Mechanism of Pumice Formation

Abstract

The purpose of the present study is to make clear the mechanism of pumice formation in order to investigate the thermodynamical conditions at volcanic eruption. J. Verhoogen investigated theoretically the mechanism of ash formation in the conditions of expansion of gas bubbles on the basis of hydro- and thermodynamical point of view. For the present purpose, the formation of gas bubbles in the molten magma would become to be essential. From this stand point, the conditions of pressure and temperature for bubble formation were calculated by using the homogeneous nucleation theory, which was justified by the laboratory experiment. A thin plate of pitchstone was heated in several ways in the electric furnace from room temperature up to 1100℃. At almost 900℃, gas bubbles began to form in the specimen, which are photographed at various stages. Nucleation rate of such gas bubbles was calculated on the basis of the experiment, and as the result, it agrees quite well with that derived from the theory. In reality, the pumice would be formed by the sudden decrease of pressure and temperature from the molten magma, however, the present experiment was made in one atmospheric pressure. Therefore, it is necessary to expose suddenly the specimen from the state of considerable pressure and temperature to one atmospheric pressure.