The oceanic crust is primarily composed of volcanic rocks, and its composition can be generally described as follows:
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Basalt: The dominant rock type in the oceanic crust is basalt, which is a mafic (rich in magnesium and iron) igneous rock. Basalt forms from the rapid cooling of lava at the ocean floor and is often found in the form of pillow lavas, which result from underwater volcanic activity.
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Gabbro: Below the basalt layer, oceanic crust typically includes gabbro, which is a coarse-grained igneous rock that forms from the slow crystallization of magma beneath the surface. Gabbro is also mafic in composition and is often found in the deeper portions of the oceanic crust.
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Sediments: The oceanic crust is overlain by a layer of sediments, which can include clay, silt, sand, and various biological material such as shells and coral fragments. These sediments accumulate over time from the remains of marine organisms, as well as from terrestrial sediments that wash into the ocean.
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Minerals: The mineral composition of oceanic crust primarily includes plagioclase feldspar, pyroxene, and olivine, reflecting its mafic nature.
Overall, the oceanic crust is thinner than continental crust, averaging about 7 kilometers (4.3 miles) in thickness, and it tends to be denser, which is primarily due to its basaltic composition.