Seismic data, dredging, and drilling have shown that the oceanic crust is divided into three parts. The top layer consists of terrigenous or pelagic sediment that averages about 0.5 kilometer in thickness. These are typical sediments from the abyssal plain. The middle layer is composed of pillowed basalt flows and underlying, steeply dipping basaltic dikes. It is thought the dikes were the feeders that supplied the lava for the overlying flows. The middle layer is about 2 kilometers thick. The bottom layer is about 5 kilometers thick and thought to consist of swarms of basalt dikes and sill‐like gabbro intrusions. This bottommost section of oceanic crust lies on top of the mantle.
This interpretation for the bottom layer is based on the detailed study of ophiolites, which are mafic rock sequences at the earth's surface that are believed to be pieces of ancient oceanic crust that were thrust onto the continent (obduction) during mountain‐building. Larger, more complete ophiolite sequences show the same general layering as that shown by drilling and seismic studies of the ocean crust: pillowed lavas, vertical basalt dikes, and massive, coarsegrained gabbro. The combined thickness of the dikes and the gabbro units in big ophiolite complexes is about the same thickness as that indicated by seismic studies for the bottom layer.
In a complete ophiolite sequence the gabbro may be underlain by ultramafic rocks such as peridotite. If ophiolites are truly oceanic crust, this contact could then represent the Moho. The peridotite would then be a segment of the uppermost part of the mantle.