It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a geochemical term rather than a plate tectonic term. 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.

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Sial has a lowerdensity(2700–2800kg/m3) than 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. TheConrad discontinuityhas been proposed as the boundary, but little is known about it, and it doesn't seem to match the point of geochemical change.[6]Instead, the boundary has been arbitrarily set at a mean density of 2800kg/m3.

Because of the large pressures, over geologic time, the sima flows like a veryviscousliquid, so, in a real sense, the sial floats on the sima, inisostatic equilibrium. Mountains extend down as well as up, much likeicebergson the ocean;so that on the continentalplatesthe sial runs between 5km and 70km deep.

sial is the lower layer of earth crust , made up of silica and aluminium