<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> <P> As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus . The amount of melt produced depends only on the temperature of the mantle as it rises . Hence most oceanic crust is the same thickness (7 ± 1 km). Very slow spreading ridges (<1 cm yr half - rate) produce thinner crust (4--5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust . An example of this is the Gakkel Ridge under the Arctic Ocean . Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust . An example of this is Iceland which has crust of thickness ~ 20 km . </P>

How much of the earth's surface is oceanic crust