Deposit temperature and sedimentation process of Aso-4 pyroclastic flow deposit

Large-scale pyroclastic flows sometimes travel long distances. There are various debates about the long-distance movement mechanism of pyroclastic flows. In particular, the influence of the sea is still under debate. The steam generated by the interaction between sea surface and the pyroclastic flow promotes the current of the pyroclastic flow. However, as the propulsion is not fixed to one direction on a flat sea surface, the propulsion force is considered to be decreased. The Aso-4 pyroclastic flow erupted about 90,000 years ago and the deposits are observed in Yamaguchi Prefecture that has a distance of more than 120 km from the source. In the process of travelling, it should have overcome obstacles such as Seto Inland Sea and topographical heights. It is a demanding research target for elucidating the movement mechanism of pyroclastic flows. To discuss the effects of the sea on the Aso-4 pyroclastic flow, we measured the deposition temperature using charcoal in Aso-4 pyroclastic flow deposits (PFD) in Yamaguchi Prefecture. Charcoal are often found in Aso-4 PFD. The temperatures of the Aso-4 PFD were measured from the reflectances and the Raman spectroscopy thermometers using them. The samples were taken from the Aso-4 PFD at three localities; Setobara, Hanaka, and Matsubara, (126.7 km, 136.7 km and 145.3 km from Nakadake crater, Aso volcano, respectively). The reflectance (Rm) measurement result Rm = 0.83-1.29 %. Based on the results, the heated temperatures were estimated to be 330-522 ℃ in the Setobara, and Hanaka and 288-429 ℃ in the Matsubara. The results of Raman spectroscopy showed that the D and G bands were measured. Based on these results, the heated temperature was estimated to be 192-324 °C in Matsubara and 151-319 °C for Hanaka. Compared to the estimated temperature of 337 ℃ for Aso-4 pyroclastic flow charcoal reported in Saga Prefecture, Kyushu (80.3 km from Nakadake crater, Aso volcano), no significant decrease of the temperature beyond the Seto Inland Sea was observed.