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 of minerals with the highest melting temperatures. There are two end member types of partial melting. In equilibrium fusion, the liquid melt continuously reacts with the residual crystals , changing composition until the whole rock has melted. In fractional fusion, the melted material is separated from the remaining solid rock as it is produced. Fractional fusion leads to differentiation of chemical components in the melt, and to creation of different rock types from the same magmatic source.
Earthquake wave velocities and travel paths through the earth's interior suggest that the outer core is the only fully liquid layer of our planet. However, the outer core is composed entirely of iron , and is not a possible source of siliceous magma. Magmatic source areas are thus confined to areas of the upper mantle and lower crust that seismic shear waves indicate to be almost entirely solid. Only a very small portion (<5%) of the rock in magmatic source areas is thought to be liquid. Partial melts migrate upward from their source areas to intermediate staging areas, or magma chambers, in the middle and upper crust before erupting from volcanoes, or cooling to form intrusive igneous plutons. Magmas are generated by partial melting in a number of present-day plate tectonic settings, including subduction zones, mid-ocean ridges, and hot spots. The granitic continental interiors, called continental shields or cratons, probably formed above ancient subduction zones, or by melting at the base of the crust during the Precambrian and Paleozoic Eras when more heat was escaping from the inner earth.
See also Geothermal gradient; Phase state changes