Elucidation of contributing factors affecting the ζ-potential of volcanic rocks

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ζ-potential of several rock samples from various volcanoes was measured by using streaming potential method. Effects of surface conductivity was also taken into accounted for calculating ζ-potential. This procedure is necessary because surface current on the sample cell is not negligible when compared to the current through pore. Pressure gauge has been implemented to the instrumentation system in the second year.In general, ζ-potential of rock samples show strong dependency on pH of the circulating fluid. Four of the seven samples showed positive ζ-potential under the natural condition without pH control. Such rock samples with positive ζ-potential have already been reported by Hase et al. (2004) which measured the ζ-potential of samples from Aso Volcano. The present study has revealed that such 'anomalous' rocks are not very rare.The author investigated the relationship between major elements and ζ-potential of the rock samples in order to elucidate the contributing factors affecting the polarity of the ζ-potential. However, the correlation between each element and ζ-potential does not seem to be very clear. It was suggested that there should be some hidden factor other than the composition of rock sample itself. Further investigation including mineral composition and dissolved component in the fluid is necessary.Another contribution of the present research is the field measurement of electrical resistivity of Tarumae Volcano, Hokkaido. Our magnetotelluric measurement has revealed that there is a very conductive body just beneath the summit crater of the volcano. This result supports the hydrothermal circulation model that is inferred from self-potential anomaly on the volcano.