高温における火山岩の弾性波速度と熱膨張の測定

抄録

Elastic wave velocity and thermal expansion of four kinds of volcanic rocks at high temperatures were measured. An increasing elastic wave velocity with a rise in temperature was observed in dacite from Showa-shinzan volcano and also in basalt from Mihara volcano, while a decreasing wave velocity was observed in andesite from Aso volcano. The stepwise increase in wave velocity of thermally treated Showa-shinzan dacite was observed at a certain temperature between 200℃ and 3000℃, while this stepwise velocity inclease was not observed in a virgin specimen in the first heating run. The wave velocity of Hakone dacite decreased with a rise in temperature from room temperature to 150℃ and then it increased at higher temperatures and again decreased stepwise at temperatures above 573℃. Thermal expansion of rock specimens was measured by a dilatometer. The remarkable difference of thermal expansion curves was related to the crystalline forms of free silica in rocks, namely quartz or cristobalite. The temperature variation of porosity was obtained from the difference between the measured and the calculated thermal expansion curves. An increase in wave velocity is generally correlated to the decrease in porosity. The inversion of a alpha to beta cristobalite is related to the stepwise increase in wave velocity in Showa-shinzan dacite.