Bulk and particle strain analysis in high-temperature deformation experiments

Experimental data alone are not sufficient to describe the rheology of deformed geomaterials. To fully characterize a material's rheological properties, independent verification of deformation mechanisms is required. Here, we use standard image analysis techniques to semi-quantify the physical changes in experimentally deformed cores of soda-lime silica glass beads and rhyolite ash previously described by Quane and Russell [Quane, S.L., Russell, J.K., 2005a. Welding: insights from high-temperature analogue experiments. J. Volcanol. Geotherm. Res. 142, 67–87]. The properties we measure by image analysis include porosity, radial bulging and particle elongation. The image analysis measurements combined with digital output from the experiments allow us to determine the amount of total axial and radial strain accumulated by the bulk sample (εb) and by individual particles (εp). We demonstrate that these metrics of strain are nearly equal to the one-dimensional strain recorded by the deformation apparatus (εm) and sample shortening (εs), confirming that all strain introduced by the deformation apparatus is being transferred into both the bulk sample and individual particles. We also show that εb is manifest as two discrete components: axial (εa) and radial (εr) strain. We use these independent components of strain accumulation to show that, despite having nearly identical strain–time and stress–strain deformation paths, glass bead cores and rhyolite ash cores have strikingly different mechanisms of strain accumulation. In the higher porosity rhyolite ash cores, axial strain dominates, implying that, under the conditions present, natural glassy particulate geomaterials deform almost entirely by porosity loss.