By HIROYUKI TAKEI/ Staff Writer
April 5, 2022 at 07:10 JST
Tomohiro Tsuji points to part of the pyroclastic flow deposits, comprising a stack of orange-hued and dark-gray geological formations, in Yamaguchi’s Tokuji-Yunoki district on March 16. (Hiroyuki Takei)
YAMAGUCHI—An eruption of Mount Aso in Kumamoto Prefecture 90,000 years ago created a pyroclastic flow that traveled 170 kilometers north to the Sabagawa river here, the longest such distance recorded in Japan, a scientist said.
Tomohiro Tsuji, an assistant professor of geology with the Yamaguchi University graduate school, said on March 16 that deposits from the pyroclastic flow were discovered last December at a construction site along a prefectural highway in the Tokuji-Yunoki district of Yamaguchi city.
Rolling geological formations with orange and dark-gray hues were found stacked on top of another for a total thickness of 20 to 50 centimeters.
An analysis of the glassy components and mineral crystals contained in the deposits revealed characteristics of the “Aso-4,” the fourth caldera-forming eruption of Mount Aso that occurred 90,000 years ago.
Charred pieces of wood were also found mixed in the deposits, indicating the temperature of the pyroclastic flow exceeded 300 degrees.
The Aso-4, one of the largest known eruptions in Japan, covered the entire archipelago with volcanic ash. The ash reached a depth of 30 cm in Shikoku and 15 cm in Hokkaido.
The pyroclastic flows of hot gas and volcanic ash reached inland areas of Yamaguchi Prefecture, where deposits have been discovered at many spots along the Fushinogawa and Kotogawa rivers.
Previously, the longest distance set by a pyroclastic flow was 166 km after the Aso-4 that traveled to a location in Hagi, also in Yamaguchi Prefecture.
The latest finding is located in the upper reaches of the Sabagawa river, 20 km northeast of the previously easternmost location where the flow reached in Yamaguchi’s Niho district.
The pyroclastic flow likely went over the Notanidao Pass, also in the city, which currently stands about 320 meters above sea level, Tsuji said.
Pyroclastic flows sometimes travel over the sea. But this particular flow remained hot in Yamaguchi Prefecture, indicating that water did not separate Mount Aso, now on the southernmost main island of Kyushu, from the prefecture at the time, the geologist added.
“I was surprised to learn the pyroclastic flow traveled this far, which shows it was so powerful,” Tsuji said.
Tsuji said he will conduct additional research and search for deposits further upstream along the Sabagawa river.
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