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San'in Kaigan NP, Hyogo
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A Genbudo basalt sample was magnetized south and upward—nearly opposite today’s field. Matuyama compared it with rocks from 36 sites and showed reversed-magnetized rocks were older, evidence that Earth’s magnetic field had flipped.
The familiar Japanese rock name 玄武岩 (genbugan) was coined in 1884 by geologist Bunjiro Koto, inspired by Genbudo. A world-famous type of volcanic rock carries this local place-name in Japanese.
Locals call Genbudo basalt “Nadaishi.” Its sturdy, handy-to-stack pieces went into traditional stone walls and, in everyday kitchens, served as weights for pressing pickled vegetables.
After the 1925 North Tajima earthquake devastated Kinosaki, basalt quarried at Genbudo was used to strengthen the Ootani River’s banks. The local rock became part of the town’s recovery—and its familiar riverside scenery.
In 1807, Confucian scholar Shibano Ritsuzan named the quarry Genbudo after Genbu, a mythical turtle-and-snake guardian. The rock’s polygonal joints reminded him of the creature’s shell.
The dramatic openings are old quarries, not natural caves: people cut away basalt and left striking columns behind. Genbudo itself once had 4 openings; the 1925 earthquake buried one and joined others, changing its outline.
Genbudo’s preservation had an unlikely helper: railway contractor Furukawa Hisakichi donated money after local official Kobayashi Masayoshi organized a preservation group. The group bought land and improved access; a station opened across the river in 1912.
Seiryudo has a second name, Ongakudō—the “Music Cave.” The San’in Kaigan Geopark notes its natural acoustic effect and that performers from different musical genres have held concerts there.
The park has 5 named quarry faces but draws on 4 Chinese directional guardians: Genbu, Seiryu, Byakko and Suzaku. Suzaku—the southern guardian—gets both a North and a South cave.
At Seiryudo, the stone columns are clues to cooling lava, not blocks stacked by hand. As lava cooled and contracted about 1.6 million years ago, cracks grew inward from its surface, forming the striking ranks.
Byakko Cave’s columns run sideways, and their cross-sections are thinner than at Genbudo. Since slower-cooling lava tends to form thicker columns, the fine joints point to faster cooling near the flow’s edge.
South Suzaku’s rough, mostly unjointed rock marks the front of the ancient lava flow. It chilled too quickly to form orderly columns, and small holes in the stone mark places where volcanic gas escaped.
Matuyama’s 1929 paper on magnetic reversals initially drew little attention and was later forgotten. It resurfaced in the 1960s and became one of Japan’s most frequently cited papers for a time.
In 2022, Genbudo was named among the first 100 IUGS Geological Heritage Sites. It was selected from 181 candidates across 56 countries; only 2 sites in Japan made that inaugural list.
