Origin and mode of emplacement of lithic-rich breccias at Aso Volcano, Japan: Geological, paleomagnetic, and petrological reconstruction

Kuniyuki Furukawa, Koji Uno, Taro Shinmura, Masaya Miyoshi, Tatsuo Kanamaru, Hiroo Inokuchi

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


Takajosan breccia rocks are distributed around the southwestern caldera rim of the Aso Volcano in Japan. They are characterized by coarse lithic breccias with a pumiceous matrix. The proximal coarse lithic breccias are divided into the lower massive unit and the upper stratified unit. The lower massive lithic breccias tend to transform laterally into tuff breccias and pumiceous lapilli tuffs. Paleomagnetic results showed that all of the deposits were deposited at high temperatures of 175-560°C. This was also supported by geological characteristics such as spatter clasts, clasts with a bread-crust texture, and weakly welded parts. These features clearly show that the deposits originated from pyroclastic density currents (PDCs). The dense lithic-rich lithofacies, low vesicularity of pumice, lack of plinian fall deposits, and radial distribution indicate that the deposits were derived from boil-over PDCs rather than plinian column-collapse PDCs. The SiO2 contents of the matrix glasses of the proximal lower massive breccia showed a progressive decrease from the bottom toward the upper part. We interpret that this chemical variation corresponds to chemical zonation of the magma chamber. This indicates that the massive deposits aggraded progressively from the base upwards (progressive aggradation), rather than through en masse freezing. The vertical lithofacies changes of the proximal breccias from the lower massive to the upper stratified units indicate that a sustained current in a quasi-steady state switched to an unsteady current with the progression of the volcanic activity.

Original languageEnglish
Pages (from-to)22-31
Number of pages10
JournalJournal of Volcanology and Geothermal Research
Publication statusPublished - Apr 15 2014


  • Aso
  • Breccia
  • Paleomagnetism
  • Progressive aggradation
  • Pyroclastic density current

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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