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Earth's crust is a thin shell on the outside of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, accounting for less than 1% of Earth's volume. It is the top component of the
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
, a division of Earth's layers that includes the crust and the upper part of the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
. The lithosphere is broken into
tectonic plates This is a list of tectonic plates on Earth's surface. Tectonic plates are pieces of Earth's crust 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is ...

tectonic plates
whose motion allows heat to escape from the interior of the Earth into space. The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spok ...
and so is significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the
Mohorovičić discontinuity The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of ...
, a boundary defined by a contrast in
seismic Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following perio ...

seismic
velocity. The temperature of the crust increases with depth, reaching values typically in the range from about to at the boundary with the underlying mantle. The temperature increases by as much as for every kilometer locally in the upper part of the crust


Composition

File:Elemental abundances.svg, 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 in ) are not the heaviest, but are rather the siderophile (iron-loving) elements in the
Goldschmidt classification The Goldschmidt classification, developed by Victor Goldschmidt Victor Moritz Goldschmidt (January 27, 1888 in Zürich – March 20, 1947 in Oslo Oslo ( , also , , rarely ) is the Capital city, capital and List of towns and cities in Nor ...
of elements. These have been depleted by being relocated deeper into Earth's core. Their abundance in
meteoroid A meteoroid () is a small rocky or metallic body in . Meteoroids are significantly smaller than s, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as or . Most are fragments from s or as ...
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: #
Oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also called "Pacific Islander(s)" Places *Oceanic, British Columbia, a settlement on Smith Island, Br ...
: to thick and composed primarily of denser, more
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composit ...
rocks, such as
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
,
diabase Diabase (), also called dolerite () or microgabbro, is a mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a so ...

diabase
, and
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusion, intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a crystallinity, holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coa ...

gabbro
. #
Continental Continental may refer to: Places * Continent * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * Continental (album), ''Continental'' (album), an alb ...
: to thick and mostly composed of less dense, more
felsic In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
rocks, such as
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cool ...

granite
. The average thickness of the crust is about to . Because both 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 of
isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Prope ...

isostasy
). As a result, the continents form high ground surrounded by deep ocean basins. The continental crust has an average composition similar to that of
andesite Andesite ( or ) is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of so ...

andesite
, though the composition is not uniform, with the upper crust averaging a more felsic composition similar to that of
dacite Dacite () is a formed by rapid solidification of that is high in and low in s. It has a fine-grained () to texture and is intermediate in composition between and . It is composed predominantly of and . Dacite is relatively common, occurri ...
, while the lower crust averages a more mafic composition resembling basalt. The most abundant
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s in
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
's
continental crust Continental crust is the layer of igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...
are
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity ...
s, which make up about 41% of the crust by weight, followed by
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
at 12%, and
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming Silicate minerals#Inosilicates, inosilicate minerals found in many Igneous rock, igneous and metamorphic rock, metamorphic rock (geology), rocks. Pyroxenes have t ...
s at 11%. All the other constituents except water occur only in very small quantities and total less than 1%. Continental crust is enriched in
incompatible elementIn petrology Image:LvMS-Lvm.jpg, A volcanic lithic fragment (geology), sand grain seen under the microscope, with plane-polarized light in the upper picture, and cross polarized light in the lower picture. Scale box is 0.25 mm. Petrology (from the ...
s compared to the
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic ocean crust and much enriched compared to the underlying mantle. The most incompatible elements are enriched by a factor of 50 to 100 in continental crust relative to primitive mantle rock, while oceanic crust is enriched with incompatible elements by a factor of about 10. Estimates of average density for the upper crust range between 2.69 and 2.74 g/cm3 and for lower crust between 3.0 and 3.25 g/cm3. In contrast to the continental crust, the oceanic crust is composed predominantly of pillow lava and sheeted dikes with the composition of
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate ...
basalt, with a thin upper layer of sediments and a lower layer of
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusion, intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a crystallinity, holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coa ...

gabbro
.


Formation and evolution

Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun. It formed via accretion, where
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s 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. This crust was likely repeatedly destroyed by large impacts, then reformed from the
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
ocean left by the impact. None of Earth's primary crust has survived to today; all was destroyed by erosion, impacts, and
plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
over the past several billion years. Since then, Earth has been forming secondary and tertiary crust, which correspond to oceanic and continental crust respectively. Secondary crust forms at mid-ocean spreading centers, where partial-melting of the underlying
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
yields
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic 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. That means that old crust must be destroyed somewhere 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 new ocean crust and destroying old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old. In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years and have been found in the Narryer Gneiss Terrane in
Western Australia Western Australia (abbreviated as WA) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...

Western Australia
, in the
Acasta Gneiss The Acasta Gneiss is a tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic In geology, felsic is an adjective describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 200 ...

Acasta Gneiss
in the
Northwest Territories The Northwest Territories (commonly abbreviated as NT or NWT; french: Territoires du Nord-Ouest) is a federal territory A territory is an administrative division Administrative division, administrative unitArticle 3(1). , country subd ...

Northwest Territories
on the
Canadian Shield The Canadian Shield (french: Bouclier canadien ), also called the Laurentian Plateau, is a large area of exposed Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of History of the Ea ...

Canadian Shield
, and on other
craton A craton (, , or ; from el, κράτος ''kratos'' "strength") is an old and stable part of the continental lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial p ...
ic regions such as those on the Fennoscandian Shield. Some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane. Continental crust is tertiary crust, formed at subduction zones through recycling of subducted secondary (oceanic) crust. The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years.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, Oxford Most crustal rocks formed before 2.5 billion years ago are located in
craton A craton (, , or ; from el, κράτος ''kratos'' "strength") is an old and stable part of the continental lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial p ...
s. Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in Earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense
orogeny Orogeny is the primary mechanism by which mountains are formed on continents. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. This leads to both structural deformation Deformation ...
; these periods coincide with the formation of the
supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...
s such as
Rodinia Rodinia (from the Russian родить, ''rodit'', meaning "to beget, to give birth", or родина, ''rodina'', meaning "motherland, birthplace") was a Neoproterozoic supercontinent that assembled 1.1–0.9 billion years ago and broke up 750 ...

Rodinia
,
Pangaea Pangaea or Pangea () was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology) ...

Pangaea
and
Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (ge ...

Gondwana
. The crust forms in part by aggregation of
island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries (such as the Ring of Fire). Most island arcs originate on oceanic crust and have resulted from the descent of the litho ...

island arc
s including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant
lithospheric A lithosphere ( grc, wikt:λίθος#Ancient Greek, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geol ...
mantle.


See also

* Brittle-Ductile Transition Zone *
Solid EarthSolid earth refers to "the earth beneath our feet" or '' terra firma'', the planet's solid surface and its interior. It contrasts with the Earth's fluid envelopes, the atmosphere and hydrosphere (but includes the ocean basin), as well as the bio ...
*
Structure of Earth The internal structure of Earth, structure of the solid Earth, or simply structure of Earth refers to concentric spherical layers subdividing the Solid earth, i.e., excluding Earth's atmosphere File:Atmosphere gas proportions.svg, Compositi ...


References


External links

* {{Authority control Structure of the Earth