Oceanic Crust

the upper part of the oceanic lithosphere down to the MOHOROVICIC DISCONTINUITY. It is formed of several layers commencing with a top layer of sediment which may be thin or absent (as over oceanic ridges). However, near to continental shelves, sediment may accumulate in thicknesses up to 2 or 3 kilometres. Beneath this is a layer of LAVAS (basaltic) and dykes which together are about 2 km thick. The next layer is approximately 5 km (3 miles) thick and its composition is similar to that of gabbro, a coarse-grained basic igneous rock. The base of this layer approaches the MANTLE in composition. These layers seem to remain remarkably constant between ocean basins.

Taken from Dictionary of Science