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The volcanic hot springs in the prefecture include Okuhida Hot Spring Village's hot springs such as Shin-Hotaka Onsen, Hirayu Onsen, Shin-Hirayu Onsen, Fukuji Onsen, Tochio Onsen, as well as Nigorigo Onsen, Shirakawa Onsen, and Hirase Onsen, which draws water from there, and Gero Onsen.
The hot springs of Okuhida Hot Spring Village are believed to be powered by the active Yake-dake volcano, and all of them produce high-temperature hot springs. Hirayu Onsen even has a source that exceeds 100°C.
The spring quality includes sodium-bicarbonate chloride springs, sulfur-containing sodium-bicarbonate chloride springs, sulfur-containing sodium-chloride bicarbonate springs, simple sulfur springs, and simple hot springs. Some simple sulfur springs in Shin-Hotaka Onsen and Hirayu Onsen are rare in the prefecture for their milky white appearance.
In places like Hirayu Onsen, you can observe the formation of biomats (microbial films) near the hot spring outlets, and biofilms that appear as oil films on the surface of the bathwater filled with hot springs. These can be seen throughout the hot spring town. Some people mistake the oil film for floating oil, but when touched, it doesn't feel sticky like oil, proving it's not oil. These biomats and biofilms are not harmful substances but rather a testament to the natural hot springs, enhancing the hot spring experience.
In the Kamijigane area of Shin-Hirayu Onsen, a large amount of tufa has accumulated due to the hot springs that once erupted. This tufa produces so-called leaf stones, where leaves are coated with calcium carbonate. The tufa has been dated using carbon isotopes from the wood fragments it contains, and it is estimated to have formed around the Nara period or slightly before or after.
Okuhida Hot Spring Village is known nationwide for its abundant hot water supply, boasting scenic open-air baths that utilize the high-temperature and abundant hot water.
Shirakawa Village's Shirakawa Onsen and Hirase Onsen are hot springs powered by the Hakusan volcano. In the Jigokudani area of Shirakawa, the hydrothermal alteration zone of the Hakusan volcano spreads, and you can observe hot spring phenomena such as fumaroles emitting high-temperature steam, natural hot spring eruptions, and the formation of hot spring flowers.
Hot spring eruptions are recognized in small quantities from various places, with spring temperatures ranging from 19.2°C to 92.2°C (at an air temperature of 23.8°C) and pH values varying from 4.2 to 6.7. Although there are no registered acidic springs in the prefecture, weakly acidic hot springs exist in the great nature of Jigokudani.
In the areas where hot springs erupt or gas is emitted, hot spring flowers precipitated from the hot springs, sublimated sulfur flowers from the gas, and iron alum are formed. The entire hydrothermal alteration zone is filled with the smell of hydrogen sulfide.
At the shore of Shirakawa's Hakusui Lake, hot springs (sulfur-containing sodium-chloride springs) are created by passing water through high-temperature steam erupting from underground. The created hot springs are used in open-air baths at Shirakawa Onsen and are also piped about 13 km away to Hirase Onsen for use.
Nigorigo Onsen in Gero City's Kosaka Town is located at the 7th station of the Gifu Prefecture side of the Ontake Mountain climbing trail, and it is the highest hot spring town in Japan that operates year-round.
Powered by the Ontake volcano, the spring quality includes sodium-calcium bicarbonate sulfate springs, which are rare in the prefecture for their high content of mirabilite (sodium sulfate). The presence of iron also gives the water a brownish color.
In the valleys near Nigorigo Onsen, there are places where hot springs naturally erupt. Although they are relatively low in temperature and not abundant in quantity, you can recognize the hot spring eruptions by the formation of hot spring flowers such as sulfur, calcium carbonate, and calcium sulfate on the valley floors and slopes.
On the right bank of the Zaimoku Falls near Nigorigo Onsen, a large-scale terraced tufa dome, about 30 meters wide and 20 meters long, has been formed by hot spring deposits. It is one of the largest natural tufa domes in Japan.
Most of the sources are alkaline simple springs, but many contain a relatively high amount of baking soda, making the skin feel smooth when bathing. There is also a faint smell of hydrogen sulfide, adding to the hot spring atmosphere.
The heat source of Gero Onsen is not clearly understood, but it is suggested that the Yuga-mine volcanic body, which is extremely close to the Ontake volcano and is thought to have erupted about 100,000 years ago, has not completely cooled underground and may be the heat source for Gero Onsen.
In April 2004, the "Gero Hot Spring Museum," a rare museum specializing in hot springs, opened in Gero Onsen. It displays various materials related to the science and culture of hot springs, provides detailed explanations, and collects hot spring-related books and papers, playing a significant role in the promotion of hot springs.
Other hot springs in the prefecture are mostly non-volcanic. Many of the historically old non-volcanic hot springs are cold mineral springs with spring temperatures below 25°C.
In Gero City's Kosaka Town, Yuyama Onsen and Shimojima Onsen, and in Gero City's Norimasa, Norimasa Onsen, carbon dioxide-containing sodium bicarbonate springs and carbon dioxide-containing sodium chloride springs erupt. These are natural carbonated waters, traditionally called "cider springs."
Gifu City's Mitado Shrine Hot Spring and Nagara River Onsen, which draws water from there, Hida City's Furukawa Town's Tanbo no Yu (Keta Onsen), and Takayama City's Asahi Town's Akigami Onsen are simple iron (II) springs (bicarbonate type), and the water turns brown in the bath due to the high iron content. When it first erupts, it is nearly transparent, but it oxidizes when exposed to air, causing the brown color to deepen over time.
The hot springs distributed from the southern Kamo district to the Tono region originally erupt from granite bodies containing radioactive elements, making most of them simple radioactive springs.
The number of hot springs in the prefecture has increased rapidly over the past 30 years. Many municipalities have drilled hot springs, and public day-use hot spring facilities have been established in various places.
Many of these hot springs are obtained by deep drilling over 1000 meters to pump up deep-seated thermal water underground. Most of them are low-temperature springs with spring temperatures ranging from 25°C to 42°C.
The spring quality includes simple springs, sodium bicarbonate springs, sodium bicarbonate chloride springs, sodium chloride bicarbonate springs, and sodium chloride springs, with relatively similar spring qualities containing baking soda and salt as components. Especially in hot springs with a high baking soda content, the skin tends to feel smooth when bathing.
The day-use bathing facility "Himeshaga no Yu" at Shimojima Onsen intermittently erupts a sodium bicarbonate spring rich in iron and carbon dioxide. At the place where the hot spring water overflows from the source, a large amount of tufa containing iron precipitates, allowing you to witness the formation of so-called "leaf stones" and rimstone.