Introduction to the Landforms and Geology of Japan

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The 1888 Eruption of Bandai-san and Colored Lakes

Water colors of the Goshikinuma lakes

The 1888 eruption resulted in making many lakes including Lake Hibara, Lake Onogawa, Lake Akimoto, and the Goshikinuma lakes at the foot of Bandai-san.

The Goshikinuma lakes consist of dozens of lakes and swamps. Major lakes include the Bishamon-numa, Aka-numa, Midoro-numa, Tatsu-numa, Benten-numa, Ruri-numa, Ao-numa, and Yanagi-numa. The term “numa” in Japanese refers to a marsh, swamp, or pond. These ponds are about 4 to 13 meters deep and 100 to 4000 meters around. The Goshikinuma lakes are characterized by various colors, turquoise blue, blue-green, reddish brown and so on. Bishamon-numa, Benten-numa, Ruri-numa, and Ao-numa have a beautiful turquoise blue or blue blended with white. Lake water colors vary depending on the reflection of blue sky and green of tree leaves, weather conditions, substances in the water, and deposits and water plants on the bottom of lake. The turquoise blue of the ponds is attributed to white suspension of allophane colloid particles and short wavelength light (blue) diffused by the fine particles. Allophane is a poorly-crystalline, hydrous aluminosilicate clay mineral (Al2SiO5·H2O). This mineral is commonly found in volcanic ash soil.

Ao-numa
Photo 3: Ao-numa [Another window]

The major ponds are interconnected by two drainage systems. In the first drainage system, water flows from Ruri-numa through Ao-numa, Benten-numa, Tatsu-numa, Midoro-numa, Bishamon-numa into the Nagase River. In the second drainage system, water flows from Yaroku-numa through Chichi-numa, Haha-numa, Yanagi-numa into the first system at the upper stream of the Tatsu-numa. Aka-numa does not belong to either of the two systems, provided with spring water. This pond has a reddish color because the water is rich in iron and manganese and oxidized iron attaches to stalks of water plants.

Ruri-numa
Photo 4: Ruri-numa [Another window]

Since the ponds drain into the Nagase River, the ponds cannot keep their colors unless substances creating the colors such as allophane are continuously provided to the drainage systems. There is a pond, Aka-numa, in the crater of Bandai-san. The water color of this pond is not blue but reddish brown. This is the color of deposits yielded by insolubilization of iron (III) ions. Stones in the pond are stained color of iron rust by the deposits (Photo 5). Aka-numa in the crater is considered a headspring of the Goshikinuma lakes.

This “Aka-numa” is the same name as the pond in the Goshikinuma lakes mentioned above but the Japanese character of Aka-numa in the crater is different from that of another Aka-numa. “Aka-numa” in the crater is “銅沼” in Japanese, that in Goshikinuma is “赤沼”. “銅” refers to copper, “赤” to red. 

Aka-numa
Photo 5: Aka-numa in the crater [Another window]

The water is pH3, sulfuric acid, and rich in iron (10 to 30 mg/l), manganese (10 mg/l), and aluminum (150 mg/l). The sulfuric acid is produced from oxidized sulfur compounds ejected from fumaroles in the crater. The acidic water erodes rock, releasing substances including iron, manganese, and aluminum from the rock into the water. In addition to these substances, the pond contains high levels of molecular silica. Allophane is not produced in strong acidic water although it is a compound of aluminum and silica. Groundwater running from Aka-numa in the crater flows into Ruri-numa about 3.3 km from Aka-numa via Midori-numa. Since water of Ruri-numa is pH 4.5 to 5.0, the water from Aka-numa is thought to be neutralized on the way to Ruri-numa. There are hot springs of which alkalescence water contain sodium, chlorides, and hydrogen carbonate ions around Bandai-san. Therefore, the influx of alkaline hot spring water into the water accounts for the neutralization. It is known that chloride ions in spring water abruptly increase in a ski area between Aka-numa in the crater and Ruri-numa. Alkaline hot spring water is likely to flow into the water running from Aka-numa in or around the ski area. Most iron have deposited in Aka-numa in the crater because the spring water at the ski area is very poor in iron. (Chiba, 1988 and 1989)

The process of producing allophane in the Goshikinuma lakes is as follows: acidic water containing aluminum and silica, which are materials of allophane, flows out from Aka-numa in the crater of Bandai-san. Most iron in the water deposits in and near Aka-numa. The acidic water is neutralized by mixing alkaline hot spring water on the way to Ruri-numa, and then allophane is produced by combination of aluminum with silica.

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