Temporal change in rock-magnetic properties of volcanic ashes ejected during a 1-year eruption event: a case study on the Aso Nakadake 2019–2020 eruption

Abstract

We investigated temporal changes in the rock-magnetic properties of volcanic ash ejected from the Aso Nakadake volcano during a sequence of ash eruptions from 2019 to 2020. For 39 volcanic ash samples, magnetic hysteresis parameters, including saturation magnetization (Ms), saturation remanent magnetization (Mrs), coercivity (Bc), and coercivity of remanence (Bcr), were obtained. Curie temperature (Tc) of the samples was also estimated using thermomagnetic analyses. Titanium-rich and -poor titanomagnetites were the dominant magnetic minerals in the volcanic ash, of which the titanium-rich phase was dominant. Systematic magnetic measurements of the volcanic ash ejected during the 1-year eruption event indicate that temporal changes in the hysteresis parameters occurred throughout the event. These temporal changes suggest that the Mrs/Ms and Bc values of the volcanic ash increased considerably during several periods. The clear increases in Mrs/Ms and Bc, associated with the central peak in FORC diagrams, indicate that non-interacting single-domain grains increased. For these high Mrs/Ms and Bc samples, thermal demagnetizations of 3-axis IRM show that the low unblocking-temperature component up to 250–300 °C has apparently higher coercivity, suggesting that the above-mentioned, non-interacting single-domain grains are Ti-rich titanomagnetite. Interestingly, the high Mrs/Ms and Bc values were synchronous with observations of volcanic glow. These results suggest that changes in the magnetic properties of titanomagnetite grains in volcanic ash reflect changes in physical conditions from the vent to the conduit of the volcano. Graphical Abstract: [Figure not available: see fulltext.]