DEFINITION OF OCEANIC CRUST

The distinctive type of crust that is produced

at mid-ocean ridges and underlies the ocean basins is known as

oceanic crust. About 70 percent of the Earth’s surface is covered

by oceanic crust. It is rich in iron and magnesium minerals

such as olivine and pyroxene and is also known as sima or

simatic crust. Most oceanic crust is 3–6 miles (5–10 km) thick,

thickening considerably by cooling from where it is created by

magmatic upwelling at the mid-ocean ridges as part of the

seafloor spreading process. Oceanic crust has an average density

of 3.0 grams per cubic centimeter, and an average seismic Pwave

velocity of 3.85 miles per second (6.2 km/s).

Since most oceanic crust is produced by broadly similar

processes, it has a characteristic general layered structure that

can be recognized on the basis of seismic refraction studies.

Geologists have compared the seismic velocities of rock samples

from ophiolites and oceanic crust and concluded that

many are broadly similar. This observation has been used to

suggest that ophiolites are pieces of oceanic crust tectonically

emplaced onto the continents and also to use the geology of

ophiolites to infer details about the deep parts of oceanic

crust. Layer 1 of the oceanic crust consists of deep-sea sediments,

underlain by layer 2A, consisting of up to several kilometers

of pillow basalts and mafic dikes. Layer 2B, where

present, is interpreted to represent a sheeted dike complex

that may also be several kilometers thick, underlain by Layer

3, consisting of gabbro and gabbro cumulate rocks. These

layers are underlain by a layer marking a sharp increase in

seismic velocity known as the seismic Moho, interpreted from

ophiolite studies to represent the transition from cumulate

gabbros to denser cumulate ultramafic rocks. Since there is a

sharp seismic boundary at this transition, but the processes of

crystal accumulation from a melt are similar across the

boundary, a separate and deeper petrological Moho is distinguished.

This is marked at the boundary from crystal cumulate

rocks to mantle rocks from which the melts that formed

the overlying crust were extracted. This petrological Moho is

not determinable seismically but only from ophiolite or deep

drilling studies.

See also DIVERGENT OR EXTENSIONAL BOUNDARIES; OCEAN

BASIN; OPHIOLITES; PLATE TECTONICS.

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