<P> Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate . It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates . The crust overlies the solidified and uppermost layer of the mantle . The crust and the solid mantle layer together constitute oceanic lithosphere . </P> <P> Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium . It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter . </P> <P> The crust uppermost is the result of the cooling of magma derived from mantle material below the plate . The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas . As the lavas cool they are, in most instances, modified chemically by seawater . These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions . But most magma crystallises at depth, within the lower oceanic crust . There, newly intruded magma can mix and react with pre-existing crystal mush and rocks . </P>

Where is the youngest oceanic crust in the pacific ocean