Sial

 ( Petrology ) 

In geology, sial is an antiquated Blend word for the composition of the upper layer of Earth's crust, namely rocks rich in aluminium . It is sometimes equated with the continental crust because it is absent in the wide ,Continental crust has been defined as That type of the Earth’s crust which underlies the continents and the continental shelves: it is equivalent to the sial.

but 'sial' is a geochemistry term rather than a plate tectonics term. As these elements are less dense than the majority of Earth's elements, they tend to be concentrated in the upper layer of the crust.

The uppermost layer of the crust is called the sial, consisting of silicate and aluminium (Si = silicate, Al = aluminium). On average, the thickness of the sial is till 25 km from the surface. The continents are composed mainly of lighter rock material formed from silicon and aluminium, so the sial is thick over the continents and very thin or absent on the ocean floor, especially the Pacific Ocean. Average density of the sial is 2.7 g/cm³.

Geologists often refer to the rocks in this layer as felsic, because they contain high levels of feldspar, an aluminium silicate mineral series. However, the sial "actually has quite a diversity of rock types, including large amounts of basaltic rocks."

The name 'sial' was taken from the first two letters of silicon dioxide and of aluminium oxide. The sial is often contrasted to the 'sima' (another antiquated blended term), the next lower layer in Earth, which is rich in silica and magnesium and is often exposed in the ocean basins; and the nickel-iron alloy planetary core, sometimes referred to as the 'Nife'. These geochemical divisions of Earth's interior (with these names) were first proposed by Eduard Suess in the 19th century. This model of the outer layers of Earth has been confirmed by petrographic, gravimetric, and seismic evidence.

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Properties The sial has a lower density (2700–2800 kg/m³) than the sima, which is primarily due to increased amounts of aluminium, and decreased amounts of iron and magnesium. The base of the sial is not a strict boundary, the sial grades into the denser rocks of the sima. The Conrad discontinuity has been proposed as the boundary, but little is known about it, and it doesn't seem to match the point of geochemical change. Instead, the boundary has been arbitrarily set at a mean density of 2800 kg/m³.

Because of the large pressures, over geologic time, the sima flows like a very viscosity liquid, so, in a real sense, the sial floats on the sima, in isostasy.

Edwin Michael Bridges (1990). 9780521383431, Cambridge University Press. ISBN 9780521383431

Mountains extend down as well as up, much like on the ocean; so that on the continental plate tectonics, the sial runs between 5 km and 70 km deep.

Luis Lliboutry (2025). 9781852331153, Springer-Praxis. ISBN 9781852331153

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See also

• Asthenosphere
• Eduard Suess
• KREEP
• Lithosphere
• Earth's mantle

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Sources

• 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.

Categories: Petrology, Structure Of The Earth

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