IvErgent Plate Boundaries In The Oceans The Midocean Ridge System

Some continental rifts may evolve into midocean ridge-spreading centers. The world's best example of where this transition can be observed is in the Ethiopian Afar, where the East African continental rift system meets juvenile oceanic spreading centers in the Red Sea and Gulf of Aden. Three plate boundaries meet in a wide plate boundary zone in the Afar, including the African/Arabian boundary (Red Sea spreading center), the Arabian/Somalian boundary (Gulf of Aden spreading center), and the African/ Somalian boundary (East African rift). The boundary is a complex system known as an RRR (rift-rift-rift) triple junction. The triple junction has many complex extensional structures, with most of the Afar near sea level, and isolated blocks of continental crust such as the Danakil horst isolated from the rest of the continental crust by normal faults.

The Red Sea has a young, or juvenile, spreading center similar in some aspects to the spreading center in the middle of the Atlantic Ocean. Geologists recognize two main classes of oceanic spreading centers, based on characteristics of the shapes of their surfaces (geomorphology) and elevation or topography. These different types are formed in spreading centers with different spreading rates, with slow spreading rates, 0.2-0.8 inches per year (0.5-2 cm/yr), on Atlantic-type ridges, and faster rates, generally 1.5-3.5 inches per year (4-9 cm/yr), on Pacific-type ridges.

Atlantic-type ridges are characterized by a broad, 900-2,000-mile (1,500-3,000-km) wide swell in which the seafloor rises 0.6-1.8 miles (1-3 km) from abyssal plains at 2.5 miles (4.0 km) below sea level to about 1.7 miles (2.8 km) below sea level along the ridge axis. Slopes on the ridge are generally less than 1°. Slow, or Atlantic-type, ridges have a median rift, typically about 20 miles (30 km) wide at the top to 0.6-2.5 miles (1-4 km) wide at the bottom of the long, deep medial rift. Many constructional volcanoes are located along the base and inner wall of the medial rift. Rugged topography and many faults forming a strongly block-faulted slope characterize the central part of Atlantic-type ridges.

Pacific-type ridges are generally 1,250-2,500 miles (2,000-4,000 km) wide, and rise 1.2-1.8 miles (2-3 km) above the abyssal plains, with 0.1° slopes. Pacific-type ridges have no median valley but many shallow earthquakes, high heat flow, and low gravity in the center of the ridge, suggesting that magma may be present at shallow levels beneath the surface. Pacific-type ridges have much smoother flanks than Atlantic-type ridges.

The high topography of both types of ridges shows that they are underlain by low-density material and are floating on this hot substrate. Geologists call this mechanism of making mountains isostatic compensation. New magma upwells beneath the ridges and forms small magma chambers along the ridge axis. The magma in these chambers crystallizes to form the rocks of the oceanic crust that gets added (in approximately equal proportions) to both diverging plates. The crust formed at the ridges is young, hot, and relatively light, so it floats on the hot underlying asthenosphere. As the crust ages and moves away from the ridge, it becomes thicker and denser, and subsides; this explains the topographic profile of the ridges. The rate of thermal subsidence is the same for fast- and slow-spreading ridges (a function of the square root of the age of the crust), explaining why slow-spreading ridges are narrower than fast-spreading ridges.

Abundant volcanoes, with vast outpourings of basaltic lava, characterize the centers of the midocean ridges. The lavas are typically bulbous-shaped forms called pillows, as well as tubes and other, more massive flows. The ridge axes are also characterized by high heat flow, with many thermal vents marking places where seawater has infiltrated the oceanic crust and made its way to deeper levels, where it is heated by coming close to the magma, then rises again to vent on the seafloor. Many of these vents precipitate sulfide and other minerals in great quantities, forming chimneys called black smokers that may be many tens of feet (several meters) tall. These chimneys have high-temperature metal- and nutrient-rich water flowing out of them (at temperatures of several hundred degrees Celsius), with the metals precipitating once the temperature drops on contact with the cold seawater outside the vent. These systems may cover parts of the oceanic crust with layers of sulfide minerals. Unusual primitive communities of sulfide-reducing bacteria, tube worms, and crabs have been found near several black smoker vents along midocean ridges. Many scientists believe that similar settings may have played an important role in the early appearance and evolution of life on the planet.

Geophysical seismic refraction studies in the 1940s and 1950s established that the oceanic crust exhibits seismic layering similar in many places in the oceans. Seismic layer one consists of sediments, layer two is interpreted to be a layer of basalt 0.6-1.5 miles (1-2.5 km) thick, and layer three is approximately four miles (6 km) thick and interpreted to be crystal cumulates, underlain by the mantle. Some ridges and transform faults expose deeper levels of the oceanic lithosphere. These typically include a mafic dike complex, thick sections of gabbro, and ultramafic cumulates. In some places rocks of the mantle are exposed, typically consisting of strongly deformed ultramafic rocks that have had a large amount of

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Formation of oceanic crust and lithosphere at midocean ridges. Magma forms by partial melting in the asthenosphere and upwells to make a magma chamber beneath the ridge axis. As the plates move apart, dikes intrude upward from the magma chamber and feed the lava flows on the surface. Heavy crystals settle out of the magma chamber and form layers of crystal cumulates on the magma chamber floor.

magma squeezed out of them. These unusual rocks are called depleted harzburgite tectonites.

As the plates move apart, the pressure on deep underlying rocks is lowered, which causes them to rise and partially melt by 15-25 percent. Basaltic magma is produced by partially melting the peri-dotitic mantle, leaving a residue-type of rock in the mantle known as harzburgite. The magma produced in this way moves up from deep within the mantle to fill the gap opened by the diverging plates. This magma forms a chamber of molten or partially molten rock that slowly crystallizes to form a coarse-grained igneous rock known as gabbro, which has the same composition as basalt. Before crystallization some of the magma moves up to the surface through a series of dikes and forms the crustal-sheeted dike complex, and basaltic flows. Many of the basaltic flows have distinctive forms, with the magma forming bulbous lobes known as pillow lavas. Lava tubes are also common, as are fragmented pillows formed by the inward explosive collapse (implosion) of the lava tubes and pillows. Back in the magma chamber other crystals grow in the gabbroic magma, including olivine and pyroxene, and are heavier than the magma, so they sink to the bottom of the chamber. These crystals form layers of dense minerals known as cumulates. Beneath the cumulates the mantle material from which the magma was derived becomes progressively more deformed as the plates diverge and form a highly deformed ultramafic rock known as a harzburgite or mantle tectonite. This process can be seen on the surface in Iceland along the Reykjanes Ridge.

Much of the detailed information about the deep structure of oceanic crust comes from the study of ophiolites, which are interpreted to be on-land equivalents of oceanic crust tectonically emplaced on the continents during the process of convergent tectonics and ocean closure. Studies of ophiolites have confirmed the general structure of the oceanic crust as inferred from the seismic reflection and refraction studies and limited drilling. Numerous detailed studies of ophiolites have allowed unprecedented detail about the structure and chemistry of inferred oceanic crust and lithosphere to be completed, and as many variations as similarities have been discovered. The causes of these variations are numerous, including differences in spreading rate, magma supply, temperature, depth of melting, tectonic setting (arc, forearc, back arc, midocean ridge, etc.), and the presence or absence of water. The ocean floor, however, is still largely unexplored, and scientists know more about many other planetary surfaces than is known about Earth's ocean floor.

The mid-Atlantic ridge rises above sea level on the North Atlantic island of Iceland, lying 178 miles (287 km) off the coast of Greenland and 495 miles (800 km) from the coast of Scotland. Iceland has an average elevation of more than 1,600 feet (500 m) and owes its elevation to a hot spot that is interacting with the midocean ridge system beneath the island. The mid-Atlantic ridge crosses the island from southwest to northeast, and has a spreading rate of 1.2 inches per year (3 cm/yr), with the mean extension oriented toward an azimuth of 103°. The oceanic Reykjanes ridge and sinistral transform south of the island rises to the surface and continues as the Western Rift Zone. Active spreading is transferred to the Southern Volcanic Zone across a transform fault called the south Iceland seismic Zone, then continues north through the Eastern Rift Zone. Spreading is offset from the oceanic Kolbeinsey ridge by the dextral Tjornes fracture zone off the island's northern coast.

During the past 6 million years the Iceland hot spot has drifted toward the southeast relative to the North Atlantic, and the oceanic ridge system has made a succession of small jumps so that active spreading has remained coincident with the plume of hottest and therefore weakest mantle material. These ridge jumps have caused the active spreading to propagate into regions of older crust that have been remelted, forming unusual alkalic and even silicic volcanic rocks that are deposited unconform-ably over older oceanic (tholeiitic) basalts. Active spreading occurs along a series of 5-60 mile (10-100 km) long zones of fissures, graben, and dike swarms, with basaltic and rhyolitic volcanoes rising from central parts of fissures. Hydrothermal activity is intense along the fracture zones, with diffuse faulting and volcanic activity merging into a narrow zone within a few miles beneath the surface. Detailed geophysical studies have shown that magma episodically rises from depth into magma chambers located a few miles below the surface, then dikes intrude the overlying crust and flow horizontally for tens of miles to accommodate crustal extension of several to several tens of feet over several hundred years.

Many Holocene volcanic events are known from Iceland, including 17 eruptions of Hekla from the Southern Volcanic zone. Iceland has an extensive system of glaciers and has experienced a number of eruptions beneath them that cause water to infiltrate the fracture zones. The mixture of water and magma induces explosive events including Plinian eruption clouds, phreo-magmatic, tephra-produc-

ing eruptions, and sudden floods known as jokul-hlaups induced when the glacier experiences rapid melting from contact with magma. Many Icelanders have learned to use the high geothermal gradients to extract geothermal energy for heating and to enjoy the many hot springs on the island.

See also African geology; convergent plate margin processes; plate tectonics; transform plate margin processes.

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