Electric resistivity of volcanic ash is one of important physical property to estimate influences of volcanic ash fall on social activities such as railway operation. Therefore, to clarify factors that determines electric resistivity of volcanic ash, we developed a new equipment to measure the electric resistivity of volcanic ash, and applied it to the six samples of volcanic ash collected in Japan. According to the test result, the resistivity ranges from 106 Ωm to 108 Ωm or more in dry condition and 10 to 100 Ωm in the wet condition with the water content of 20%. We have reconfirmed that the resistivity of volcanic ash strongly depends on the water content.
Furthermore, to understand why the resistivity differs even at the same water content, we compared the obtained resistivity to chemical composition of the volcanic ashes and the ionic composition of the suspension of 10 g volcanic ashes mixed with 100 ml of pure water. As a result, sulfur content of the volcanic ashes ranges from 0.00 to 7.64 wt.%, and pH and total ion equivalent concentration of the suspension are 4 - 6.5 and 0.1 - 100 meq/L, respectively. We have revealed that the volcanic ashes with higher sulfur content produces the suspension with lower pH and the higher total ion equivalent concentration, showing the lower resistivity in the wet condition.
These results implicate that the sulfur in volcanic ash is oxidized to generate hydrogen ions, which promote the dissolution of ionic compounds contained in volcanic ash, resulting in a higher total ion equivalent concentration of suspension and lower resistivity of volcanic ash in the wet condition. Thus, we concluded that the influence of volcanic ash on electric apparatuses depends on the sulfur content and the water content of the volcanic ash.
