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 viscous liquid, so, in a real sense, the sial floats on the sima, in isostatic equilibrium. Mountains extend down as well as up, much like icebergs on the ocean; so that on the continental plates the sial runs between 5 km and 70 km deep. Sial has a mean density of 2.7-2.8 grams per cubic centimeter.
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