It refers to the composition of the upper layer of the earth's crust rock which are rich in silcon and aluminium minerals
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In geology, the term 'sial' refers to the composition of the upper layer of the Earth's crust, namely rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins,[1] but "sial" is a geochemical term rather than a plate tectonic term.[2] As these elements are less dense than the majority of the earth's elements, they tend to be concentrated in the upper layer of the crust.
The uppermost layer of the crust is called sial consisting of silicate and Aluminium (Si =silicate, Al= aluminium).On an average, the thickness of 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 sial is 2.7 gm/cc.
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."[3]
The name 'sial' was taken from the first two letters of silica and of alumina. The sial is often contrasted to the 'sima,' the next lower layer in the Earth, which is often exposed in the ocean basins; and the nickel-iron alloy core, sometimes referred to as the "Nife". These geochemical divisions of the Earth's interior (with these names) were first proposed by Eduard Suess in the 19th century. This model of the outer layers of the earth has been confirmed by petrographic, gravimetric, and seismic evidence.
Answer:
"Sial" refer to the composition of upper layer of the earths crust rock which are rich in silicon and aluminum minerals