Variations in pore structure in subaerial lava flows at Nishinoshima, Japan, inferred from physical properties

Abstract

Nishinoshima, a volcanic island in the Izu–Ogasawara arc, Japan, has erupted intermittently since activity restarted in 2013, alternating primarily between effusive lava flows and explosive eruptions. An understanding of pore structure in lava flow clues to the volcanic processes that accompany eruption, including the physical conditions during effusion, the efficiency of gas escape, and seawater circulation within the edifice. We investigated the relationships between various physical properties of the lava flows on Nishinoshima to characterize their internal pore structure. Laboratory measurements of porosity, density, permeability, electrical resistivity, and P-wave velocity were conducted at ambient temperature and atmospheric pressure on andesitic lava blocks collected by land-based surveys conducted in 2016, 2019, and 2021. The measured resistivities and P-wave velocities vary with porosity, although the trends are different in different lithofacies (vesicular, dense, and foliated blocks). By comparing these variations with theoretical models, they can be attributed to differences in connectivity and geometry of pore space in the different lithofacies. These lithofacies-dependent pore structures contribute to the observed variations in permeability, which can vary by as much as five orders of magnitude for a given porosity. The differences in pore structure and permeability may be related to post-eruption processes, where shear deformation during lava flow emplacement flattened and elongated the bubbles, leading to low aspect ratios, smaller apertures, and higher connectivity in the foliated lavas. Our data and interpretations could assist in estimating the internal pore structures at Nishinoshima through geophysical surveys, although further investigation incorporating submarine samples is necessary.