Hydrothermal vents are openings 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 where tectonic plates separate, ocean basins, and hotspots. When hot water from these vents spreads into the ocean, it creates clouds of particles called hydrothermal plumes. Over time, these vents can form rocks and mineral deposits.
Hydrothermal vents form because the Earth has a lot of heat and water inside its crust. Under the ocean, these vents can create structures called black smokers or white smokers, which release many different elements into the ocean. This helps shape the chemical balance of the ocean. Compared to most deep ocean areas, places near hydrothermal vents have more life. These areas support complex ecosystems powered by chemicals in the vent water. Bacteria and archaea that use chemicals from the vents form the base of the food chain, supporting animals like giant tube worms, clams, limpets, and shrimp. Scientists believe similar vents may exist on Jupiter’s moon Europa and Saturn’s moon Enceladus. Some think ancient vents might have existed on Mars.
Scientists have suggested that hydrothermal vents may have played a key role in the beginning of life on Earth. The conditions in these vents can help form molecules needed for life. Some studies show that certain types of vents, like alkaline vents or those with supercritical CO₂, might be better at creating these molecules. However, how life began is still a topic of debate among scientists, and many different ideas exist.
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 areas where two tectonic plates are moving apart, and new ocean floor is created.
The water that comes out of hydrothermal vents on the seafloor is mostly seawater that flows into the vent system near volcanic areas through cracks and porous rock or sediment. Some of the water also comes from magma that rises from deep within the Earth. On land, water in fumarole and geyser systems is mostly meteoric water and groundwater that seep into the system from the surface. This water can also include some metamorphic water, magmatic water, and sedimentary brine released by magma. The amounts of each type of water vary depending on the location.
The water around these vents is usually about 2°C (36°F), but water from the vents can be much hotter, ranging from 60°C (140°F) up to 464°C (867°F). At such depths, the pressure is so high that water can exist as a liquid or as a supercritical fluid. The critical point for pure water is 375°C (707°F) at 218 atmospheres. However, when salt is added to the water, the critical point increases. For example, seawater with 3.2% salt has a critical point of 407°C (765°F) and 298.5 bars, which is about 2,960 meters (9,710 feet) below sea level. If hydrothermal fluid with this salt level vents above 407°C (765°F) and 298.5 bars, it is supercritical. The salt content in vent fluids can vary due to changes in the crust. For example, fluid with 2.24% salt has a critical point at 400°C (752°F) and 280.5 bars. Therefore, water from the hottest parts of some vents can become a supercritical fluid, which has properties between those of a gas and a liquid.
Examples of supercritical venting have been found at several locations. At Sister Peak, a vent in the Comfortless Cove Hydrothermal Field, low-salt vapor-type fluids are released. While sustained supercritical venting was not observed, a brief release of 464°C (867°F) fluid 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, a vent site called Beebe, located about 5,000 meters (16,000 feet) below sea level, has shown continuous supercritical venting at 401°C (754°F) with 2.3% salt.
Although supercritical conditions have been observed at multiple sites, it is still unclear what role, if any, supercritical venting plays in processes like hydrothermal circulation, mineral deposits, chemical exchanges, or life.
Hydrothermal vent chimneys begin to form when the mineral anhydrite is deposited. Over time, sulfides of copper, iron, and zinc form in the gaps of the chimneys, making them less porous. Some vents have grown as much as 30 cm (1 foot) per day. In 2007, an exploration near Fiji found that these vents are a major source of dissolved iron, which is important in the iron cycle.
Black smokers and white smokers
Some hydrothermal vents create tall, chimney-like structures. These chimneys form when minerals dissolved in hot vent fluid mix with cold seawater. When extremely hot water from the vent meets the cold ocean water, the minerals form solid particles that build up the chimneys. Some chimneys can grow as tall as 60 meters (200 feet). One example is "Godzilla," a chimney on the Pacific Ocean floor near Oregon that reached 40 meters (130 feet) before collapsing in 1996.
A black smoker is a type of hydrothermal vent found on the ocean floor, usually in the bathyal zone (most commonly between 2,500 and 3,000 meters (8,200 to 9,800 feet) deep), but also in shallower and deeper areas. Black smokers look like dark, chimney-like structures that release a cloud of black material. These vents release particles rich in sulfur-containing minerals, called sulfides. Black smokers form in groups 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, especially sulfides, from the crust. When the hot water meets cold ocean water, the minerals form solid particles, creating black, chimney-like structures around each vent. Heat from the water helps minerals harden, making the chimneys grow thicker over time. Over time, the metal sulfides deposited by black smokers can form large deposits of valuable minerals. For example, hydrothermal fluids near the Rainbow Vent Field in the Azores contain 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 a deep diving vehicle called ALVIN from the Woods Hole Oceanographic Institution. Today, black smokers are found 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, discovered in 2008 near the Mid-Atlantic Ridge between Greenland and Norway at 73°N. 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 vents 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 often 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 usually represent vents that are close and far from the main heat source, respectively. White smokers are more common in later stages of hydrothermal fields as the heat source moves farther away (due to cooling magma) and the fluids become more similar to seawater. The minerals from white smokers are rich in calcium and form deposits of sulfate minerals (like barite and anhydrite) and carbonates.
Hydrothermal plumes
Hydrothermal plumes are mixtures of fluids that form when hot water from hydrothermal vents mixes with the surrounding ocean water. These plumes often have different physical and chemical properties compared to normal seawater, such as higher temperatures, different densities, and varying levels of chemicals like pH and ions. These differences create conditions that support chemical reactions, such as those involving the transfer of electrons (oxidation-reduction reactions) and the formation of solid substances (precipitation reactions).
Hydrothermal vent fluids are much hotter than ocean water, often reaching temperatures between 40 and over 400 degrees Celsius, compared to the cold ocean water at about 4 degrees Celsius. Because hot water is less dense than cold water, it rises upward due to buoyancy, forming a plume. This rising stage is called the "buoyant plume" phase. As the plume rises, forces from the movement of the plume and surrounding water create turbulence, mixing the hot fluid with seawater. This mixing gradually dilutes the plume until it becomes the same density as the surrounding water. At this point, the plume stops rising and begins to spread sideways across the ocean, a stage known as the "nonbuoyant plume" phase.
Chemical reactions happen as the plume evolves. Seawater is usually oxidizing, meaning it tends to cause other substances to lose electrons, while hydrothermal vent fluids are often reducing, meaning they tend to cause substances to gain electrons. When these fluids mix with seawater, reduced chemicals like hydrogen gas, hydrogen sulfide, methane, iron, and manganese react. In fluids with high levels of hydrogen sulfide, dissolved metals like iron and manganese form dark-colored minerals, such as those seen in "black smokers." Over time, these metals may also oxidize, forming insoluble minerals like iron and manganese (oxy)hydroxides. Scientists use terms like "near field" and "far field" to describe different stages of plume evolution: the "near field" refers to areas where metals are still undergoing oxidation, while the "far field" refers to areas where oxidation is complete.
Scientists use chemical tracers to locate hydrothermal vents. These tracers should not react with other substances, so changes in their concentration are due only to dilution. Noble gases like helium are useful tracers because hydrothermal vents release higher amounts of helium-3, a rare type of helium found only inside Earth. This creates unusual helium levels in seawater that indicate hydrothermal activity. Another noble gas, radon, can also help track plumes. Radon is radioactive, and its decay can help determine the age of a plume when combined with helium data. Radon-222 is especially useful because it has the longest half-life among radon isotopes, about 3.82 days. Other substances like hydrogen gas, hydrogen sulfide, methane, and metals such as iron and manganese may also indicate hydrothermal activity, but they are reactive and less reliable as tracers.
Hydrothermal plumes play a key role in ocean chemistry. Hydrothermal 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 these metals enter the ocean, physical and chemical processes determine their fate. Based on energy interactions, iron and manganese should form solid precipitates in seawater, but their suspension in water can be influenced by organic compounds and tiny particles.
Iron and manganese often have the highest concentrations in acidic vent fluids and are biologically important, especially iron, which is a limited nutrient in the ocean. Their far-field transport through organic compounds may help distribute these metals across the ocean. Hydrothermal vents also release other metals like molybdenum, which may have been important in Earth's early ocean chemistry and the origin of life. However, iron and manganese can also remove other metals and nutrients like phosphorus, vanadium, arsenic, and rare earth elements from seawater by adsorbing them onto their surfaces. While hydrothermal plumes may add iron and manganese to the ocean, they can also remove other elements, acting as both a source and a sink for different substances.
Biology of hydrothermal vents
Life has usually been thought to depend on energy from the sun, but deep-sea organisms near hydrothermal vents do not have access to sunlight. Therefore, biological communities around these vents must rely on nutrients found in chemical deposits and fluids from the vents. Earlier, scientists who study the ocean floor believed vent organisms depended on marine snow, like other deep-sea life. This would mean they relied on plants and, indirectly, the sun. Some vent organisms do eat this "rain," but such a system would support only a small number of life forms. However, hydrothermal vent areas have far more organisms than the surrounding seafloor, with densities up to 10,000 to 100,000 times greater.
Hydrothermal vents are a type of ecosystem where life uses chemical reactions instead of sunlight for energy. These ecosystems depend on bacteria that use chemicals like hydrogen sulfide to create food. The water from vents contains many dissolved minerals, supporting large populations of these bacteria. These bacteria convert sulfur compounds into organic material through a process called chemosynthesis.
Hydrothermal vents also provide iron to the ocean, which helps phytoplankton grow. The oldest known vent-related ecosystem is the Figueroa Sulfide from the Early Jurassic period in California. This ecosystem depends on the vent for energy, unlike most surface life that uses sunlight. Some vent organisms rely on oxygen from photosynthetic life, while others do not need oxygen.
Chemosynthetic bacteria form thick mats that attract smaller animals like amphipods and copepods, which eat the bacteria directly. Larger animals, such as snails, shrimp, crabs, tube worms, fish, and octopuses, form a food chain based on predator-prey relationships. Common groups of organisms near vents include annelids, gastropods, and crustaceans, with large bivalves, tube worms, and shrimp making up much of the nonmicrobial life.
Siboglinid tube worms, which can grow over 2 meters tall, are often found near vents. They lack mouths and digestive systems, absorbing nutrients from bacteria inside their bodies. Each ounce of tube worm tissue contains about 285 billion bacteria. These worms have red plumes that contain hemoglobin, which carries hydrogen sulfide to the bacteria. In return, the bacteria provide the worms with carbon compounds. Two species found near vents are Tevnia jerichonana and Riftia pachyptila. A community called "Eel City" is mainly made up of the eel Dysommina rugosa and is located near the Nafanua volcanic cone in American Samoa.
Over 100 gastropod species have been identified near vents, and more than 300 new species have been discovered. Some species are closely related to those in distant vent areas. Scientists believe that before the North American Plate moved over a mid-ocean ridge, all vent life was part of one region. Separation of these areas led to species evolving differently. Similar traits found in vent life support the theory of evolution.
Although life is sparse at such depths, black smokers are the center of ecosystems. Sunlight does not reach these areas, so organisms like archaea and extremophiles use heat, methane, and sulfur compounds from vents to create energy through chemosynthesis. Larger animals like clams and tube worms eat these microbes. These microbes also deposit minerals into the vent, completing the life cycle.
A type of bacteria called Chlorobiaceae was found near a black smoker in the ocean off Mexico at a depth of 2,500 meters. These bacteria use the faint light from the vent for photosynthesis instead of sunlight. This is the first known organism to use non-sunlight for photosynthesis.
New species are often found near vents. The Pompeii worm (Alvinella pompejana), which can survive up to 80°C, was discovered in the 1980s. The scaly-foot gastropod (Chrysomallon squamiferum), found in 2001, uses iron sulfides for its body armor instead of calcium carbonate. The extreme pressure at these depths helps stabilize the iron sulfide. This armor may protect the worm from predators.
In 2017, scientists found evidence of possible ancient life in hydrothermal vent deposits in Quebec, Canada. These fossils may be as old as 4.28 billion years, shortly after Earth formed.
Hydrothermal vent ecosystems have large amounts of life, but this depends on relationships between organisms. Unlike shallow or land-based vents, deep-sea vents rely on symbiosis between animals and bacteria. Since sunlight does not reach these depths, organisms use chemical energy instead of sunlight. Bacteria convert chemicals like hydrogen sulfide into food for the host animals. However, hydrogen sulfide is toxic to most life, which surprised scientists when they discovered thriving vent ecosystems in 1977. Researchers now study how these bacteria help animals survive the toxicity.
Discovery and exploration
In 1949, a deep water survey found unusually hot saltwater in the center of the Red Sea. Later studies in the 1960s confirmed the presence of hot, 60 °C (140 °F) saltwater and related metal-rich muds. The hot water was coming from an active crack in the ocean floor. The highly salty water was not suitable for life. These brines and muds are now being studied as possible sources of valuable metals.
In June 1976, scientists from the Scripps Institution of Oceanography collected the first evidence of underwater hydrothermal vents along the Galápagos Rift, a branch of the East Pacific Rise, during the Pleiades II expedition using the Deep-Tow seafloor imaging system. In 1977, the first scientific papers about hydrothermal vents were published by Scripps scientists. Researcher 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 return the next year for direct observations using the DSV Alvin.
Chemosynthetic ecosystems near the Galápagos Rift hydrothermal vents were first observed in 1977 by marine geologists funded by the National Science Foundation. The lead scientist for the submersible study was Jack Corliss of Oregon State University. Corliss and Tjeerd van Andel from Stanford University studied the vents and their ecosystems while diving in the DSV Alvin, a research submersible operated by the Woods Hole Oceanographic Institution (WHOI). Other scientists on the expedition included Richard (Dick) Von Herzen and Robert Ballard of WHOI, Jack Dymond and Louis Gordon of Oregon State University, John Edmond and Tanya Atwater of the Massachusetts Institute of Technology, Dave Williams of the U.S. Geological Survey, and Kathleen Crane of Scripps Institution of Oceanography. This team published their findings about the vents, life forms, and vent fluid composition in the journal Science. In 1979, biologists led by J. Frederick Grassle of WHOI returned to the same location to study the biological communities discovered two years earlier.
High-temperature hydrothermal vents, called "black smokers," were discovered in spring 1979 by Scripps scientists using the submersible Alvin. The RISE expedition explored the East Pacific Rise near 21° N to test geophysical mapping of the seafloor and find another hydrothermal field 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 sulfide mineral mounds discovered by the French CYAMEX expedition in 1978. Before diving, Robert Ballard identified temperature differences in the water using a deep-towed instrument. 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 hydrothermal vent field with a biological community similar to the Galápagos vents. On April 21, William Normark and Thierry Juteau discovered high-temperature vents emitting black mineral particles from chimneys, called "black smokers." Macdonald and Jim Aiken attached a temperature probe to Alvin to measure the water temperature at the vents, recording the highest temperatures ever measured at deep-sea hydrothermal vents (380±30 °C). Analysis of black smoker material and chimneys showed that iron sulfide is a common mineral in the "smoke" and chimney walls.
In 2005, Neptune Resources NL, a mineral exploration company, received 35,000 km of exploration rights over the Kermadec Arc in New Zealand’s Exclusive Economic Zone to search for seafloor massive sulfide deposits, which may contain lead-zinc-copper sulfides formed by modern hydrothermal vent fields. In April 2007, a vent in the Pacific Ocean near Costa Rica, named the Medusa hydrothermal vent field, was discovered. The Ashadze hydrothermal field (13°N on the Mid-Atlantic Ridge, -4200 m elevation) was the deepest known high-temperature hydrothermal field until 2010, when a hydrothermal plume from the Beebe site (18°33′N 81°43′W, -5000 m elevation) was found by scientists from NASA Jet Propulsion Laboratory and Woods Hole Oceanographic Institution. This site is located on the 110 km long, ultraslow spreading Mid-Cayman Rise within the Cayman Trough. In early 2013, the deepest known hydrothermal vents were discovered in the Caribbean Sea at nearly 5,000 meters (16,000 ft) depth.
Oceanographers are studying the volcanoes and hydrothermal vents of the Juan de Fuca mid-ocean ridge, where tectonic plates are moving apart. Hydrothermal vents and other geothermal features are currently being explored 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 located within the Earth's plates, such as at hotspot volcanoes. In 2009, scientists identified about 500 active submarine hydrothermal vent fields. Approximately half of these were seen directly on the ocean floor, while the other half were suspected based on signs in the water or deposits found on the seafloor.
Rogers et al. (2012) identified at least 11 different regions where hydrothermal vent systems are found.
Exploitation
Hydrothermal vents sometimes create valuable mineral resources through the formation of large sulfide deposits on the seafloor. The Mount Isa orebody in Queensland, Australia, is an example of such deposits. Many hydrothermal vents contain metals like cobalt, gold, copper, and rare earth metals, which are important for making electronic devices. Hydrothermal activity on ancient seafloors formed Algoma-type banded iron formations, which are a source of iron ore.
In the mid-2000s, rising prices for base metals led mineral exploration companies to focus on extracting resources from hydrothermal vents on the seafloor. This process could reduce costs. In Japan, where most minerals are imported, there is strong interest in mining seafloor resources. In 2017, Japan Oil, Gas and Metals National Corporation (JOGMEC) conducted the world’s first large-scale mining of hydrothermal vent deposits using the Research Vessel Hakurei. This operation took place at the Izena hole/cauldron vent field in the Okinawa Trough, a region with 15 confirmed vent fields.
Two companies are currently working to begin mining seafloor massive sulfides (SMS). Nautilus Minerals is preparing to extract 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 attached to a remotely operated vehicle (ROV). In 2007, Neptune Minerals used a modified suction pump on an ROV to collect SMS samples.
Seafloor mining could harm the environment. Dust from mining machines might affect filter-feeding organisms, and mining could cause vents to collapse or release methane ice. It might also trigger underwater landslides. Mining tools could create noise and human-made light, which could harm deep-sea organisms. These organisms live in dark, deep areas and are sensitive to sound. Sudden noise from machines might damage their hearing or disrupt communication between them. Light from mining tools and surface ships could harm deep-sea life, as some species are adapted to darkness. Surface lights might also disorient seabirds, causing them to collide with objects or become exhausted.
Three mining waste processes—side cast sediment release, dewatering, and sediment disturbance—could create sediment plumes, which are clouds of particles in the water. Side cast sediment release involves moving material aside during mining, which might form plumes on the seafloor. Dewatering involves releasing water from ships, which might carry heavy metals like copper and cobalt into the water. Sediment disturbance occurs when mining tools move across the seafloor, causing plumes.
These processes could release heavy metals into the water, changing ocean chemistry or harming marine life. Increased sediment release might smother organisms on the seafloor, disrupt feeding, or block gas exchange. Excess sediment could also speed up sedimentation rates, further affecting the environment.
Conservation
The conservation of hydrothermal vents has been a topic of intense debate among ocean scientists for the past 20 years. Some experts say that scientists may be the main cause of harm to these rare habitats. Efforts have been made to create rules for how scientists study vent sites, but there is no official international agreement that legally requires scientists to follow these rules.
A major issue in protecting hydrothermal vent ecosystems is deep-sea mining. Four types of minerals are being considered for commercial use: manganese nodules, cobalt-rich crusts, seafloor massive sulfides, and phosphorite nodules. Seafloor massive sulfides near hydrothermal vents are a key focus of discussion. As previously explained, black smokers create large amounts of sulfides, mainly iron sulfides like pyrite. This leads to high sulfide deposits near these vents. A central challenge in conservation is deciding how to use newly mined sulfides in a way that is sustainable, while considering the effects of mining on these ecosystems.
The impacts of mining deep-sea minerals are not fully understood because hydrothermal vent ecosystems are highly active and constantly changing. Mining an active vent area would depend on the regrowth of chemosynthetic bacteria, which are essential for producing energy from vent fluids. Large-scale studies on the effects of mining have not been conducted, but research on how vent ecosystems recover after volcanic events has provided some insights. These studies show that bacteria can return to an area in 3–5 years, while larger animals take about 10 years to return. However, the types of species present after recovery often change, with new species moving into the area. This shift in biodiversity may harm critically endangered species that live in extreme deep-sea conditions, such as mollusks. More research is needed to understand the long-term effects of mining on these ecosystems.
Shallow hydrothermal vents are also being studied as models for understanding climate change in extreme environments. Scientists track changes in specialized organisms that live near vents to study effects like ocean acidification, rising temperatures, and heavy metal pollution. How deep-sea mining might affect these climate change studies is an area for future research.
Geochronological dating
Scientists use common methods to determine the ages of hydrothermal vents by dating sulfide minerals, such as pyrite, and sulfate minerals, such as baryte. These methods include radiometric dating and electron spin resonance dating. Each method has its own limitations, assumptions, and challenges. Some general challenges include the need for highly pure minerals to be tested, the specific age ranges each method can measure, the risk of heating causing older mineral ages to be erased, and the presence of multiple mineral formation events that create a mix of ages. In areas where minerals formed in different stages, electron spin resonance dating typically provides an average age of all the minerals, while radiometric dating often reflects the age of the youngest mineral layer due to the way parent atoms decay. These factors explain why different 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
Scientists who study Earth's chemistry and biology, like Rogers et al. (2012), have found some hydrothermal vent areas, but the exact locations of these vents in the deep ocean are still not fully understood. Most of the ocean floor remains unexplored, with less than 1% of it well known. Most hydrothermal vents currently identified by scientists are found along mid-ocean ridges. Understanding where these systems are located helps scientists study how they form, as many theories about their creation involve seismic activity, especially near volcanic areas.
During the Paleocene and Eocene periods, seismic activity caused by the splitting of continents led to the release of gases, liquids, and sediments from Earth's interior. This event formed large craters on top of sills, which are layers of igneous rock created when magma moves between existing rock layers. These craters on the ocean floor contain groups of hydrothermal vents. Features of these vents include sedimentary layers that slope inward and structures made of sandstone, such as 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 funnel-shaped sides. 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 chemical changes create conditions that support life, such as thermophiles and other organisms.