海洋地壳是岩石圈的一部分，由密度较大的硅镁质岩石构成，偏向碱性，与大陆地壳相比，硅酸盐较缺乏，密度也较大，平均密度约3.0（大陆地壳2.7），由于密度较大，根据大陆均衡学说，海洋地壳无法像大陆地壳般在地幔之上浮得那么高。

主要是由玄武岩组成。海洋地壳的厚度约在5至10公里之间，地球内部由于热的作用产生对流，岩浆上升处，是在地表张裂板块，产生分离板块边缘（divergent boundaries），中洋脊是为代表，该地区会有许多浅的、正断层（张裂作用）式的小地震。大部分情况下，和板块碰撞时隐没，因此地质年龄也较年轻，现存的海洋地壳年龄都在200百万年之内。在中洋脊由深部岩浆加进来，所产生的是为海洋板块，在浅部都是玄武岩，深部则为辉长岩。

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 comprise 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.