Ocean ridge system

The ocean ridge system is a complex chain of undersea volcanic mountains found in all the oceans. These mountains contain rift valleys along their axes, which are believed to be spreading centers from which continental motion takes place. All existing evidence, such as volcanic activity, the flow of heat from within the Earth, and various types of faulting and rifting, supports modern theories about seafloor spreading, tectonic plates, and continental drift.

Seafloor Spreading

The ocean ridge system is a complex chain of mountains about 80,000 kilometers long that winds through the ocean basins. These mountain ranges vary from a few hundred to a few thousand kilometers in width and have an average relief of 0.6 kilometers. Studies of the seafloor indicate that ocean ridges are found in every major ocean basin. They are composed of basalt and covered by various types of sediments. Many ridges have narrow depressions extending thousands of kilometers along their axes. Heat probes lowered into these rifts indicate much higher temperatures than on the flanks of the ridges. Another significant finding has been that the rocks that make up the seafloor are much younger than those that make up the continents. This finding countered the pre-1960s belief that the rocks of the ocean basin were more ancient than those of the continents.

In the 1960s, the theory of seafloor spreading suggested that new seafloor is constantly being added by volcanic activity at the ocean ridges. The theory of plate tectonics proposes that the Earth's crust is divided into several major plates. These plates extend down into the Earth's mantle to a hot, semi-molten zone known as the asthenosphere. Since the rock that composes the tectonic plates is less dense than the rock that forms the mantle, the plates may be considered to be floating on the asthenosphere. Because the interior of the Earth is much hotter than the surface, a flow of heat toward the surface is a constant process. This heat transfer method, called convection, is a density current type. Material such as molten rock or hot air or water is less dense than the same material in a cooler state. As a result, it flows upward, with cooler material filling in the space below. When the hot material reaches a higher level and releases its heat, it, too, returns to the depths to be reheated. This process, along with the relatively low density of the tectonic plates, causes the plates to drift apart.

The location and symmetry of the mid-ocean ridges, especially in the Atlantic and Indian Oceans, suggest the configuration of the continents before they began to drift apart. The modern theories of plate tectonics and seafloor spreading are in basic agreement with the theory of continental drift as proposed by German geophysicist Alfred Wegener in 1915. However, modern ideas regarding the mechanics of drift differ. The basic concept of seafloor spreading at the oceanic ridges proposes that tension cracks form in the crust at these spreading centers. Molten rock from the mantle flows upward through these fissures, forming the volcanic ridges and creating new seafloor. As the fissures widen, new crustal material moves away on both sides, creating additional new seafloor. In this way, the mid-ocean ridges such as the Mid-Atlantic Ridge, the East Pacific Rise, the Antarctic Rise, and the Carlsberg Ridge of the Indian Ocean were formed. The spreading away from the oceanic ridges takes place at about 2.5 centimeters per year in the Atlantic and about 8 to 12 centimeters per year in the Pacific.

As new seafloor is being created at the ridges, old seafloor is being destroyed at continental margins. Here, old seafloor is subducted beneath continental plates. At these converging plate boundaries, old seafloor is forced downward into the mantle, where it undergoes remelting. This molten rock may return to the surface through cracks or fissures in the overlying rock. Volcanic action is the result of this material reaching the Earth's surface. Because old seafloor is destroyed in this manner, it is now understood why rocks that make up the ocean floor are relatively young. Deep-seafloor drilling projects in the Atlantic and Indian Oceans have failed to find rock or fossil samples older than the Jurassic period of Earth's history, which ended about 145.5 million years ago.

Rift Zones

The present ocean ridges show large offsets in some areas. This phenomenon results from transform faulting. As spreading of the plates took place at divergent boundaries, fracture zones developed at right angles to the axes of the ridges. Displacements of the ridges along these faults produced the observed structure.

The narrow rift valleys are believed to have been caused by downfaulting along divergent plate boundaries. Some rift zones, such as the one associated with the Carlsberg Ridge of the northern Indian Ocean, link up with continental rift zones. This rift has been shown to be connected with the African rift zone. Studies conducted during the 1970s indicate that extensive amounts of volcanism and seismic activity exist along rift zones.

Hot Springs and Smokers

Also found along oceanic ridges are hot-water springs. These springs, or smokers, had been predicted, but direct evidence for them was not gathered until the early 1960s when metal-bearing sediments were discovered on the East Pacific Rise. The first actual observation of a smoker occurred in 1980 by the deep-diving research submarine Alvin crew. The Alvin was part of an underwater research program conducted near the Galápagos spreading center in the eastern Pacific. Researchers were surprised to find a rather extensive plant and animal community living near the smoker at a depth too great for photosynthesis to be a factor. Clams as long as thirty centimeters were found, as well as white crabs and tube worms some three meters long.

Smokers have been found to emit great quantities of sulfur-enriched waters. Dissolved within the acidic water are various types of metals. These metals are dissolved from the rocks as the superheated water moves toward the surface. Metals such as copper, zinc, iron, silver, and gold are extracted and concentrated into a supersaturated fluid. When this fluid is discharged from a smoker at the seafloor, it will precipitate to form an ore body if cooling takes place rapidly. These massive sulfide deposits are sometimes made permanent when volcanic eruptions cover them with basalt flows.

Mid-Atlantic Ridge

Although most of the knowledge regarding the nature of the sea bottom has been gained since the 1960s, the Mid-Atlantic Ridge system has been known to exist since echo-sounding studies were done after World War I. By 1960, it had been determined that the Mid-Atlantic Ridge was continuous with other oceanic ridge systems worldwide. The same cannot be said of the rift system that was discovered in the Mid-Atlantic Ridge. Profiles taken across ridge systems do not always indicate such rift depressions. This anomaly can probably be explained by considering the possibility that these rifts have been filled with volcanic material and, therefore, may go undetected.

A rather extensive study was conducted along the Mid-Atlantic Ridge, about 200 miles south of the Azores, in 1974. It found that the rift valley is bordered by a series of steep slopes that appear to be high-angle faults. Many open vertical fissures were observed, some as much as eight meters across. Although no active volcanism was observed during this study, mounds of pillow lava along the fissures indicated that volcanism had occurred. The positioning of the faults and fissures indicated that the Mid-Atlantic Ridge is spreading east and west from its central axis. Further research confirmed this slow-spreading ridge. Each year, it spreads two to five centimeters (0.8 to 2 inches), forming a Grand Canyon-sized trench.

East Pacific Rise

The East Pacific Rise, which is part of the system of mid-ocean ridges, stands some two to three kilometers above the ocean floor and is thousands of kilometers wide. The slopes in this area are not as great as those of the Mid-Atlantic system, but the mechanism of ridge formation is the same. It has been suggested that part of the East Pacific Rise extends under the North American continent and that the San Andreas fault may be part of this system. Evidence supporting this possibility is that the rate of displacement along the fault is comparable to the rate of ridge spreading on the East Pacific Rise near the Gulf of California. The East Pacific Ridge spreads much faster than the Mid-Atlantic, expanding six to sixteen centimeters (three to six inches) annually.

Study of the Ocean Floor

The ocean floor has been investigated using sonar since the early twentieth century. Using this technique, sound waves from a device are sent into the sea in all directions. As these waves strike objects, they are reflected back to the source. By analyzing the reflections, scientists are able to determine the nature of the seafloor. This technique was refined during World War II for the purpose of locating enemy submarines. By the early 1950s, new maps and charts of the seafloor had been made using sonar.

In the 1960s, a plan to drill a deep hole through the crust of the Earth to the mantle was proposed. Since this deep hole was intended to intersect the Mohorovičić discontinuity (named after Andrija Mohorovičić, the Croatian geologist who discovered it), the project was called Mohole. Given that the crust of the Earth is much thinner under the ocean basins than it is under the continents, the ocean was the most logical location to drill. Although the six-kilometer-deep hole was never drilled, several preliminary holes were. Samples taken from this drilling revealed the presence of gray, claylike sediments overlying dark, heavy basalts.

Project Mohole was later replaced by a plan to drill many holes at various locations into the rock beneath the ocean depths. This project, under the direction of various American oceanographic institutes, became known as the Deep Sea Drilling Project. Much of the drilling was done from the deck of the now-retired Glomar Challenger of the company Global Marine. More than six hundred holes were drilled in the Atlantic, Pacific, and Indian Oceans and the Mediterranean Sea, revealing a wealth of data on the nature and evolution of the ocean basins. The Deep Sea Drilling Project continued studying the ocean floor under several program names in the early twenty-first century before becoming the International Ocean Discovery Program (IODP) in 2013. Combined, these programs are the longest-running international scientific research effort to understand the Earth.

In the past, ocean researchers had to gather data from the decks of ships by lowering various tools to the bottom to gather samples—there was a need for direct observation by the oceanographer. In 1948, the first minisubmarine, or bathyscaphe, made an uncrewed dive to 759 meters. In 1954, a crewed vessel was taken to a depth of 4,050 meters. During the 1960s, interest in these submersibles grew quickly in the United States. Since then, submersibles such as the Aluminaut and Alvin have been used in various deep-sea research projects. Autonomous, remotely operated, and human-occupied underwater vehicle technology continues to evolve to aid researchers in locating, viewing, and collecting information from the ocean floor and has been used for on-site studies of the rift zone of the Mid-Atlantic Ridge and of the hot-spring smokers of the East Pacific Rise.

Another technique, marine seismology, has been employed to study the ocean ridges. Oil companies have used seismic studies to explore for oil deposits on continents, and it was only a matter of time before this technology was adapted for the study of the ocean depths. The process involves making a sound explosion in the sea using an air gun. The rapid release of compressed air produces the sound waves. The waves then travel through the water to the bottom, where they are reflected by seismometers located on the ocean floor. The data are collected, and a computer generates a seismogram. This technique has been used to find active magma chambers in rift zone areas.

Economic Resources

Studies of submarine rift areas that have revealed the hot-water springs and smokers have shown these areas to offer potential economic riches. Smokers discharge water that contains sulfur in solution as well as various types of metals such as copper, zinc, nickel, lead, gold, cobalt, selenium, molybdenum, antimony, and silver. These metals precipitate to the sea bottom and, if they are cooled rapidly enough, form deposits. If the ore bodies are covered with volcanic material shortly after their deposition, they become protected from the erosive processes of seawater. In time, these deposits drift from oceanic spreading centers to become parts of continents. Many mineral deposits formed in just this manner have been mined since ancient times.

In the late 1950s, metal-rich sediments were first found along mid-ocean ridges. By the late 1960s, metalliferous muds were found at the bottom of the Red Sea, the Gulf of California, and the East Pacific Rise near the Galápagos Islands and south of Baja, California. Beginning in 1977, marine geologists could observe the formation of metal deposits while observing smokers from the research submarine Alvin. Interest in ridge deposition of metals centers on the processes of metallogenesis. Mining such deposits continues to be of interest to scientists. The copper deposits in the area of the Galápagos spreading center alone are worth billions.

Principal Terms

asthenosphere: a zone of rock within the mantle that has plastic flow properties attributable to intense heat

basalt: a heavy, dark-colored volcanic rock

converging plates: a tectonic plate boundary where two plates are pushing toward each other

divergent plates: a tectonic plate boundary where two plates are moving apart

metallogenesis: the process by which metallic ores are formed

sediments: the solid fragments of rock that have been eroded from other rocks and then transported by wind or water and deposited

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