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]
Oceanic crust Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.[12]