Around 90,000 years ago, a huge eruption occurred at the Aso volcano. It was an event of unimaginable scale, and the largest in its history. This eruption caused huge pyroclastic flows (fast-moving currents of hot volcanic gas, ash, and rocks) to smother nearly the entirety of the area that makes up today's Bungo Ono City.
The pyroclastic flows spread to cover most of Kyushu, partially even crossing over the sea to reach today's Yamaguchi Prefecture on the main island of Japan. The pyroclasts accumulated several tens of meters high in the plains of central Bungo Ono, with some of it melting from its own heat before cooling and hardening to form into a type of rock known as ignimbrite (welded tuff).
Shrinking as it cooled, it formed countless vertical cracks. This formed what we call columnar joints. These joints of ignimbrite can be seen throughout the Oita Bungoono Geopark.