They gloss over a lot with the term "crustal differentiation". This page (intro of a book) explains a bit about how magma gets separated into silicate-rich and

They gloss over a lot with the term "crustal differentiation". This page (intro of a book) explains a bit about how magma gets separated into silicate-rich and -poor fractions: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magmatic-differentiation https://www.sciencedirect.com/topics/earth-and-planetary-sci... .