Incomplete melting of parent rock, characteristically producing a melt whose chemical composition differs from that of the parent material. It is thought that partial-melting processes play a major role in generating more-defined liquids from less-evolved ones, so that many basalts may be the result of partial melting in the (ultrabasic) upper mantle, and many granites may have derived partly or completely from the partial melting of continental crust (anatexis). Partial melting preferentially enriches melts with incompatible elements. In a subduction zone, rocks of intermediate composition may form (e.g. andesites). With increasing temperature and pressure, the subducted oceanic crust (of basic composition) first undergoes metamorphism and then begins to melt or release watery fluids; this material rises into the overlying mantle, which may also begin to melt, giving rise to intermediate magma.
Subjects: Earth Sciences and Geography.