Mihara Yama is the active central cone of O Shima Volcano on the island of O Shima, Tokyo Prefecture, Japan. The outer slopes of O Shima Volcano compose most of the island. Their crest is a somma, about 600 km in altitude and 3 km in diameter, which encloses a caldera. Mihara Yama rises from the southern part of the caldera to an altitude of 755m. It is a truncated cone with a crater 800 m in diameter. Before the eruption in 1950 the crater contained an inner pit approximately 300 m in diameter and 165 m deep.
Mihara Yama erupted on July 16, 1950, after 10 years of quiescence. Activity continued until September 24. Eruption resumed on February 4, 1951, and continued until April 2, after which activity was intermittent until the eruption ceased on June 28.
In 1950 molten basalt was ejected explosively accompanied at times by flows. Two successive cinder cones formed from the accumulation of ejecta, some of which were hurled as high as 500 m. Lava filled the inner pit and crater, and a small amount overflowed into the caldera. About 52,000,000 metric tons of material was erupted.
In 1951 six successive small cones developed. Beginning February 21 the principal cone formed. Lava overflowing the crater rim solidified mostly as aa and covered nearly 2 square km of the caldera floor. The intermittent activity during the final 3 months consisted of nine brief periods of eruption, a few of which were relatively violent. Between eruptions and especially after the final eruption the crater floor subsided. In 1951 about 20,000,000 metric tons of material was erupted.