This melt is intermediate in composition. That is, its composition is higher in BRS than the original rock. If time and conditions allow, the fractionation process can continue and the intermediate rock produced during the first fractionation can fractionate into a felsic magma, leaving behind a crystal residue more mafic than the intermediate rock. One fraction becomes more mafic rich on Bowen's Reaction Series , the other more felsic. Bowen's original idea was that fractionation occurred during the crystallization process.
Crystallization begins with minerals highest in the reaction series.
Because these minerals have the highest specific gravity they settle to the bottom of the magma chamber by gravity settling . Also, because these first formed minerals are high in Ca, Mg, and Fe, they take these elements to the bottom with them in greater quantities than their average composition in the original melt. The remaining melt is thus depleted in Ca, Mg, and Fe, and has a composition lower in the reaction series. Thus, the original magma of one composition is divided into two fractions. There are practical problems with fractional crystallization.