Hydrothermal Vents

Hydrothermal vents are openings on the sea floor where hot water is released. Hydrothermal water forms when cold seawater percolates down into the seabed rock and toward the underlying mantle, where hot rocks heat it. This interaction changes both rocks and water. Deposits rich in metals form around these vents, and many ore deposits originally developed in similar environments. Numerous organisms live around hydrothermal vents, supported by bacteria that derive their energy from the reduction of sulfide to sulfate.

Characteristics

The late 1970s discovery of hydrothermal vents and the unique communities that live around them astounded scientists. Studies of these features have provided important and exciting research opportunities for biologists, geologists, and marine chemists. Almost all of the deep-sea floor is cold and quiet, but this is not true near hydrothermal vents. In contrast to the homogeneous and stable environment of the deep sea, hydrothermal vents are constantly changing, and scientists have observed many changes in hydrothermal vents and their associated communities in the relatively short period that they have been studied. Hydrothermal vents are easily the most spectacular features, both geologically and biologically, on the floor of the abyss. There are almost certainly many more hydrothermal vents remaining to be discovered along midocean ridges, where hydrothermal vents are concentrated.

Hydrothermal vents are hot springs on the ocean floor, the “exhaust pipes” of seafloor hydrothermal systems. These vents are the most accessible parts of seafloor hydrothermal systems, driven by heat sources that lie several hundred meters or kilometers beneath the ocean floor. Hydrothermal systems form where near-freezing seawater from the bottom of the ocean penetrates deep into the crust along fissures until it comes into contact with hot rock structures. The water may be heated to as high as 400 degrees Celsius before it rises back up through the crust and jets back into the deep sea at the vents. The development of hydrothermal systems requires two things: hot rock structures and a way for seawater to percolate along cracks into the hot zone and return to the surface. How vigorous the resulting hydrothermal system is depends on both these factors. The hottest rock structures are found at midocean ridges and other submarine volcanic centers such as the Loihi seamount off the coast of Hawaii, where the frequent eruption of basalt lavas maintains conditions that are most favorable for developing and maintaining hydrothermal systems.

Yet it is not enough for hot rock to lie beneath the surface; there must also be a way for seawater to be brought into contact with the hot rock and kept there until the water is sufficiently heated. This requires the right configuration of fractured hot rock: If the cracks are too wide or the hot rock is not deep below the sea floor, the water will not be heated sufficiently before it returns to the surface. Similarly, if the cracks are too narrow, insufficient water will flow through the system to develop a robust hydrothermal system, or the channels may be easily blocked by minor seismic or tectonic movements or mineral deposits. The sensitivity of deep-sea hydrothermal systems to the sub-seafloor circulation geometry may be why hydrothermal events experience rapid changes and are generally short-lived features.

Seafloor hydrothermal systems are similar to hot springs or geysers found on the continents in that both form as hot rock heats and chemically modifies cold water. Seafloor hydrothermal systems differ, however, in their much higher temperatures. The boiling temperature of water increases as pressure increases, and pressures on the sea floor are several hundred times greater than at the surface. This relationship between pressure and the temperature at which a liquid boils is well known to most cooks, who are familiar with the fact that water boils at a lower temperature at high altitude, where the pressure is lower than it does at sea level. Hydrothermal systems on land, such as those of Yellowstone National Park, have a maximum water temperature of not much more than about 100 degrees Celsius. At greater temperatures, the water is superheated and “flashes,” or turns instantly to steam. This makes a geyser such as Old Faithful erupt, as the water in contact with the hot underlying rock turns to steam and violently forces out the colder, overlying water. The higher pressure of the sea floor prohibits flashing, however, and allows water to be heated to much higher temperatures.

Although most hydrothermal systems are found along the midocean ridges, where magma lies not more than a few kilometers beneath the ocean floor, hydrothermal water is not heated by magma. Water heated by magma would be much closer to magmatic temperatures of 1,200 degrees Celsius. Instead, the water is heated as it passes through hot but solid rock, which is heated by underlying magma. The upper limit of about 400 degrees Celsius for seafloor hydrothermal systems may reflect the maximum rock temperature at which fractures can form and remain open, or it may reflect the separation of the fluid into immiscible fluids. It may be that at temperatures much higher than 400 degrees Celsius, the basaltic rock slowly flows, closing any fractures that have formed. Another possibility is that, at pressures corresponding to typical depths of midocean ridges (about 300 bars), seawater separates into two fluids at about 400 degrees Celsius. One phase is enriched in salt relative to seawater, while the other fluid contains a lower salt concentration than seawater. The more concentrated phase will be the denser phase, which may be why it is not found among waters issuing from hydrothermal vents. About one-third of the total heat lost from the planet’s interior through the sea floor is lost due to seafloor hydrothermal systems.

Metal Deposits

The seawater moving through a seafloor hydrothermal system is chemically changed as it is heated and alters the rocks through which it passes. The hot rock absorbs all of the magnesium and sulfate in the seawater, and large amounts of metals such as manganese, cobalt, copper, zinc, and iron are lost from the hot rock to the circulating water. Magnesium is so completely removed from seawater that the depletion of this element in seawater can only be explained by seafloor hydrothermal activity. Oxygen-rich seawater is transformed into oxygen-poor hydrothermal water, and a high concentration of metals can be dissolved in such water. Seawater, for example, contains negligible magnesium and iron, whereas hydrothermal waters contain up to 1 millimole of magnesium and 6 millimoles of iron per liter.

When hot, chemically transformed water returns to the ocean at hydrothermal vents, the result is often spectacular. Near the vent, the hot, oxygen-poor, hydrogen-sulfide-rich hydrothermal water mixes with cold, oxygen-rich, hydrogen-sulfide-poor seawater. This may cause the metals in the hydrothermal fluid to precipitate. Some of it may precipitate in fractures just beneath the sea floor. These “stockwork” deposits are most likely to be preserved, and many ore deposits represent ancient stockwork deposits. Some metals precipitate around the vent itself, forming metal-rich chimneys. These chimneys are typically composed of an outer layer of anhydrite and inner deposits of copper-iron sulfides. These chimneys can have fantastic shapes, resembling spires, columns, cones, and beehives. Scientists visiting these chimneys give them fanciful names: “nail,” “fir tree,” and “moose” are names given to vent features from one hydrothermal field. Chimneys have been found to be up to 30 meters in diameter and up to 45 meters tall. Their growth rates can be 1 meter per year, and vent chimneys are thus among the fastest-changing of all geologic phenomena. Fast-growing vent chimneys rapidly become unstable and collapse, then rise again. Cycles of growth and collapse continue as long as the hydrothermal fluids continue to issue from the vent.

Metals that do not precipitate as stockwork or vent chimneys rise with the hydrothermal water issuing from the vent. The hydrothermal water exits the vents in a jet stream due to the pressure exerted from below. Because this water is so hot, it is less dense than seawater, and the jet forms the base of a hydrothermal plume. The plume becomes larger as it rises because cold seawater mixes turbulently with the hydrothermal water, and these waters quickly become well-mixed. The hottest hydrothermal fluids (those heated to 300 degrees Celsius or more) are sufficiently metal-rich and acidic to precipitate large quantities of microscopic grains of iron sulfides, zinc sulfides, and copper sulfides upon mixing with seawater. Much of this precipitates as the plume rises, producing a sulfide “cloud.” The rapid precipitation of sulfides darkens the hydrothermal plume, giving it the appearance of smoke. Those vents releasing the hottest fluids have the greatest sulfide precipitation density and are often called “black smokers.” In contrast, vents releasing cooler water (100 to 300 degrees Celsius) do not contain sufficient sulfide or metals in solution to cause this effect. Instead, mixing these hydrothermal waters with seawater causes white particles of silica, anhydrite, and barite to form, forming a white cloud in the mixing plume and giving rise to the name “white smoker.” White smokers may reflect mixing just below the surface, with the result that most of the metals associated with white smoker vents may be deposited just below the sea floor. Because the original hydrothermal vent waters are cooler and thus denser, plumes from white smokers do not rise as far as those from black smokers before obtaining neutral buoyancy and spreading laterally.

Mixing progressively dilutes the hydrothermal water until the mixed water cools to the point that it has the same density as ambient seawater and no longer rises. A tremendous amount of seawater must be mixed with the hydrothermal fluids before the plume attains neutral buoyancy. This may involve 10,000 or even 100,000 times as much seawater as hydrothermal water. During dilution, the mixture rises tens to hundreds of meters to a level of neutral buoyancy, eventually spreading laterally as a distinct hydrographic and chemical layer, recognizable hundreds or even thousands of kilometers away from the hydrothermal vent. The continued settling of hydrothermal iron and magnesium from these layers is the source of most of the metals in manganese nodules of the abyssal sea floor, far away from the midocean ridges.

Vast quantities of metals are deposited as sulfides around hydrothermal vents, especially as stockwork and in sediments around chimneys. These are particularly rich in iron, copper, zinc, and lead. The exact composition of these deposits reflects several controlling factors, including the temperature of the hydrothermal fluid, water depth, composition of the source rock, and flow regime in the portion of the hydrothermal system that lies beneath the sea floor. Many of the world’s great ore deposits seem to be fossil seafloor hydrothermal systems, with the possible difference being that these may have mostly formed at convergent plate margins instead of divergent plate margins, where most modern hydrothermal systems are known. This may be why ancient massive sulfide deposits are typically much larger than modern ones.

Study of Hydrothermal Vents

To a marine biologist, hydrothermal vents are the oases of the deep-sea floor. In contrast to most of the ocean floor, which supports few animals, hydrothermal vents teem with life. The analogy to a desert oasis falls short because there is much more life around a hydrothermal vent. Life in most of the deep sea is scarce because food is scarce. There is no sunlight, so plants, the basis of most food chains, cannot grow. In contrast, food is abundant around hydrothermal vents; often, life is so crowded that the animals obscure the sea floor.

Vent fields can be divided into three biotic zones reflecting the distance from the hydrothermal vents: the vent opening, the near field, and the periphery. The bulk of the biomass is at the vent openings, where the density of life is so great that it appears to be limited by space, not food. Around hydrothermal vents on the East Pacific Rise and Galápagos Rift, vent openings are dominated by endosymbionts such as tube worms, clams, and mussels. These endosymbionts can grow to a great size; some tube worms are 1 meter long with 3-meter tubes, while clams up to 30 centimeters long are common. Other animals, including limpets, polychaete worms, bresiliid shrimp, crabs, and fish, live with these. Vent chimneys at several Mid-Atlantic Ridge vents are almost entirely populated by bresiliid shrimp.

Autotrophic bacteria are the primary producers around hydrothermal vents. These use sulfur to convert carbon dioxide, water, and nitrate into essential organic substances in a fashion similar to how plants use sunlight during photosynthesis. This process is called “chemosynthesis.” Autotrophic bacteria live within the subsurface plumbing system of the vents, on the sea floor, and suspended in and around the hydrothermal plume itself, sometimes in such abundance that they color the water a milky blue or carpet the sea floor in white or bright yellow mats. Some of these bacteria can tolerate inordinately high temperatures, up to 110 degrees Celsius or more. All the other life around a hydrothermal vent ultimately feeds off the autotrophic bacteria. Some autotrophic bacteria are symbiotic, living within larger vent animals in a mutually beneficial relationship. The bacteria provide food, and the animals provide essential inorganic nutrients.

The flow of water around hydrothermal vents controls the distribution of life. Because cold water mixes with the hydrothermal plume, cold seawater flows in from all directions to converge on the rising plume. This means that the sulfur on which the autotrophic bacteria depend is not distributed around the vent. Similarly, the animals that feed or depend on the bacteria cannot survive away from the vent opening. The near field is mostly populated by suspension feeders, animals that capture bacteria, and other organisms that drift away from the vent opening. These animals are presumed to live as close as possible to the vent but are forced to maintain a certain distance because of the toxic effects of very high concentrations of heavy metals. Animals living on the vent periphery include scavengers and other types that sustain themselves from bacteria that settle out of the hydrothermal plume.

Most of the animals that live around hydrothermal vents live nowhere else except in other sulfur-rich, reducing environments, such as in the rotting carcasses of whales or in “cold seeps,” where cold, chemically altered seawater percolates up through the sea floor. Many vent animals may be ancient, originating in Mesozoic or earlier times. These animals may have been insulated from the effects of surface catastrophes, such as the meteor collision at Chicxulub believed to have killed off at least 75 percent of all life on the planet, including the dinosaurs, about 65 million years ago.

There are several fascinating features about the life around hydrothermal vents. One of the most intriguing is the suggestion that this environment is similar to what existed when life was just developing about 4 billion years ago. It is entirely possible that the first autotrophic life-forms were chemosynthetic, not photosynthetic. In the twenty-first century, scientists continue to make discoveries about hydrothermal vents. In 2022, scientists discovered a field of hydrothermal vents on the Knipovich Ridge off the coast of Svalbard, a Norwegian island. In 2024, scientists announced the discovery of five new hydrothermal vents in the Eastern Tropical Pacific Ocean. The discovery of new ecosystems in volcanic cavities beneath hydrothermal vents on the East Pacific Rise allowed researchers to deduce that life exists above and below the sea floor. 

Principal Terms

basalt: a typical volcanic rock of the ocean floor with a relatively low silica content

convergent plate margin: an area where two tectonic plates impinge upon each other with pressure directed toward the area of contact; typically, in such regions, the lithosphere is being returned to the mantle at a subduction zone, forming volcanic “island arcs” and associated hydrothermal activity

divergent margin: an area where two tectonic plates are moving away from each other; where the crust and lithosphere form by seafloor spreading

endosymbiont: an animal-hosting autotrophic bacterium, with both host and bacterium enjoying the benefits of symbiosis

hydrostatic pressure: the pressure resulting from an overlying, continuous column of water, approximately 1 bar for every 10 meters of water

magma: molten rock generated by melting in the Earth’s mantle

midocean ridge: a region of the sea floor where new oceanic crust is created by seafloor spreading

millimole: a universal relative quantity equal to one thousandth of a mole

mole: the quantity, in grams, of any pure material numerically equivalent to the atomic or molecular weight of that material

ophiolite: a section of oceanic crust and upper mantle that has been thrust out of the ocean floor and up onto the continental crust

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