earth's crust under oceanic is composed of this rock type
Oceanic crust 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[1] as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.[2]
Mid-ocean ridge Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of basalt (known as MORB for mid-ocean ridge basalt) and below it, gabbro.[1] They are formed by two oceanic plates moving away from each other. Hydrothermal vents are a common feature at oceanic spreading centers. [2]