A process known as partial melting generates the molten rock, known as magma, that cools to form crystalline rocks in the earth's outer compositional layer, or its crust. The terms "partial melting," "partial fusion," and "anatexis" refer to processes that create a magmatic melt from a portion of a solid rock less than the whole. Because most crystalline, or igneous, rocks in the earth's crust are composed of a number of silicate minerals that melt at different temperatures, and of minerals with heterogeneous crystal lattices, almost all magmas are generated by partial melting.

Incongruent melting occurs over a range of temperatures; the mineral components with the lowest melting temperatures melt first, and the minerals with the highest melting temperatures melt last. Partial melts are thus enriched in the chemical components of minerals with lower melting temperatures, and the remaining unmelted portion of the rock is composed...