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- Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate.
- The crust and the solid mantle layer together comprise oceanic lithosphere.
- Oceanic crust is the result of erupted mantle material originating from below the plate, cooled and in most instances, modified chemically by seawater.
- This occurs mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions.
- It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however it is denser, having a mean density of about 2.9 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.
- 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 of ophiolites, comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging and drilling.
- Layer 1 is on an average 0.4 km thick.
- It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge.
- Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks.
- It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.
- These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements.
- The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.
- As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus.
- Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust.
- In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense.
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