Why Are Scientists Exploring the Submarine Ring of Fire?
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 on how to achieve sustainable use of resources, links to our maritime history, and information to help 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 and results from the motion of large pieces of the Earth’s crust known as tectonic plates. This volcanic activity is often associated with “hydrothermal systems” or seafloor hot springs, where heat and chemicals from the interior of volcanoes are vented into the ocean. This process supports unique biological communities. These communities are typically based on chemosynthetic food chains, and some are highly productive. Microbial organisms that are essential to these food chains have unusual adaptations that allow them to survive extreme chemical and physical conditions, including enzymes previously unknown to science. These enzymes are believed to have a high potential for developing new natural products of interest to industrial and medical research. Vent fluids typically have high concentrations of metals that quickly precipitate in cold ocean waters. This process may be directly linked to the formation of ores and concentrated deposits of gold and other precious and exotic metals.
Plate Tectonics and the Submarine Ring of Fire
Tectonic plates consist of 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.
Where tectonic plates slide horizontally past each other, the boundary between the plates 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. View animations of different types of plate boundaries at:
A convergent plate boundary is formed when tectonic plates collide more or less head-on. Usually one of the converging plates moves beneath the other, a process 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. View the 3-dimensional structure of a subduction zone at:
Where tectonic plates are moving apart, they form a divergent plate boundary. At divergent plate boundaries, 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:
The Pacific Ocean Basin lies on top of the Pacific Plate. To the east, new crust is formed at the oceanic spreading ridges between the Pacific Plate and the North American and South American Plates. To the west, the Pacific Plate converges against the Philippine Plate. Where the two plates converge, the Pacific Plate is forced beneath the Philippine Plate, creating the Marianas Trench (which includes the Challenger Deep, the deepest known area of the Earth’s oceans). 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 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 off the western coasts of North and South America, and older crust is recycled to the lower mantle at the convergent plate boundaries of the western Pacific.
For more information on plate tectonics, visit the NOAA Learning Objects Web site (http://www.learningdemo.com/noaa/). Click on the links to Lesson 1 for interactive multimedia presentations and Learning Activities on Plate Tectonics.
Volcanism and Hydrothermal Vents Along the Mariana Arc
Volcanoes associated with convergent plate boundaries are very different from those found near the divergent plate boundaries of oceanic spreading ridges. Volcanoes at divergent plate boundaries tend to be linear and look like long, low ridges. These volcanoes generally do not erupt explosively, and their lava has a relatively primitive composition since it comes more or less directly from the Earth’s interior. Arc volcanoes, on the other hand, tend to be isolated and cone-shaped, and often erupt as violent explosions. Their lava rises through a longer path on its way to the surface, and its composition may be significantly different from lava at the source. See the satellite and sonar survey animation of the Mariana Arc Volcanic Chain at:
Seawater entering the permeable ocean crust in the vicinity of volcanoes is subjected to increased heat and pressure, and dissolves a variety of gases, metals and other materials from the surrounding rock. These conditions cause many metals to be concentrated by a thousand to a million times their concentration in normal seawater. When the hydrothermal fluid is vented into cold ocean water, some dissolved substances precipitate out of solution, forming metal deposits, “chimneys,” and “black smokers.” 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. 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 explored along the Mariana Arc.
Previous Expeditions to the Submarine Ring of Fire
While portions of the Submarine Ring of Fire have been mapped and sampled during the past several decades, other areas are virtually unexplored and there has been very little dedicated study of submarine hydrothermal systems. Beginning in 2002, a series of Ocean Exploration expeditions have undertaken systematic investigations in some of these areas. 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/).
In 2003 and 2004, the 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 expeditions also deployed Autonomous Underwater Hydrophones to monitor undersea seismicity and marine mammal activity. See http://www.oceanexplorer.noaa.gov/explorations/03fire/logs/feb22/feb22.html for more about these instruments and the ocean’s SOFAR region that “traps” sound and allows it to be heard at extremely long distances.
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 2006 Submarine Ring of Fire Expedition is focused on interdisciplinary investigations of the hydrothermal and volcanic processes on the submarine volcanoes along the Mariana Arc. Because these sites are virtually unexplored, the questions are basic, and include:
• What biological organisms are associated with Mariana Arc volcanoes and hydrothermal sites?
• What are the chemical and geological characteristics of these sites?
• What is the surrounding topography and water circulation?
To answer these questions, scientists will
• Collect samples, observations, and measurements for biological, chemical, and geological investigations;
• Measure ocean currents; and
• Map the seafloor in the vicinity of selected submerged volcanoes.
In addition, the expedition will recover and deploy seafloor instruments to monitor seismic activity, as well as transponders to assist with underwater navigation.
As on previous expeditions to the Mariana Arc, the 2006 Submarine Ring of Fire Expedition will make extensive use of a remotely operated underwater vehicle (ROV) – an unoccupied, highly maneuverable underwater robot operated by a person aboard a surface vessel. JASON II/Medea, the ROV selected for this expedition, is a two-part ROV system capable of diving to a depth of 6500 m. The JASON II vehicle carries most of the tools, instruments, and observation equipment, while Medea acts as a buffer between JASON II the support ship. This arrangement stops the tether to the support ship from tugging on the ROV as the ship rises and falls with motion of the sea. This arrangement means that Jason II is very maneuverable and stable. Medea is equipped with down-looking cameras, and provides a bird’s eye view of JASON II during ROV operations. JASON II has seven cameras and associated lights that can take a series of digital images which can be merged to form a large mosaic picture of the seafloor. JASON II also has a portable multibeam sonar system that can create highly detailed maps; manipulator arms that can maneuver instruments or collect samples; a “slurp pump” that can be used to suck up mud, sand, microbial mats, and fast-moving animals such as shrimp, crabs and squat lobsters; and can be equipped to take water samples as well as measure temperature, conductivity and depth. The JASON II/Medea system is accompanied by a control van, a tool van, and a crew consisting of three pilots, three navigators, three engineers and two data processors. For more information about JASON II/Medea, visit http://www.whoi.edu/marops/vehicles/jason/index.html.
In addition to the ROV, a variety of other devices will be used to explore the volcanoes and hydrothermal vent systems of the Mariana Arc. A torpedo-shaped instrument called a CTD will be used to collect data on seawater conductivity, temperature, and depth. These data can be used to determine salinity of the seawater which is a key indicator of different water masses. A CTD may be deployed by itself, or may be attached to a submersible or a larger water sampling array known as a rosette, or carousel (for more information on CTDs and rosettes visit http://oceanexplorer.noaa.gov/technology/tools/sonde_ctd/sondectd.html).
An Acoustic Doppler Current Profiler (ADCP) will be used to measure the speed and direction of ocean currents using the principle of “Doppler shift.” This principle is based on the familiar phenomenon experienced when a passing train blows its whistle. As the train approaches, the whistle’s pitch seems to rise; then as the train moves away the pitch becomes lower. The ADCP uses the fact that the change in pitch is proportional to the train’s speed, and emits a sequence of high frequency pulses of sound that scatter off of moving particles in the water. Depending on whether the particles are moving toward or away from the ADCP, the pitch of the signal bounced back is either higher or lower. Particles moving away from the instrument produce a return signal with a lower pitch, while particles moving toward the instrument produce a return signal with a higher pitch. Since the particles move at the same speed as the water that carries them, the frequency shift is proportional to the speed of the water, or current. The ADCP emits and receives acoustical signals in four different directions, so that current direction can be computed using trigonometric relationships between the signals. The ADCP measures the current at many different depths simultaneously, making it possible to determine the speed and direction of the current from the surface of the ocean to the bottom. For more information on the ADCP, visit http://oceanexplorer.noaa.gov/technology/tools/acoust_doppler/acoust_doppler.html
A multibeam sonar system will be used to make bathymetric maps and three-dimensional images of the seafloor. Multibeam sonars send out multiple, simultaneous sonar beams in a fan-shaped pattern that is perpendicular to the ship’s track. This allows the seafloor on either side of the ship to be mapped at the same time as well as the area directly below (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information).
Plankton nets and bottom grabs will be used to collect biological samples, in addition to those collected using the ROV. Finally, a gravimeter will be used to measure variations in gravity that could results from variations in Earth’s density at different locations.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.