The vesiculation of dissolved volatiles in the magma could be a driving mechanism of magma ascent that might cause an episodic and abrupt occurrence of volcanic eruption. The bulk density of magma is significantly reduced by the formation and expansion of bubbles in the magma, and it is formulated as a function of pressure under the assumption that the solubility of the volatile component follows the Henry’s law and that bubbles change their volume like an ideal gas. A simple expression of ascent velocity associated with buoyancy force is applicable to various mechanisms of magma migration involving fracture, viscous resistance, pipe and permeable flows in the crust. Magma is assumed to stay first in a magma reservoir under a gravitationally stable condition. The vesiculation may be accidentally initiated in some part of magma by the decrease of magma pressure due to elevated mamga level, or crustal stress changes. Resultant bubble inclusion reduces the bulk magma density so that the magma begins to move upward subject to buoyancy. The ascent motion as well as the vesiculation process is accelerated at shallower depth with decreasing pressure. This process of magma ascent is accompanied by a significant expansion of the magma volume, and finally leads to a volcanic eruption on the crustal surface.
