Hagi Geopark: Hagi Castle Town Area
The area around Hagi was shaped by three distinct periods of geological activity. Between 100 and 30 million years ago, there was a huge upsurge in magma beneath the Amurian plate. Beginning 20 million years ago, magma beneath the seabed slowly emerged over a period of 8 million years. From 2 million to 8,800 years ago, the area around Hagi was formed by island arc magma. Even by the standards of the Japanese archipelago, which is particularly seismically active, Hagi has a long history of volcanic upheaval.
Between approximately 1 million and 1.3 million years ago, a series of volcanic eruptions created the Aono Volcano Group. The eruption that created Mt. Nosaka also dammed a river, forming a massive lake. Eventually the lake burst its banks, and the water flowed into a river to the north, creating the Abu River that we see today. The mud that collected on the lakebed was gradually washed downriver, and sand from the seabed was pushed up by the waves, forming a delta where the river met the sea.
It was on this delta of mud and sand that Hagi Castle Town was built. Much of the stone used in the construction of Hagi Castle Town was hewn from igneous rock on Mt. Kasayama. Andesite rock, skillfully extracted by stoneworkers, was ferried across the bay in small boats to the town. Once ashore, the rock was further shaped on the northeast side of Mt. Shizuki before being used to construct the castle walls. Many of the rocks that were abandoned mid-process can still be seen along the shore at the foot of Mt. Kasayama today.
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