Oceanic crust is the part of Earth's
lithosphere that surfaces in the
ocean basins. 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, having a mean density of about 3.3
grams per
cubic centimeter.
Composition
Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of
ophiolites (sections of oceanic crust that are preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by
submersibles, dredging (especially from
ridge crests and
fracture zones) and drilling. Oceanic crust is significantly simpler than
continental crust and generally can be divided in three layers.
- 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......
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