Sial

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typical Sial material, a Precambrian granite showing potassium feldspar (felsic) matrix

For the Sial tribe of Pakistan, see Sial (tribe).

Sial or SiAl is the name for the upper layer of the Earth's 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 silicates and aluminium minerals. When the sial comes to the surface it is typically 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 plates the sial runs between 5 km and 70 km deep. Because in a real sense the sial floats on the 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' which is the name for the lower layer of the Earth's crust, which is exposed in the ocean basins.

[edit] Petrology

The sial has a lighter density (2700 - 2800 kg/m³) than the 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 basalts of the sima. The dividing line, called the Conrad discontinuity, has been arbitrarily set at a mean density of 2800 kg/m³.

[edit] 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.