Continental crust is the layer ofigneous,metamorphic, andsedimentary rocks that forms thegeological continents and the areas of shallow seabed close to their shores, known ascontinental shelves. This layer is sometimes calledsial because its bulk composition is richer inaluminium silicates (Al-Si) and has a lower density compared to theoceanic crust,[1][2] calledsima which is richer inmagnesium silicate (Mg-Si) minerals. Changes inseismic wave velocities have shown that at a certain depth (theConrad discontinuity), there is a reasonably sharp contrast between the morefelsic upper continental crust and the lower continental crust, which is moremafic in character.[3]
Most continental crust is dry land above sea level. However, 94% of theZealandia continental crust region is submerged beneath thePacific Ocean,[4] withNew Zealand constituting 93% of the above-water portion.
The continental crust consists of various layers, with a bulk composition that isintermediate (SiO2 wt% = 60.6).[5] The average density of the continental crust is about 2.83 g/cm3 (0.102 lb/cu in),[6] lessdense than the ultramafic material that makes up themantle, which has a density of around 3.3 g/cm3 (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3 (0.10 lb/cu in). At 25 to 70 km (16 to 43 mi) in thickness, continental crust is considerably thicker than oceanic crust, which has an average thickness of around 7 to 10 km (4.3 to 6.2 mi). Approximately 41% of Earth's surface area[7][8] and about 70% of the volume ofEarth's crust are continental crust.[9]
Because the surface of continental crust mainly lies above sea level, its existence allowed land life toevolve from marine life. Its existence also provides broad expanses of shallow water known asepeiric seas andcontinental shelves where complexmetazoan life could become established during earlyPaleozoic time, in what is now called theCambrian explosion.[10]
All continental crust is ultimately derived from mantle-derived melts (mainlybasalt) throughfractional 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.[11] These processes occur primarily atmagmatic arcs associated withsubduction.
There is little evidence of continental crust prior to 3.5Ga.[12] About 20% of the continental crust's current volume was formed by 3.0 Ga.[13] There was relatively rapid development onshield areas consisting of continental crust between 3.0 and 2.5 Ga.[12] During this time interval, about 60% of the continental crust's current volume was formed.[13] The remaining 20% has formed during the last 2.5 Ga.
Proponents of a steady-state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapidplanetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation ofcratons (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics).[14] However, this is not generally accepted.[15]
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 grandsupercontinent cycle.[16]
There are currently about 7 billion cubic kilometres (1.7 billion cubic miles) 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 insubduction 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 theHimalayas or theAlps). For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedlyrecycled oceanic crust; the oldest intact crustal fragment is theAcasta Gneiss at 4.01Ga, whereas the oldest large-scale oceanic crust (located on thePacific plate offshore of theKamchatka Peninsula) is from theJurassic (≈180Ma), although there might be small older remnants in theMediterranean Sea at about 340 Ma.[17] Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history.[8][18]
The height of mountain ranges is usually related to the thickness of crust. This results from theisostasy associated withorogeny (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.[19] The thinnest continental crust is found inrift zones, where the crust is thinned bydetachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of theAtlantic Ocean, for example) are termedpassive margins.
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 recentigneous 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.
Continental crust is produced and (far less often) destroyed mostly byplate tectonic processes, especially atconvergent 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 volcanicisland arcs,seamounts 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.[20] 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.[14]
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.[21] 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.[22] The growth of continental crust appears to have occurred inspurts of increased activity corresponding to five episodes of increased production through geologic time.[23]
| Shells | |
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| Global discontinuities | |
| Regional discontinuities | |
