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Continental crust

 
Continental Crust

 
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Continental crust
 
 
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The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continent
Continent

A continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents ? they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia ....
s and the areas of shallow seabed close to their shores, known as continental shelves
Continental shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during Ice age such as the current epoch by relatively shallow seas and Bay....
. This layer is sometimes called sial
Sial

In geology, the sial is the upper layer of the Earth's crust made of rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a Geochemistry term rather than a Plate tectonics term....
 due to more felsic
Felsic

Felsic is a term used in geology to refer to silicate minerals, magma, and rock which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
, or granitic
Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
, bulk composition, which lies in contrast to the oceanic crust
Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
, called sima
Sima (geology)

Sima is the name for the lower layer of the Earth's crust . This layer is made of rocks rich in silicates and magnesium minerals. Typically the sima when it comes to the surface is basalt, so sometimes this layer is called the 'basalt layer' of the crust....
 due to its mafic
Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
, or basaltic
Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
 (also called mafic
Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
) rock.






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Topo
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continent
Continent

A continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents ? they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia ....
s and the areas of shallow seabed close to their shores, known as continental shelves
Continental shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during Ice age such as the current epoch by relatively shallow seas and Bay....
. This layer is sometimes called sial
Sial

In geology, the sial is the upper layer of the Earth's crust made of rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a Geochemistry term rather than a Plate tectonics term....
 due to more felsic
Felsic

Felsic is a term used in geology to refer to silicate minerals, magma, and rock which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
, or granitic
Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
, bulk composition, which lies in contrast to the oceanic crust
Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
, called sima
Sima (geology)

Sima is the name for the lower layer of the Earth's crust . This layer is made of rocks rich in silicates and magnesium minerals. Typically the sima when it comes to the surface is basalt, so sometimes this layer is called the 'basalt layer' of the crust....
 due to its mafic
Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
, or basaltic
Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
 (also called mafic
Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
) rock. (Based on the change in velocity of seismic wave
Seismic wave

Seismic waves are waves that travel through the Earth or other elastic body, for example as the result of an earthquake, explosion, or some other process that imparts forces to the body....
s, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.)

Consisting mostly of granitic rock, continental crust has a density of about 2.7g/cm3 and is less dense than the material of the Earth's mantle
Mantle (geology)

The mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers....
, which consists of mafic rock. Continental crust is also less dense than oceanic crust
Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
, though it is considerably thicker; mostly 35 to 40 km versus the average oceanic thickness of around 7-10 km. About 40% of the Earth's surface is now underlain by continental crust.

Importance

Because continental crust mostly 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 shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during Ice age such as the current epoch by relatively shallow seas and Bay....
 where complex metazoan life could become established during early Paleozoic
Paleozoic

The Paleozoic or Palaeozoic Era is the earliest of three geology Era of the Phanerozoic Eon . The Paleozoic spanned from roughly , and is subdivided into six period ; from oldest to youngest they are: the Cambrian, Ordovician, Silurian, Devonian period, Carboniferous, and Permian...
 time. If Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
 were like the other silicate planet
Planet

A planet , as 2006 definition of planet by the International Astronomical Union , is a celestial body orbiting a star or Stellar evolution#Stellar remnants that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared the neighbourhood of planetesimals....
s and lacked the duality of oceanic and continental crust, our planet would be a very different place and the evolution of Homo sapiens and civilization
Civilization

A civilization is a society or culture group normally defined as a complex society characterized by the practice of agriculture and settlement in towns and city....
 would have been impossible.

Forces at Work

In contrast to the relative permanence of continental crust, the size, shape, and number of continents is constantly changing, as different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle
Supercontinent cycle

The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory is constantly being reconfigured....
. Today there are ~7 billion cubic kilometers of continental crust, but in the past there may have been fewer or more. The relative permanence of continental crust contrasts with the short life of oceanic crust. As a consequence of the density difference, when active margins of continental crust meet oceanic crust in subduction zones, the oceanic crust is typically subducted back into the mantle. Because of its relative low density, continental crust is only rarely subducted or re-cycled back into the mantle (for instance, where continental crustal blocks collide and overthicken, causing deep melting). For this reason the oldest rocks on Earth are within the craton
Craton

A craton is an old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years....
s or cores of the continents, rather than in repeatedly recycled oceanic crust
Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
; the oldest continental rock is the Acasta Gneiss
Acasta Gneiss

The Acasta Gneiss is a Rock outcrop of Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada. It was found in 1989 and was named for the nearby Acasta River east of Great Slave Lake, some 350 km north of Yellowknife, Northwest Territories....
 at 4.01 Ga
Annum

Annum is one form of the Latin noun meaning year, not a form normally used for derivatives in modern languages: the accusative case Grammatical number of the second declension grammatical gender noun annus , anni ....
, while the oldest oceanic crust is no older than Jurassic
Jurassic

The Jurassic is a geologic period that extends from about annum to  Ma, that is, from the end of the Triassic to the beginning of the Cretaceous....
 (~180 Ma
Annum

Annum is one form of the Latin noun meaning year, not a form normally used for derivatives in modern languages: the accusative case Grammatical number of the second declension grammatical gender noun annus , anni ....
). Continental crust and the rock layers that lie on and within it are thus the best archive of Earth history.

The height of mountain ranges is usually related to the thickness of crust. This results from the isostasy
Isostasy

Isostasy is a term used in geology to refer to the state of gravity equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density....
 associated with orogeny
Orogeny

Orogeny refers to natural mountain building, and may be studied as a tectonic structural event, as a geographical event, and a chronological event: orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust, and happen within a specific period of time....
 (mountain formation). The crust is thickened by the compressive forces related to subduction
Subduction

In geology, subduction is the process that takes place at convergent boundary by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge....
 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
Rift

In geology, a rift is a place where the Earth's Crust and lithosphere are being pulled apart and is an example of extensional tectonics.Typical rift features are a central linear downdropped geologic fault segment, called a graben, with parallel normal faulting and rift-flank uplifts on either side forming a rift valley, where the rift r...
 zones, where the crust is thinned by detachment faulting
Detachment fault

Detachment faulting is associated with large-scale Lithosphere extensional tectonics. Detachment Geologic fault often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade Metamorphic rock footwalls....
 and eventually severed, replaced by oceanic crust
Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
. The edges of continental fragments formed this way (both sides of the Atlantic Ocean
Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions; with a total area of about 106.4 million square kilometres . It covers approximately one-fifth of the Earth's surface....
, for example) are termed passive margin
Passive margin

A passive margin is the transition between oceanic crust and continental crust which is not an active plate Continental margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust....
s.

The high temperatures and pressures at depth, often combined with a long history of complex distortion, mean that much of the lower continental crust is metamorphic - the main exception to this being recent igneous intrusion
Intrusion

In geology, an intrusion is a body of igneous rock that has crystallized from molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto , the Roman mythology of the underworld....
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.

Today there are ~7 billion cubic kilometers of continental crust, but in the past there may have been less or more. Today continental crust is produced and destroyed mostly by plate tectonic processes, especially at convergent plate boundaries
Convergent boundary

In plate tectonics, a convergent boundary or convergent plate boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide....
. 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 arcs, seamount
Seamount

A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000?4,000 meters depth....
s or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is also lost, due to erosion and sediment subduction
Subduction

In geology, subduction is the process that takes place at convergent boundary by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge....
, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones. Many aspects of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently than 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 (Taylor and McLennan, 1995). 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 graphic at ).

See also

  • Sial
    Sial

    In geology, the sial is the upper layer of the Earth's crust made of rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a Geochemistry term rather than a Plate tectonics term....
  • Oceanic crust
    Oceanic crust

    Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or Sima ....
  • Craton
    Craton

    A craton is an old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years....
  • Terrane
    Terrane

    A terrane in geology is a fragment of crustal material formed on, or broken off from, one tectonic plate and Accretion ? "Suture " ? to crust lying on another plate....


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