[S-VC30] Volcanic and igneous activities, and these long-term forecasting

[SVC30-10] Reappraising the Eruptive Age and Classification of Holocene Volcanic Products around Nakadake Crater, Aso Volcano Using Paleomagnetic and Rock Magnetic Methods

*Chisato Anai¹, Yasuo Miyabuchi², Mitsuru Utsugi¹, Shin Yoshikawa¹, Nobutatsu Mochizuki³, Hidetoshi Shibuya³, Takahiro Ohkura¹ (1.Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 2.Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, 3.Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University)

Keywords:Aso Nakadake, paleomagnetic secular variation (PSV)

Nakadake volcano, the current active center of the Aso central cones (Kyushu), is one of the most active volcanoes in Japan. It has been active since ca. 22-21 ka, and has formed the old edifice (22-21 ka), the young edifice (around 5 ka) and a younger pyroclastic cone (up to the present). The lava flows from the young edifice spread on the flank of the cone several times in recent 5 ka. These lavas are supposed to give stratigraphic markers for constructing the eruptive history of Nakadake volcano, but the similarity in chemical composition and lithology hampers distinguishing and correlating them. We have conducted a paleomagnetic study to distinguish and correlate the lavas of different ages since the paleomagnetic secular variation (PSV) provides a high-resolution age information. If lava units have a temporal difference of more than 50 years, they could be distinguished by their paleomagnetic directions. The samples were collected from 14 lava flows and 6 agglutinates layers (welded scoria-fall deposits) and were subjected to the paleomagnetic and rock-magnetic measurements. These samples appear to be influenced by chemical alteration in the surface of the outcrop by sulfides of volcanic gases. To check a rock-magnetic effect of the chemical alteration of the lavas and agglutinates, thermomagnetic analyses were made on chip samples from the top (surface of rock) and bottom (inside of rock) of the collected paleomagnetic cores. The thermomagnetic analyses indicate that the core top and bottom samples show the same behaviors, in spite of the difference in color, and the carrier of magnetization of each core is either titanium rich (titanium content, x, is about 0.6) or poor (x is about 0.15 to 0.3) titanomagnetites. The natural remanent magnetization of each sample shows a simple, single vector component in alternating field demagnetization experiments, which well defines the primary component. Site mean directions can be divided into three different direction groups. These data suggest that the eruption producing lava flows and/or agglutinates occurred at three different ages. One of the direction group is not consistent with the directions of the eruptive ages of Nakadake young edifice assigned from the previous stratigraphic studies. Comparing these directions with the paleomagnetic secular variation curve drawn from basaltic volcanoes in the Aso central cones, the ages of the direction groups can be assigned to around 6.0-4.3 ka and 3.5 ka. The present result will contribute not only to the volcanology, but also to the disaster prevention and disaster risk reduction.