Oceanic crust is produced wherever hot mantle material comes into contact with the surface of the solid earth, so in a way it can be thought of as an atmospheric weathering product of the mantle. In this way the oceanic crust reflects much about the composition and motion of the underlying mantle. The oceanic crust, excluding the overlying sediments, is uniformly composed of rocks having a basaltic composition. It is however, found by worldwide seismic refraction experiments to consist of three layers of increasing velocity downwards. These differences reflect the differing origins of each layer, and their mechanical states.
All oceanic crust is produced at spreading centers by a very specialized process.
Hot mantle pyrolite in a thermal convection boundary layer partially melts near the surface due to the decrease in pressure. The exsolved basalt magma cools into gabbroic intrusions below 2 km depths, while large quantities erupt at the sea bed to flow into pillow lavas . The top of the magma chamber is being continually intruded by dikes taking magma to the surface. Because this is occurring at a spreading center, each dike is always split by a new one, forming a vast stack of vertically-oriented dikes called a sheeted dike complex . Sea water circulates into the cooling oceanic crust to depths of 3 km, driven by the heat of the magma intrusion.