known as hydrothermal fluid. Buoyant, hot hydrothermal fluid rises toward the sea floor in a concentrated zone of upflow to exit from the sea floor at temperatures ranging from 50°F (10°C) to greater than 750°F (400°C), depending on the degree of cooling and of mixing with seawater during the ascent. If the sea floor is shallow enough and the fluid hot enough, the solution may boil; but it usually does not because of the pressure of overlying seawater.

Hydrothermal fluid that mixes extensively with seawater below the sea floor surface may reach the sea floor as warm springs, with temperatures of 50–86°F (10–30°C). This outflow is usually detectable as cloudy or milky water, but the flow is slow and no mineral deposits accumulate except for some hydrothermal staining or oxidation of sea floor basalts. When hotter, relatively undiluted hydrothermal fluid reaches the sea floor, it is still buoyant with respect to seawater, so that the hot solution rises out of cracks in the sea floor at velocities up to about 6 ft (2 m) per second, mixing turbulently with seawater as it rises. Mixing of hydrothermal fluid with seawater leads to precipitation of minerals from solution, forming mineral deposits at the exit from the sea floor and so-called smoke, tiny mineral particles suspended in the rising plume of fluid. Black smoker vents are distinguished by the presence of such large quantities of minute mineral particles that the plumes become virtually opaque.

Perhaps the most striking feature of sea-floor hydrothermal vents is their dense biologic communities. Vent faunas tend to be dominated by mollusks, annelids, and crustaceans, whereas faunas on nonvent hard-bottom habitats consist predominantly of cnidarians, sponges, and echinoderms. Biologically, vents are among the most productive ecosystems on Earth. Sulfide from hydrothermal fluids provides the energy to drive these productive systems. Whereas most animal life depends on food of photosynthetic origin (inorganic carbon converted to useful sugars by plants using energy from the Sun), the animals at hydrothermal vents obtain most or all of their food by a process of chemosynthesis. Chemosynthesis is accomplished by specialized bacteria residing in hydrothermal fluids, in mats on the sea floor, or in symbiotic relationships with other organisms. The bacteria convert inorganic carbon to sugars by mediating the oxidation of hydrogen sulfide, thereby exploiting the energy stored in chemical bonds. A few vent animals are also known to use methane gas as a source of energy and carbon. The physical and chemical conditions at hydrothermal vents would be lethal to most marine animals, but vent species have adapted to the conditions there.

In a remarkable discovery, it was shown that chemosynthetic microbes known as Archaea are flushed from cavities deep within the Earth's crust by hydrothermal and volcanic activity. These microbes are hyperthermophilic (hot-water-loving) and thrive at temperatures exceeding 90°C (194°F). It is now suspected that an entire community of such microbes inhabits the rocks deep within the water-saturated portions of the Earth's crust.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

hydrothermal vent

[‚hī·drə¦thərm·əl ′vent]
A hot spring on the ocean floor, found mostly along mid-oceanic ridges, where heated fluids exit from cracks in the earth's crust. Iron, sulfur, and other materials precipitate from these waters to form dark clouds. Also known as black smoker.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.