The crust, the uppermost layer of the solid Earth, is a region of interaction between surface processes brought about by the heat of radioactive reactions deep in the Earth. It is the most complex layer of the lithosphere in its physical and chemical nature.
The Earth's crust contains a wide variety of rock types, ranging from sedimentary rocks dominated by single minerals, such as sandstone (which is mainly silica) and limestone (which is mainly calcite), to the mineral-chemical mixtures of igneous rocks such as basalt lavas and granite intrusions.
The crust is divided into ocean crust and continental crust. The average height of the two differs by about 4.5km and the difference in their average total thickness is more exaggerated (continental crust is about 40km thick, and oceanic crust about 7km).
The boundary between the crusts and the mantle is almost everywhere defined sharply by the Mohorovicic seismic discontinuity. But there the similarity between oceanic and continental crust ends: they contrast strongly in structure, composition, average age, origin and evolution.
Vertical sections of both types of crust have been studied in zones of uplift caused by colliding tectonic plates. Combined with seismic evidence, these sections provide a unified view of crustal structure and composition.
Oceanic Crust
Seismic studies of the ocean crust and upper mantle have identified four separate layers characterized by downward increases in wave propagation velocity, density and thickness.
The upper two layers were studied by the Deep Sea Drilling Project which ran from 1968 tom 1984, whereas all that was known about the third and fourth layers had come only from ophiolites - uplifted ocean crust sections that are exposed on the Earth's surface. In 2004 the original drilling project evolved through the ODP and was transformed into the Integrated Ocean Drilling Program.
The top layer of the ocean crust, with an average thickness of 0.5km, comprises sedimentary muds (pelagic clays). They include the finest particles that were eroded from continents, and biochemically-precipitated carbonate and siliceous deposits.
The bottom three layers are made up of igneous materials formed during ocean-ridge processes. The chemical composition of these layers is that of basic igneous rocks, but their physical characteristics vary.
The second layer, with an average thickness of 1.7km, consists of basalt pillow lavas that were originally quenched by seawater when they erupted onto the sea floor. At the boundary between the second and third layers, the lavas give way to sheeted complexes of almost vertical dykes. These features, which have an average vertical thickness of 1.8km, are followed by a thick (3km) sequence of layered, coarse-grained, intrusive gabbros that must have cooled and crystallized slowly at a depth, with early formed crystals segregating into layers.
The bottom layer includes an upper portion of layered peridotite which grades downwards into unlayered mantle peridotite.
Both layered peridotites and gabbros probably represent a fossilized magma chamber, which was originally created by the partial melting of the mantle beneath an ocean ridge. Molten material was probably ejected from the chamber roof, forming dykes that fed the pillow lava eruptions of the second layer. The Mohorovicic discontinuity lies between the two deepest layers.
Continental Crust
In terms of seismic structure, the Earth's continental crust is much less regular than the ocean crust. A diffuse boundary called the Conrad discontinuity occurs between the upper and lower continental crusts at a depth of between 15 and 25km.
The upper continental crust has a highly variable topmost layer which is a few kilometers thick and comprises relatively unmetamorphosed volcanic and sedimentary rocks. Most of the sedimentary rocks were laid down in shallow marine conditions and subsequently uplifted. Beneath this superficial layer of the upper crust, most of the rock is similar in composition to granodiorite or diorite and is made up of intermediate, coarse-grained intrusive, igneous rocks.
The total thickness of the upper continental crust reaches a maximum of about 25km in zones of recent crustal thickening caused by igneous activity (such as the Andes mountain range in South America) and by tectonic over-thrusting during collision (such as the Alps and Himalayas in Europe and Asia).
This crust is of minimum thickness (about 15km) in the ancient continental cratonic shield areas, where igneous rocks themselves have been metamorphosed to form granite gneisses.
The lower continental crust extends down to the Mohorovicic discontinuity and comprises denser rocks that may otherwise have a similar composition to those of the upper crust. They include intermediate igneous rocks that have suffered intense metamorphism at high pressures, resulting in the growth of dense minerals and basic igneous rocks at lower degrees of metamorphism. This region is the least well-known, most inaccessible part of the Earth's crust.
Age and Growth of Earth's Crust
All of the ocean crust is approximately the same age (less than 180 million years) because it is continuously created at ocean ridges and destroyed at destructive plate margins. In contrast, continental crust yields a spectrum of ages ranging back over most of the Earth's history.
Ancient Precambrian igneous and sedimentary rocks occur, frequently in a strongly metamorphosed and deformed state, in stable ancient cratons such as the Canadian and Baltic shields.
Cratons are large masses of Precambrian rock that have been unaffected by later mountain-building. Younger rocks surround them, developed around the cratons by processes analogous to those at modern active plate margins in which partial melting of the sinking ocean plate yields magmas that rise to build mountain ranges like the Andes.
Continents may be considered to grow by a two stage partial melting process: when the mantle beneath ocean ridges melts to form ocean crust, and when oceanic crust beneath destructive plate margins melts to form continental crust. Such processes have probably occurred throughout the Earth's history but at a declining rate because the vigor of mantle convection and plate motions depends on the output of radioactive heat sources, which is declining.