Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC36] Volcanic and igneous activities, and these long-term forecasting

Sun. May 26, 2019 10:45 AM - 12:15 PM A07 (TOKYO BAY MAKUHARI HALL)

convener:Teruki Oikawa(GSJ, National Institute of Advanced Industrial Science and Technology), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Daisuke MIURA(Department of Physical Science, Graduate School of Science, Osaka Prefecture University), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Chairperson:Nobuo Geshi(GSJ/AIST), Daisuke Miura

11:00 AM - 11:15 AM

[SVC36-08] The initial fine ash deposit of the Shikotsu caldera-forming eruption at ca. 45 ka

*Chiharu Tomijima1, Daisuke MIURA1, Shimpei Uesawa2, Mizuho Amma-Miyasaka3 (1.Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 2.Central Research Institute of Electric Power Industry, 3.Graduate School of Sciences, Hokkaido University)

Keywords:Caldera, transport system, eruption dynamics

We have studied a thin and fine-grained ash deposit that occurs at the bottom of a massive plinian fall unit from Shikotsu caldera, southwest Hokkaido, Japan. This fine ash deposit could be a sign of the initial stage for a catastrophic caldera-forming eruption. The ash is extensively observed at the large number of outcrops for the Shikotsu plinian fall deposit. It is believed that the Shikotsu caldera-forming eruption (SCFE) took place at ca. 42–45 ka.
The lowermost part of the fall unit is divided into two units, A1 and A2. The unit A1 is a well-sorted, thin and fine-grained ash deposit, whereas the unit A2 is constituted by four layers with the alternation of ash and pumice fall deposits. The unit A1 further comprises two sub-layers: the lower layer is a thin (~4 mm thick) and very fine-grained ash layer with sheet-like carbide and the upper one is a crystal rich fine ash layer (~30 mm thick). While the total thickness of A2 is gradually decreasing as far from the source caldera, the thickness of A1 was almost constant everywhere, which extends throughout a distance up to 70 km from the source caldera.
Dry sieve analyses at 0,5 interval between -1 and +4 in φ scale were carried out on 8 samples of A1 deposit. The results of grain-size distribution show a bit of change between 1.26 and 2.25 in Mdφ and between 0.38 and 0.83 in σφ. These changes of Md φ and σφcorrespond to distance from 11 km to 70 km, indicating that A1 is quite well sorted and homogeneous grain-sized deposit in its long-distance transportation. These findings suggest that the unit A1 is a well-sorted, widely dispersed and high temperature dilute flow deposit. It seems to be different from the normal pyroclastic surge deposits and may be resulted from a kind of critical mode of eruption just prior to the caldera-forming plinian eruption.