Arima Onsen is one of Japan’s three ancient hot springs, located in Kita Ward, Kobe City, Hyogo Prefecture. The trio of ancient hot springs, known as the “Nihon San Koyu,” also includes Dogo Onsen in Matsuyama City, Ehime Prefecture, and Shirahama Onsen in Shirahama Town, Nishimuro District, Wakayama Prefecture.
Among the three, Dogo Onsen is renowned as Japan’s oldest hot spring, with a history dating back approximately 1,300 years, as indicated by records found in the Kojiki and Nihon Shoki, ancient Japanese texts. The source of Dogo Onsen springs from a depth of 1,200 meters underground, and the water quality is categorized as a simple spring.
Shirahama Onsen, situated in Shirahama Town, Wakayama Prefecture, is a relatively newer addition to the trio, with a history of around 1,200 years. The source of Shirahama Onsen emerges from a depth of 1,000 meters underground, and its water quality is also classified as a simple spring, similar to Dogo Onsen.
Now, Arima Onsen is a hot spring located in Kobe City, Hyogo Prefecture. Its history is estimated to be around 1,300 years ago, similar to Dogo Onsen, based on descriptions from the Nara period. However, Dogo Onsen, mentioned in the Kojiki, is considered Japan’s oldest, overshadowing Arima Onsen, whose description is found in the Nihon Shoki.
The source of Arima Onsen springs from a depth of 1,000 meters underground and consists of two types: the “Kin-no-yu” (Golden Spring) and the “Gin-no-yu” (Silver Spring).
The Golden Spring is characterized by its reddish-brown color and thick cloudy water. It contains about 1.5 times the salinity of seawater, with reputed benefits for muscle pain, joint pain, neuralgia, and high heat retention and moisturizing effects. Additionally, it is said to be effective for anemia and sensitivity to cold due to its high iron content.
On the other hand, the Silver Spring is a clear, transparent water that contains carbon dioxide and radium.However, why does Arima Onsen have two types of springs? The fact that Arima Onsen springs without any connection to volcanic magma is not well-known.
The Mystery of Arima Onsen①：Why does hot spring water exist in an area without any volcanoes nearby?
Many of Japan’s hot springs are of volcanic origin, with nearby volcanoes contributing to their formation. However, Arima Onsen and surrounding areas such as Takarazuka, Takedao, and Nada Onsen in Kobe belong to a distinct non-volcanic category known as “Arima-type hot springs.”
The characteristics of Arima-type hot springs include the emergence of hot springs with high temperatures and the water composition being chloride or bicarbonate hydrogen ion springs.
So, how do Arima-type hot springs, which are not of volcanic origin, come into existence, and what is the mechanism that brings the hot spring water to the surface? The answer lies in the interaction of four major tectonic plates supporting the Japanese archipelago: the Pacific Plate, the Philippine Sea Plate, the Eurasian Plate, and the North American Plate.
Beneath Arima Onsen, there exists high-temperature seawater generated during the subduction process of the Philippine Sea Plate. This high-temperature seawater undergoes heating within the plate due to internal heat, mixing with underground water under high-temperature and high-pressure conditions, producing the hot spring water of Arima Onsen.
While the water quality of volcanic hot springs varies significantly based on the components of magma and heated water, Arima Onsen is a rare type of hot spring directly connected to the plate’s heat, resulting from the unique interaction of seawater with the plate’s heat during the subduction process. The composition of the hot spring water is significantly influenced by the components of the geological strata through which seawater passes.
The Mystery of Arima Onsen②：The reason for the existence of two types of spring qualities
Arima Onsen, being a non-volcanic hot spring, lacks the typical characteristics associated with volcanic hot springs, such as the smell of sulfur and acidity. Additionally, it contains fewer components commonly found in volcanic hot springs, such as metasilicic acid. This results in a gentle and skin-friendly hot spring, with the potential for enhancing the complexion, making it a cherished destination for therapeutic bathing since ancient times.
Why does Arima Onsen have two types of spring qualities, gold and silver?
The dual qualities of the Kin-no-yu (Golden Spring) and Gin-no-yu (Silver Spring) in Arima Onsen are believed to be influenced significantly by the composition of the geological strata through which underground water passes.
The Kin-no-yu (Golden Spring) contains a high concentration of iron and carbon dioxide. The iron content results from the reaction of underground water with minerals containing iron within the geological strata as it passes through. Additionally, carbon dioxide is generated through the decomposition of carbonate minerals such as limestone and dolomite within the strata due to heat.
The combination of iron and carbon dioxide in the Kin-no-yu produces a reddish-brown cloudy water. Iron is believed to have benefits for promoting circulation and improving skin, while carbon dioxide is associated with fatigue recovery and beauty effects.
As for the Gin-no-yu (Silver Spring), it contains a high concentration of radium and carbon dioxide. Radium is generated when radioactive substances like uranium and thorium within the geological strata undergo decay. Carbon dioxide is also produced similarly to the Kin-no-yu, through the heat-induced decomposition of carbonate minerals like limestone and dolomite. The Gin-no-yu, resulting from the mixture of radium and carbon dioxide, is a colorless, transparent, and refreshing water. Radium is believed to have benefits for circulation promotion and skin improvement, while carbon dioxide is associated with fatigue recovery and beauty effects.
Arima Onsen also features a spring known as “Kin Gin Kon-yu,” which has characteristics intermediate between the Golden and Silver Springs. Why not give it a try and experience its unique qualities?
The Mystery of Arima Onsen③：The Mystery of Spring Output Volume
However, there remains an unsolved mystery in Arima Onsen – the consistent spring output volume. The source of Arima Onsen maintains a nearly constant spring output throughout the year. This phenomenon is believed to be attributed to the formation of the hot spring, where seawater mixes in from deep underground.
The Mystery of Arima Onsen④：The Mystery of Beauty Components
Furthermore, the hot springs of Arima contain elements such as iron, carbon dioxide, radon, and other components believed to be associated with skin-beautifying effects. However, the detailed mechanism behind the relationship between the spring output volume of Arima Onsen and its skincare benefits remains yet to be fully elucidated.
Conclution
Arima Onsen is situated above the Arima-Takatsuki Fault, known as the Arima-Takatsuki Tectonic Line, which is recognized as the epicenter of the Osaka-North earthquake. In other words, Arima Onsen is a unique hot spring that owes its existence to seismic activity. Earthquakes are natural threats and sources of fear, yet the hot springs created by these seismic events have the extraordinary ability to soothe both the mind and body, promoting healing and restoring health. Going forward, Arima Onsen will undoubtedly continue to be an irreplaceable hot spring, offering solace and well-being to people.
