Quantitative measurement of bubble textures in pumice clasts using a digital stereo microscope with low-angled ring illumination

<jats:title>Abstract</jats:title><jats:p>Quantitative analysis of bubble textures in a large number of volcanic pyroclasts is critical to investigating the eruption dynamics in a volcanic conduit. Here, we used a digital stereo microscope with low-angled ring illumination (DSM-LaRI) to measure bubble textures on unpolished cutting surfaces of pumice clasts. As the DSM-LaRI enhances brightness contrast between the bubbles (pores) and the matrix, we easily obtained the two-dimensional data on the size and shape of bubbles by image analysis. The DSM-LaRI imaging provided the distributions of size and shape of bubbles at least 50 µm across. We applied the DSM-LaRI to analyze more than 1000 pumice clasts from the 232 AD Taupo eruption and measured the mean bubble radius (<jats:inline-formula><jats:alternatives><jats:tex-math>$$\overline{R}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>R</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math></jats:alternatives></jats:inline-formula>) and the mean deformation degree (<jats:inline-formula><jats:alternatives><jats:tex-math>$$\overline{D}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math></jats:alternatives></jats:inline-formula>) in the individual clasts. The distribution of <jats:inline-formula><jats:alternatives><jats:tex-math>$$\overline{R}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>R</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math></jats:alternatives></jats:inline-formula> and <jats:inline-formula><jats:alternatives><jats:tex-math>$$\overline{D}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math></jats:alternatives></jats:inline-formula> in each layer showed a distinctive difference between the fallout and the flow deposits. These quantitative data are consistent with a qualitative classification in a previous study. Although the new DSM-LaRI method has the disadvantage of the low spatial resolution, it allows for the analysis of a large number of pumice clasts in a short time, which can address larger scale heterogeneity, by efficiently generating a large representative suite of bubble size and shape data to link bubble textures to conduit processes. This provides vital information for quantitatively modeling eruption dynamics.</jats:p>