Oceanic crust is formed at the mid-ocean ridge crest by extrusive and intrusive solidification of basaltic magmas. These processes result in a uniform, layered ocean crust that is approximately 5 km thick throughout the ocean basins. The layers are:
A surface layer of extruded pillow basalts.
A layer of sheeted basaltic dikes, resulting from solidification of magma in vertical conduits.
Two layers of gabbros representing solidification of material from the magma chamber and accumulation of crystals on the bottom of the magma chamber.