The Science of Onsen: Inside Japan’s Hot Springs

One of Japan’s most popular and beloved attractions, for both tourists and residents alike is the onsen. These hot spring baths have been around in one form or another pretty much since the beginning of recorded history in Japan. Much has been written and said down the years about their rejuvenative and healing qualities as well as a number of intense and sometimes heated debates over what actually constitutes an onsen in the purest sense.

Firstly let’s look at how an onsen actually comes into being. Given its close proximity to a number of tectonic plates, Japan not only suffers frequent earthquakes, but it also has a relatively high level of volcanic activity. However, whilst it would be very difficult to argue that earthquakes and volcanic eruptions are ever a good thing, it is this heightened level of volcanicity that gives birth to the geothermal energy necessary to create onsens. Thanks to this unique blending of geographical and geological factors Japan has among the most abundant resources of natural hot spring water in the world. Over the centuries, the places where these natural waters are concentrated have been identified and a number of “onsen resorts” have blossomed across the country.

Photo by mizoguchi.coji on Flickr.

Throughout Japan, many of the more popular “Onsen Resort” areas such as Dougo in Ehime Prefecture, and Hakone, in Kanagawa, near Tokyo actually house dozens of individual onsens, each with their own hotels, inns and unique characteristics and quirks.

However, whilst the composition of each onsen is unique, how they come into being is largely the same process for them all.

As you may recall from your high school geography class, our Earth is composed of many layers. The top layer, on which we stand now, is known as the Earth’s crust. The Earth’s crust is somewhere in the region of 32-35 kilometres thick. Below the crust rests the mantle, and it is in this region, which is largely composed of molten rock, that onsen waters find their origin. In volcanic regions like Japan, surface water can seep down from the earths surface and crust towards the mantle. Here as the water comes into contact with the increasingly hotter and hotter rocks in the lower crust it is heated and as some of the water evaporates into steam this builds pressure which ultimately forces the water back up to the surface in a heated form. The areas where these geo-thermally heated waters finally re-emerge at the surface are where our beloved onsens eventually come to be.

Photo : Kazuhisa OTSUBO on Flickr

Now that we know where onsens come from the next question is how can these simple pools of hot water acquire such seemingly mystical healing qualities?

Again, a little bit of geological context is required.

If water is hotter, then it can dissolve and retain more of the solid elements around it. The earth’s crust from which the waters of the onsen come forth is extremely rich in minerals. As the hot waters rise from deep below the Earth’s surface these minerals are absorbed into the water, making it very mineral rich, far more so than conventional surface water. The term “natural mineral water” is one that drinks companies throw around far too simplistically these days, however the waters of the average onsen really are “mineral water” in its truest sense.

Photo : katsuuu 44 on Flickr

One of Japan’s other great sources of fascination to mineralogists is the diversity of minerals to be found in the rocks around the country. In turn, this vast array of minerals, leads to an even greater diversity in the types of onsen on offer and in the health benefits to be garnered from bathing in each of these different types.

The different composition of each onsen offers unique benefits and as such if you are suffering from a specific condition you may want to consider visiting a particular onsen catering for that ailment. I should add at this point that research in the actual healing qualities of onsen is still very basic and as such I can’t give any guarantees as to their effectiveness. So if you’ve still got a bad back after an hour in an onsen, please don’t sue me!

Photo : MIKI Yoshihito on Flickr

Onsens with a high sulphuric content (Iou-Onsen in Japanese) have a number of supposed medicinal benefits. Among these are: reduced blood pressure, softening of previously hardened arteries, improved circulation and relief for conditions such as arthritis and rheumatism.

Bicarbonate Onsens, in particular those rich in sodium bicarbonate, are good for treating muscular discomfort, fatigue and chronic skin conditions such as eczema.

Iron Onsens, those with an abundance of iron carbonate or melantertite, have been said to help those suffering from anemia or menopausal issues. However, when looking for this kind of onsen, it is best to find one that is as close to the source as possible, as exposure to oxygen reduces the effectiveness of the iron onsen considerably.

Salt Onsens have a consistency and texture similar to seawater. They are said to improve muscle and joint pains and in some cases even improve fertility. Also, as the high salt content inhibits the evaporation of sweat on your body, they can also improve your resistance to cold temperatures.

Onsens with a high level of acidity are especially good at treating severe skin conditions and their highly stimulating effect on the body is also said to be beneficial to sufferers of diabetes, however you should be careful to ensure that you wash your body with ordinary water afterwards to prevent abrasion and irritation of your skin.

Carbon dioxide Onsens are my personal favourite. Rich in carbonic acids, these onsens are quite common across Japan and have a very soothing warming feeling on the body, even at a relatively low temperature. This warming effect improve circulation, heart condition, and a host of other related problems. They are also said to boost fertility and even help with weight loss through accelerating your metabolism.

Yudomari seaside onsen
Photo : JoshBerglund19 on Flickr

With so many different types of onsen to choose from, I highly recommend you do some research before you head out. Possible medical and health benefits are one thing, but it’s also important to find an onsen that has the right ambience and atmosphere for you.

Do you want a simple inn or a luxury resort?
Are you looking for a mountain retreat or somewhere close to the city?
Do you want to enjoy simple Japanese food and drink or do you want a banquet fit for The Emperor himself?
Japan’s onsens are as luxurious and relaxing as they are diverse.
So what are you waiting for?

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