assimilation-fractional crystallization

Assimilation in geology refers to the process where a magma incorporates and chemically alters surrounding rocks. This occurs when the hot magma melts or reacts with the wall rocks it encounters. As the magma crystallizes, it can also react with these incorporated foreign materials, changing the overall composition of the magma.1

Fractional crystallization is another process that modifies magma. As magma cools, minerals crystallize and separate from the liquid. Early-formed, denser minerals like olivine and pyroxene can sink and be removed, leaving behind a liquid that is progressively enriched in silica and other elements.2

When assimilation and fractional crystallization happen together, it's called AFC. This combined process is thought to be responsible for the formation of certain rock types, such as andesites from basalts. Crystal formation itself begins with nucleation, where ions, atoms, or molecules gather to form a solid seed, followed by growth as more particles join.13