Composition of the Continental Crust

Oxide	Percent
SiO2	60.6
Al2O3	15.9
CaO	6.4
MgO	4.7
Na2O	3.1
Fe as FeO	6.7
K2O	1.8
TiO2	0.7
P2O5	0.1

Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number. The rarest elements in the crust (shown in yellow) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated deeper into the Earth's core. Their abundance in meteoroid materials is higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.

This table shows the abundance of elements in Earth's crust. Numbers show percentage in mass. The continental crust has an average composition similar to that of the igneous rock, andesite. Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. Although the continental crust comprises only about 0.6 weight percent of the silicate Earth, it contains 20% to 70% of the incompatible elements.

All the other constituents except water occur only in very small quantities, and total less than 1%. Estimates of average density for the upper crust range between 2.69 g/cm³ and 2.74 g/cm³ and for lower crust between 3.0 g/cm³ and 3.25 g/cm³.

A more detailed and comprehensive list of elemental composition for the upper crust is given in the main article abundance of elements in Earth's crust.

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Key-word:

1. composition, 2. mantle;

Moon’s Crust

A particularly large meteorite is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a “lunar magma ocean.” Plagioclase feldspar crystallized in large amounts from this magma ocean and floated towards the surface. The cumulate rocks form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite): the lower part of the crust may contain a higher percent of ferromagnesian minerals such as the pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase. The underlying mantle is denser and is also olivine-rich.

The thickness of the crust ranges between about 20 and 120 km. Crust on the far side of the moon averages about 12 km thicker than that on the near side. Estimates of average thickness fall in the range from about 50 to 60 km. Most of this plagioclase-rich crust formed shortly after formation of the moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after the formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of crustal formation or deformation due to plate tectonics.

Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although the radius of the Moon is only about a quarter that of Earth, the lunar crust has a significantly greater average thickness. This thick crust formed almost immediately after formation of the Moon. Magmatism continued after the period of intense meteorite impacts ended about 3.9 billion years ago, but igneous rocks younger than 3.9 billion years make up only a minor part of the crust.

Дата добавления: 2015-11-16