Earth's crust is its thick outer shell ofrock, comprising less than one percent of the planet'sradius andvolume. It is the top component of thelithosphere, asolidified division ofEarth's layers that includes thecrust and the upper part of themantle.[1] The lithosphere is broken intotectonic plates whose motion allows heat to escape the interior of Earth into space.
The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made ofperidotite and is therefore significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at theMohorovičić discontinuity, a boundary defined by a contrast inseismic velocity.
The temperature of the crust increases with depth,[2] reaching values typically in the range from about 700 to 1,600 °C (1,292 to 2,912 °F) at the boundary with the underlying mantle. The temperature increases by as much as 30 °C (54 °F) for everykilometer locally in the upper part of the crust.[3]
Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of the atomic number. The rarest elements in the crust (shown inyellow) are not the heaviest, but are rather the siderophile (iron-loving) elements in theGoldschmidt classification of elements. These have been depleted by being relocated deeper into Earth's core. Their abundance inmeteoroid materials is higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.
The crust of Earth is of two distinct types:
Continental: 25–70 km (about 15–44 mi) thick and mostly composed of less dense, morefelsic rocks, such asgranite. In a few places, such as theTibetan Plateau, theAltiplano, and the easternBaltic Shield, the continental crust is thicker (50–80 km (31–50 mi)).
The average thickness of the crust is about 15–20 km (9.3–12.4 mi).[5]
Because both the continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. The surface of the continental crust is significantly higher than the surface of the oceanic crust, due to the greater buoyancy of the thicker, less dense continental crust (an example ofisostasy). As a result, the continents form high ground surrounded by deep ocean basins.[6]
The continental crust has an average composition similar to that ofandesite,[7] though the composition is not uniform, with the upper crust averaging a more felsic composition similar to that ofdacite, while the lower crust averages a more mafic composition resembling basalt.[8] The most abundantminerals inEarth'scontinental crust arefeldspars, which make up about 41% of the crust by weight, followed byquartz at 12%, andpyroxenes at 11%.[9]
All the other constituents except water occur only in very small quantities and total less than 1%.[11]
Continental crust is enriched inincompatible elements compared to thebasaltic ocean crust and much enriched compared to the underlying mantle. The most incompatible elements are enriched by a factor of 50 to 100 in the continental crust relative to primitive mantle rock, while oceanic crust is enriched with incompatible elements by a factor of about 10.[12]
The estimated average density of the continental crust is 2.835 g/cm3, with density increasing with depth from an average of 2.66 g/cm3 in the uppermost crust to 3.1 g/cm3 at the base of the crust.[13]
In contrast to the continental crust, the oceanic crust is composed predominantly of pillow lava and sheeted dikes with the composition ofmid-ocean ridge basalt, with a thin upper layer of sediments and a lower layer ofgabbro.[14]
Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun. It formed via accretion, whereplanetesimals and other smaller rocky bodies collided and stuck, gradually growing into a planet. This process generated an enormous amount of heat, which caused early Earth to melt completely. As planetary accretion slowed, Earth began to cool, forming its first crust, called a primary or primordial crust.[15] This crust was likely repeatedly destroyed by large impacts, then reformed from themagma ocean left by the impact. None of Earth's primary crust has survived to today; all was destroyed byerosion, impacts, andplate tectonics over the past several billion years.[16]
Since then, Earth has been forming a secondary and tertiary crust, which correspond to oceanic and continental crust, respectively. Secondary crust forms at mid-oceanspreading centers, where partial-melting of the underlyingmantle yieldsbasaltic magmas and new ocean crust forms. This "ridge push" is one of the driving forces of plate tectonics, and it is constantly creating new ocean crust. Consequently, old crust must be destroyed, so opposite a spreading center, there is usually a subduction zone: a trench where an ocean plate is sinking back into the mantle. This constant process of creating a new ocean crust and destroying the old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old.[17]
The average age of Earth's current continental crust has been estimated to be about 2.0 billion years.[20] Most crustal rocks formed before 2.5 billion years ago are located incratons. Such an old continental crust and the underlying mantleasthenosphere are less dense than elsewhere on Earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intenseorogeny, which coincide with the formation of thesupercontinents such asRodinia,Pangaea andGondwana. The crust forms in part by aggregation ofisland arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyantlithospheric mantle. Crustal movement on continents may result in earthquakes, while movement under the seabed can lead to tidal waves.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 14.ISBN978-0-521-88006-0.
^Levin, Harold L. (2010).The Earth through time (9th ed.). Hoboken, N.J.: J. Wiley. pp. 173–174.ISBN978-0-470-38774-0.
^R. L. Rudnick and S. Gao, 2003, Composition of the Continental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 1–64 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, OxfordISBN0-08-043751-6
^ABUNDANCE OF ELEMENTS IN THE EARTH’S CRUST AND IN THE SEA,CRC Handbook of Chemistry and Physics, 97th edition (2016–2017), p. 14–17
^Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993).Manual of mineralogy : (after James D. Dana) (21st ed.). New York: Wiley. pp. 221–224.ISBN0-471-57452-X.
^Christensen, Nikolas I.; Mooney, Walter D. (June 10, 1995). "Seismic velocity structure and composition of the continental crust: A global view".Journal of Geophysical Research: Solid Earth.100 (B6):9761–9788.Bibcode:1995JGR...100.9761C.doi:10.1029/95JB00259.
^P. J. Patchett and S. D. Samson, 2003, Ages and Growth of the Continental Crust from Radiogenic Isotopes. In The Crust (ed. R. L. Rudnick) volume 3, pp. 321–348 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, OxfordISBN0-08-043751-6
^A. I. S. Kemp and C. J. Hawkesworth, 2003, Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 349–410 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, OxfordISBN0-08-043751-6
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