低温下の岩石の熱膨張について

Abstract

Thermal expansion of several rocks down to 80K was measured with a dilatometer under dry and wet conditions at atmospheric pressure. According to the results of thermal expansion measurements, the rocks could be classified into two groups: granitic and non-granitic. For the non-granitic rocks in this study, i.e., tuff, sandstone, andesite, and basalt, the thermal expension coefficient increased in proportion to porosity but in inverse proportion to density, Young's modulus, and the strength. On the other hand, for granitic rocks (two granites and quartz-diorite), the thermal expansion coefficient increased in proportion to density and Young's modulus, but decreased with an increase in porosity. These differences are attributable to the pore and crack shape within the rocks. Low aspect ratio cracks are the main cause of granitic rock porosity, and they cancel the thermal expansion of the whole rock because they can provide space for the expansions of the surrounding grains. On the contrary, high aspect ratio pores, mainly within non-granitic rocks, only negligibly affect the thermal expansion of the whole rock.When the rock was saturated with water, the thermal expansion coefficient became higher than that under a dry condition. As the thermal expansion coefficient of ice is several times higher than those of rocks, the ice in pores and cracks contributes to the increase in the thermal expansion coefficient. And in the case of granitic rocks, this is partly because the ice existing in the crack increases its net aspect ratio and reduces the space available for the expansion of the mineral grains. The large and complicated hysteresis patterns of wet rocks seen upon thermal cycling are due to temperature differences between the freezing and melting of water in the rock pores and cracks.