The oceanic crust is composed mostly of which substance. why?

It is composed of several layers, not including the overlying sediment. The topmost layer includes lavas made of basalt.

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 centimeteras opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.