Genbudo Cave

Genbudo Cave is a set of exposed basalt outcrops near Toyooka in Hyogo Prefecture, Japan, formed from lava that erupted and cooled roughly 1.6 million years ago. As the lava mass cooled slowly and evenly, it contracted into the tall, distinctive hexagonal columns of columnar basalt for which the site is known, the same jointing process that shapes other basalt sites in this section such as Giant's Causeway and Fingal's Cave. The site holds particular scientific significance because Japanese geophysicist Motonori Matuyama used its magnetized basalt in the 1920s as part of the evidence for geomagnetic reversal, the discovery that Earth's magnetic field has flipped polarity in the geological past; the reversed-polarity epoch that includes the site's formation is now named the Matuyama Chron in his honor. Quarrying at the site through the 20th century exposed several named rock faces including Genbu-do, Seiryu-do, Byakko-do and Sujaku-do, each named for one of the four traditional Chinese guardian deities. This entity documents Genbudo Cave specifically as a geological formation, a columnar basalt site whose magnetized rock helped establish the discovery of geomagnetic reversal.