Introduction to Landforms and Geology of Japan

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Columnar Joints and Landscapes

Jogasaki Coast
Jogasaki Coast

The Jogasaki Coast about 9 km long was formed by lava flows in the eastern Izu Peninsula (location). Well developed columnar joints are found at Hashidate near the southernmost Jogasaki Coast. The landforms consist of cliffs with columns and wave cut benches with hexagonal patterns like those of Tsumekizaki, but the columns are much larger, 70 to 150 cm wide. The columns become narrow and unclear at the end of the bench (sea side). Columnar joints are also exposed at Kosaitsuna and Saitsuna to the north of Hashidate. There is a sea cave formed by collapse of columns at Kosaitsuna.

 

 

Jogasaki Coast (Hashidate)
Photo 13: Columnar jointing at Hashidate

Wave cut bench at Saitsuna
Photo 14: Wave cut bench at Saitsuna

Sea cave at Kosaitsuna 
Photo 15: Sea cave at Kosaitsuna

The lava flows of the Jogasaki Coast are basaltic andesite and were spouted in a volcanic activity 4000 years ago. This activity formed Omuroyama (scoria cone) to the west of the coast. Since the lava flows are young, their morphology and structure are well preserved in the coast. The Jogasaki Coast, therefore, is a good observation point of lava flow. For example, you can walk on the rough surface of lava flow, which is a characteristic of aa lava and massive lava, at Kannongane and Igaigane. Wrinkles remain on the surface, the heights of which are 0.3 to 0.5 meters and the intervals are 2 to 3 meters. Wrinkles are formed perpendicular to the flowing direction when a lava flow reaches on a gentle slope from the steep and at the forward end of lava flow. At Nicho, the texture of lava is glassy and porous, different from that of lava in Hashidate. Rapid cooling made the lava glassy, and gas emitted from the lava produced the pores.

Lava at Kannongane
Photo 16: Lava at Kannongane

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