Oceanic crust is the uppermost layer of the oceanic portion of thetectonic plates. It is composed of the upper oceanic crust, withpillow lavas and adike complex, and thelower oceanic crust, composed oftroctolite,gabbro andultramaficcumulates.^[1][2] The crust overlies the rigid uppermost layer of themantle. The crust and the rigid upper mantle layer together constitute oceaniclithosphere.

Oceanic crust is primarily composed ofmafic rocks, orsima, which is rich in iron and magnesium. It is thinner thancontinental crust, orsial, generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.^[3]

The crust uppermost is the result of the cooling of magma derived frommantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidifiedcrystal mush derived from earlier injections, forming magma lenses that are the source of thesheeted dikes that feed the overlying pillow lavas.^[4] As the lavas cool they are, in most instances, modified chemically by seawater.^[5] These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known asflood basalt eruptions. But mostmagmacrystallises at depth, within thelower oceanic crust. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.^[6]

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses ofophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of theseismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor bysubmersibles, dredging (especially fromridge crests andfracture zones) and drilling.^[7] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers.^[8] According tomineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.^[9]

• Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidatedsediments, usually thin or even not present near themid-ocean ridges but thickens farther away from the ridge.^[10] Near the continental margins sediment isterrigenous, meaning derived from the land, unlike deep sea sediments which are made of tinyshells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenoussediments transported byturbidity currents.^[11]
• Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystallinebasalt usually in the form ofpillow basalt, and layer 2B – 1.5 km thick layer composed ofdiabasedikes.^[12]
• Layer 3 is formed by slow cooling ofmagma beneath the surface and consists of coarse grainedgabbro andcumulateultramafic rocks.^[13] It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.^[14]

The most voluminousvolcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-potassiumtholeiitic magmas. These rocks have low concentrations of large ionlithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highlyincompatible elements. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridgehot spots such as surroundings ofGalapagos Islands, theAzores andIceland.^[15]

Prior to theNeoproterozoic Era 1000Ma ago the world's oceanic crust was moremafic than present-days'. The more mafic nature of the crust meant that higher amounts of water molecules (OH) could be stored thealtered parts of the crust. Atsubduction zones this mafic crust was prone to metamorphose intogreenschist instead ofblueschist at ordinaryblueschist facies.^[16]

Oceanic crust is continuously being created at mid-ocean ridges. Ascontinental plates diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges.^[17]

As the mantle rises it cools and melts, as the pressure decreases and it crosses thesolidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr⁻¹ half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is theGakkel Ridge under theArctic Ocean. Thicker than average crust is found aboveplumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this isIceland which has crust of thickness ~20 km.^[18]

The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it.^[19] The oceanic lithospheresubducts at what are known asconvergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.^[20]The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as theWilson Cycle.

The oldest large-scale oceanic crust is in the westPacific and north-westAtlantic — both are about up to 180-200 million years old. However, parts of the easternMediterranean Sea could be remnants of the much olderTethys Ocean, at about 270 and up to 340 million years old.^[21][22][23]

The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in thebasalt. A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge.^[24] New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock,its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity.

• Continental crust
• Lithosphere
• Mohorovičić discontinuity
• Plate tectonics
• Seabed 2030
• Seafloor depth versus age

• Marshak, Stephen (2005).Earth: Portrait of a Planet. pp. 41–87.
• McDuff, Russell E.; Heath, G. Ross."Ocean 540: Oceanic Lithosphere; Plate Tectonics; Seafloor Topography". School of Oceanography, University of Washington. Retrieved9 August 2009.