Genbudo Park

Home to the five beautiful basalt Genbudo Caves with over 1.6 million years of geological history, Genbudo Park was designated in 1931 as a National Natural Monument by the Japanese Government due to their significant role in the discovery of reverse magnetization, and the impressive caves are also part of the San’in Kaigan UNESCO Global Geopark, making it a perfect educational outdoor trip for visitors to the area.

Genbudo, Japanese for Basalt Cave and can also be interpreted as the Black Tortoise Cave, consists of 5 caves made of columnar joints that formed over 1.6 million years ago after the magma from a volcanic eruption cooled, contracted, and formed vertical cracks.

These curvaceous columns have a distinctive polygonal shape and a honeycomb pattern. The five caves are each named after four Chinese mythological creatures Genbu (Black Tortoise), Seiryu (Azure Dragon), Byakko (White Tiger) and Suzaku (Phoenix). Two caves are named after the Phoenix, North Phoenix and South Phoenix.

Before becoming a park and museum for the public to enjoy, the Genbudo Caves were utilized as a quarry during the Edo Period (1603~1867), resulting in the recessed areas of the cave that are still viewable to this day. Locals discovered that the rocks could easily be used for building materials, and visitors can still see these basalt rocks just about everywhere during one’s visit to the area, including in the park’s walkways.

The basalt rocks also hold scientific and historical importance. In 1926, the Japanese geophysicist Motonori Matuyama was the first to surmise that the Earth’s magnetic field had undergone reversals in the past, a theory made here when he discovered that the polar magnetic alignment of this basalt is the opposite of the Earth’s current magnetic polarity. Evidence was found in Genbudo that sometime during the formation of this basalt the Earth had experienced a reverse of magnetic polarity.