草津白根山の噴出物中の粘土鉱物とその噴火形態

抄録

Phyllosilicates were present in the volcanic ashes from several 1982-1983 eruptions of Kusatsu-Shirane Volcano . Pyrophyllite, kaolin and smectite were present as major clay minerals. Hydrothermally altered rocks collected from the western flank showed that, the basement rocks of this volcano were altered to form silica minerals, alunite and various clay minerals such as pyrophyllite, kaolin and smectite. The presence of pyrophyllite in the volcanic ashes indicates that the explosions probably have taken at a depth corresponding to the basement rocks. On the other hand, samples of air-fall tephra from 1897 and 1925 eruptions contained little or no pyrophyllite suggesting that these explosions likely occurred in the volcanic rocks of Kusatsu-Shirane Volcano. Based on the distribution and amounts of clay minerals in the volcanic ejectas, the depth of explosions during repeated eruptions may be inferred.