Have you ever witnessed the moment a hot spring bursts forth? Hot water erupts from the ground, and white steam rises. This is the result of an astonishing phenomenon occurring beneath the earth.
How do hot springs emerge? What happens underground? Let's delve into the mechanism.
Three Conditions for Hot Springs to Emerge
Three conditions are necessary for hot springs to emerge.
- Water: The presence of water underground.
- Heat: A heat source to warm the water.
- Passage: A pathway for the water to reach the surface.
When these three conditions are met, a hot spring can emerge.
Condition 1: Water (Groundwater)
The source of hot springs is rainwater or snowmelt.
When it rains, water seeps into the ground. Water that penetrates deep underground becomes groundwater, which is the raw material for hot springs.
Japan is a country with abundant rainfall, about twice the world average annual precipitation. This plentiful rainwater gives rise to abundant hot springs.
Condition 2: Heat (Heat Source)
There are two types of heat sources that warm groundwater.
Volcanic Heat Source (Magma)
Beneath volcanoes lies magma, with temperatures ranging from about 800 to 1,200°C.
Magma directly heats the groundwater, or high-temperature steam and gases released from magma heat the groundwater.
Many volcanic hot springs are formed this way, often characterized by high temperatures and rich mineral content.
Non-Volcanic Heat Source (Geothermal Gradient)
Even in regions without volcanoes, hot springs can emerge. The heat source here is the geothermal gradient.
The geothermal gradient is the phenomenon where temperature increases as you go deeper underground, averaging about 3°C per 100 meters.
Thus, at a depth of 1,000 meters, the temperature is about 30°C higher than at the surface, and at 2,000 meters, about 60°C higher.
Groundwater that penetrates deep underground is heated by this geothermal gradient, resulting in non-volcanic hot springs.
Condition 3: Passage (Faults and Fissures)
Heated groundwater needs a passage to reach the surface.
Faults and Fissures
Underground, there are faults and fissures. Earthquakes and crustal movements create gaps in the rock layers.
Heated groundwater rises to the surface through these faults and fissures, which serve as outlets for hot springs.
Pressure
Heated groundwater is under high pressure, which pushes the water to the surface.
Especially in volcanic hot springs, high-temperature, high-pressure steam pushes the water up. Some springs, like geysers, erupt periodically.
The Mechanism of Volcanic Hot Springs
Let's explain the mechanism of volcanic hot springs step by step.
- Rainwater Penetrates Underground: Rainwater seeps into the ground, becoming groundwater.
- Magma Heats Groundwater: Magma beneath the volcano heats the groundwater.
- Volcanic Gases Dissolve: Volcanic gases like hydrogen sulfide and carbon dioxide dissolve into the groundwater.
- Pressure Builds Up: The heated groundwater builds up pressure.
- Eruption Through Faults and Fissures: High-pressure groundwater erupts through faults and fissures to the surface.
Through this process, volcanic hot springs are born.
The Mechanism of Non-Volcanic Hot Springs
The mechanism of non-volcanic hot springs is slightly different.
- Rainwater Penetrates Deep Underground: Rainwater penetrates to depths of several thousand meters.
- Heated by Geothermal Gradient: At great depths, the temperature rises due to the geothermal gradient, heating the groundwater.
- Dissolution of Underground Minerals: Minerals from underground rocks dissolve into the groundwater.
- Long Ascent Time: Heated groundwater takes a long time to ascend through faults and fissures.
- Emergence at the Surface: After hundreds to thousands of years, it emerges at the surface.
Non-volcanic hot springs take longer than volcanic ones, but their mineral content is not diluted.
Types of Hot Spring Sources
There are several patterns in how hot springs emerge.
Natural Eruption Springs
Hot springs that naturally erupt due to underground pressure, requiring no pumps.
Like the Yubatake in Kusatsu Onsen, they continuously flow, offering the most luxurious hot spring experience.
Drilled Springs
Hot springs where wells are drilled to pump up groundwater.
Drilling can reach depths of several hundred to several thousand meters, with pumps used to extract the water.
Geysers
Hot springs that erupt at regular intervals.
Steam accumulates underground, and when pressure builds up, it erupts all at once. The "Old Faithful" geyser in Yellowstone, USA, is famous.
In Japan, Beppu Onsen in Oita Prefecture has geysers.
The Age of Hot Springs
Hot springs have an "age," which is the time from when water penetrates underground to when it emerges at the surface.
Volcanic Hot Springs: Typically a few years to a few decades, relatively "young."
Non-Volcanic Hot Springs: Hundreds to thousands of years, "old."
The "Golden Spring" of Arima Onsen is thought to be seawater heated deep underground about a million years ago. It emerges after an unimaginably long time.
Hot Springs as a Renewable Resource
Hot springs are a renewable resource.
As long as rainwater and snowmelt continue to seep underground, hot springs will continue to emerge. As long as volcanic activity persists, volcanic hot springs will continue to flow.
However, excessive extraction should be avoided. If groundwater is depleted, hot springs will dry up.
With proper management, hot springs can flow forever. Sustainable use is essential to preserve Japan's hot spring culture.
Listening to the Sound of Hot Springs
Have you ever heard the sound of hot springs emerging?
Sounds like "gurgle" and "hiss" are the sounds of hot springs emerging from underground. High-pressure steam and gases erupt with the water.
Hearing these sounds allows you to feel the "living hot springs" and experience the phenomenon occurring underground through your senses.
Conclusion
Hot springs are a phenomenon where rainwater seeps underground, is heated, and emerges at the surface. Magma from volcanoes or the geothermal gradient heats the groundwater.
Volcanic hot springs emerge in a few years to a few decades. Non-volcanic hot springs take hundreds to thousands of years to emerge.
Hot springs are a renewable resource. As long as rainwater continues to seep underground, hot springs will continue to flow. With proper management, they can be enjoyed forever.
When you witness the moment a hot spring emerges, remember the astonishing phenomenon occurring underground. Enjoy and appreciate this gift from Japan's earth.
