In Sect. 2.8, we learned that magma vesiculation, associated with decompression, induces decrease in water content in the silicate melt, leading to rise in the melting point of silicates. Therefore, for natural volcanic eruptions, when magma (which is saturated with water at depths) at a constant temperature spouts to the surface along with vesiculation, it becomes a supercooled state in most cases. In this chapter, we will summarize a point of similarity and a point of difference between cooling crystallization and decompression-vesiculation-induced crystallization (hereafter referred to as decompression-induced crystallization) from a thermodynamic perspective. Next, in cases where vesiculation progresses in equilibrium with decompression, we will explain a crystallization process. In this case, water content in melt is equivalent to solubility with respect to pressure. Then, because of the influence of bubble nucleation and growth processes, we will examine the case in which water content can change in disequilibrium with pressure. In the latter case, the rate of change in water content is greatly different between the disequilibrium and quasi-equilibrium regimes. Because there is no difference between both cases in the facts that crystallization undergoes crystal nucleation and growth, we use knowledge on cooling crystallization.