Using X-ray tomography as a tool to better quantify bubble network characteristics in volcanic tephra in three-dimensions

Eruption-style studies of active volcanoes suggest that volatile pressure build-up and volatile degassing rates in volcano conduit magmas determine volcanic explosivity. Explosive eruptions appear to occur when the viscosity and permeability of the magma are high and low, respectively, leading to gas pressure build up within the magma. Conversely, effusive eruptions tend to occur when gas can more easily escape from magma due to high magma permeabilities and/or low viscosities. Permeabilities and the ability of magma to degas depend on how well-connected and extensive the percolating bubble networks are within the magma. Currently, one method to determine bubble connectivity in the volcano conduit uses quenched volcanic ejecta as an analogue to observe bubble network formation just below the magma fragmentation threshold. We employ X-ray tomography at the Advanced Light Source synchrotron facility at Lawrence Berkeley National Laboratory to obtain high-resolution, three-dimensional (3D) X-ray tomography images of these quenched pumice clasts to then determine pumice permeability numerically. Numerical determinations of permeability, as opposed to measurements using a permeameter, are critical when, for example, numerical upscaling methods are to be applied to the clasts for permeability determinations at the volcano conduit scale. The highest resolution scans we have used at the ALS is 4.1 microns to keep tomography files at a size that are manageable to process. However, some of the inter-bubble walls are thinner than the scanning resolution. If the inter-bubble walls are thinner than the given resolution, tomographic scans will likely not detect these walls. This will result in a misrepresentation of bubble connectivity, macroscopic pumice clast permeabilities, and associated volatile degassing rates. Therefore, for studies of volcanic eruption dynamics, it is critical to accurately define these thin bubble walls in pumice clasts during 3D image post-processing. Here we present a method for re-introducing the thin, undetected bubble walls during post-processing of X-ray tomography scans and how pumice permeability is affected by this method.