<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 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 . </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> <P> 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 (sections of oceanic crust that are thrust onto and preserved on the continents), 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 (especially from ridge crests and fracture zones) and drilling . Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers . According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle . </P>

Explain the difference in geologic age of the continents and the ocean floor