The color systematics of volcanic ash fall samples in estimating eruption sequences: A case study of the 2017–2018 eruption at Shinmoe-dake, Kirishima volcano, Southwest Japan.

Abstract The color of pyroclasts is fundamental because it is affected by the processes and properties of the magma that derive the diversity in eruption style, as well as particle morphology, chemistry, and petrological characteristics. Currently, it is fundamental to derive the componentry of pyroclasts or the ratio of each component (CR) to monitor ongoing eruptions. However, the color description of pyroclasts and the derivation of the componentry have not yet been fully established to a robust quantitative standard. The derivation of the CR, as well as other petrological analyses, is too laborious and time-consuming to introduce as a sustainable monitoring method. In this study, we introduce spectroscopic colorimetry to rapidly and quantitatively describe the color of eruptive products as a proxy for petrological analyses and to derive the CR based on an objective clear standard for ash particle classification. We performed color spectroscopy of bulk and sieved ash samples and analyzed the componentry of the major size fraction for time series samples in the waxing stage of the 2017–2018 Shinmoe-dake eruption, Kirishima volcano, Southwest Japan. We found that the color of the bulk ash changed systematically with the evolution of componentry. This temporal color change was due to an increase in the amount of vesicular particles with clear glass against dark angular lava particles, as well as a grain size change, which we interpret as an indication of transition from phreatic/phreatomagmatic to magmatic eruption. Subsequently, the color of the ash changed when the amount of compositionally different lava particles increased gradually, coinciding with a shift toward a more dominant effusion of lava. As the lava effusion continued, a slight reddening of the ash, indiscernible to the naked eye, was clearly detected by the spectrometer before the onset of intermittent Vulcanian eruptions. We interpreted this to be due to oxidation caused by a decrease in ascent speed and formation of a stagnant caprock at a shallow depth of the conduit, which accumulated overpressure for the Vulcanian explosion. The rapid and objective derivation of the color values and componentry may help sustainable quasi-real-time monitoring of the eruptive materials of ongoing eruptions.