The liquid state extends from the melting point to the boiling point. Beyond the critical point fluids with liquid-like densities transition continuously to fluids with gas-like densities. A liquid close to its freezing temperature may differ significantly in such properties as viscosity, microscopic structure and chemical behavior from a liquid of the same composition near its boiling or critical points. For this reason it is convenient to define a melt as a liquid that is at, or very near, its freezing point. A melt is therefore saturated, or nearly so, in a solid phase (or assemblage) of broadly similar bulk composition. The exact meaning of “broadly similar” will remain undefined, but will become clear from the context of this and the following chapter, in which we will discuss electrolyte solutions. There is a parallel between this definition of melt and that of vapor, which is a gas that is at equilibrium with its liquid.

This chapter focuses on the ways in which melts form in planetary interiors. Because several excellent and up-to-date textbooks on igneous petrology are available (see, Winter, 2001; McBirney, 2006; Philpotts & Ague, 2009), and the research literature in the field is vibrant, I will not discuss processes of magma evolution and crystallization. There is no point in repeating here what is explained in much greater detail elsewhere. It is important to recall that a magma is an assemblage of melt, suspended solids and dissolved volatiles.