It is largely made up of magma which has risen to the surface, cooled and solidified. Although a complete section of oceanic crust has not yet been drilled, there are estimations of composition based on analyses of ophiolites, comparison of 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 it can be divided in three layers. Layer 1 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 organisams, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents. [1] Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystaline basalt usually in form of pillow basalt, and layer 2B – 1.5 km thick layer composed of diabase dikes. 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.