Continental crust: Difference between revisions
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The '''continental crust''' is the layer of [[granite|granitic]] and [[sedimentary]] rock which forms the [[continent]]s and the areas of shallow seabed close to their shores, known as [[Continental shelf|continental shelves]]. It is less [[dense]] than the material of the Earth's [[Mantle (geology)|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. As a consequence, when active margins of continental crust meet oceanic crust in [[subduction zone]]s, the oceanic crust is [[subducted]].<br> |
The '''continental crust''' is the layer of [[granite|granitic]] and [[sedimentary]] rock which forms the [[continent]]s and the areas of shallow seabed close to their shores, known as [[Continental shelf|continental shelves]]. It is less [[dense]] than the material of the Earth's [[Mantle (geology)|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. As a consequence, when active margins of continental crust meet oceanic crust in [[subduction zone]]s, the oceanic crust is [[subducted]].<br> |
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In spite of its relative low density, continental crust can be subducted or re-cycled back into the mantle. For this reason the oldest rocks on Earth are within the "[[craton]]s" or cores of the continents, rather than in repeatedly recycled [[oceanic crust]]. |
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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 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. |
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The thinnest continental crust is found in [[rift]] zones, where the crust is thinned by [[fault (geology)|detachment faulting]] and eventually severed, replaced by [[oceanic crust]]. The edges of continental fragments formed this way are termed [[passive margin]]s. |
The thinnest continental crust is found in [[rift]] zones, where the crust is thinned by [[fault (geology)|detachment faulting]] and eventually severed, replaced by [[oceanic crust]]. The edges of continental fragments formed this way are termed [[passive margin]]s. |
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It is not clear whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. About 40% of the Earth's surface is now underlain by continental crust. |
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==References== |
==References== |
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Saal, A.L., Rudnick R.L., Ravizza G.E. & Hart S.R., 1998. ''Re-Os [[isotope geochemistry|isotope]] evdence for the composition, formation and age of the lower crust.'' Nature, 39317, 1998. |
Saal, A.L., Rudnick R.L., Ravizza G.E. & Hart S.R., 1998. ''Re-Os [[isotope geochemistry|isotope]] evdence for the composition, formation and age of the lower crust.'' Nature, 39317, 1998. |
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von Huene, R. and D.W. Scholl, 1991. "Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust." |
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Reviews of Geophysics, 29, 279-316. |
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Revision as of 16:42, 29 January 2006
The continental crust is the layer of granitic and sedimentary rock which forms 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. As a consequence, when active margins of continental crust meet oceanic crust in subduction zones, the oceanic crust is subducted.
In spite of its relative low density, continental crust can be subducted or re-cycled back into the mantle. 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 not clear whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. About 40% of the Earth's surface is now underlain by continental crust.
References
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.
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.