Columnar Joints and Landscapes
Scenic spots of columnar jointing
There are many volcanic rock outcrops accompanied with columnar jointing in Japan. This section introduces six scenic spots of columnar jointing: Genbudo (Hyogo), Tsumekizaki, Kawazu Nanadaru, Jogasaki Coast, Hinomisaki, and Takachihokyo.
Genbudo in northern Hyogo Prefecture is famous for excellent curved joints (location below). This place was a quarry. The Genbudo lava is basaltic, about 100 m thick (in this area) and erupted 1.6 million years ago. There are five caves (exposures of columnar joints): Genbudo, Seiryudo, Byakkodo, Kita-suzakudo, and Minami-suzakudo. Major caves are Genbudo and Seiryudo.
Genbudo consists of vertical columns in the lower part and columns curved upward in the upper part. Almost horizontal columns are found in the right upper part. The upper part borders on the lower part with a discontinuity plane. Such plane may suggest a temporal gap between the layers. However, it is thought that there is no temporal gap between the upper and lower parts because no oxidized zone indicating a temporal gap is found in the border zone. The discontinuity resulted from differential cooling lava flow (Suwa et al., 1991).
Photo 2: Genbudo
The Seiryudo outcrop, 40 m wide and 33 m high exhibits graceful curves converging upward. Since this appearance is associated with a dragon soaring up, this cave was named “Seiryu-do”. “Seiryu” means “(blue) dragon” and “do” means “cave” in Japanese. Cross sections of truncated columns show hexagonal patterns in the right side of the outcrop. The columns are 20 to 50 cm wide and have platy joints perpendicular to the elongated direction of columns at 7 to 20 cm intervals.
Photo 3: Seiryudo
In Byakkodo, horizontal columns can be easily observed. The columns are narrower than the vertical columns in Genbudo. The size of column depends on cooling rates of lava as mentioned above. Therefore, these columns cooled more rapidly than those in Genbudo. The lava in Byakkodo is thought to be near the forward terminal of the lava flow.
Photo 4: Byakkodo
Photo 5: Cross sections of columns
The cross sections of columns are mainly hexagonal, but other polygons
such as pentagon and heptagon are found.
Suwa et al., 1991, counted
each type of columns in some outcrops and showed the percentage of each
type as follows: 66% hexagon, 21% pentagon, 10% heptagon, and 2%
octagon. There are no triangle, tetragon, nonagon, and more polygons in
the outcrops. The relative sizes of cross sections do not vary
Genbudo is a historical place in terms of geology. Japanese name of basalt, “Genbu-gan” derives from the lava of Genbudo (“gan” means rock in Japanese). Moreover, Matsuyama Motonori discovered the reversed polarity known as the Matsuyama Reverse Epoch (2.48-0.73 Ma) in paleomagnetic stratigraphy, in his study of natural remanent magnetization of the Genbudo lava (published in 1929).
Location: Akaishi, Toyooka City, Hyogo Prefecture (On
the Google map)
1) Genbudo Station on JR Sanin Main Line. It takes about 2 hours and 40 minutes from Kyoto using a limited express train. Take a boat (300 yen/one way, 3 min.) to cross the river because Genbudo is on the other side. The boat service may be temporally unavailable due to weather, etc., so ask Genbudo Museum (0796-23-3821).
2) Kinosaki Onsen Station on JR Sanin Main Line. Take a taxi (10 minutes from the station). Also you can hire a bicycle at the information office of accommodation in front of the station. 30 minute cycling to Genbudo
[Names written in Japanese]
Kinosaki Onsen Eki (station): 城崎温泉駅