Crust (geology)

Earth cutaway from core to exosphere.

In geology, a crust is the outermost layer of a planet, part of its lithosphere. Planetary crusts are generally composed of a less dense material than its deeper layers. The crust of the Earth is composed mainly of basalt and granite. It is cooler and more rigid than the deeper layers of the mantle and core.

On stratified planets, such as Earth, the lithosphere is floating on fluid interior layers. Because of convection in the plastic, although non-molten, upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move. Oceanic crust is different from continental. The oceanic crust (sima) is 5 to 10 km thick and is composed primarily of basalt. The continental crust (sial) is 20-70 km deep and is composed of a variety of less dense rocks. The crust's temperature ranges from the air temperature to about 900°C near the upper mantle. In all Earth's crust occupies less than 1% of Earth's volume.

Origin of the Earth's crust

Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.4 billion years ago. The primordial crust was very thin, and was likely recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system. There is a theory that the Moon was formed by one such very large impact.

The Earth has likely always had some form of basaltic oceanic crust, but there is evidence it has also had continental style crust for as long as 3.8 to 3.9 billion years. The oldest crust on Earth is the Narryer Gneiss Terrane in Western Australia at 3.9 billion years, and certain parts of the Canadian Shield and the Fennoscandian Shield are also of this age.

The majority of the current Earth's continental crust was formed primarily between 4.6 billion years and 3.9 billion years ago, in the Hadean. The vast majority of rocks of this age are located in cratons where the crust is up to 70km thick. The lower density of the continental crust as compared to the oceanic crust prevents it being destroyed by subduction. Crust formation is linked to periods of intense orogeny or mountain building; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms not so much by accumulation of granite and metamorphic fold belts, but by depletion of the mantle to form buoyant lithospheric mantle.

Composition of the crust

The common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. F. W. Clarke has calculated that a little more than 47% of the Earth's crust consists of oxygen. It occurs principally in combination as oxides, of which the chief are silicon, aluminium, iron, calcium, magnesium, potassium and sodium oxides. Silica is a major constituent of the crust occurring as the silicate minerals, which are the most common minerals of igneous and metamorphic rocks. From a computation based on 1672 analyses of all kinds of rocks Clarke arrived at the following as the average percentage composition by mass:

Plates in the crust of the earth, according to the plate tectonics theory

Oxide	Percent
SiO2	59.71
Al2O3	15.41
CaO	4.90
MgO	4.36
Na2O	3.55
FeO	3.52
K2O	2.80
Fe2O3	2.63
H2O	1.52
TiO2	0.60
P2O5	0.22
total	99.22

All the other constituents occur only in very small quantities, and total less than 1%.

See also

• Continental drift
• Plate tectonics

External links

• USGS Crust Thickness Map