Hydrothermal vents are cracks on the ocean floor where hot water heated by the Earth's heat flows out. These vents are often found near areas with volcanic activity, such as mid-ocean ridges, ocean basins, and hotspots. When hot water from vents mixes with the ocean, it creates clouds of particles that spread through the water, called hydrothermal plumes. These vents also form rocks and mineral deposits as the hot water cools and releases minerals.
Hydrothermal vents exist because Earth has a lot of water and is geologically active. Under the ocean, these vents can create structures called black smokers or white smokers, which release many different elements into the ocean. These elements help shape the chemical processes in the ocean. Compared to most deep-sea areas, regions near hydrothermal vents are more biologically active. They support unique ecosystems where bacteria and archaea that use chemicals instead of sunlight form the base of the food chain. These microbes support other life, such as giant tube worms, clams, limpets, and shrimp. Scientists believe similar vents may exist on Jupiter’s moon Europa and Saturn’s moon Enceladus. Some evidence suggests ancient vents might have existed on Mars.
Some scientists think hydrothermal vents may have played a role in the beginning of life on Earth. The conditions in these vents can help form molecules needed for life. Certain types of vents, like those with alkaline water or supercritical carbon dioxide, might be especially good at creating these molecules. However, how life began is still a topic of debate, and scientists have many different ideas about it.
Physical properties
Hydrothermal vents in the deep ocean usually form along mid-ocean ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. These are places where two tectonic plates are moving apart, and new ocean floor is created.
The water that comes out of seafloor hydrothermal vents is mostly seawater that flows into the system through cracks and porous layers of rock near volcanic areas. It also includes some water from magma that rises from deep within the Earth. On land, water in systems like fumaroles and geysers is mainly rainwater and groundwater that seep into the ground from the surface. This water can also contain some water from metamorphic rocks, magma, or salt-rich fluids released by magma. The amounts of these types of water vary depending on the location.
At these deep ocean depths, the surrounding water is about 2°C (36°F). However, water from hydrothermal vents can be much hotter, ranging from 60°C (140°F) to as high as 464°C (867°F). Because of the high pressure at these depths, water can exist as a liquid or as a supercritical fluid at these temperatures. The critical point of pure water is 375°C (707°F) at a pressure of 218 atmospheres.
Adding salt to water increases the temperature and pressure needed to reach the critical point. For example, seawater with 3.2% salt has a critical point at 407°C (765°F) and 298.5 bars, which is about 2,960 meters (9,710 feet) below sea level. If a hydrothermal fluid with this salt level vents at temperatures above 407°C (765°F) and 298.5 bars, it becomes a supercritical fluid. The salt content of vent fluids can vary widely due to processes in the Earth’s crust. Lower salt fluids have critical points at lower temperatures and pressures than seawater but higher than pure water. For example, a fluid with 2.24% salt has a critical point at 400°C (752°F) and 280.5 bars. This means that water from the hottest parts of some hydrothermal vents can be a supercritical fluid, which has properties between a gas and a liquid.
Examples of supercritical venting have been found at several locations. At Sister Peak (Comfortless Cove Hydrothermal Field, 4°48′S 12°22′W, depth 2,996 m or 9,829 ft), low-salt vapor-type fluids are released. While sustained supercritical venting was not observed, a brief release of 464°C (867°F) exceeded supercritical conditions. A nearby site, Turtle Pits, vents low-salt fluid at 407°C (765°F), which is above the critical point for that salt level. In the Cayman Trough, the Beebe vent site (the deepest known hydrothermal site at ~5,000 m or 16,000 ft below sea level) has shown sustained supercritical venting at 401°C (754°F) with 2.3% salt.
Although supercritical conditions have been observed at multiple sites, the importance of supercritical venting for processes like hydrothermal circulation, mineral deposits, chemical exchanges, or life is not yet fully understood.
The early stages of hydrothermal vent chimneys begin with the formation of the mineral anhydrite. Over time, sulfides of copper, iron, and zinc deposit in the gaps of the chimneys, making them less porous. Vent chimneys can grow as fast as 30 cm (1 ft) per day. A 2007 study of deep-sea vents near Fiji found these vents to be a major source of dissolved iron, which is important for the iron cycle in the ocean.
Black smokers and white smokers
Some hydrothermal vents create tall, tube-shaped structures. These structures form from minerals that are dissolved in the hot water coming from the vent. When this extremely hot water mixes with the very cold ocean water, the minerals form solid particles that build up the height of the structures. Some of these structures can grow as tall as 60 meters (200 feet). An example is a vent called "Godzilla," located on the deep seafloor near Oregon in the Pacific Ocean. It reached a height of 40 meters (130 feet) before it collapsed in 1996.
A black smoker is a type of hydrothermal vent found on the ocean floor, usually in the bathyal zone (most often between 2,500 and 3,000 meters (8,200 to 9,800 feet) deep), but also in shallower or deeper areas. These vents look like black, chimney-like towers and release a dark cloud of material. Black smokers typically release particles rich in sulfur-containing minerals, such as sulfides. They form in groups covering hundreds of meters when superheated water from beneath Earth's crust rises through the ocean floor (temperatures can exceed 400°C (752°F)). This water carries dissolved minerals from the crust, especially sulfides. When it meets cold ocean water, many of these minerals form solid deposits, creating black, chimney-like structures around each vent. The chimneys grow thicker because heat helps minerals solidify. Over time, the metal sulfides deposited can form large ore deposits. Some black smokers near the Azores on the Mid-Atlantic Ridge are especially rich in metals. For example, water from the Rainbow Vent Field contains up to 24,000 μM of dissolved iron.
Black smokers were first discovered in 1979 on the East Pacific Rise by scientists from the Scripps Institution of Oceanography during the RISE Project. They were studied using the deep-sea vehicle ALVIN from the Woods Hole Oceanographic Institution. Today, black smokers are known to exist in the Atlantic and Pacific Oceans, usually at depths of about 2,100 meters (6,900 feet). The northernmost black smokers are a group of five called Loki's Castle, found in 2008 by scientists from the University of Bergen at 73°N on the Mid-Atlantic Ridge between Greenland and Norway. These vents are of interest because they are in a more stable part of Earth's crust, where tectonic activity is less common, making hydrothermal vent fields rare. The deepest known black smokers are in the Cayman Trough, located 5,000 meters (3.1 miles) below the ocean surface.
White smoker vents release lighter-colored minerals, such as those containing barium, calcium, and silicon. These vents usually have cooler plumes because they are farther from their heat source.
Black and white smokers can exist together in the same hydrothermal field, but they generally form near and far from the main heat source, respectively. However, white smokers are more common in later stages of hydrothermal fields when the heat source becomes farther away due to magma solidifying. At this stage, the water in these vents is mostly seawater, and the minerals deposited are rich in calcium, forming deposits of sulfate minerals like barite and anhydrite, as well as carbonates.
Hydrothermal plumes
Hydrothermal plumes are types of fluid that form when hot fluids from hydrothermal vents are released into the water above the seafloor. These fluids often have different physical properties, such as temperature and density, and chemical properties, such as pH and the presence of certain ions, compared to seawater. These differences create chemical and physical changes that allow reactions like oxidation-reduction and precipitation to occur.
Hydrothermal vent fluids are much hotter than seawater, often reaching temperatures between 40 and over 400 degrees Celsius, while seawater near the seafloor is about 4 degrees Celsius. Because hot fluids are less dense, they rise through the water due to buoyancy, forming a hydrothermal plume. This rising stage is called the "buoyant plume" phase. As the plume rises, movement between the plume and seawater creates turbulence that mixes the two fluids, gradually diluting the plume. Eventually, the plume becomes neutrally buoyant, meaning it no longer rises but instead spreads sideways across the ocean, sometimes for thousands of kilometers. This stage is called the "nonbuoyant plume" phase.
Chemical reactions happen as the plume evolves. Seawater is typically oxidizing, while hydrothermal fluids are usually reducing. When these fluids mix, chemicals like hydrogen gas, hydrogen sulfide, methane, and metals such as iron and manganese react. In fluids with high hydrogen sulfide, metals like iron and manganese form dark-colored sulfide minerals, such as those seen in "black smokers." Over time, these metals may oxidize, forming insoluble minerals like iron and manganese (oxy)hydroxides. The area near the vent where metals are actively oxidizing is called the "near field," while the area where oxidation is complete is called the "far field."
Scientists use chemical tracers in hydrothermal plumes to locate deep-sea vents. Effective tracers should not react much, so changes in their concentration are due only to dilution. Noble gas helium is a useful tracer because hydrothermal vents release higher amounts of helium-3, a rare isotope from Earth's interior, compared to seawater. This creates unusual helium isotope patterns in seawater that indicate hydrothermal activity. Another tracer is radon, a radioactive gas. Radon-222, which has a half-life of about 3.82 days, can help determine the age of hydrothermal plumes when combined with helium data. Other substances like hydrogen gas, hydrogen sulfide, methane, and metals such as iron and manganese are also found in hydrothermal fluids but are less useful as tracers because they react easily.
Hydrothermal plumes play a key role in how hydrothermal systems affect ocean chemistry. These vents release many trace metals, including iron, manganese, chromium, copper, zinc, cobalt, nickel, molybdenum, cadmium, vanadium, and tungsten, many of which are important for life. Once released, these metals are influenced by physical and chemical processes. Based on scientific principles, iron and manganese should form insoluble precipitates in seawater, but organic compounds and the formation of colloids or nanoparticles can keep them dissolved far from the vent.
Iron and manganese often have the highest concentrations in acidic hydrothermal fluids and are biologically important, especially iron, which is a limiting nutrient in the ocean. Their transport through organic complexation may help move these metals across the ocean. Hydrothermal vents also release other biologically important metals like molybdenum, which may have been significant in the early development of Earth's oceans and the origin of life. However, iron and manganese precipitates can also remove trace metals from seawater. The surfaces of iron (oxy)hydroxide minerals can absorb elements like phosphorus, vanadium, arsenic, and rare earth metals from seawater. While hydrothermal plumes may add metals like iron and manganese to the ocean, they can also remove other metals and nutrients like phosphorus, acting as a net sink for these elements.
Biology of hydrothermal vents
Life on Earth has usually been thought to depend on energy from the sun. However, deep-sea organisms near hydrothermal vents do not have access to sunlight. Therefore, biological communities around these vents rely on nutrients from chemical deposits and fluids found in the vent areas. Scientists once believed that vent organisms depended on marine snow, which is organic material that falls from the ocean surface. This would mean they relied on plants and the sun for energy. While some vent organisms do consume marine snow, this alone would not support large numbers of life. In fact, hydrothermal vent zones have far more organisms than the surrounding sea floor, with densities up to 100,000 times greater.
Hydrothermal vents are a type of ecosystem where life is fueled by chemical compounds instead of sunlight. This process is called chemosynthesis. Vent communities can support large amounts of life because many organisms depend on chemosynthetic bacteria for food. The water from hydrothermal vents contains dissolved minerals that support large populations of these bacteria. These bacteria use sulfur compounds, such as hydrogen sulfide, to create organic material through chemosynthesis. Hydrogen sulfide is toxic to most organisms, but the bacteria convert it into energy.
Hydrothermal vents also play a role in ocean life beyond their immediate surroundings. They provide iron to phytoplankton, which are tiny plants that form the base of ocean food chains.
The oldest known example of a biological community linked to a hydrothermal vent is the Figueroa Sulfide from the Early Jurassic period in California. This ecosystem depends on the vent itself for energy, unlike most life on Earth, which relies on sunlight. However, some vent organisms use oxygen produced by photosynthetic organisms, while others do not need oxygen.
Chemosynthetic bacteria grow into thick mats that attract small animals like amphipods and copepods, which feed directly on the bacteria. Larger animals, such as snails, shrimp, crabs, tube worms, fish, and octopuses, form a food chain above these primary consumers. Common groups of organisms near hydrothermal vents include annelids, gastropods, and crustaceans. Large bivalves, vestimentiferan worms, and "eyeless" shrimp make up most of the nonmicrobial life.
Siboglinid tube worms, which can grow over 2 meters tall, are often found near hydrothermal vents. These worms lack mouths and digestive systems and absorb nutrients from bacteria inside their bodies. Each ounce of tube worm tissue contains about 285 billion bacteria. The worms’ red plumes contain hemoglobin, which carries hydrogen sulfide to the bacteria. In return, the bacteria provide the worms with carbon compounds. Two species found at vents are Tevnia jerichonana and Riftia pachyptila. One community, called "Eel City," is dominated by the eel Dysommina rugosa and is located near Nafanua volcanic cone in American Samoa.
Over 100 gastropod species have been identified near hydrothermal vents, and more than 300 new species have been discovered. Many of these species are "sister species" found in different vent areas. Scientists believe that before the North American Plate moved over the mid-ocean ridge, all vent life in the eastern Pacific was part of one region. This movement created barriers that led to the evolution of different species in separate areas. Similar adaptations seen across different vents support the theory of natural selection and evolution.
Although life is sparse at great ocean depths, black smokers are the centers of ecosystems. Sunlight does not reach these depths, so organisms like archaea and extremophiles use heat, methane, and sulfur compounds from black smokers to create energy through chemosynthesis. More complex life, such as clams and tube worms, feed on these organisms. These base organisms also deposit minerals into black smokers, completing the life cycle.
A species of phototrophic bacterium, part of the Chlorobiaceae family, was found near a black smoker off Mexico’s coast at a depth of 2,500 meters. This bacterium uses the faint light from the smoker for photosynthesis instead of sunlight. This is the first known organism to use non-solar light for photosynthesis.
New species are frequently discovered near black smokers. For example, the Pompeii worm Alvinella pompejana, which can survive temperatures up to 80°C, was found in the 1980s. The scaly-foot gastropod (Chrysomallon squamiferum), discovered in 2001 near the Kairei hydrothermal vent in the Indian Ocean, uses iron sulfides for its hardened body parts instead of calcium carbonate. The extreme pressure at this depth may help stabilize these materials for biological use. This armor likely protects the worm from predators.
In 2017, scientists found evidence of what may be Earth’s oldest life forms. Fossilized microorganisms in hydrothermal vent deposits in Quebec, Canada, date back to about 4.28 billion years ago, shortly after Earth formed.
Hydrothermal vent ecosystems have large amounts of life, but this depends on symbiotic relationships. Unlike shallow-water and land hydrothermal systems, deep-sea vent ecosystems rely on partnerships between animals and chemosynthetic bacteria. Since sunlight does not reach these depths, organisms cannot use photosynthesis. Instead, bacteria convert chemicals like sulfide into energy. These bacteria transform inorganic molecules into organic ones that the host animals use for food. Sulfide is highly toxic, but scientists were surprised to find thriving life at vents in 1977. This life depends on bacteria living inside the gills of vent animals. Scientists now study how these bacteria help animals survive the toxic conditions.
Discovery and exploration
In 1949, a study of the deep Red Sea found unusually hot brines in the central area. Later research in the 1960s confirmed the presence of hot, 60 °C (140 °F) brines and related mineral-rich muds. These hot liquids came from an active crack under the ocean floor. The high salt content of the water made it difficult for living things to survive. Scientists are now studying these brines and muds to see if they contain valuable metals that could be mined.
In June 1976, scientists from the Scripps Institution of Oceanography found the first evidence of underwater hydrothermal vents along the Galápagos Rift, a part of the East Pacific Rise, during the Pleiades II expedition. They used a special underwater imaging system called Deep-Tow. In 1977, the first scientific reports about hydrothermal vents were published by Scripps researchers. Scientist Peter Lonsdale shared photos taken by deep-towed cameras, and graduate student Kathleen Crane shared maps and temperature data. Scientists placed devices at the site, called "Clam-bake," to help return for further study the next year using the submersible DSV Alvin.
In 1977, scientists from the National Science Foundation directly observed ecosystems near hydrothermal vents at the Galápagos Rift. Jack Corliss of Oregon State University led the study. Corliss and Tjeerd van Andel from Stanford University used the DSV Alvin to examine the vents and their ecosystems on February 17, 1977. Other scientists on the expedition included researchers from the Woods Hole Oceanographic Institution, Oregon State University, the Massachusetts Institute of Technology, the U.S. Geological Survey, and the Scripps Institution of Oceanography. They published their findings in the journal Science. In 1979, biologists led by J. Frederick Grassle of the Woods Hole Oceanographic Institution returned to the same area to study the life forms found earlier.
High-temperature hydrothermal vents, called "black smokers," were discovered in spring 1979 by scientists from the Scripps Institution of Oceanography using the submersible Alvin. The RISE expedition explored the East Pacific Rise near 21° N to test mapping techniques and find new hydrothermal fields beyond the Galápagos Rift. The expedition was led by Fred Spiess and Ken Macdonald, with participants from the U.S., Mexico, and France. The dive area was chosen based on findings from the French CYAMEX expedition in 1978, which discovered sulfide mineral mounds on the seafloor. Before diving, Robert Ballard used a deep-towed instrument to find temperature differences in the water. The first dive targeted one of these areas. On April 15, 1979, during a dive to 2,600 meters, Roger Larson and Bruce Luyendyk found a vent field with life similar to the Galápagos vents. Later, on April 21, William Normark and Thierry Juteau discovered high-temperature vents that released black mineral particles from chimneys, known as black smokers. Scientists measured temperatures at these vents, which reached 380±30 °C, the highest recorded at that time. Analysis showed that iron sulfide minerals form the "smoke" and chimney walls.
In 2005, Neptune Resources NL, a mining company, received permission to explore 35,000 km of the Kermadec Arc in New Zealand’s Exclusive Economic Zone for seafloor sulfide deposits, which could be a new source of lead, zinc, and copper. In 2007, scientists discovered a hydrothermal vent field off Costa Rica’s coast, named the Medusa vent field after the mythological figure. The Ashadze hydrothermal field, located at 13°N on the Mid-Atlantic Ridge at -4200 m depth, was the deepest known high-temperature vent field until 2010. That year, scientists from NASA and Woods Hole Oceanographic Institution found a hydrothermal plume at the Beebe site, located on the Mid-Cayman Rise in the Cayman Trough at -5000 m depth. In 2013, the deepest known hydrothermal vents were found in the Caribbean Sea at nearly 5,000 meters (16,000 feet) depth.
Oceanographers are currently studying volcanoes and hydrothermal vents on the Juan de Fuca mid-ocean ridge, where tectonic plates are moving apart. Hydrothermal vents and other geothermal features are also being studied in Bahía de Concepción, Baja California Sur, Mexico.
Distribution
Hydrothermal vents are found along the edges where Earth's plates meet, but they can also be found in areas inside the plates, such as at hotspot volcanoes. As of 2009, about 500 active underwater hydrothermal vent fields were known. Around half of these had been seen directly on the ocean floor, while the other half were suspected based on signs in the water and deposits on the seafloor.
Rogers et al. (2012) identified at least 11 different regions where hydrothermal vent systems are found:
- Mid-Atlantic Ridge province,
- East Scotia Ridge province,
- northern East Pacific Rise province,
- central East Pacific Rise province,
- southern East Pacific Rise province,
- south of the Easter Microplate,
- Indian Ocean province,
- four provinces in the western Pacific and many others.
Exploitation
Hydrothermal vents sometimes create valuable mineral resources through the depositing of seafloor massive sulfide deposits. The Mount Isa mineral deposit in Queensland, Australia, is a good example. Many hydrothermal vents contain valuable metals like cobalt, gold, copper, and rare earth metals used in electronic devices. Hydrothermal venting on ancient seafloors is believed to have formed Algoma-type banded iron formations, which are important sources of iron ore.
In recent years, mineral exploration companies have focused on extracting resources from hydrothermal fields on the seafloor, especially during the mid-2000s when prices for base metals increased. This could potentially lower mining costs.
In countries like Japan, where most minerals are imported, there is a strong interest in extracting seafloor minerals. Japan’s first large-scale mining of hydrothermal vent minerals took place in August–September 2017, led by Japan Oil, Gas and Metals National Corporation (JOGMEC). The operation used the Research Vessel Hakurei and focused on the Izena hole/cauldron vent field in the Okinawa Trough, a region with 15 confirmed vent fields.
Two companies are currently preparing to mine seafloor massive sulfides (SMS). Nautilus Minerals is working on extracting minerals from its Solwarra deposit in the Bismarck Archipelago, while Neptune Minerals is in an earlier stage with its Rumble II West deposit near the Kermadec Islands. Both companies plan to use modified technology. In 2006, Nautilus Minerals successfully brought over 10 metric tons of SMS to the surface using drum cutters on a remotely operated underwater vehicle (ROV). In 2007, Neptune Minerals used a modified suction pump on an ROV to collect SMS samples.
Seafloor mining may harm the environment. Dust from mining machines could affect filter-feeding organisms. Mining might also cause vents to collapse or release methane gas, or trigger underwater landslides.
Mining tools can also create noise and light pollution. Deep-sea organisms live in very quiet, dark environments and have sensitive hearing. Sudden noise from mining could harm their hearing or disrupt communication between them. Many deep-sea organisms use low-frequency sounds to communicate, so increased noise might interfere with their behavior. Mining tools also produce artificial light, which could damage the eyes of deep-sea creatures. Surface support vessels use lights at night, which might disorient seabirds and cause them to crash into objects.
Three mining waste processes—side cast sediment release, dewatering, and sediment disturbance—could create sediment plumes. Side cast sediment release involves moving material aside during mining, which might form clouds of dust on the seafloor. Dewatering involves releasing water mixed with heavy metals like copper and cobalt, which could change ocean chemistry or harm marine life. Sediment disturbance occurs when mining tools move across the seafloor, potentially smothering organisms or disrupting their feeding and gas exchange. Increased sediment can also raise the rate of sediment buildup on the seafloor, harming ecosystems.
Conservation
The protection of hydrothermal vents has been a topic of intense debate among ocean scientists for the past 20 years. Some argue that scientists themselves may cause harm to these not very common habitats. Efforts have been made to create rules for how scientists should behave when studying vent sites. While there is a general agreement on proper practices, there is no official international rule that is legally required.
Protecting hydrothermal vent ecosystems after mining an active system would depend on the return of chemosynthetic bacteria, as this fluid is the main source of energy for the vent ecosystem. It is very hard to understand how mining affects the hydrothermal vent fluid because no large-scale studies have been conducted. However, scientists have studied how these ecosystems recover after volcanic events. These studies show that it took 3–5 years for bacteria to return to an area and about 10 years for larger animals to come back. Researchers also found that the types of species changed after destruction, with some new species appearing. More research is needed to understand how long-term mining affects the recovery of species in these ecosystems.
Geochronological dating
Scientists use specific methods to determine the ages of hydrothermal vents. These methods involve dating sulfide minerals, such as pyrite, and sulfate minerals, such as baryte. Two common dating techniques are radiometric dating and electron spin resonance dating. Each method has its own limitations, requirements, and challenges. Some challenges include the need for very pure mineral samples, the specific age ranges each method can measure, and the risk of heating minerals above certain temperatures, which can erase the ages of older minerals. Additionally, when minerals form in multiple stages, it can create a mix of different ages. In such cases, electron spin resonance dating usually provides the average age of the entire mineral sample, while radiometric dating often reflects the ages of the youngest mineral layers because parent nuclei decay over time. These factors explain why different dating methods may produce different ages for the same sample and why samples from the same hydrothermal chimney can have varying ages.
History and formation of hydrothermal vents
Although some scientists, such as Rogers et al. (2012), have found locations of hydrothermal vents, the exact places where these vents form in deep ocean areas are not fully understood. Most of the ocean floor remains unexplored, with less than 1% of it well studied. Scientists currently know that most hydrothermal vents are found along mid-ocean ridges. Understanding where these systems are located is important because many theories about their formation involve seismic activity, especially near volcanic areas.
During the Paleocene and Eocene periods, seismic activity caused by continental rifting released gases, liquids, and sediments from Earth’s interior. This event created large craters above sills, which are layers of igneous rock formed when magma moves between existing rock layers. These craters on the seafloor contain groups of hydrothermal vents. Features of these vents include sedimentary layers that slope inward, as well as sandstone dykes, pipes, and breccias. These features are classified as subvolcanic intrusions, which contribute to hydrothermal activity. A study used 2D seismic reflection data to describe the structures of these systems, which are located in craters with a funnel-like shape. These structures are often called chimneys and form on top of the vents. The interaction between oceanic crust and seawater creates these systems, changing the local chemistry and forming deposits rich in different metals. These metal deposits and altered chemistry create conditions that support life, such as thermophiles and other organisms.