Here's some information, but be advised it's from the 1910 edition of Britannica and is probably out of date, but at least it's not childish. Underlying causes of differentiation The primary cause of change in the composition of a magma is cooling, which is an inevitable consequence of the magma being created and migrating from the site of partial melting into an area of lower stress - generally a cooler volume of the crust. Cooling causes the magma to begin to crystallise minerals from the melt or liquid portion of the magma. Most magmas are a mixture of liquid rock (melt) and minerals (phenocrysts). Contamination is another cause of magma differentiation. Contamination can be caused by assimilation of wall rocks, mixing of two or more magmas or even by replenishment of the magma chamber with fresh, hot magma. The whole gamut of mechanisms for differentiation has been referred to as the FARM process, which stands for Fractional crystallization, Assimilation, Replenishment and Magma mixing. [edit] Fractional crystallization of igneous rocks Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates, which changes the composition of the melt. Fractional crystallization in silicate melts (magmas) is a very complex process compared to chemical systems in the laboratory because it is affected by a wide variety of phenomena. Prime amongst these is the composition, temperature and pressure of a magma during its cooling. The composition of a magma is the primary control on which mineral is crystallized as the melt cools down past the liquidus. For instance in mafic and ultramafic melts, the MgO and SiO2 contents determine whether forsterite olivine is precipitated or whether enstatite pyroxene is precipitated. Two magmas of similar composition and temperature at different pressure may crystallize different minerals. An example is high-pressure fractional crystallizaion of granites to produce single-feldspar granite, and low-pressure conditions which produce two-feldspar granites. The partial pressure of vapor phases in silicate melts is also of prime importance, especially in near-solidus crystallization of granites.