木川田 喜一

The Kusatsu-Shirane volcano, located in the central part of Honshu, Japan, is one of the most famous active volcanoes in Japan. It has three crater lakes named Yugama, Mizugama and Karagama on its summit. Yugama, the largest and deepest one among the three, is well known as an active crater lake filled with strongly acidic water. We have monitored water chemistry of the all three crater lakes for more than 40 years. Our observations reveal that water chemistries of Mizugama and Karagama as well as that of Yugama have fluctuated reflecting the volcanic activity at least since 1960s. The fluctuation of water chemistry of Mizugama is distinctive in that the potassium content increased in and after the periods of high volcanic activity, although the mechanism of the potassium supply is still unknown.