Experimental Petrology on Pyroclastic Flows

Abstract

“Experimental petrology” is one of the useful ways of approach to the genetic study on pyroclastic flows. The temperatures of pyroclastic flows directly measured at Komagatake Volcano range from 385° to 510℃, which are probably much lower than the actual maximum temperature. The density of the pumice ranges from 0.2 to 0.8, the average being 0.51, but increases as the grain size becomes finer. However, the total pressure, say at the bottom of 10m thick flows, will not probably exceed 1.5atm. Systematic variation of bulk density and porosity is observed within a flow unit of Shikotsu pumice flows. Chemical composition of pyroclastic flows from various parts of the world is plotted in Q-Or-Ab diagram. The Japanese rocks lie closer to Q-Ab line than the others, and consequently the temperature of beginning of melting of these rocks is expected to be slightly higher than that in other rocks. Diagram to estimate the chemical composition from the indices of refraction of melted glass of the volcanic rocks is given. Thermal experiments on the pumice, obsidian, and related rocks under atmospheric pressure and under small or moderate hydrostatic pressures have been carried out. The results give clue to the estimation of the temperatures of welding in the pyroclastic flows or related subjects. Thermal effects on phenocrystic minerals in the pyroclastic flows are also discussed.