Oceanic crust is formed at constructive margins or mid ocean spreading ridges and is around 10km thick.It last around 200ma before being subducted. It is mafic in composition which means its direct origins are from the mantle and is very high in iron and magnesian.
Ths mafic melt forms two main rock types MORB or mid-ocean ridge basalt, which erupts from the spreading ridge volcanoes and gabbro which is the crystalline version of basalt formed in the magma chamber below the ridge.
It is very difficult to research the ridge , submarines and seismic surveys being the main tools. However fragments called Ophiolites ,which have been scraped off onto the surface during collision, have helped geologists stay dry and still look at oceanic crust. The surface is composed of basaltic pillow lavas beneath this are sheeted dykes which supply the lava to the surface.Beneath this section is the solidified magma chamber made from crystalline gabbro and beneath this the mantle peridotites.