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Various components contained in hot spring water easily precipitate (settle) due to rapid temperature and pressure drops after gushing out, release of carbon dioxide, oxidation due to contact with air, and changes in pH. These precipitated substances formed from components in hot spring water are collectively called "Onsenka".
The so-called "Yunohana" seen in hot spring baths is also a type of Onsenka, formed by the precipitation of components such as sulfur contained in the hot spring. Additionally, the deposits called scale that adhere to the inside of bathtubs and pipes through which hot spring water passes are also a type of Onsenka formed by the precipitation of hot spring components.
The components contained in hot springs are diverse, and various types of Onsenka are formed depending on the differences in the components. Onsenka is classified into Keika, Sekkaika, Sulfurka, Ironka (Brown Ironka), and Sulfateka, depending on the differences in the main components that make up the Onsenka.
(1) Sulfateka
Precipitated substances (Onsenka) with sulfates such as calcium sulfate (gypsum), sodium sulfate (mirabilite), and barium sulfate (barite) as the main components are specifically called Sulfateka.
From the hot spring water gushing out from the large geyser source of Tamagawa Onsen in Akita Prefecture, a sulfateka called Hokutolite, which forms a solid solution of barium sulfate and lead sulfate, is generated. Hokutolite contains trace amounts of radium and is known for its radioactivity. It is a very rare hot spring precipitate that is only found in two places in the world: Beitou Hot Spring in Taiwan and Tamagawa Onsen in Akita Prefecture, and is a representative of "hot spring minerals" formed from hot springs. Hokutolite from Tamagawa Onsen was designated as a special natural monument of Japan in 1952. Additionally, Hokutolite from Beitou Hot Spring in Taiwan is also designated as a natural monument in Taiwan.
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Hokutolite from Tamagawa Onsen (provided by Nobuyuki Sasaki) | Hokutolite from Beitou Hot Spring in Taiwan (provided by Nobuyuki Sasaki) |
(2) Keika
Precipitated substances (Onsenka) with silicic acid contained in hot springs as the main component are called Keika. This is formed by the precipitation of dissolved silicic acid (silicic acid dissolved in water) called metasilicic acid (H2SiO3) contained in hot spring water.
There are many hot spring areas nationwide that generate Keika, such as Osorezan in Aomori Prefecture, Akinomiya Onsen in Akita Prefecture, and Shiraike Jigoku in Kannawa Onsen, Oita Prefecture.
From Akinomiya Onsen in Akita Prefecture, a fish egg-shaped Keika called "Jijoukeiseki" is produced. This is a rare Keika (chalcedony) formed by the aggregation of spherical opals and chalcedony, and is also called "Buriko Stone" because its appearance resembles the eggs (Buriko) of the fish Hatahata. It is designated as a natural monument of Japan. Additionally, from the new hot spring of Tateyama Onsen in Toyama Prefecture, a Keika called Gyotekiseki, which is a transparent fish egg-shaped opal precipitated from silicic acid in hot spring water, is known to be produced.
Keika from Kannawa Onsen in Beppu, Oita Prefecture
Owned by Gero Hot Spring Museum
(3) Sekkaika
Precipitated substances made of calcium carbonate are called Sekkaika. It is the most commonly seen among hot spring precipitates. Many of the scales that adhere to the inside of pipes that carry hot spring water and bathtubs are this Sekkaika.
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Scale of Sekkaika adhering to a hot spring pipe | Scale of Sekkaika adhering to a bathtub |
(3) Sekkaika
Hot spring water that dissolves bicarbonate ions (HCO3-) and calcium ions (Ca2+) along with carbon dioxide underground precipitates as calcium carbonate when it gushes out to the surface due to decompression, causing carbon dioxide to escape into the air. The chemical reaction is as follows:
Ca2+ | + 2HCO3- | → | CaCO3 ↓ | + H2O | + CO2↑ |
Calcium ion | Bicarbonate ion | Calcium carbonate | Water | Carbon dioxide |
In places where hot springs containing bicarbonate ions and calcium ions naturally gush out to the surface, the above reaction occurs, and calcium carbonate precipitates (Sekkaika) continue to be generated, resulting in a large accumulation of Sekkaika around the gushing holes.
At Iwama Onsen in Ishikawa Prefecture, there are countless natural gushing hot spring sources along the river, and a large amount of hot spring water gushes out. Around each gushing hole, Sekkaika, a precipitate of hot springs, accumulates in a tower shape, and hot water gushes out from the top. Such tower-shaped accumulations of Sekkaika with gushing holes are called geyser towers. As Sekkaika continues to accumulate upward due to hot spring components, the position of the gushing hole gradually rises.
At Iwama Onsen, there are countless geyser towers, both large and small, with the largest reaching over 2 meters. They are designated as a special natural monument of Japan as a group of geyser towers.
At Geto Onsen in Iwate Prefecture, hot springs gush out from the slopes of the Geto River, forming a dome-shaped Sekkaika (Sekkaika dome) down to the riverbed. It is called Tengu Rock because its appearance resembles a Tengu. The Sekkaika dome is 17.6 meters high, and as a large Sekkaika deposit formed at a natural hot spring gushing site, it is designated as a natural monument of Japan.
Near the Nigori River Onsen in Gifu Prefecture, there are many places where hot springs naturally gush out, and small-scale Onsenka can be seen everywhere. Among them, on the right bank of Zaimoku Falls, there is a terraced Sekkaika dome about 20 meters long. Although not well known, it is not inferior to other Sekkaika deposits designated as natural monuments of Japan.
In places where Sekkaika is forming outdoors, in autumn, leaves fall and are coated by calcium carbonate precipitates, hardening and becoming fossil-like. In hot spring areas, this is sometimes called "leaf stone" or "leaf fossil". Of course, it is not a real fossil.
The large Sekkaika deposit at Shin-Hirayu Onsen, which produces leaf stones, has been revealed by Sekkaika dating to have accumulated around the Nara period.
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Geyser tower of Sekkaika at Iwama Onsen, Ishikawa Prefecture | Sekkaika dome at Geto Onsen, Iwate Prefecture |
(4) Sulfurka
Precipitated substances formed by the precipitation of sulfur components contained in hot springs are called Sulfurka. They are especially common in volcanic fumarole areas and the surrounding hot springs.
In sulfur springs (hydrogen sulfide type) and sulfur-containing springs (hydrogen sulfide type) that contain a lot of hydrogen sulfide, when the hot spring gushes out to the surface, the hydrogen sulfide (gas) in the hot spring water is oxidized by the oxygen in the air, precipitating as solid sulfur and forming Sulfurka.
2H2S | + O2 | → | 2S ↓ | + 2H20 |
Hydrogen sulfide | Oxygen | Sulfur | Water |
When carefully observing a bathtub filled with sulfur springs (hydrogen sulfide type), it is sometimes possible to observe that the oxidation of hydrogen sulfide is promoted at the surface where the hot spring is in contact with the air, and the fine sulfur generated forms a film-like thin layer on the surface. The film-like sulfur formed on the surface settles to the bottom of the bathtub when the hot spring is stirred.
When bathing in sulfur springs, it is sometimes possible to see white, thin "Yunohana" floating in the bathtub. Many of these "Yunohana" are Sulfurka precipitated from solid sulfur formed from hydrogen sulfide contained in the hot spring.
At Lake Usori in Osorezan, Aomori Prefecture, yellow sulfur precipitates from the hot spring water flowing into the lake, forming Sulfurka on the lakebed.
In addition to Sulfurka formed by the precipitation of components contained in hot spring water, there are also Sulfurka formed by the sublimation of hydrogen sulfide and other components contained in volcanic gases into sulfur. In fumarole areas seen around volcanoes and their surroundings, volcanic gases and steam are vigorously ejected from fumaroles. When approaching a fumarole, it is sometimes possible to see bright yellow sulfur formed by the sublimation of sulfur adhering and forming Sulfurka.
In the fumarole area of Jigokudani, Tateyama, Toyama Prefecture, sulfur continues to accumulate around the fumaroles, forming Sulfurka called fumarole towers over 2 meters high.
In fumarole areas around volcanoes, high-purity sulfur precipitated from hot springs and volcanic gases accumulates as Sulfurka, and there were once many places where sulfur was mined as sulfur mines. Well-known examples include Matsuo Mine in Iwate Prefecture, Bandai Mine in Kusatsu Onsen, Gunma Prefecture, Ogushi Sulfur Mine in Manza Onsen, Gunma Prefecture, and Kawarage Jigoku's Kawarage Sulfur Mine in Akita Prefecture.
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Sulfurka adhering around a fumarole | Fumarole tower formed by accumulated sulfur |
(5) Ironka
Precipitated substances formed by the precipitation of iron components in hot spring water as goethite (needle iron ore) are called Ironka. Because they are brown precipitates, they are conspicuous, and in nature, the presence of Ironka often indicates the gushing of hot springs.
At Ohshirakawa Jigokudani in Shirakawa Village, Gifu Prefecture, Ironka is generated where hot springs gush out, dyeing the gravel in the valley brown. Downstream, goethite solidifies small gravels in the valley, forming board-shaped Ironka called "Oniita".
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Ironka adhering to gravel in Ohshirakawa Jigokudani |