Oceanic crust

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Age of oceanic crust

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. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is more dense, having a mean density of about 3.3 grams per cubic centimeter.

1 Composition
2 Life Cycle
3 Magnetic Lines
4 References
5 See also

[edit] Composition

Above the oceanic crust is a thin layer of sediment, which may or may not be present, but which at its thickest is less than 1 km thick. Much of the sediment is clay and the shells of dead plankton. When present, the sediment is usually thin near the mid-ocean ridges and thickens farther away from the ridge. Beneath the layer of sediment are layers of basaltic rocks. Near the surface these are fine grained basalts, tending towards the coarser grained gabbros at depth. These mafic rocks are composed of elements that are denser than the granite and metamorphic rocks that are common on continents.

[edit] Life Cycle

Oceanic crust generally does not last longer than 200 million years. It is continuously being created at oceanic ridges. At these ridges, hot magma rises into the crust and cools, pushing the crust apart at the ridge. The continuous formation of new oceanic crust pushes the older crust away from the mid-ocean ridge. As it moves away from the ridge, the crust becomes cooler and denser, while the sediment may build on top of it.

The oceanic crust subducts toward the mantle at what are known as convergent boundaries. These boundaries can exist between two oceanic plates or between an oceanic and a continental plate. At oceanic plate-continental plate boundaries, the oceanic plate always subducts because the continental crust is less dense. The subduction process is why oceanic crust lasts a maximum of 200 million years. Plate tectonics is the study of these processes.

[edit] Magnetic Lines

Main article: Seafloor spreading

The oceanic crust displays an interesting pattern of parallel magnetic lines, parallel to the ocean ridges, frozen in the basalt. In the 1950’s, scientists mapped the magnetic field generated by rocks on the ocean floor. They noticed a symmetrical pattern of positive and negative magnetic lines as they moved along the ocean floor, and the line of symmetry was at the mid ocean ridge. The fact that the anomalies were symmetrical at the mid-ocean ridge was explained by the hypothesis that new rock was being formed by magma at the mid-ocean ridges, and the ocean floor was spreading out from this point. When the magma cooled to form rock, it aligned itself with the current position of the north magnetic pole of the Earth (which has reversed many times in its past) at the time of its cooling. New magma forced the older cooled magma away from the ridge. Approximately half of the new rock was formed on one side of the ridge and half on the other.