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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 area around Rome, known as Latium. Through the p ...
,
sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentatio ...

sedimentary
, and
metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of a metamorphic reaction. Abbreviat ...

metamorphic rock
s that forms the geological
continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of th ...

continent
s and the areas of shallow seabed close to their shores, known as
continental shelves
continental shelves
. This layer is sometimes called ''
sial 250 px, Typical sial material, a Precambrian granite from St. Francois Mountains">granite.html" ;"title="Precambrian granite">Precambrian granite from St. Francois Mountains, Missouri, showing the orthoclase, potassium feldspar (felsic) matrix ( ...

sial
'' because its bulk composition is richer in aluminium silicates and has a lower density compared to the
oceanic crust The oceanic crust is the uppermost layer of the oceanic portion of the 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 i ...
, called ''
sima Sima or SIMA may refer to: People * Sima (given name), a feminine name in use in Arabic countries, Persia (Iran). * Sima (surname) * Sima (Chinese surname) Places * Sima, Comoros, on the island of Anjouan, near Madagascar * Sima, Nepal, in the J ...
'' which is richer in magnesium silicate minerals and is denser. Changes in
seismic wave Seismic waves are waves The United States Naval Reserve (Women's Reserve), better known as the WAVES (for Women Accepted for Volunteer Emergency Service), was the women's branch of the United States Naval Reserve The United States Navy ...
velocities have shown that at a certain depth (the
Conrad discontinuityThe Conrad discontinuity corresponds to the sub-horizontal boundary in continental crust 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneous rock, igneous, Sedimentary rock, sedimen ...
), there is a reasonably sharp contrast between the 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 ...
upper continental crust and the lower continental crust, which is 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 ...
in character. The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6). The average density of continental crust is about , less
dense The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

dense
than the ultramafic material that makes up 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 ...
, which has a density of around . Continental crust is also less dense than oceanic crust, whose density is about . At , continental crust is considerably thicker than oceanic crust, which has an average thickness of around . About 40% of Earth's surface area and about 70% of the volume 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 the top component of the lithosphere, a division of Earth's layers that includes the Crust (geology), ...
is continental crust. Most continental crust is dry land above sea level. However, 94% of the
Zealandia Zealandia (pronounced ), also known as ( Māori) or Tasmantis, is an almost entirely submerged mass of continental crust 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneou ...
continental crust region is submerged beneath the
Pacific Ocean The Pacific Ocean is the largest and deepest 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. ...

Pacific Ocean
, with
New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon Engl ...

New Zealand
constituting 93% of the above-water portion.


Importance

Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and
continental shelves
continental shelves
where complex
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...

metazoan
life could become established during early
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popula ...
time, in what is now called the Cambrian explosion.


Origin

All continental crust is ultimately derived from mantle-derived melts (mainly
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 Extrusi ...

basalt
) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur primarily at magmatic arcs associated with
subduction Subduction is a geological process in which the oceanic 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 ...

subduction
. There is little evidence of continental crust prior to 3.5 Ga.Hart, P. J., ''Earth's Crust and Upper Mantle,'' American Geophysical Union, 1969, pp. 13–15 About 20% of the continental crust's current volume was formed by 3.0 Ga. There was relatively rapid development on
shield A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from melee weapon, close-ranged weaponry or projectiles such as arrows, ...
areas consisting of continental crust between 3.0 and 2.5 Ga. During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. There are scientific reasons (see Armstrong 1991) to believe the total volume of continental crust has remained more or less the same after the formation of Earth and that presently found age distribution is just the result of the processes leading to the formation of
craton A craton (, , or ; from el, κράτος ''kratos'' "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust (geology), crust and the uppermost mantle (geology), mantle. Having ...
s (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics).


Forces at work

In contrast to the persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand
supercontinent cycle The supercontinent cycle is the quasi-periodic aggregation and dispersal 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 co ...
. There are currently about of continental crust, but this quantity varies because of the nature of the forces involved. The relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in
subduction Subduction is a geological process in which the oceanic 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 ...

subduction
zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the
Himalayas The Himalayas, or Himalaya (; Sanskrit Sanskrit (; attributively , ; nominalization, nominally , , ) is a classical language of South Asia that belongs to the Indo-Aryan languages, Indo-Aryan branch of the Indo-European languages. It ar ...

Himalayas
or the
Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest and most extensive mountain range A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt ...

Alps
). For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust; the oldest intact crustal fragment is 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
at 4.01 Ga, whereas the oldest large-scale oceanic crust (located on the
Pacific Plate The Pacific Plate is an oceanic tectonic plate This is a list of tectonic plates on Earth's surface Earth is the third planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant th ...
offshore of the
Kamchatka Peninsula The Kamchatka Peninsula (, ''Poluostrov Kamchatka'', ) is a peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform surrounded by water on most of its border while being connected to a mainland from which it ex ...

Kamchatka Peninsula
) is from the
Jurassic The Jurassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
(≈180 Ma), although there might be small older remnants in the
Mediterranean Sea The Mediterranean Sea is a connected to the , surrounded by the and almost completely enclosed by land: on the north by and and , on the south by , and on the east by the . The Sea has played a central role in the . Although the Mediterrane ...
at about 340 Ma. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history. The height of mountain ranges is usually related to the thickness of crust. This results from the
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
associated with
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 ...
(mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision. The buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found. The thinnest continental crust is found in
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...

rift
zones, where the crust is thinned by
detachment faultImage:View_of_Doso_Doyabi_from_Wheeler_Peak.jpg, View of Doso Doyabi, Snake Range, Nevada, which was formed by detachment faulting. Detachment faulting is associated with large-scale extensional tectonics. Detachment Fault (geology), faults often hav ...
ing and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of the , for example) are termed
passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate Continental margin, margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Contin ...

passive margin
s. The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic – the main exception to this being recent
igneous intrusion , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, ...
s. Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer immediately beneath it. Continental crust is produced and (far less often) destroyed mostly by
plate tectonic 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 ...
processes, especially at convergent plate boundaries. Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Also, material can be accreted horizontally when volcanic
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,
seamount A seamount is a large geologic landform that rises from the ocean floor The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
s or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction. It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount. By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Ga ago. The growth of continental crust appears to have occurred in ''spurts'' of increased activity corresponding to five episodes of increased production through geologic time.


See also

* Digital geologic mapping *
Geologic map A geologic map or geological map is a special-purpose map A map is a symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by ...
*
Geologic province A geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt, or a number of contiguous related elements. Adjoining provinces may be simila ...
*
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 ...
*
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 ...
*
Oceanic crust The oceanic crust is the uppermost layer of the oceanic portion of the 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 i ...
* OneGeology *
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 ...
*
Terrane 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 ...


References


Bibliography

* * * * * * * * * (Diagram entitled "Model of growth of continental crust through time" by ) *


External links


Average composition of Continental CrustCrust 5.1
{{DEFAULTSORT:Continental Crust Plate tectonics Structure of the Earth Earth's crust