We propose a new method for estimating pore structures of volcanic rocks. Permeability of ascending magma controls the escape of gas, and greatly affects the style of eruption. The interconnection of bubbles in magma must play a key role in controlling the permeability. However, the mechanism of their interconnection has been poorly understood. In order to understand it, we must have a good understanding of pore structures in volcanic rocks. A volcanic rock has a wide variety of pores in size and shape. The conventional equivalent channel model is not useful for estimating pore structures from permeability. We thus have made a new permeability model for volcanic rocks. Our model is composed of a bundle of parallel identical tubes. Each tube is made of serial two circular tubes with different radii. The two radii, the length fraction of narrow tube and the separation of tubes are parameters of characterizing pore geometry. Tube radii can be estimated through microstructural observation, and other two geometrical parameters can be constrained from measured permeability and porosity. We applied this method to rock samples from Mt. Yakedake, and found that the permeability is mainly determined by narrow parts, the length fraction of which is less than 0.1. Although uncertainties are left in the estimation, we can obtain a reasonable structural image. Electrical conductivity and other physical properties can provide us with additional information to constrain geometrical parameters.

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