Continental crust

The continental crust is the layer of granitic, sedimentary and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. It is less dense than the material of the Earth's mantle and thus "floats" on top of it. Continental crust is also less dense than oceanic crust, though it is considerably thicker, averaging 20 to 80 km versus the average oceanic thickness of around 5-10 km. About 40% of the Earth's surface is now underlain by continental 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 normally not subducted or re-cycled back into the mantle (although it sometimes is). 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 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 are termed passive margins.

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 Bya continental crust contituted less than 10% of the present amount. By 3.0 Bya the amount was about 25% and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Bya (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

• Butler, Rob, Making new continents, http://earth.leeds.ac.uk/assyntgeology/extra_info/ehistory.htm Accessed 01/29/2006
• Saal, A.L., Rudnick R.L., Ravizza G.E. & Hart S.R., 1998. Re-Os isotope evdence 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 0763726427
• 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.

External link

• Average composition of Continental Crust