Latest Issue

2024 Vol.133 No.1

Genbudo in Toyooka City, Hyogo Prefecture

Genbudo was formed via erosion and mining from basalt that erupted ca. 1.6 million years ago. It has remarkable columnar joints that remind visitors of the cooling process of magma. In 1807, an Edo-period scholar Ritsuzan SHIBANO named this cave Genbudo after Genbu, an imaginary turtle-like animal, because the polygonal joints resembled the shell of a turtle. In 1884, Dr. Bunjiro KOTO of the University of Tokyo coined the rock name Genbu-gan basalt in Japanese, after the cave. In 1929, Dr. Motonori MATUYAMA of Kyoto University measured the magnetization of basalt and demonstrated that there had been a reversal of the Earth’s magnetic field (Matuyama, 1929), and also made great contributions to the development of paleomagnetology and the establishment of plate tectonics. The period of geomagnetic polarity reversal in ca. 2.58—0.77 million years ago is called the Matuyama reverse polarity chron. The last polarity reversal is known to correspond to the beginning of the Chibanian Age (Suganuma et al., 2021). In 2022, Genbudo was selected by UNESCO to be one of the top 100 outcrops in the world, and it is an important outcrop in Japanese geology. The image was taken using a 360-degree camera with permission from Genbudo Park¹⁾.

(Photograph & Explanation: Kenji KAWAI, Tomohiko SATO
and Sanefumi SHOJI; May 15, 2023)

Note

1) https://s.insta360.com/p/529abe323a3fe013f80e522cc25376f0?e=true&locale=en-us [Cited 2023/9/26].

References

Matuyama, M. (1929):On the direction of magnetisation of basalt in Japan, Tyosen and Manchuria. Proc. Imp. Acad. Japan, 5, 203-205.

Suganuma, Y., Okada, M., Head, M.J., Kameo, K., Haneda, Y., Hayashi, H., Irizuki, T., Itaki, T., Izumi, K., Kubota, Y., Nakazato, H., Nishida, N., Okuda, M., Satoguchi, Y., Simon, Q. and Takeshita, Y. (2021) :Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan. Episodes, 44, 317-347.