Why Are Scientists Exploring Submarine Volcano Systems?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers about how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species. In addition, exploration of active volcanoes provides important information on potential hazards such as risks to shipping from shallow eruptions, as well as the generation of dangerous tsunamis from large submarine eruptions and landslides.
The Submarine Ring of Fire is an arc of active volcanoes that partially encircles the Pacific Ocean Basin, including the Lau Basin, Kermadec and Mariana Islands in the western Pacific, the Aleutian Islands between the Pacific and Bering Sea, the Cascade Mountains in western North America, and numerous volcanoes on the western coasts of Central America and South America. These volcanoes result from the motion of large pieces of the Earth’s crust known as tectonic plates. Along mid-ocean ridges (also called spreading centers), tectonic plates are moving apart. Molten rock rises from Earth’s mantle into the gap between the separating plates and produces extensive lava flows. Along oceanic trenches, tectonic plates are colliding so that one plate is descending below another plate. This process is called subduction. The subducting plate is heated as it descends into Earth’s mantle, causing gases and water to be released from the heated rock. These gases and water lower the melting temperature of the hot mantle rocks, so that magma (molten rock) is produced. The magma rises and accumulates in areas called magma chambers, then erupts to form volcanoes. For additional discussion, please see “More About Plate Tectonics and the Submarine Ring of Fire,” below.
This volcanic activity releases immense quantities of heat, minerals, gases and other substances, and often produces “hydrothermal systems” or seafloor hot springs. These processes influence the entire ocean, and support unique biological communities. Many species in these communities are new to science and have a high potential for developing important new natural products for industrial and medical applications. In addition, fluids produced by volcanic activity often have high concentrations of metals that quickly precipitate in cold ocean waters, and may be directly linked to the formation of ores and concentrated deposits of gold and other precious and exotic metals.
Beginning in 2002, Ocean Exploration expeditions have undertaken systematic mapping and study of volcanoes and hydrothermal systems in previously unexplored areas of the Submarine Ring of Fire. The first of these expeditions (http://oceanexplorer.noaa.gov/explorations/02fire/) focused on Explorer Ridge, part of the seafloor spreading center about 160 km south of Vancouver Island, Canada. This expedition produced the first highly detailed maps of a major hydrothermal field known as “Magic Mountain,” and located more than 30 active vents (see http://oceanexplorer.noaa.gov/explorations/02fire/logs/magicmountain/).
The 2003 and 2004 Submarine Ring of Fire Expeditions (http://www.oceanexplorer.noaa.gov/explorations/03fire/ and http://www.oceanexplorer.noaa.gov/explorations/04fire/) explored the submarine volcanoes of the Mariana Arc, located to the north of Guam in the western Pacific. These volcanoes are the result of converging tectonic plates, in contrast to the diverging plates at Explorer Ridge. These expeditions discovered at least ten new hydrothermal sites, and were on site during an actual eruption of a volcano that spewed hot acidic fluid, molten sulfur, and chunks of volcanic ash (see http://oceanexplorer.noaa.gov/explorations/04fire/logs/april01/april01.html).
The 2005 Submarine Ring of Fire Expedition (http://www.oceanexplorer.noaa.gov/explorations/05fire/) explored the volcanoes of the Kermadec Arc, located north of New Zealand. Manned submersibles were used to explore active hydrothermal systems that had never been visited before. Some of these systems were well within the photic zone at 160-180 meters, so that chemosynthetic systems overlapped with organisms from photosynthetic systems. Iron-rich fluids venting at one volcano were accompanied by large areas (acre size) covered with actively-forming or recently-formed microbial mats.
The Submarine Ring of Fire 2006 Expedition (http://oceanexplorer.noaa.gov/explorations/06fire/welcome.html) continued exploration of submarine volcanoes along the Marina Arc. Discoveries included an actively erupting volcano, liquid carbon dioxide vents, the shallowest “black smoker” yet discovered, and more than 12 new species of chemosynthetic organisms at hydrothermal vent sites.
The New Zealand American Submarine Ring of Fire 2007 Expedition (http://oceanexplorer.noaa.gov/explorations/07fire/welcome.html) focused on the Brothers submarine volcano, a site of vigorous geothermal activity north of New Zealand along the Kermadec Arc. The expedition used an autonomous underwater vehicle named ABE (Autonomous Benthic Explorer), and produced the most comprehensive exploration of this type of submarine volcano to date. Visit http://oceanexplorer.noaa.gov/explorations/07fire/logs/aug16/aug16.html for a detailed topographic map of the Brothers caldera and links to a virtual tour of ABE’s high-resolution bathymetry.
The Northeastern Lau Basin is very unusual, because in addition to volcanic activity associated with the subduction of the Pacific Plate beneath the Indo-Australian Plate, there are also areas in which the Indo-Australian Plate seems to be pulling apart; and these areas are also rich in volcanic and hydrothermal activity. Preliminary surveys of the area between 2008 and 2011 have shown that the Northeastern Lau Basin is one of the most concentrated areas of active submarine volcanism and hot springs anywhere on Earth.
The primary objective of the Submarine Ring of Fire 2012: Northeastern Lau Basin Expedition is to explore and characterize the unique ecosystems in the NE Lau Basin through examination of their geology, chemistry, and macro- and microbiology. For additional information, please see the Mission Plan and background essays for the expedition found at (http://oceanexplorer.noaa.gov/explorations/12fire/welcome.html), and the “More About” sections below.
More About Finding Underwater Volcanoes and Hydrothermal Vents
Hydrothermal vents and underwater volcanoes cause changes to the chemistry and physical characteristics of the surrounding seawater. As this altered seawater diffuses away from vent and cold-seep sites, it produces masses of water called plumes that are distinctly different from normal seawater. Scientists search for these plumes, since they provide clues about the location of hydrothermal vents and underwater volcanoes. Redox potential is one of the key chemical characteristics used to locate plumes. When an atom or molecule loses an electron it is said to be oxidized, and when an atom or molecule gains an electron it is said to be reduced. A reducing substance is one that reduces; in other words, it donates electrons. Similarly, an oxidizing substance is a substance that oxidizes; that is, it receives electrons. A reaction in which one or more electrons are transferred between two molecules is called a redox reaction. Moving electrons can produce electric currents, so electronic instruments can be used to measure these currents. The voltage produced by redox reactions is called redox potential (or oxidation reduction potential, abbreviated as ORP). Chemosynthesis (see below) depends upon the availability of reducing substances such as hydrogen sulfide or methane that can donate electrons. Habitats in which these substances occur are called reducing habitats.
Another important change that can signal the presence of hydrothermal vents is the presence of unusually high concentrations of iron and manganese that dissolve in hydrothermal fluids while these fluids are still inside rocks below the seafloor. When these fluids enter seawater near vents, the metals precipitate and produce particles that can be detected with sensors that measure optical backscatter (abbreviated as OBS).
Instruments to measure ORP and OBS are often attached to another instrument called a CTD. CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties. Conductivity is a measure of how well a solution conducts electricity and is directly related to salinity, which is the concentration of salt and other inorganic compounds in seawater. Salinity is one of the most basic measurements used by ocean scientists. When combined with temperature data, salinity measurements can be used to determine seawater density, which is a primary driving force for major ocean currents. Often, CTDs are attached to a much larger metal frame called a rosette, which may hold water sampling bottles that are used to collect water at different depths, as well as other instruments (such as ORP and OBS sensors) that can measure additional physical or chemical properties.
Since underwater volcanoes and hydrothermal vents may be several thousand meters deep, ocean explorers usually raise and lower a CTD rosette through several hundred meters near the ocean floor as the ship slowly cruises over the area being surveyed. This repeated up-and-down motion of the towed CTD may resemble the movement of a yo-yo; a resemblance that has led to the nickname “tow-yo” for this type of CTD sampling. See: http://oceanexplorer.noaa.gov/technology/tools/sonde_ctd/sondectd.html and http://www.pmel.noaa.gov/ for more information.
More About Hydrothermal Vent Ecosystems
When seawater penetrates the permeable ocean crust in the vicinity of volcanoes, increased heat and pressure cause a variety of gases, metals and other materials to dissolve into the water from the surrounding rock. This process causes many metals to be concentrated by a thousand to a million times their concentration in normal seawater. The water does not boil because of the high pressure in the deep ocean, but may reach temperatures higher than 350° C. The resulting solutions are called hydrothermal fluids.
Hydrothermal vents are locations where hydrothermal fluids erupt through the seafloor. The temperature of the surrounding water is near freezing, which causes some of the dissolved minerals to precipitate from the solution. This makes the hot water plume look like black smoke, and in some cases the precipitated minerals form chimneys or towers. Dissolved gases may react to form other materials. At NW Rota Volcano, for example, dissolved sulfur dioxide forms sulfuric acid and elemental sulfur. At NW Eifuku Volcano, 1600 meters below the sea surface, the 2004 Ring of Fire Expedition found buoyant droplets of liquid carbon dioxide, probably formed from degassing of a carbon-rich magma.
Hydrothermal fluids also provide an energy source for a variety of chemosynthetic microbes that in turn are the basis for unique food webs associated with hydrothermal vents. One of the most exciting and significant scientific discoveries in the history of ocean science was made in 1977 at hydrothermal vents near the Galapagos Islands. Here, researchers found large numbers of animals that had never been seen before clustered around underwater hot springs flowing from cracks in the lava seafloor.
The presence of thriving biological communities in the deep ocean was a complete surprise, because it was assumed that food energy resources would be scarce in an environment without sunlight to support photosynthesis. Researchers soon discovered that the organisms responsible for this biological abundance do not need photosynthesis, but instead are able to obtain energy from chemical reactions through processes known as chemosynthesis. Photosynthesis and chemosynthesis both require a source of energy that is transferred through a series of chemical reactions into organic molecules that living organisms may use as food. In photosynthesis, light provides this energy. In chemosynthesis, the energy comes from other chemical reactions.
Energy for chemosynthesis in the vicinity of hydrothermal vents often comes from hydrogen sulfide. In chemosynthetic communities where hydrogen sulfide is present, large tubeworms known as vestimentiferans are often found, sometimes growing in clusters of millions of individuals. These unusual animals do not have a mouth, stomach, or gut. Instead, they have a large organ called a trophosome that contains chemosynthetic bacteria. Vestimentiferans have tentacles that extend into the water. The tentacles are bright red due to the presence of hemoglobin, which can absorb hydrogen sulfide and oxygen, which are transported to the bacteria in the trophosome. The bacteria produce organic molecules that provide nutrition to the tubeworm. A similar symbiotic relationship is found in clams and mussels that have chemosynthetic bacteria living in their gills.
Bacteria are also found living independently from other organisms in large bacterial mats. Many of these microbes have specific adaptations to extreme conditions; scientists found evidence for microbes living in hot spring fluids on NW Rota with a pH of 2.0 or less! Other new and unique microbes are expected to be found in association with extreme vent fluids as other sites are identified and explored in the Northeastern Lau Basin.
Visit http://www.pmel.noaa.gov/vents/home.html for more information and activities on hydrothermal vent communities.
More About Plate Tectonics and the Submarine Ring of Fire
Tectonic plates are portions of the Earth’s outer crust (the lithosphere), about 5 km thick, as well as the upper 60 - 75 km of the underlying mantle. The plates move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometers thick. Heat within the asthenosphere creates convection currents (similar to the currents that can be seen if food coloring is added to a heated container of water). These convection currents cause the tectonic plates to move several centimeters per year relative to each other.
The junction of two tectonic plates is called a “plate boundary.” Three major types of plate boundaries are produced by tectonic plate movements. If two tectonic plates collide more or less head-on they form a convergent plate boundary. Usually, one of the converging plates will move beneath the other, which is known as subduction. Deep trenches are often formed where tectonic plates are being subducted, and earthquakes are common. As the sinking plate moves deeper into the mantle, fluids are released from the rock causing the overlying mantle to partially melt. The new magma (molten rock) rises and may erupt violently to form volcanoes, often forming arcs of islands along the convergent boundary. These island arcs are always landward of the neighboring trenches. For a 3-dimensional view of a subduction zone, visit: http://oceanexplorer.noaa.gov/explorations/03fire/logs/subduction.html.
The junction of two tectonic plates that are moving apart is called a divergent plate boundary. Magma rises from deep within the Earth and erupts to form new crust on the lithosphere. Most divergent plate boundaries are underwater (Iceland is an exception), and form submarine mountain ranges called oceanic spreading ridges. While the process is volcanic, volcanoes and earthquakes along oceanic spreading ridges are not as violent as they are at convergent plate boundaries. View the 3-dimensional structure of a mid-ocean ridge at: http://oceanexplorer.noaa.gov/explorations/03fire/logs/ridge.html.
The third type of plate boundary occurs where tectonic plates slide horizontally past each other, and is known as a transform plate boundary. As the plates rub against each other, huge stresses are set up that can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California. See animations of different types of plate boundaries at:
The volcanoes of the Submarine Ring of Fire result from the motion of several major tectonic plates. The Pacific Ocean Basin lies on top of the Pacific Plate. To the east, along the East Pacific Rise, new crust is formed at the oceanic spreading center between the Pacific Plate and the western side of the Nazca Plate. Farther to the east, the eastern side of the Nazca Plate is being subducted beneath the South American Plate, giving rise to active volcanoes in the Andes. Similarly, convergence of the Cocos and Caribbean Plates produces active volcanoes on the western coast of Central America, and convergence of the North American and Juan de Fuca Plates causes the volcanoes of the Cascades in the Pacific Northwest.
On the western side of the Pacific Ocean, the Pacific Plate converges against the Philippine Plate and Australian Plate. Subduction of the Pacific Plate creates the Marianas Trench (which includes the Challenger Deep, the deepest known area of the Earth’s ocean), Kermadec Trench, and Tonga Trench. As the sinking plate moves deeper into the mantle, new magma is formed as described above, and erupts along the convergent boundary to form volcanoes. The Mariana and Kermadec Islands are the result of this volcanic activity, which frequently causes earthquakes as well. The movement of the Pacific Ocean tectonic plate has been likened to a huge conveyor belt on which new crust is formed at the oceanic spreading ridges, and older crust is recycled to the lower mantle at the convergent plate boundaries of the western Pacific.
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