Well, I think mostly it is oceanic crust well because <span>The scientist noticed a symmetrical pattern of positive and negative magnetic lines as they moved along the ocean floor, and the line of symmetry was at the mid-ocean ridge. that why it is seen as the youngest on our earth</span>
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. 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 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.
(Copied and pasted because my friend wrote this on FaceBook. Hopefully this helps!)
