Seafloor 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 result 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 slopes.
 
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 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.
 
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. 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 side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern.
 
Geographic Features
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 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, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from oceanic lithosphere to 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 oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense. 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 where 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 new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant.
seafloor spreading
Earth's newest crust is created at sites of seafloor spreading—red sites on this map.
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 result of plate tectonics in the Afar Triple Junction.
active plate margin
Noun

convergent tectonic plate boundary where an oceanic plate is crashing into a continental plate.

basalt
Noun

type of dark volcanic rock.

churn
Verb

to mix vigorously or violently.

Noun

steep wall of rock, earth, or ice.

consistent
Adjective

maintaining a steady, reliable quality.

Noun

one of the seven main land masses on Earth.

continental crust
Noun

thick layer of Earth that sits beneath continents.

Noun

the movement of continents resulting from the motion of tectonic plates.

convection current
Noun

movement of a fluid from a cool area to a warm area.

Noun

the extremely hot center of Earth, another planet, or a star.

Noun

rocky outermost layer of Earth or other planet.

dense
Adjective

having parts or molecules that are packed closely together.

diameter
Noun

width of a circle.

disprove
Verb

to prove wrong.

divergent boundary
Noun

area where two or more tectonic plates are moving away from each other. Also called an extensional boundary.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

fast spreading center
Noun

mid-ocean ridge where seafloor spreading is occuring at more than 100 millimeters (4 inches) a year.

fault
Noun

a crack in the Earth's crust where there has been movement.

fracture
Verb

to break.

frigid
Adjective

very cold.

geologic
Adjective

having to do with the physical formations of the Earth.

geologist
Noun

person who studies the physical formations of the Earth.

Noun

rock formed by the cooling of magma or lava.

Noun

outer, solid portion of the Earth. Also called the geosphere.

Noun

molten, or partially melted, rock beneath the Earth's surface.

magnetic field
Noun

area around and affected by a magnet or charged particle.

Noun

force by which objects attract or repel one another.

magnetometer
Noun

scientific instrument used to measure the presence, strength, and direction of Earth's magnetic field.

Noun

middle layer of the Earth, made of mostly solid rock.

mantle convection
Noun
slow movement of Earth's solid mantle caused by convection currents transferring heat from the interior of the Earth to the surface.
Mid-Atlantic Ridge
Noun

underwater mountain range that runs from Iceland to Antarctica.

mid-ocean ridge
Noun

underwater mountain range.

molten
Adjective

solid material turned to liquid by heat.

mountain range
Noun

series or chain of mountains that are close together.

ocean basin
Noun

depression in the Earth's surface located entirely beneath the ocean.

oceanic crust
Noun

thin layer of the Earth that sits beneath ocean basins.

Noun

a long, deep depression in the ocean floor.

Paleozoic Era
Noun

about 541-252 million years ago.

Pangaea
Noun

supercontinent of all the Earth's landmass that existed about 250 million years ago.

passive plate margin
Noun

lithospheric region where oceanic crust transitions to continental crust without faults or subduction zones.

plastic
Noun

chemical material that can be easily shaped when heated to a high temperature.

plate tectonics
Noun

movement and interaction of the Earth's plates.

predict
Verb

to know the outcome of a situation in advance.

Noun

depression in the ground caused by the Earth's crust spreading apart.

Noun

horseshoe-shaped string of volcanoes and earthquake sites around edges of the Pacific Ocean.

Noun

rift in underwater mountain range where new oceanic crust is formed.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

seawater
Noun

salty water from an ocean or sea.

Noun

solid material transported and deposited by water, ice, and wind.

slow spreading center
Noun

mid-ocean ridge where seafloor spreading is occuring at less than 55 millimeters (2 inches) a year.

subduct
Verb

to pull downward or beneath something.

subduction zone
Noun

area where one tectonic plate slides under another.

supercontinent
Noun

ancient, giant landmass that split apart to form all the continents we know today.

symmetrical
Adjective

having the same arrangement of parts on either side.

tectonic activity
Noun

movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

tectonic plate
Noun

massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

theorize
Verb

to formulate and propose a group of ideas to explain a scientific question.

transition
Noun

movement from one position to another.

transitional crust
Noun

lithospheric region where oceanic crust transitions into continental crust.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

volume
Noun

space an object occupies.