Learn why Japan has 111 active volcanoes, about 27,000 springs, and 4 plate boundaries. Discover volcanic vs non-volcanic hot springs, and why Arima stays hot without a nearby volcano.
Published: Dec 24, 2025
Learn why Japan has 111 active volcanoes, about 27,000 springs, and 4 plate boundaries. Discover volcanic vs non-volcanic hot springs, and why Arima stays hot without a nearby volcano.
Published: Dec 24, 2025
Volcanoes and hot springs are closely connected, but not all of Japan’s hot springs are formed by volcanoes alone. Near volcanoes, underground heat warms groundwater and makes hot springs more likely to form. Yet even where no volcano is nearby, some springs still emerge through plate movement and water circulation deep underground.
The short answer is this: Japan has so many hot springs because it lies in a tectonic zone where 4 plates meet, creating abundant underground heat sources, and because it gets lots of rain and snow, which means plenty of groundwater. Volcanoes are the best-known heat source, but they are not the only one. This article explains how volcanoes create hot springs, the difference between volcanic and non-volcanic springs, and why some springs emerge even without volcanoes, using numbers along the way.
The Japanese archipelago sits where 4 plates meet: the Pacific, Philippine Sea, North American, and Eurasian plates. As oceanic plates subduct beneath continental ones, crustal movement and volcanic activity intensify. That creates many places with underground heat sources.
The numbers are striking. The Japan Meteorological Agency monitors 111 active volcanoes, about 7% of the world’s roughly 1,500 active volcanoes. Hot springs are also abundant: according to the Ministry of the Environment, Japan has about 27,000 springs and around 2,900 hot spring areas, as of fiscal 2022. Add heavy precipitation to that, and the conditions are ideal for water seeping underground, warming up, and rising back to the surface.
Beneath a volcano lies a magma chamber, an intense source of heat. When groundwater enters that area, it is heated and can rise more easily as a hot spring.
Volcanic gases and minerals also dissolve into the water, making sulfur springs and acidic springs more likely. That is why hot springs in volcanic regions often have a strong sulfur smell, cloudy water, or high acidity. Places such as Kusatsu, Noboribetsu, Hakone, and Beppu are classic examples.
Hot springs are generally divided into two types by their heat source. But this classification does not determine whether a spring is good or bad.
| Main heat source | Typical composition and appearance | Examples | |
|---|---|---|---|
| Volcanic hot springs | Magma and volcanic heat | Strong character from sulfur, acidity, and volcanic gases | Kusatsu, Noboribetsu, Hakone |
| Non-volcanic hot springs | Deep underground heat, or fluids related to plate activity | Ranges from mild water to highly saline springs | Arima, deep urban hot springs |
Volcanic does not automatically mean more highly mineralized or better. Some people find volcanic springs too strong, while many prefer gentler non-volcanic waters. For travelers, it is more practical to focus on the spring type, temperature, and intensity than on whether the water is volcanic.
The deeper underground you go, the hotter it gets. In general, temperature rises by about 3°C every 100 m, a phenomenon called the geothermal gradient. Even without a volcano, drilling deeper can sometimes reach warmed groundwater. The overall mechanism of how hot springs emerge is explained in more detail in How Hot Springs Rise.
Another representative example of high-temperature, highly saline water in a place without a volcano is Arima Onsen. There is no active volcano nearby, so why does such hot, mineral-rich water emerge there? According to a 2020 Kobe University study, the relatively young and hot Philippine Sea Plate subducting from the Nankai Trough releases water at a depth of about 60 km, and that high-temperature fluid rises to the surface through faults. Springs like this are called Arima-type hot springs. In other words, having a hot spring does not necessarily mean a volcano is nearby.
Because it lies in a tectonic zone where 4 plates meet, creating many underground heat sources, and because it also has abundant rainfall and groundwater. There are about 27,000 springs nationwide, according to the Ministry of the Environment.
Because the deeper underground you go, the hotter it becomes, rising about 3°C every 100 m. There are also examples like Arima Onsen, where high-temperature fluids from subducting plates rise to the surface.
There is no simple answer. Volcanic springs tend to have stronger character, while non-volcanic springs are often gentler. The best choice depends on the water type and temperature.
The Japan Meteorological Agency monitors 111 active volcanoes, about 7% of the world total.
Volcanoes and hot springs are closely linked because underground heat warms groundwater and affects mineral composition. But Japan has so many hot springs not only because of volcanic activity, but also because it sits where 4 plates meet and has abundant groundwater. Even without a volcano, high-temperature water can emerge, as in Arima Onsen. Understanding this mechanism makes it easier to appreciate the differences in smell, color, and temperature you encounter at hot spring resorts.
Volcanoes and hot springs are closely connected, but not all of Japan’s hot springs are formed by volcanoes alone. Near volcanoes, underground heat warms groundwater and makes hot springs more likely to form. Yet even where no volcano is nearby, some springs still emerge through plate movement and water circulation deep underground.
The short answer is this: Japan has so many hot springs because it lies in a tectonic zone where 4 plates meet, creating abundant underground heat sources, and because it gets lots of rain and snow, which means plenty of groundwater. Volcanoes are the best-known heat source, but they are not the only one. This article explains how volcanoes create hot springs, the difference between volcanic and non-volcanic springs, and why some springs emerge even without volcanoes, using numbers along the way.
The Japanese archipelago sits where 4 plates meet: the Pacific, Philippine Sea, North American, and Eurasian plates. As oceanic plates subduct beneath continental ones, crustal movement and volcanic activity intensify. That creates many places with underground heat sources.
The numbers are striking. The Japan Meteorological Agency monitors 111 active volcanoes, about 7% of the world’s roughly 1,500 active volcanoes. Hot springs are also abundant: according to the Ministry of the Environment, Japan has about 27,000 springs and around 2,900 hot spring areas, as of fiscal 2022. Add heavy precipitation to that, and the conditions are ideal for water seeping underground, warming up, and rising back to the surface.
Beneath a volcano lies a magma chamber, an intense source of heat. When groundwater enters that area, it is heated and can rise more easily as a hot spring.
Volcanic gases and minerals also dissolve into the water, making sulfur springs and acidic springs more likely. That is why hot springs in volcanic regions often have a strong sulfur smell, cloudy water, or high acidity. Places such as Kusatsu, Noboribetsu, Hakone, and Beppu are classic examples.
Hot springs are generally divided into two types by their heat source. But this classification does not determine whether a spring is good or bad.
| Main heat source | Typical composition and appearance | Examples | |
|---|---|---|---|
| Volcanic hot springs | Magma and volcanic heat | Strong character from sulfur, acidity, and volcanic gases | Kusatsu, Noboribetsu, Hakone |
| Non-volcanic hot springs | Deep underground heat, or fluids related to plate activity | Ranges from mild water to highly saline springs | Arima, deep urban hot springs |
Volcanic does not automatically mean more highly mineralized or better. Some people find volcanic springs too strong, while many prefer gentler non-volcanic waters. For travelers, it is more practical to focus on the spring type, temperature, and intensity than on whether the water is volcanic.
The deeper underground you go, the hotter it gets. In general, temperature rises by about 3°C every 100 m, a phenomenon called the geothermal gradient. Even without a volcano, drilling deeper can sometimes reach warmed groundwater. The overall mechanism of how hot springs emerge is explained in more detail in How Hot Springs Rise.
Another representative example of high-temperature, highly saline water in a place without a volcano is Arima Onsen. There is no active volcano nearby, so why does such hot, mineral-rich water emerge there? According to a 2020 Kobe University study, the relatively young and hot Philippine Sea Plate subducting from the Nankai Trough releases water at a depth of about 60 km, and that high-temperature fluid rises to the surface through faults. Springs like this are called Arima-type hot springs. In other words, having a hot spring does not necessarily mean a volcano is nearby.
Because it lies in a tectonic zone where 4 plates meet, creating many underground heat sources, and because it also has abundant rainfall and groundwater. There are about 27,000 springs nationwide, according to the Ministry of the Environment.
Because the deeper underground you go, the hotter it becomes, rising about 3°C every 100 m. There are also examples like Arima Onsen, where high-temperature fluids from subducting plates rise to the surface.
There is no simple answer. Volcanic springs tend to have stronger character, while non-volcanic springs are often gentler. The best choice depends on the water type and temperature.
The Japan Meteorological Agency monitors 111 active volcanoes, about 7% of the world total.
Volcanoes and hot springs are closely linked because underground heat warms groundwater and affects mineral composition. But Japan has so many hot springs not only because of volcanic activity, but also because it sits where 4 plates meet and has abundant groundwater. Even without a volcano, high-temperature water can emerge, as in Arima Onsen. Understanding this mechanism makes it easier to appreciate the differences in smell, color, and temperature you encounter at hot spring resorts.