Sial

Typical rock belonging to the sial, a granite of the Precambrico of the St. Francis Mountains, Missouri, showing the potassium feldespato matrix (felsic)

Sial is a term, now obsolete, that designates the rocks that form the fundamental part of the continental crust, located on darker and denser rocks that also emerge on the ocean floor and form the chasm The term refers to the fact that they are rocks based on silicates characterized by the abundance of aluminum, instead of the magnesium that distinguishes the rocks of the chasm.

Eduard Suess invented the terms sima and sial, and Alfred Wegener proposed the variant so for the latter as more convenient. l, avoiding confusion with the Latin (and Spanish) term salt, and his modification was successful. Both concepts have lost significance and, although they describe a real situation, with a discontinuous sial lying on a continuous chasm, they have no place in modern theories that describe the structure of the Earth and explain its dynamism. The concepts were very important, however, in the theory of Continental Drift, proposed by Wegener, who mistakenly imagined the continents as masses of sial sliding over the chasm.

Properties

Sial has a lower density (2600 - 2800 kg/m³) than sima, mainly due to a higher amount of aluminum, and a lower amount of iron and magnesium. The lower limit of the sial layer is not a strict limit, the sial floats on the denser rocks of the abyss. The Conrad discontinuity has been proposed as a limit, but little is known about it, and it does not appear to coincide with the point of geochemical change. Instead, the limit has been arbitrarily set at a mean density of 2800 kg/m³.

Due to the great pressures, over geologic time, the chasm layer flows as a highly viscous liquid, so in a real sense, the sial floats in the chasm, in isostatic equilibrium. The mountains they extend up and down, just like icebergs in the ocean; so that on the continental part of the plates the SIAL layer is between 5 km and 70 km deep.