0manago volcano (2375m), situated about 120 km north of Tokyo is a member of the Nikko volcano group. The volcanic activity of this mountain can be divided into main and later stages. Between these two stages, there was a period of quiescence. In the main stage, a lava rich in cognate inclusions erupted. This lava was viscous and formed a steep-sided dome which constitutes now almost bulk of the mountain. The activity of later stage began with minor eruptions of scoriaceous materials from a newly opened top crater. This activity was followed by a eruption of lava flows from the same crater, which covered western and eastern slopes of the dome. The lava was so fluid that it formed thin lava flows about 1-2m thick even on the gentle slope at the foot of the mountain. At least two lava streams flowed down on each of west and east slopes. The rocks of two stages differ in petrographic feature and chemical composition. The lava of the main stage is olivine-bearing augite-hypersthene dacite of the calc-alkali rock series and that of the later stage is olivine-augite-hypersthene and esite of the tholeiitic one. The order of emption of calc-alkali and tholeiitic rock types in Omanago volcano is reverse to those of two stratovolcanoes; Nyoho-Akanagi and Nantai of the Nikko volcano group in which the lavas of main stage are tholeiitic and those of later stage are calc-alkaline.
