<P> Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor . Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling . Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers . According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle . </P> <Ul> <Li> Layer 1 is on an average 0.4 km thick . It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge . Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents . </Li> <Li> Layer 2 could be divided into two parts: layer 2A--0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B--1.5 km thick layer composed of diabase dikes . </Li> <Li> Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks . It constitutes over two - thirds of oceanic crust volume with almost 5 km thickness . </Li> </Ul> <Li> Layer 1 is on an average 0.4 km thick . It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge . Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents . </Li> <Li> Layer 2 could be divided into two parts: layer 2A--0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B--1.5 km thick layer composed of diabase dikes . </Li>

The oldest sediments on earth are found at the center of ocean basins