抄録
We observed volcanic ash of phreatic eruptions from Ontake, Tokachi, Kurikoma, and Azuma volcanoes. Various alteration conditions are interpreted from the alteration mineral assemblages. The difference in the alteration mineralogy is related to the frequency of magma eruptions. The volcanoes with high frequencies of magma extrusions tend to discharge the ash with the monotonous mineral assemblages of the low-T advanced argillic alteration, whereas those with low frequencies are related to the assemblages from porphyry-type hydrothermal systems.
