Mid-Ocean Ridges

The mid-ocean ridge is an interconnected system of undersea volcanoes that meander over the Earth like the raised seams on a baseball. It is a continuous 40,000-mile (60,000-kilometer) seam that encircles Earth and bisects its oceans. The mid-ocean ridge represents an area where, in accordance with plate tectonic theory, lithospheric plates (also called tectonic plates) move apart and new crust is created by magma (molten rock) pushing up from the mantle . The mid-ocean ridge system is an example of a divergent (rather than a convergent or transform) plate boundary.

The mid-ocean ridge system has been understood only since the development and acceptance of plate tectonic theory in the 1960s. Four major scientific developments spurred the formation of the theory: (1) demonstration of the young age of the ocean floor; (2) confirmation of repeated reversals of Earth's magnetic field in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated recycling of the oceanic crust; and (4) precise documentation that Earth's earthquake and volcanic activity is concentrated along subduction zones and mid-ocean ridges.

Ridge Characteristics

Mid-ocean ridges have different shapes (morphology) depending on how fast they are spreading, how active they are magmatically and volcanically, and how much tectonic stretching and faulting is taking place. Scientists believe that the most likely reason for the different morphologies is due to the strength of the ocean crust at these different sites, and how cold and brittle the upper part of the tectonic plate is.

Ridge Types.

There are two types of mid-ocean ridges: fast-spreading and slow-spreading. Fast-spreading ridges like the northern and southern East Pacific Rise have smoother topography at the ridge crest, and look somewhat like domes. They have relief of 100 to 200 meters (328 to 656 feet). The East Pacific Rise moves at an average of 15 centimeters (5.9 inches) per year.

Slow-spreading ridges like the Mid-Atlantic Ridge have large, wide, rift valleys, sometimes as wide as 10 to 20 kilometers (6 to 12 miles) and very rugged terrain at the ridge crest that can have relief of up to 3.2 kilometers (2 miles). The Mid-Atlantic Ridge moves at an average of 2.5 centimeters (1 inch) per year.

Fast-spreading ridges are "hotter," meaning that more magma is present beneath the ridge axis, and that more volcanic eruptions occur. Because the plate under the ridge crest is hotter, scientists think that the plate responds to the divergent spreading process more fluidly, and that the ridge behaves like hot taffy being pulled apart. In this scenario, the ridge crest does not have a chance to subside (sink or settle).

At slower spreading ridges, the seafloor behaves more like a cold chocolate bar—when pulled, it cracks and breaks to form ridges and valleys. As the sheets of oceanic crust move away from the mid-ocean ridge, the rock is cooled and thus becomes heavier. After about 200 million years, the cooled lithospheric plate has become heavier than the asthenosphere that it rides over, and it sinks, thereby producing a subduction zone.

Fracture Zones.

Mid-ocean ridges do not form straight lines but are instead offset in many places by fracture zones, or transform faults. Fracture zones are thought to occur due to zones of weakness in the pre-existing continent before it was rifted apart. Most mid-ocean ridges are divided into hundreds of segments by fracture zones. Along the Mid-Atlantic Ridge, fracture zones occur at an average interval of 55 kilometers (34 miles). As the Mid-Atlantic Ridge is some 16,000 kilometers (10,000 miles) long, it is divided by fracture zones into about 300 distinct segments. The ridge crest and its associated faults are the locus of nearly all shallow earthquakes occurring in mid-ocean areas.

Water and Minerals.

Ocean water is constantly percolating through fissures (cracks) at the mid-ocean ridge. Downward-convecting cold ocean water meets the hot new crust far below the surface, and many types of metals such as sulfur, copper, zinc, gold, and iron are transferred to the water. This hot, mineral-laden water gushes back up through the cracks, forming hydrothermal vents. As the hot water, which can reach temperatures of 371°C (700°F), escapes from the vents and comes in contact with the near-freezing water of the ocean bottom, the metals quickly precipitate out of solution. The results are surging black clouds of particle-rich water called black smokers, which often erupt out of tall chimneys of previously deposited solidified mineral.^* Because so much metal is spewed out, hydrothermal vents have been responsible for many of the world's richest ore deposits. These unique features also are found to harbor a diverse array of deep-ocean life.

see also Hot Springs on the Ocean Floor; Life in Extreme Water Environments; Mineral Resources from the Ocean; Ocean-Floor Bathymetry; Plate Tectonics; Volcanoes, Submarine.

Larry Gilman

and K. Lee Lerner

Bibliography

Coulomb, J. Seafloor Spreading and Continental Drift. Dordrecht, Netherlands: D. Reidel Publishing Co., 1972.

Nicolas, A. The Mid-Oceanic Ridges. Berlin, Germany: Springer Verlag, 1995.

Thurman, Harold, and Elizabeth Burton. Introductory Oceanography, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

^* See "Hot Springs on the Ocean Floor" for a photograph of a black smoker.

mid-ocean ridge A long, linear, elevated, volcanic structure often lying along the middle of the ocean floor. Such ridges tend to occupy central positions because the oceans have formed by the symmetrical spreading of two lithospheric plates from the ridge sites. Ocean ridges occur in all the Earth's oceans, but may be offset from a central position, e.g. the E. Pacific ridge, where one side of the oceanic crust is being consumed along a subduction zone. At mid-ocean ridges, ocean floor is being formed. At the centre there is a rift valley, formed as discrete segments, bordered by high mountains on both sides. At a fast-spreading ridge (opening at up to 15 cm a year) the crust is smoother, with flat lavas flowing from fissures, than at a slow-spreading ridge (about 2 cm a year), where the median valley contains a chain of small volcanoes linked by fissure eruptions.