<P> Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate . 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 (er 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 . </P> <P> The crust is the result of erupted mantle material originating from below the plate, cooled and 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 . </P>

Earth's crust under oceanic is composed of this rock type