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
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......