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

Abstract We investigated temporal changes in the magnetic properties of volcanic ash ejected from the Aso Nakadake volcano during a sequence of ash eruptions from 2019 to 2020. Titanium-rich titanomagnetite and titanium-poor titanomagnetite were the dominant magnetic minerals in the samples where titanium-rich titanomagnetite was more dominant. From the rock magnetic measurements, parameters such as the saturation remanent magnetization (Mrs), saturation magnetization (Ms), coercivity (Bc), and titanium content estimated from the Curie temperature (Tc) were extracted and checked for their temporal changes. The magnetic behavior of the magnetic minerals was confirmed by the increasing values of Mrs/Ms and Bc at several periods. The samples with higher values of Mrs/Ms and Bc included titanomagnetite with a low Tc (high titanium content). The clear increase in Mrs/Ms and Bc suggests that the ratio of the single-domain volume fraction increased, indicating that the titanomagnetite particles became finer in size. Interestingly, the periods of high Mrs/Ms and Bc were synchronized with observations of the volcanic glow. These results suggest that changes in the magnetic properties of volcanic ash reflect changes in physical and/or thermal conditions from the vent to the conduit.