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Guest: Japan Geology Notes

    My Japanese friend; Hirotoshi Nishiwaki, who has learnt geomorphology and physical geography at University, has visited many places of geological interest, this time he visited Akagi Volcano, just to the north-east of the city of Maebashi in Gunma Prefecture in Central Japan.

      The following notes and photos are from Hirotoshi Nishiwaki:

Akagi volcano:
    Akagi Volcano is a large stratovolcano, which extends 20 kilometers east-west and 30 kilometers north-south with an area of 500 square kilometers. This volcano consists of a broad foot with gentle slopes, bodies with steep slopes, a caldera on its summit and central cones in the caldera. The main body is made of an old stratovolcano that is well eroded and a younger stratovolcano of which the landforms on its surface are well preserved. The old body was formed by the eruption of basaltic-andesite lava, and the younger body is made of mainly thick andesite lava and lava domes. The foot of the volcano is younger than this young body, which consists of andesite-dacite pyroclastic deposits and debris flow deposits produced from the pyroclastic deposits. The caldera 3 by 4 km in diameter was formed during the last activity of Akagi Volcano. The caldera lakes and the lava domes are distributed in the caldera.
   The activity of Akagi Volcano is thought to begin over 300,000 years ago and to form the stratovolcano until about 220,000 years ago. The large eruptions of pyroclastic flows started from 130,000 to 140,000 years ago and continued until about 50,000 years ago, which formed the caldera on the summit at the last stage of this activity. The forming of lava domes and pumice flows occurred in the caldera 32,000 years ago. There is no reliable record of eruptions afterward.
   According to a field guide book of the volcanoes in the Kanto area, you can see many volcanic features at Akagi Volcano: lava domes, caldera and caldera lake, lacustrine sediments, tephra from other volcanoes and lava around the summit, pyroclastic flow deposits - pumice and welded tuff, mudflow deposits, fan of the volcano foot and its deposits, and stones in streams around the volcano.
   This volcano is in Gunma Prefecture on the way from Tokyo to Nagano. From my house in Yokohama to Akagi, the distance is about 170km. I made it a one day trip, so I visited three sites. It was a quite fine spring day. I had a nice trip. First I went into the caldera of Akagi and climbed Jizodake that is a lava dome in the centre of the caldera. It was easy to get the top. It took about 20 to 30minutes. I found a ropeway, but it didn't work. I didn't know why.
   I got wonderful view on the top (see photos: Akagi Ono, Akagi Kono). You can look out over the caldera and volcanoes around Akagi from the top. There seems to be some good walking tracks in the caldera. I walked around the caldera lake and explosion caldera (Kono). Although it was the weekend, there were few people, so it was peaceful. The season when I visited was too early for tender green spring growth yet. I think that if you were to go there when it is the season of fresh spring growth or red leaves of autumn, it must be beautiful.
   There is a visitor centre at the foot of Jizodake. There you can see displays of the geology, the vegetation and animals of Akagi. The explanations were not so detailed, and all of them were written in Japanese.
   I heard an interesting thing; when I was walking near the lake, I talked to a man who was doing road maintenance. He said that a hot spring had been tapped near Kono. The hot water spouted out so much from the well that it was running into the river. As a result, a fisherman's association complained about it. He guessed the reason that the hot water was too rich in salt content. So the well was plugged to stop it flowing into the river.
   Next I went to Tanashita to see outcrops of welded tuff and waterfalls. This area was interesting. (Photos: Tanashita Fudodaki, outcrops of welded, and a stratum of conglomerate (Akagi fudoreki)). There is also a Shinto shrine. So you can see a traditional Japanese cultural landscape.
   By the way, I found a small museum that is called "Conodont Kan (kan is Japanese)" in Omama near Akagi. This museum introduces the nature, history and folkways of Omama in addition to conodonts, which are very small fossils. The town is known as the birthplace for the studying of conodonts in Japan, because the first discoverer of conodonts in Japan, Shingo Hayashi, was from Omama. That is why the local government named this museum "Conodont Kan". The building of the museum, which was a bank, is very old, built in 1921. It is a precious historic western building in Taisho era. You can see the photos of the building at the web site of the museum (The text in this site is written in Japanese): Visit the Museum site at: Conodont Kan

Photo 1 Akagi Ono: This picture is from the top of the Jizodake. Ono is the caldera lake. The highest mountain by Ono is a part of stratovolcano in the first stage.
	Photo 2 Akagi Konuma: This picture is of the way to the top of the Jizoudake. Konuma is the caldera lake. The mountain around Konuma is a lava dome.
Photo 3 Tanashita Fudodaki: There is a waterfall barely visible in the upper centre of the photo. The rock is welded tuff. This fall is called "medaki", "me" means female and "daki" means fall.
	Photos 4 & 5 Tanashita fudodaki This fall is not so high but is beautiful. The fall is the one of the 100 best falls in Japan. The rock is welded tuff. This fall is called "odaki", "o" means male.
Photo 6 Akagi Fudoreki: Most of these conglomerate boulders are andesite; it is thought that the conglomerates were derived from Akagi Volcano.
	Photo 7 Akagi Welded tuff: A welded columnar jointed tuff of Tanashita
Acknowledgement: To Hirotoshi Nishiwaki

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