Seafloor spreading

Seafloor spreading is a geologic process in which tectonic plates, large slabs of Earth's lithosphere, split apart from one another. Seafloor spreading and other tectonic activity processes are the result of mantle convection, which 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 one another, 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 those fractures and spills onto the crust. The bubbled-up magma is cooled by frigid seawater to form igneous rock. The 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 the place that the southern Indo-Australian plate forms a boundary with the Antarctic plate.

Seafloor spreading is not consistent at all mid-ocean ridges. Slowlyspreading ridges are the sites of tall, narrow underwater cliffs and mountains. Rapidlyspreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow-spreading ridge. It spreads 2-5 cm (0.8-2 in.) 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 ridge. It spreads about 6-16 cm (3-6 in.) every year and has no ocean trench 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 ridgesm the actual site of seafloor spreading. The age, density, and thickness of oceanic crust increases 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 most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s.

They discovered that the magnetism of the ocean floor around mid-ocean ridges was divided into matching “stripes” on both sides of the ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma cools. Scientists determined that the same process form the perfectly-symmetrical stripes on both sides of a mid-ocean ridge. The continual process of seafloor spreading separates the stripes in an orderly pattern.

Geographic features

Oceanic crust slowly moves away from mid-ocean ridges and the sites of seafloor spreading. As oceanic crust moves, it becomes cooler, denser, and thicker. Eventually, older oceanic crust encounters a tectonic boundary with continental crust. In some cases, oceanic crust encounters an active plate margin, which is an actual plate boundary in which 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 Pacific Ring of Fire, the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific Ocean Basin.

In other cases, oceanic crust encounters passive plate margins, which are not plate boundaries but areas in which a single tectonic plate changes gradulally from oceanic lithosphere to continental lithosphere. Passive margins is not the site of a faults or subduction zone. Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will 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 by 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. Geologists predict that seafloor spreading will eventually separate the two continents completely and join the Red and the Mediterranean Seas.

Mid-ocean ridges and seafloor spreading can also influence sea levels. As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes denseer. That 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 tothe shallower oceans and higher sea levels of 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. That 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.

Seafloor spreading is just one part of plate tectonics. Subduction is another and happens when tectonic plates crash into, rather than spread apart from, one another. 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 new crust, and subduction destroys old crust. The two forces roughly balance each other and so the shape and the diameter of the Earth remain practically constant.

Spreading at a mid-ocean ridge

Diagram of oceanic ridge

World Distribution of Mid-Oceanic Ridges: the big picture.

Age of oceanic crust: youngest (red) is along spreading centres

Seafloor spreading happens at the bottom of an ocean as tectonic plates move apart. The seafloor moves and carries continents with it. At ridges in the middle of oceans, new oceanic crust is created. The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges.^[1]

At the Mid-Atlantic Ridge (and other places), material from the upper mantle rises through the faults between oceanic plates. It forms new crust as the plates move away from each other. The new crust then slowly moves away from the ridge. It is a place of earthquakes and volcanoes. Seafloor spreading helps explain continental drift in plate tectonics. At oceanic trenches, seafloor crust slides down and under continental crust.

Earlier theories (such as by Alfred Wegener) of continental drift were that continents "plowed" through the ocean. The modern accepted idea is that the ocean floor itself moves and carries the continents with it as it expands from a mid-ocean ridge. The phenomenon is caused by convection in the weak upper mantle, the asthenosphere.^[2][3]

Additionally, spreading rates determine if a ridge is a fast, intermediate, or slow. As a general rule, fast-spreading ridges have a rate of more than 9 cm/year. Intermediate ridges have a spreading rate of 4-9 cm/year, and slow-spreading ridges have a rate less than 4 cm/year. ^[1]

Mid-ocean ridge

A mid-ocean ridge is an underwater mountain system. It consists of mountain chains and a rift valley running along its spine, formed by plate tectonics. A mid-ocean ridge marks the boundary between two tectonic plates that are moving apart and is made by a divergent boundary.

The world's mid-ocean ridges are connected and form a single global mid-oceanic ridge system that is part of every ocean. The mid-oceanic ridge system is the longest continuous mountain range in the world. At 65,000 km (40,400 mi), It is several times longer than the Andes, the longest continental mountain range. The total length of the oceanic ridge system is 80,000 km (49,700 mi).^[4]

Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of basalt and gabbro.

The rocks making up the crust below the seafloor are youngest at the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of decompression melting in the underlying mantle.^[5]

The oceanic crust is made up of rocks much younger than the Earth itself: oceanic crust in the ocean basins is everywhere less than 200 million years old. The crust is in a constant state of "renewal" at the ocean ridges. Moving away from the mid-ocean ridge, ocean depth progressively increases; the greatest depths are in ocean trenches. As the oceanic crust moves away from the ridge axis, the peridotite in the underlying mantle cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere.

Slow-spreading ridges like the Mid-Atlantic Ridge have large, rift valleys that are sometimes as wide as 10-20 km, and very rugged terrain at the ridge crest. By contrast, fast-spreading ridges like the East Pacific Rise are narrow, sharp incisions surrounded by generally flat topography, which slopes away from the ridge over many hundreds of miles.