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.

Why Japan Has So Many Hot Springs

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.

Why Volcanoes Make Hot Springs More Likely

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.

Volcanic vs Non-Volcanic Hot Springs

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.

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.

Hot Springs Can Exist Without Volcanoes: The Arima Example

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.

Common Misconceptions

• All hot springs in Japan come from volcanoes ... In reality, there are many non-volcanic hot springs as well. There is no definitive national statistic showing the exact ratio of volcanic to non-volcanic springs, but springs unrelated to volcanoes, such as Arima-type and deep groundwater-type springs, are found throughout Japan.
• Volcanic hot springs are always more valuable ... Value is not uniform. Many people prefer mild non-volcanic waters, and mineral strength alone does not determine quality.

FAQ

Why does Japan have so many hot springs?

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.

Why are there hot springs in places without volcanoes?

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.

Which is better, volcanic or non-volcanic hot springs?

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.

How many volcanoes are there in Japan?

The Japan Meteorological Agency monitors 111 active volcanoes, about 7% of the world total.

Summary

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.

Sources

• Japan Meteorological Agency, What Is an Active Volcano?
• Ministry of the Environment, Hot Spring Usage Status
• National Research Institute for Earth Science and Disaster Resilience, Hi-net, Plate Tectonics
• Kobe University, Unraveling the Origin of High-Temperature Spring Water in Arima Onsen (2020)
• Kanagawa Prefectural Hot Springs Research Institute, Volcanic and Non-Volcanic Hot Springs