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Just What Makes a Japanese Onsen?

What is Required to be Considered an “Onsen”?

Hot Springs

Hot springs, called onsen in Japanese, are pools of steaming-hot water that bubbles out of the ground. They can be found all over Japan and are defined by the Japanese Hot Spring Law (onsenhou). Simply put, an onsen meets two important criteria: the water is at least 25 degrees Celsius when it surfaces and it contains more than a defined amount of natural mineral components.
Why are onsen hot? Why do they have a certain chemical composition? The answers to these questions largely lie in geology. Here are the three principal ways that onsen are formed.

1.Volcanic Hot Springs

This type of onsen is formed by the heat and gasses from volcanic magma.

1. At a relatively shallow point within the layers of a volcano (generally 1–10 km deep), you find a chamber formed when magma from deep in the earth rises up and gathers. This may have been heated to well over 1,000 degrees Celsius!

2. Rainwater and snow sink deep into the earth and is heated by the magma chamber.

3. This underground water rises and, when it gets near the surface, can burst out through channels and cracks as a naturally flowing spring. Alternatively, channels near the surface can be drilled down to and an onsen created.

2.Non-Volcanic Hot Springs (Deep Groundwater Type)

These are onsen that are formed by rainwater and snow heated by geothermal mechanisms other than volcanoes.

1. Rainfall and snow sink deep into the earth.

2. As you move underground, the temperature rises about 2–3 degrees Celsius roughly every 100 meters. For example, if it is 20 degrees at the surface, by the time you are 1,000 meters underground the temperature will be 40-50 degrees! This heat warms the water that has sunk underground.

3. Alternatively, magma that has cooled becomes bands of very hot rocks deep underground. These can also heat underground water.

4. This heated water then rises and gushes out as natural springs. It can also be drilled down to in order to create an onsen.

3.Non-Volcanic Hot Springs (Fossilized Seawater Type)

These are onsen, hot springs with such a high salt composition that they can be considered onsen even if they are cooler than 25 degrees.

1. Due to the movement of tectonic plates—and other activity that dates back millennia—ancient seawater is trapped in channels in the earth and becomes fossilized seawater.

2. This fossilized seawater is heated by hot rocks and other geothermal energy.

3. Heated fossilized seawater rises to the surface and emerges as natural springs. It can also be drilled down to in order to create an onsen.

Now you know the three main types of onsen. Next time you are relaxing in a hot spring, you can imagine the searing heat produced by volcanic magma or the fantastical world of deep ancient seawater caverns that create a high-salt onsen. The more you know, the more ways you have to enjoy the hot spring experience!
Next time you are able to visit an onsen, why not check how the hot springs were formed?