Partial melting

Partial melting occurs when some minerals in a rock melt while others remain solid, producing magma and a residual solid with a changed composition.

Solidus: The boundary below which a material is entirely solid at a given pressure. Melting begins when a rock crosses its solidus.
Phase diagram: A graph showing which phases of a material are stable under different physical conditions. Phase diagrams map solid, liquid, and mixed states as temperature and pressure change.
Magma: Molten or partly molten rock beneath Earth’s surface, commonly containing crystals and dissolved gases. The melt produced by partial melting becomes magma when it gathers and moves.
Residual mantle: Mantle rock left after melt has been extracted from it. It is the solid residue of mantle partial melting.
Fractional crystallization: The evolution of magma as crystals form and are separated from the remaining liquid. It changes magma composition after melting, rather than producing melt from solid rock.
Liquidus: The boundary above which a material is entirely liquid at a given pressure. Between the solidus and liquidus, rock is only partly molten.
Mineral: A naturally occurring crystalline solid with a defined chemical composition or range of compositions. Different minerals in one rock have different melting behaviors.
MORB: Basalt erupted at mid-ocean ridges, formed chiefly from melting of upwelling mantle. Its chemistry records partial melting beneath spreading ridges.
Melt extraction: The separation and movement of liquid melt away from its solid source rock. Extraction preserves compositional differences created during partial melting.
Assimilation (geology): The incorporation of surrounding rock into magma, changing the magma’s composition. Assimilation adds crustal material to magma instead of selectively melting its source.

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