The document summarizes a field trip report on Aso Volcano in Kyushu, Japan. [1] A 90,000-year record of eruptions from Aso Volcano has been reconstructed from tephra layers, with 36 distinct silicic pumice-fall deposits providing key markers. [2] Post-caldera activity has produced over 100 cubic km of material at an average rate of 1.5 cubic km per 1000 years. [3] The largest eruption was 30,000 years ago and produced over 2 cubic km of material.

1. FIELD TRIP TO ASO VOLCANO
Kyushu, Japan
Momikura sensei (Kiso-Jiban), Kyi Khin, Fujii SATOMI (Kyushu SPC), 2002

2. INTRODUCTION
• A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan,
could be studied to understand the post-caldera eruptive history of the volcano. Post-caldera
central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption
(90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic
sequence preserved above the caldera-forming stage deposits reaches a total thickness of
100 m near the eastern caldera rim.
• The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic
pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence.
Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at
intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be
correlated with lava flows or an edifice in the western part of the central cones, although the
other silicic tephra beds were erupted at unknown vents, which are probably buried by the
younger products from the present central cones. Most of silicic eruptions produced deposits
smaller than 0.1 km3, but bulk volumes of two silicic eruptions producing the Nojiri pumice
(84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km3 (VEI 5). The
largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This
catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km3).
• Total tephra volume in the past 90,000 years is estimated at about 18.1 km3 (dense rock
equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is
calculated at about 112 km3, which is six times greater than the former. Therefore, the
average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at
about 1.5 km3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.
Yasuo Miyabuchi (2009) A 90,000-year tephrostratigraphic framework of Aso
Volcano, Japan, Sedimentary Geology, Vol. 220, pp. 169-189.

3. Ring of Fire

4. Scenic Views of ASO Caldera

5. Hot and Cool POND?: A Window from the below of Earth

6. Aerial view of ASO VOLCANO, KYUSHU, JAPAN
Courtesy of Yasuo Miyabuchi (2009) A 90,000-year tephrostratigraphic framework
of Aso Volcano, Japan, Sedimentary Geology, Vol. 220, pp. 169-189.

7. Volcaniclastic stratigraphy (Tephrostratigraphy): Tephra ASO1 to 4

8. Photographs of representative
tephra sequences above the Aso-4
pyroclastic-flow deposit (ca. 90 ka)
at sites A9746 (A) and A9831 (B).

9. Aerial View of Mt. ASO volcano, One of the largest active volcano in Japan

10. Geological Map of ASO VOLCANO and its surrounding (Geological Survey of Japan)

11. Geological Map done by Momikura sensei, 2002

12. Large Volcanic bomb

13. Tephrostratigraphy

14. ASO-4 Member: Volcaniclastics with pumice and rock fragments

15. Scours and casts
ASO – 4B
(Ash Falls)
ASO-4A
(Volcaniclastic Flows)

16. ASO-3B : Volcaniclatic Fall with pumice and rock fragments

17. ASO-4A
Ash Fall
ASO-3B

18. ASO-4A
Ash Fall
ASO-3B

19. ASO-4B
ASO-4A
ASO-3

20. ASO-4 : Surge and ash fall in subaqueous condition

21. ASO-4A : Clastic Flows
ASO-3

22. ASO-3- ASO-4 : Surge
ASO-2 –ASO-3: Ash Fall (Clayey Fall)

23. ASO-3
ASO-2 and 3 Interval
ASO-2

24. ASO-4
ASO-3

25. ASO-2

26. ASO_2 : with Volcanic Bomb (Bread crust)
ASO-3 : Debris Flows

27. Hohi Andesite
Volcanic breccia
ASO-1 (Andesitic Volcanic Rock)
Hohi Andesite

28. ASO Tephrostratigraphy, Momikura, 1972

29. LEARNING IS BEAUTIFUL
BUT
KNOWING IS MOST BEAUTIFUL
MOMIKURA, 2002