Continental crust

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The thickness of the Earth's crust (km).

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima due to its mafic, or basaltic rock. (Based on the change in velocity of seismic waves, 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, which consists of mafic rock. Continental crust is also less dense than oceanic crust, 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 where complex metazoan life could become established during early Paleozoic time. If Earth were like the other silicate planets 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 would have been impossible.

Forces at Work

In contrast to the relative permanence of continental crust, the size, shape, and number of continents are constantly changing; different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle. There are currently about 7 billion cubic kilometers of continental crust, but this quantity varies due to 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 zones, the oceanic crust is typically subducted back into the mantle. Continental crust is only rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the 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 continental rock is the Acasta Gneiss at 4.01 Ga, while the oldest oceanic crust is no older than Jurassic (~180 Ma). 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 associated with orogeny (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 zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of the Atlantic Ocean, for example) are termed passive margins.

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 intrusions. 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 continental crust is produced and destroyed mostly by plate tectonic processes, especially at convergent plate boundaries. 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, seamounts 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, 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 Butler).

References

• Armstrong, R.L. 1991. "The Persistent Myth of Crustal Growth." Australian Journal of Earth Sciences 38, 613-630
• Bowring, S A. & Williams, I. S. Priscoan, 1999. "4.00-4.03 Ga orthogneisses from northwestern Canada." Contributions to Mineralogy and Petrology 134, 3-16.
• Butler, Rob, Making new continents Accessed 01/29/2006
• Cogley, J.G., 1984. "Continental Margins and the Extent and Number of Continents." Reviews of Geophysics and Space Physics 22, 101-122.
• Condie,K.C. 2002. "The supercontinent cycle: are there two patterns of cyclicity?" Journal of African Earth Sciences 35, 179–183
• Clift, P., and Vannuchi, P., 2004. Controls on Tectonic Accretion versus Erosion in Subduction Zones: Implications for the Origin and Recycling of the Continental Crust". Reviews of Geophysics 42, RG2001, doi:10.1029/2003RG000127.
• Saal, A.L., Rudnick R.L., Ravizza G.E. & Hart S.R., 1998. Re-Os isotope evidence for the composition, formation and age of the lower crust. Nature, 39317, 1998.
• Taylor and McLennan, 1995, Model of growth of continental crust through time in John Victor Walther, 2005, Essentials Of Geochemistry, Jones & Bartlett, ISBN 0-7637-2642-7
• von Huene, R. and D.W. Scholl, 1991. "Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust." Reviews of Geophysics, 29, 279-316.

See also

• Sial
• Oceanic crust
• Craton
• Terrane

External links

• Average composition of Continental Crust
• Crust 5.1
• Evolution of the Continental Crust