Sial
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2007-09-13T10:43:45Z
Vsmith
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Reverted edits by [[Special:Contributions/202.133.44.5|202.133.44.5]] ([[User talk:202.133.44.5|talk]]) to last version by Trusilver
[[Image:Precambrian Granite St Francis Mountains Missouri.jpg|thumb|right|250px|typical Sial material, a [[Precambrian]] [[granite]] showing [[Orthoclase|potassium feldspar]] ([[felsic]]) [[Matrix (geology)|matrix]]]]
{{for|the Sial tribe of Pakistan|Sial (tribe)}}
'''Sial''' or '''''SiAl''''' is the name for the upper layer of the Earth's [[Crust (geology)|crust]], which is also known as the ''[[continental crust]]'' because it is absent in the wide oceanic basins. This layer is made of rocks rich in [[Silicate|silicates]] and [[aluminium]] minerals. When the sial comes to the surface it is typically [[granite|granitic]], so sometimes this layer is called the 'granitic layer' of the crust. Geologists often refer to the rocks in this layer as [[felsic]]. On the continental [[Plate tectonics|plates]] the sial runs between 5 km and 70 km deep. Because in a real sense the sial floats on the [[Sima (geology)|sima]], mountains extend down as well as up, much like [[icebergs]] on the ocean, hence the great variation in depth. See [[Isostasy]].
The name 'sial' was taken from the first two letters of silica and of aluminium. Comparable is the name '[[Sima (geology)|sima]]' which is the name for the lower layer of the Earth's crust, which is exposed in the ocean basins.
==Petrology==
The sial has a lighter density (2700 - 2800 kg/m<sup>3</sup>) than the [[Sima (geology)|sima]], which is due to increased amounts of aluminium, and decreased amounts of iron and magnesium. At the base of the sial it grades into the [[Basalt|basalts]] of the sima. The dividing line, called the Conrad discontinuity, has been arbitrarily set at a mean density of 2800 kg/m<sup>3</sup>.
==References==
*Bates, R.L., and Jackson, J.A., (1987) ''Glossary of geology'' American Geological Institute, Alexandria, Virginia.
*Dilek, Y. and Newcomb, S. (eds.) (2003) ''Ophiolite Concept and the Evolution of Geological Thought'' Geological Society of America Special Paper 373, Boulder, Colorado.
==See also==
* [[KREEP]]
* [[Sima (geology)]]
* [[Nife]]
[[Category:Petrology]]
[[Category:Structure of the Earth]]
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