Oceanic crust Oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however it is denser,
Geology of the Iberian Peninsula a thinned piece of crust from another continent called the Meguma terrane. There are five units in the stack. At the lowest level are high pressure, low temperature metamorphosed rocks. Second is an ophiolite. Third is the lower part of a continental crust which has been metamorphosed to high temperature with high pressure. Fourth is a layer of sediments derived from weathering land with low-grade metamorphism. There is also an underlying Ediacaran and early Paleozoic layer called the autochthenous sequence. Metamorphism of the allochthenous nappe occurred 390–380 Ma in the Middle Devonian. This is possibly from the Rheic Ocean. Finally,