Abstract
After clarifying that the composition of the 1950–1951 lava flow of Mihara Volcano, O-sima, Japan, is completely in the liquid state above 1200°C and at oxygen partial pressures below 10−4 atm, the most probable oxygen partial pressure of the magma from Mihara Volcano was estimated to be 10−8.0 atm. On the basis of the present results as well as those available in the literature, the oxygen partial pressure of the typical tholeiitic basalt magma was deduced to be nearly 10−8 atm.