What is the oceanic crust?

The oceanic crust is the outermost layer of Earth's lithosphere found under the oceans. It is created at spreading centers on oceanic ridges , which occur at divergent plate boundaries. Oceanic crust is thinner and denser than continental crust , averaging about 6 km (4 miles) in thickness with a density of about 2.9 to 3 grams per cubic cm. It comprises several layers, including basalt lavas, sheeted dikes, and gabbros. The oceanic crust lies atop Earth’s mantle. The study of ophiolites , or slices of the ocean floor thrust above sea level by plate tectonics, helps scientists understand the structure and composition of the oceanic crust.

How is oceanic crust different from continental crust?

Oceanic and continental crusts are the outermost layers of Earth's lithosphere, but they differ in composition, density, and thickness. Oceanic crust, found under the oceans, is basaltic, denser (about 2.9 to 3 grams per cubic cm), and thinner, averaging about 6 km thick. Continental crust is granitic, less dense (about 2.7 grams per cubic cm), and much thicker, typically around 40 km thick. The density difference causes the less-dense continental crust to float higher on the mantle, resulting in different average elevations.

Where is oceanic crust located on Earth?

Oceanic crust is the outermost layer of Earth's lithosphere found under the oceans. It is created at spreading centers on oceanic ridges , which occur at divergent plate boundaries. Examples of oceanic plates include the Pacific Plate, as well as the oceanic crust between North America and the Mid-Atlantic Ridge on the North American Plate. Oceanic crust differs from continental crust because it is thinner, denser, younger, and has a different chemical composition. New oceanic crust is consistently formed, which means that older crust must be destroyed elsewhere. This occurs at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends, or is subducted, beneath another.

How is new oceanic crust formed?

New oceanic crust forms at divergent plate boundaries through a process called seafloor spreading. As plates move apart, pressure release causes partial melting of the underlying mantle. This generates basaltic magma, which is buoyant and wells up to the surface. The magma cools and solidifies, creating new crust. This process is accompanied by volcanic activity and shallow earthquakes as the crust repeatedly rifts and heals. The age of the oceanic crust increases with distance from the spreading center, with seafloor-spreading rates varying from 0.1 cm to 17 cm per year.

What minerals and rocks are commonly found in oceanic crust?

Oceanic crust, the outermost layer of Earth's lithosphere under the oceans, is primarily composed of basalt and gabbro . The topmost layer includes basalt lavas, often as pillow lavas and sheet flows. These basalts are rich in iron, magnesium, and plagioclase . Below the lava, sheeted dikes of basaltic composition act as magma pathways. Deeper in the crust, gabbros with coarser mineral grains are found. The lower gabbro layer contains olivine , an iron-magnesium silicate. The mantle rock beneath the oceanic crust consists of peridotite , mainly olivine, with some pyroxene and amphibole.

How does oceanic crust change as it moves away from a mid-ocean ridge?

As oceanic crust moves away from a mid-ocean ridge , where new crust is formed from upwelling magma, it undergoes several changes as part of the process of seafloor spreading . The oceanic crust cools and becomes denser with age, causing it to sink. As the crust moves away from the ridge, sediments accumulate on top of the basaltic lavas. The depth of the ocean floor increases as the crust ages due to thermal contraction. The oceanic crust can eventually subduct under continental plates or younger oceanic crust at convergent plate boundaries.

oceanic crust

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Images & Videos

Cross-sectional diagram of Earth's outer layers, showing oceanic and continental crust at the surface, the rigid lithosphere, the partially melted asthenosphere, and transitions from upper to lower mantle, with labeled depths and rock properties.

Crew members aboard a drilling ship inspecting a rock core during a scientific expedition that succeeded for the first time in drilling through the upper oceanic crust.

Cross-sectional diagram illustrating plate tectonics, showing oceanic and continental crust moving over the asthenosphere. It highlights divergent boundaries at oceanic ridges where crust forms, convergent boundaries at trenches with subduction zones where crust is destroyed, and a transform boundary with a fault. Features such as island arcs, rising magma, and hotspots are labeled.

A world map uses color bands to show the age of Earth's oceanic crust in millions of years, with red marking the youngest crust along mid-ocean ridges and blue representing the oldest crust farther from spreading centers.

Diagram showing seafloor spreading at a midocean ridge, with arrows indicating tectonic plates moving apart. Magma rises to form new seafloor, and bands of normal and reversed magnetic polarity are labeled on either side of the ridge.

How does plate tectonics change Earth's surface?

oceanic crust

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Top Questions

What is the oceanic crust?
The oceanic crust is the outermost layer of Earth's lithosphere found under the oceans. It is created at spreading centers on oceanic ridges, which occur at divergent plate boundaries.

Oceanic crust is thinner and denser than continental crust, averaging about 6 km (4 miles) in thickness with a density of about 2.9 to 3 grams per cubic cm. It comprises several layers, including basalt lavas, sheeted dikes, and gabbros. The oceanic crust lies atop Earth’s mantle.

The study of ophiolites, or slices of the ocean floor thrust above sea level by plate tectonics, helps scientists understand the structure and composition of the oceanic crust.
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How is oceanic crust different from continental crust?
Oceanic and continental crusts are the outermost layers of Earth's lithosphere, but they differ in composition, density, and thickness. Oceanic crust, found under the oceans, is basaltic, denser (about 2.9 to 3 grams per cubic cm), and thinner, averaging about 6 km thick. Continental crust is granitic, less dense (about 2.7 grams per cubic cm), and much thicker, typically around 40 km thick. The density difference causes the less-dense continental crust to float higher on the mantle, resulting in different average elevations.
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Where is oceanic crust located on Earth?
Oceanic crust is the outermost layer of Earth's lithosphere found under the oceans. It is created at spreading centers on oceanic ridges, which occur at divergent plate boundaries. Examples of oceanic plates include the Pacific Plate, as well as the oceanic crust between North America and the Mid-Atlantic Ridge on the North American Plate.

Oceanic crust differs from continental crust because it is thinner, denser, younger, and has a different chemical composition. New oceanic crust is consistently formed, which means that older crust must be destroyed elsewhere. This occurs at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends, or is subducted, beneath another.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How is new oceanic crust formed?
New oceanic crust forms at divergent plate boundaries through a process called seafloor spreading. As plates move apart, pressure release causes partial melting of the underlying mantle. This generates basaltic magma, which is buoyant and wells up to the surface. The magma cools and solidifies, creating new crust. This process is accompanied by volcanic activity and shallow earthquakes as the crust repeatedly rifts and heals. The age of the oceanic crust increases with distance from the spreading center, with seafloor-spreading rates varying from 0.1 cm to 17 cm per year.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What minerals and rocks are commonly found in oceanic crust?
Oceanic crust, the outermost layer of Earth's lithosphere under the oceans, is primarily composed of basalt and gabbro.

The topmost layer includes basalt lavas, often as pillow lavas and sheet flows. These basalts are rich in iron, magnesium, and plagioclase. Below the lava, sheeted dikes of basaltic composition act as magma pathways.

Deeper in the crust, gabbros with coarser mineral grains are found. The lower gabbro layer contains olivine, an iron-magnesium silicate. The mantle rock beneath the oceanic crust consists of peridotite, mainly olivine, with some pyroxene and amphibole.
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How does oceanic crust change as it moves away from a mid-ocean ridge?
As oceanic crust moves away from a mid-ocean ridge, where new crust is formed from upwelling magma, it undergoes several changes as part of the process of seafloor spreading.

The oceanic crust cools and becomes denser with age, causing it to sink. As the crust moves away from the ridge, sediments accumulate on top of the basaltic lavas. The depth of the ocean floor increases as the crust ages due to thermal contraction. The oceanic crust can eventually subduct under continental plates or younger oceanic crust at convergent plate boundaries.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

oceanic crust, the outermost layer of Earth’s lithosphere that is found under the oceans and formed at spreading centres on oceanic ridges, which occur at divergent plate boundaries.

Oceanic crust is about 6 km (4 miles) thick. It is composed of several layers, not including the overlying sediment. The topmost layer, about 500 metres (1,650 feet) thick, includes lavas made of basalt (that is, rock material consisting largely of plagioclase [feldspar] and pyroxene). Oceanic crust differs from continental crust in several ways: it is thinner, denser, younger, and of different chemical composition. Like continental crust, however, oceanic crust is destroyed in subduction zones.

The lavas are generally of two types: pillow lavas and sheet flows. Pillow lavas appear to be shaped exactly as the name implies—like large overstuffed pillows about 1 metre (3 feet) in cross section and 1 to several metres long. They commonly form small hills tens of metres high at the spreading centres. Sheet flows have the appearance of wrinkled bed sheets. They commonly are thin (only about 10 cm [4 inches] thick) and cover a broader area than pillow lavas. There is evidence that sheet flows are erupted at higher temperatures than those of the pillow variety. On the East Pacific Rise at 8° S latitude, a series of sheet flow eruptions (possibly since the mid-1960s) have covered more than 220 square km (85 square miles) of seafloor to an average depth of 70 metres (230 feet).

Below the lava is a layer composed of feeder, or sheeted, dikes that measures more than 1 km (0.6 mile) thick. Dikes are fractures that serve as the plumbing system for transporting magmas (molten rock material) to the seafloor to produce lavas. They are about 1 metre (3 feet) wide, subvertical, and elongate along the trend of the spreading centre where they formed, and they abut one another’s sides—hence the term sheeted. These dikes also are of basaltic composition. There are two layers below the dikes totaling about 4.5 km (3 miles) in thickness. Both of these include gabbros, which are essentially basalts with coarser mineral grains. These gabbro layers are thought to represent the magma chambers, or pockets of lava, that ultimately erupt on the seafloor. The upper gabbro layer is isotropic (uniform) in structure. In some places this layer includes pods of plagiogranite, a differentiated rock richer in silica than gabbro. The lower gabbro layer has a stratified structure and evidently represents the floor or sides of the magma chamber. This layered structure is called cumulate, meaning that the layers (which measure up to several metres thick) result from the sedimentation of minerals out of the liquid magma. The layers in the cumulate gabbro have less silica but are richer in iron and magnesium than the upper portions of the crust. Olivine, an iron-magnesium silicate, is a common mineral in the lower gabbro layer.

The oceanic crust lies atop Earth’s mantle, as does the continental crust. Mantle rock is composed mostly of peridotite, which consists primarily of the mineral olivine with small amounts of pyroxene and amphibole.

Investigations of the oceanic crust

Knowledge of the structure and composition of the oceanic crust comes from several sources. Bottom sampling during early exploration brought up all varieties of the above-mentioned rocks, but the structure of the crust and the abundance of the constituent rocks were unclear. Simultaneously, seismic refraction experiments enabled researchers to determine the layered nature of the oceanic crust. These experiments involved measuring the travel times of seismic waves generated by explosions (such as dynamite blasts) set off over distances of several tens of kilometres. The results of early refraction experiments revealed the existence of two layers beneath the sediment cover. More sophisticated experiments and analyses led to dividing these layers into two parts, each with a different seismic wave velocity, which increases with depth. The seismic velocity is a kind of fingerprint that can be attributed to a limited number of rock types. Sampled rock data and seismic results were combined to yield a model for the structure and composition of the crust.

Study of ophiolites

crustal generation and destructionThree-dimensional diagram showing crustal generation and destruction according to the theory of plate tectonics; included are the three kinds of plate boundaries—divergent, convergent (or collision), and strike-slip (or transform).

Great strides in understanding the oceanic crust were made by the study of ophiolites. These are slices of the ocean floor that have been thrust above sea level by the action of plate tectonics. In various places in the world, the entire sequence of oceanic crust and upper mantle is exposed. These areas include, among others, Newfoundland and the Pacific Coast Ranges of California, the island of Cyprus in the Mediterranean Sea, and the mountains in Oman on the southeastern tip of the Arabian Peninsula. Ophiolites reveal the structure and composition of the oceanic crust in astonishing detail. Also, the process of crustal formation and hydrothermal circulation, as well as the origin of marine magnetic anomalies, can be studied with comparative clarity. Although it is clear that ophiolites are of marine origin, there is some controversy as to whether they represent typical oceanic crust or crust formed in settings other than an oceanic spreading centre—behind island arcs, for example.

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The age of the oceanic crust does not go back farther than about 200 million years. Such crust is being formed today at oceanic spreading centres. Many ophiolites are much older than the oldest oceanic crust, demonstrating continuity of the formation processes over hundreds of millions of years. Methods that may be used to determine the age of the crustal material include direct dating of rock samples by radiometric dating (measuring the relative abundances of a particular radioactive isotope and its daughter isotopes in the samples) or by the analyses of fossil evidence, marine magnetic anomalies, or ocean depth. Of these, magnetic anomalies deserve special attention.

A marine magnetic anomaly is a variation in strength of Earth’s magnetic field caused by magnetism in rocks of the ocean floor. Marine magnetic anomalies typically represent 1 percent of the total geomagnetic field strength. They can be stronger (“positive”) or weaker (“negative”) than the average total field. Also, the magnetic anomalies occur in long bands that run parallel to spreading centres for hundreds of kilometres and may reach up to a few tens of kilometres in width.

oceanic crust

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Investigations of the oceanic crust

Study of ophiolites