Magmas are commonly created at exceedingly high temperatures (1400^oC ). At those temperatures, it is impossible for the atoms and ions to combine because the energies of their collisions exceed the forces of attraction between them. As temperatures drop, however, the energy of these collisions decreases, attractive forces begin to dominate and minerals begin to form. 

The first materials to form, to crystallize into a solid phase, are primarily compounds of iron, magnesium, and calcium. Once the crystallization process begins, there exist both a liquid phase and a solid phase of individual crystals floating in this liquid. As calcium, iron and magnesium are removed from the molten phase by crystallization, there is less and less of these materials in the melt and this increases the concentration of the other elements such as sodium, potassium and aluminum. In other words, the chemical composition of the magma changes. Bowen’s reaction series essentially lists the sequence of formation of igneous minerals. 

As the cooling processes continue, because the iron-rich, solid crystals are denser than the surrounding liquid melt they settle toward the floor of the magma chamber. Over time, this continuous process of chemical change in the melt, called magmatic differentiation, gives rise to an ever changing liquid phase whose composition becomes increasingly richer in aluminum, potassium, and silica. 

This has direct implications for what type of material will erupt out of a volcano. If some of this magma rises to the surface before any extensive differentiation has taken place, the lava which is extruded is rich in iron, magnesium, and calcium. If, on the other hand, the lava erupts later after much differentiation has taken place, it will be richer in lighter elements. Differentiation therefore, explains not only how a magma's composition can change over time, but also how a single original magma can give rise to lavas of differing compositions. 

Cooling also controls crystal size. If molten material is cooled slowly, the molecules of the minerals which are solidifying from the liquid have a long time to join when cooling is slow, they will form large crystals, large enough to be visible (phaneritic). If, however, cooling is very rapid, there is insufficient time for the molecules to join into large structures, and the crystals will remain small, too small to be seen with the naked eye (aphanitic). On the extreme end of the scale, if the lava is chilled so rapidly that the molecules are scattered randomly, no crystals will form, and the material will be non-crystalline or glassy.