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Arima Onsen — photo: 663highland · CC BY 2.5 · Wikimedia Commons

How are onsen heated? Volcanoes, deep rock and the boiler behind Japan's hot springs

Nobody heats most onsen water, at least not at first. The ground does, in one of two ways: a volcano close underneath, or the plain fact that rock gets hotter the deeper you go. Some of the hottest and most famous springs in Japan, including Arima near Kobe, have no volcano anywhere near them. And a large share of baths are warmed by an ordinary boiler on top of all that, which is allowed and posted on the wall. This guide explains the three sources of heat, shows how differently they play out across the country, and tells you how to work out which one is under the bath you are sitting in.

The short answer

• Volcanic heat. Rain and snowmelt soak into the ground near a volcano, meet hot rock and volcanic gas a few kilometres down, and come back up hot. This is where the boiling, smelling, milky springs are.
• Heat from depth, no volcano. Away from volcanoes, rock still warms by roughly 3 °C for every 100 metres. Water that sinks deep enough along faults comes back warm. In parts of western Japan, very old water squeezed out of the sinking ocean plate rises from far deeper still, and it can arrive close to boiling.
• Heat added by people. Where the water comes up cool, the bath heats it. The notice by the bath says 加温 (kaon), heated, when it has been done.
• Can an onsen be cold? Yes. Under Japanese law a spring counts as an onsen if it is 25 °C or warmer or carries enough minerals, so a cold mineral spring is still an onsen. 237 of the 1,167 towns in our records with a measured temperature come out below 25 °C.

Volcanic springs

Japan has 111 active volcanoes, and the hottest, most dramatic springs sit on or beside them. Water near a volcano does not need to go deep to get hot, and it picks up volcanic gas on the way, which is why these springs are so often sulfur water, very acidic, or both. Some examples from our records:

• Kusatsu, below Kusatsu-Shirane in Gunma: sources from 44.9 to 94.5 °C, pH as low as 1.0, and about 32,300 litres a minute.
• Tamagawa in Akita: 98 °C and pH 1.05, about 9,000 litres a minute from one source.
• Noboribetsu in Hokkaido, beside the steaming valley of Jigokudani: 45 to 90 °C, with six of the ten water types in our records.
• Beppu in Oita and Unzen in Nagasaki, where steam comes out of the ground in the middle of town.

You can usually tell a volcanic town before you reach the bath: steam vents, the smell of eggs, yellow sulfur crusts on the rocks, and names with 地獄 (jigoku), "hell", for the boiling pools. More on how these regions compare is in where Japan's hot springs are.

Springs with no volcano

The rest of Japan's hot water has nothing to do with magma. There are two kinds.

Water warmed by deep rock. The ground warms steadily with depth, on average about 3 °C per 100 metres, so a kilometre down it is roughly 30 °C warmer than at the surface. Rain that sinks along faults and cracks to that depth comes back warm enough to bathe in. This is the ordinary, quiet onsen of the plains and old spa towns: clear water, little smell, usually simple or salty. Dōgo in Shikoku, one of the oldest bathhouses in the country, is non-volcanic, at 42 to 51 °C.

Very old water from very deep. A handful of springs in western Japan are far too hot to explain this way. Arima in Hyōgo reaches 98 °C at its hottest source, with no active volcano in the region. Geologists now think water like Arima's was carried down with the Philippine Sea plate as it slides under Japan, driven out of the rock deep underground, and pushed up along faults. That is why it is very salty and rich in iron, the brown kinsen "gold" water, even though the coast is nowhere near. Scientists call this kind of water "Arima-type", after the town.

The Kii Peninsula in Wakayama has no active volcano either, and it is one of the hot spots of Japan. In our records 5 of Wakayama's 12 towns with a temperature reach 42 °C: Yunomine at 92 °C, Nanki-Shirahama up to 85 °C, Kawayu at 73 °C, Yukawa at 60 °C and Ryūjin at 48 °C. The heat is thought to come from hot fluid 10 to 15 km down, rising along deep fractures; exactly how it gets there is still studied. See Yunomine and Shirahama.

Drilled water and the boiler

Many onsen today have not waited for water to rise on its own. A bore hole one or two kilometres deep reaches warm water almost anywhere in Japan, which is how cities and plains got their own onsen. Tokyo's dark kuro-yu is the best-known example; see onsen in Tokyo.

Where a bore hole or a natural spring comes up cool, the bath heats it. How common that is depends a lot on where you are. Counting towns in our records with a measured temperature:

• Hokkaido: 97 of 147 come out at 42 °C or hotter, and 21 below 25 °C.
• Gunma: 25 of 48 at 42 °C or hotter, 8 below 25 °C.
• Oita: 19 of 27 at 42 °C or hotter.
• Ehime: 3 of 17 at 42 °C or hotter, 8 below 25 °C.
• Saitama: 2 of 17 at 42 °C or hotter, 7 below 25 °C.
• Chiba: none of 15 reaches 42 °C, and 10 are below 25 °C.

Chiba has no volcano at all, and it shows: almost every onsen bath there is a cold mineral spring that has been heated. That is not a lesser onsen in law, and the water is still the spring's water, but if a steaming source is what you came for, it is worth knowing before you book. The same holds for most of Kanto's plain and much of Shikoku.

How to tell which one you are in

The signs on the wall answer it. Look for two of them in the changing room:

• 温泉分析書 (onsen bunsekisho), the spring analysis. The line 泉温 (sen'on) is the temperature at the source. Above about 42 °C, the ground did the heating. Below 25 °C, the bath is heated. In between, it depends.
• The notice on what has been done to the water. 加温 有 (kaon ari) means heated; 加水 有 (kasui ari) means water added, usually to cool a very hot source. Both are explained in onsen water types.

The analysis also hints at the origin. A low pH, sulfur or a smell of eggs points to a volcano. Clear, salty water in a town with no mountains around it is usually deep water. Brown iron water that is also very salty, far from any volcano, is the Arima kind. How the source and bath temperatures relate is in how hot onsen water is.

Things not to assume

• Not every onsen is volcanic. Some of the oldest and hottest are not.
• Heated does not mean fake. A heated cold spring is still a spring, posted honestly. Water that is recirculated and heated is a different question; see is onsen water clean.
• Hotter at the source does not mean hotter in the tub. A 98 °C source is cooled before anyone gets in. Tub temperature is set by the people running the bath.
• Do not touch the source. Water at a vent, a boiling pool or a pipe outlet can scald. Only bathe where bathing is set up.

How we wrote this

The temperatures, pH, flow and water types come from our own records of 1,538 onsen towns, 1,167 of them with a measured source temperature; we take the hottest source in each town. The explanations of where the heat comes from follow Japan's environment ministry guidance on hot springs and published geological work on Arima-type water and the Kii Peninsula springs. The 3 °C per 100 m figure is a broad average and varies by place. Nothing here is a claim about health effects.

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