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We know that the composition of the crust is SiAl and SiMa and the one of mantle is mostly ophiolites, but why? Which are the geological processes that make this difference exist?

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  • $\begingroup$ For a start. Crust has been differentiated from Mantle. en.m.wikipedia.org/wiki/Igneous_differentiation $\endgroup$
    – user12525
    Jun 22, 2019 at 18:28
  • $\begingroup$ By the way brucite is fairly common at Mantle. It is not all silicates. $\endgroup$
    – user12525
    Jun 22, 2019 at 18:33
  • $\begingroup$ An ophiolite is in fact an oceanic crustal fragment not part of the mantle at all. $\endgroup$
    – Ash
    Jun 23, 2019 at 13:17

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Generally speaking , Earth's interior is classified using two models, the mechanical and chemical model. In the chemical model, the layers are crust, mantle and core. These layers organized while Earth started to cool after formation at the stage known as planetary differentiation. Heavier materials such as Fe And Ni sunk to the core, while the lighter materials like Al and Si stayed at the outer surface of the "ball". Read more about Planetary differentiation if you are interested

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Differentiation is often spoken of as occurring between mantle and core, but it may also occur between crust and mantle and even within the crust.

Tectonic activity leads to this differentiation between the rocky layers of Earth. While we think of this process as mixing crust and mantle together, that mixing is not uniform: denser rock is favored for subduction from crust into mantle and downards through the mantle. Therefore over time lighter rock is favored for the top, or the crust, and even within upper/outer regions of the crust (continental versus ocean crust).

This sorting of lighter rock to the top due to nonuniform tectonic mixing then impacts the chemistry of the crust, or at least of the uppermost crust. Felsic rock such as granite, generally containing alkali metals and aluminum (and to some extent also pure silica), is less dense than mafic rock such as basalt which, basically, contains all the other common metals. So felsic rock is most prominent in the upper crust. With the felsic rock come alkali metals and aluminum.

More subtly, mafic rock is also variable in density, with calcium-rich rock being less dense than more purely magnesium-iron silicates. So calcium is also enriched in our crust — and also in shells and in our bones and teeth that are made from this crust.

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