How is the difference in crust and mantle composition explained? 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?
Generally speaking , Earth interior is classified using two models , the mechanical and chemical model . the chemical model 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 like 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