As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (quick-freezing refractories, like olivine and pyroxene) crystallize out first, leaving minerals with the lowest melting points or solubilities (quick-melting fusibles, like silica) behind in the melt to freeze out last.
Latent heat associated with phase change is released by the crystallization of refractories, replacing heat lost by conduction to the surrounding country rocks, lost to melting of country rock, and lost to the assimilation of fusibles in the country rock. Fusibles enter and refractories leave the melt at characteristic temperatures and pressures, and these exchanges tend to occur at specific depths along the ascent. The remaining melt loses volume as it rises, rendering its fusibles increasingly concentrated. Thus, exchanges within ascending magma leave behind a trail of solid refractories and country rock alterations.
Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates, and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. The crystals of cumulate rocks are typically cemented by residual magmatic fluids.
▪ Bowen's Reaction Series
[links: animations: fractional crystallization/magmatic settling; webpages: Kurt Hollocher's webpage on Greenland's Eocene Skaergaard Intrusion has a gallery of excellent photographs of microrhythmic layering (and other interesting phenomena); Mining: Rock Formation]