Seafloor Spreading
Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.
2Seafloor Spreading
For upon |Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.
Ocean Floor
Learn more about ocean trenches, mid-ocean ridges, and other features of the seafloor with this Nat Geo video.
Mantle Convection
Seafloor spreading is a result of mantle convection. Mantle convection is the slow, churning motion of solid and molten material in Earth’s mantle. The two theories about how mantle convection works—the “layered mantle” and “whole mantle” hypotheses—are displayed on this diagram.
Seafloor Spreading
As magma bubbles up at sites of seafloor spreading, it is cooled by frigid seawater and becomes Earth’s newest oceanic crust. Most of this new igneous rock is basalt, like this pillow lava that bubbled up at the East Pacific Rise near the Galapagos Islands.
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Oceanic Crust
Seafloor spreading helps create mid-ocean ridges, giant underwater mountain ranges that develop where tectonic activity is driving plates apart from each other. This lovely map highlights where tectonic plates are separating at mid-ocean ridges.
East Pacific Rise
Mid-ocean ridges circle the globe like “seams on a baseball,” according to NASA. Islands, such as Iceland (part of the Mid-Atlantic Ridge) and these islands off the southwest coast of Mexico are part of those “seams.”
Iceland’s the Mid-Atlantic Ridge
Slow-spreading centers have steep cliffs and plunging ocean trenches. This rocky outcrop is part of the slow-spreading Mid-Atlantic Ridge, which is just above sea level on the island of Iceland. To the left is the eastern edge of the North American continent. On the right is the western edge of Eurasia. (Fast spreading centers, such as the East Pacific Rise, have more gentle slopes and lack deep trenches.)
Passive Margins
The oceanic crust becomes thicker and denser as it moves away from the spreading center. As it reaches a continental boundary, it encounters either an active plate margin (a convergent boundary where an oceanic plate crashes into a continental plate) or a passive plate margin (where a single tectonic plate transitions from the oceanic crust to the continental crust). In this map of passive plate margins, note that seafloor created at the Mid-Atlantic Ridge will ultimately encounter passive plate margins, while seafloor created at the East Pacific Rise will likely encounter a subduction zone near the coast of South America—the site of (very) active plate boundaries.
Red Sea
As seafloor spreading rends tectonic plates from each other, it can create new ocean basins—such as the Red Sea, which was created as the African and Asian plates were torn apart over millions of years. Today, the triangle-shaped Sinai Peninsula (at the top of this beautiful NASA image) links the two continents. Eventually, Africa and Asia will split entirely and the Mediterranean and Red Seas will merge.
1Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.
Seafloor spreading and other tectonic activity processes are the results of mantle convection. Mantle convection is the slow, churning motion of Earth’s mantle. Convection currents carry heat from the lower mantle and core to the lithosphere. Convection currents also “recycle” lithospheric materials back to the mantle.
Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor.
Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fill these fractures and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock. This rock (basalt) becomes a new part of Earth’s crust.
Mid-Ocean Ridges
Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor. The Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate. 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 cliffs and mountains. Rapidly spreading ridges have a much more gentle slope.
The Mid-Atlantic Ridge, for instance, is a slow-spreading center. It spreads 2-5 centimeters (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon. The East Pacific Rise, on the other hand, is a fast-spreading center. It spreads about 6-16 centimeters (3-6 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 the mid-ocean ridge—the actual site of seafloor spreading. The age, density, and thickness of the oceanic crust increase with distance from the mid-ocean ridge.
Geomagnetic Reversals
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 the newest oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s. What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching “stripes” on either side of the ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling.
Scientists determined that the same process formed the perfectly symmetrical stripes on both sides of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern.
Geographic Features
The oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, denser, and thicker. Eventually, the older oceanic crust encounters a tectonic boundary with the continental crust.
In some cases, the oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where the oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquakes and volcanoes. 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, the oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from the oceanic lithosphere to the continental lithosphere.
Passive margins are not sites of faults or subduction zones. 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. Today, only the Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will completely separate the two continents—and join the Red and Mediterranean Seas.
Mid-ocean ridges and seafloor spreading can also influence sea levels. As the oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes denser. This increases the volume of the ocean basin and decreases the sea level. For instance, a mid-ocean ridge system in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea—contributed to shallower oceans and higher sea levels in the Paleozoic era. Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years.
Seafloor spreading disproves an early part of the theory of continental drift. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity.
Keeping Earth in Shape
Seafloor spreading is just one part of plate tectonics. Subduction is another. Subduction happens when tectonic plates crash into each other instead of spreading apart. At subduction zones, the edge of the denser plate subducts, or slides, beneath the less dense one. The denser lithospheric material then melts back into the Earth’s mantle.
Seafloor spreading creates a new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant.
Earth’s newest crust is created at sites of seafloor spreading—red sites on this map.
Map courtesy NOAA
Triple Junctions
Seafloor spreading and rift valleys are common features at “triple junctions.” Triple junctions are the intersection of three divergent plate boundaries. The triple junction is the central point where three cracks (boundaries) split off at about 120° angles from each other.
In the Afar Triple Junction, the African, Somali, and Arabian plates are splitting from each other. The Great Rift Valley and Red Sea (a major site of seafloor spreading) are the results of plate tectonics in the Afar Triple Junction.
The article was originally published here.
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