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Fractional crystallization changes a residual geological melt by removing crystals as they form. Different minerals accept different elements into their structures, so the removed solid generally has a different composition from the remaining liquid. Successive removal can progressively enrich components that enter the early crystals weakly.

Settling, flotation or flow through a crystal-rich region can separate the phases. The resulting cumulate rock and residual melt need not have the same bulk chemistry. This is a cooling and separation process; partial melting instead starts by producing liquid from an incompletely melted rock.

The proportion of material present as geological melt, with a mass or volume convention specified.

Partial melting produces liquid while some of the original rock remains solid. Its minerals and chemical components do not all enter the liquid in equal proportions. The extracted geological melt can therefore have a very different composition from its source, leaving a chemically modified residue.

Heating, adiabatic decompression melting or addition of a component such as water by flux melting can cross the relevant solidus. Melt fraction and extraction history influence both the resulting magma and the remaining mantle. Observing one erupted composition consequently does not uniquely determine the source rock or its temperature.