Addressing the elephant in the room: combined experimental and numerical approaches for scaling to volcanic conditions

Volcanic eruptions produce pyroclasts that range from microns to meters, and produce edifices and deposits that extend for kilometers. Characterization of crystal and vesicle textures on millimeter to centimeter scale samples are commonly used to interpret and quantify magmatic storage, transport conditions, and eruptive processes, despite being divorced from their initial context. Due to practical challenges, our understanding of these micro-scale textures has so far been constrained on the basis of experiments limited to millimeters to a few centimeters in total sample size, upon which numerical simulations and empirical models can be calibrated. In this work, we present results from a natural, microlite-bearing, mildly banded, rhyolitic obsidian which was heated to induce ~60 vol% vesiculation. The sample expanded primarily in one direction, along a confined cylinder to impose shear, from an initial size of 14.5 x 14.5 cm to a final experimental size of 15.0 x 36.5 cm. X-ray computed tomography at a resolution of 33 μm/pixel reveals a rich variety of textures including macro-pores up to 1.5 cm in diameter, regions of high vesicularity juxtaposed against denser regions with smaller pores, evidence of differences in vesiculation history between bands with variable initial volatiles, and densification along the sheared margins. This experiment provides new constraints on texture at the decimeter scale, and places individual sub-volumes on the centimeter scale into their broader context, allowing for analysis of shear history and connectivity on neighboring regions. On the basis of these observations, we validate multi-scale numerical simulations of coupled bubble growth, suspension-scale flow, and fluid percolation, improving our reliability in upscaling to volcanic conditions. Comparison of sample textures with the simulated bubble and fluid pressure, temperature, and strain histories results in a comprehensive picture of intra-sample gas transport and segregation, and reveals the complex vesiculation behavior of initially heterogeneous material.