Variations in Volcanic Ash Electrical Conductivity as a Function of Grain Size Distribution, Water Content, and Temperature

Volcanic lightning during explosive eruptions is a common phenomenon, and volcanic ash can be hazardous to electrical infrastructure following eruption, yet the characteristics of volcanic ash that influence its electrical conductivity are poorly understood. The physical properties of ash, specifically, the shape and size of ash grains, may factor into charge generation and transmission in both eruptive plumes and following deposition. This work compares conductivity measurements to grain size distribution and shape parameters (e.g., aspect ratios, solidity) from several volcanic centers: Five minimally processed ash samples collected from explosive eruptions in Alaska, U.S.A. and a set of homogenized (with respect to grain size and shape) ash samples. Although all ash samples are electrically insulating, homogenized samples provide maximum conductivity measurements that range over roughly four orders of magnitude (10-9 - 10-13 S/cm) and non-homogenized Alaska samples range over roughly six orders of magnitude (10-9 - 10-15 S/cm) at 25 ° and 25% relative humidity. Results suggest that particle size distribution controls ash conductivity, as samples with a wide distribution of particle sizes, due to a combination of fine and coarse grains, have the highest conductivity values. Since ash within eruption columns and plumes will be subjected to variable temperatures and exposed to both magmatic and atmospheric volatiles, we show the rate at which ash conductivity will increase due to water saturation and increasing temperature. Results of this study place important constraints on the physical properties of volcanic ash that will contribute to lightning generation and hazards to electrical infrastructure following explosive volcanic eruptions.