<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> <P> The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low - potassium tholeiitic magmas . These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements . There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland . </P>

The lithosphere of the earth is generally thinner at or near what type of plate boundaries