This bubbled-up magma is cooled by frigid seawater to form igneous rock. Seafloor spreading occurs along mid-ocean ridge s—large mountain range s rising from the ocean floor. The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate.
Seafloor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliff s and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center. It spreads centimeters. The East Pacific Rise, on the other hand, is a fast spreading center.
It spreads about centimeters inches every year. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop! The newest, thinnest crust on Earth is located near the center of mid-ocean ridge—the actual site of seafloor spreading.
The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century. Basalt, the once- molten rock that makes up most new oceanic crust , is a fairly magnetic substance, and scientists began using magnetometer s to measure the magnetism of the ocean floor in the s.
Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern. Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick.
Eventually, older oceanic crust encounters a tectonic boundary with continental crust. In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquake s and volcano es. Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire , the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin.
In other cases, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transition s from oceanic lithosphere to continental lithosphere.
Passive margins are not sites of fault s or subduction zone s. Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate on the east coast of North America or the Eurasian plate on the west coast of Europe.
New geographic features can be created through seafloor spreading. The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other. Eventually, geologist s predict, seafloor spreading will completely separate the two continent s—and join the Red and Mediterranean Seas.
Mid-ocean ridges and seafloor spreading can also influence sea level s. As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense. This new crust is gradually pushed away from the ridge by more new crust, eventually traveling the across the plate — a process called seafloor spreading — and back into the Earth's interior at a subduction zone, where one tectonic plate dives beneath another. The speed of crust formation varies from ridge to ridge: Some fast-spreading ridges produce up to 6 inches 15 centimeters of new crust per year, while slower-spreading ridges creep along at just 2 inches 5 cm per year.
Matthew Rioux, a researcher at MIT, analyzed pieces of ocean crust from the East Pacific Rise , a mid-ocean ridge 1, miles 1, kilometers off the west coast of South America that is one of the fastest-spreading ridges in the world.
By looking at the minerals within the crust, Rioux and his colleagues could get an idea of the environment in the "mush zone," which is part liquid magma, part crystallized rock, and determine the ages of different parts of the rock. Scientists have thought that magmas that form new crust at fast-spreading ridges rise up from the depths, quickly crystallize, and then push away from the ridge to form new ocean floor.
The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean, slowly spreading at a rate of 2 to 5 centimeters 0. In contrast, the East Pacific Rise spreads fast, at rates of 6 to 16 centimeters 3 to 6 inches per year. Due to the fast spreading rates, there is no rift valley in the Pacific, just a smooth volcanic summit with a crack along the crest that is much smaller than the Atlantic rift valley.
Despite being such prominent feature on our planet, much of the mid-ocean ridge system remains a mystery. While we have mapped about half of the global mid-ocean ridge in high resolution, less than one percent of the mid-ocean ridge has been explored in detail using submersibles or remotely operated vehicles.
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