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From Gifu! Hot Spring Museum Episode 5: Unique Ways Hot Springs Emerge

There are various forms in which hot springs naturally emerge on the surface. When they emerge from places like cracks in rocks, if the amount of water is small, it flows out gently, but if a large amount emerges, it can accumulate at the outlet, forming a small pond. Additionally, when the water is boiling hot or when a large amount of carbon dioxide contained in the hot spring escapes, it bubbles up, creating what is called a "bubble spring."
When hot water and steam erupt together with great force like a fountain, this hot spring phenomenon is called a geyser. If there is hot water containing high-temperature steam underground and the outlet hole (pore) of the hot spring is small, the pressure in the space where the underground hot water exists increases, causing the hot spring to erupt vigorously with a loud "whoosh" sound and splashes. The Jigokudani Geyser in Shibu Onsen, Nagano Prefecture, continues to erupt hot water and steam with a loud noise. It was designated as a national natural monument in 1927. In Oyasu Gorge, Akita Prefecture, there is a geyser called "Great Geyser," where high-temperature steam and hot water erupt horizontally with a loud noise from various places on the cliff.

Geyser at Jigokudani, Shibu Onsen, Nagano Prefecture	"Great Geyser" at Oyasu Gorge, Akita Prefecture

Among geysers, those that erupt at regular intervals are called intermittent geysers. The mechanism of periodic eruptions in intermittent geysers is explained by theories such as the cavity theory and the vertical pipe theory.
The cavity theory explains intermittent geysers as follows: There is a cavity underground where groundwater accumulates, and the groundwater in the cavity is heated by heat from the surroundings (or deep underground). As the temperature of the groundwater rises, it eventually boils, causing hot water and steam to erupt violently from the surface vent. After the eruption, the groundwater in the cavity is depleted, and the pressure decreases. After a while, groundwater accumulates again, is heated, and erupts. This cycle repeats, resulting in periodic eruptions. The period of eruption of an intermittent geyser varies depending on the size of the cavity, the rate at which groundwater accumulates, and the amount of heat applied.
The vertical pipe theory suggests that the underground of an intermittent geyser is a vertical pipe where groundwater accumulates, and when the lower groundwater is heated and boils, it rises through the pipe and erupts hot water and steam from the surface vent. After the eruption, groundwater accumulates again at the bottom of the pipe, repeating eruptions at regular intervals. It is said that large-scale intermittent geysers with long eruption periods can often be explained by the cavity theory, while small-scale intermittent geysers with short eruption periods can often be explained by the vertical pipe theory. The "Benten" intermittent geyser at Fukiage Onsen in Miyagi Prefecture erupts hot water over 100°C to a height of more than 15 meters every 15 minutes. Other well-known intermittent geysers include the intermittent geyser at Kawamata Onsen in Tochigi Prefecture, the "Tornado Hell" at Beppu Kannawa Onsen in Oita Prefecture, and the Jigokusawa Intermittent Geyser Group in Miyagi Prefecture. The Mesugama and Osugama intermittent geysers at Onikobe in Miyagi Prefecture were designated as special national natural monuments in 1952, but according to the author's research, they have now completely disappeared.

"Benten" Intermittent Geyser at Fukiage Onsen, Miyagi Prefecture