<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> <P> Oceanic crust is continuously being created at mid-ocean ridges . As plates diverge at these ridges, magma rises into the upper mantle and crust . As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it . The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges . </P>

What is the temperature of the earth's oceanic crust