Partial melting

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.

Chemical modification of residual rock by extraction of melt. In mantle peridotite, depletion commonly changes mineral proportions and trace-element contents, and can accompany volatile loss. Compositional buoyancy and water content must be assessed separately from thermal density changes when explaining a long-lived craton.

Concentration of melt transport toward a narrower extraction zone, affecting how melt production is converted into oceanic crust thickness.

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

Melting during ascent when a hot rock parcel crosses its solidus as pressure falls, without requiring external heating of the ascending parcel.

Melting induced by addition of components, especially water, that lower a rock’s solidus.