A spring in which groundwater is warmed above average temperatures by geothermal forces and dissolves to the surface.
Hot spring water rises to the surface from deep underground along faults and fractures, such as regular fractures in igneous rocks, some of which are direct surface discharges, while others are horizontally distributed within shallow permeable strata and discharge from the fault, fracture, and permeable strata and the geologic framework that contains them.
Hot springs are closely related to the geothermal temperature of the area where they are expected to occur, and the geothermal temperature exhibits a characteristic pattern that varies with elevation, depth, and geology. Inherently affected by the ground temperature, the temperature gradually decreases with increasing depth until it reaches the constant temperature layer, where there is no change in temperature and the annual difference in temperature is less than 0.1℃.
At depths above the constant temperature zone, the water temperature increases proportionally with depth, and the rate of warming in the subsurface usually increases by 1℃ every 30 to 35 meters. Hot springs are generally found in volcanic areas. Hot springs also form in magma-containing igneous rocks, and there are also hot springs that are not related to volcanoes or igneous rocks.
Hot springs are formed when virgin water contained in the magma seeps to the surface along a thermal gradient, or when rainwater enters deep underground, heats up, becomes circulating water, and seeps to the surface. There are four possible sources of heat for this groundwater.
First, geothermal heat, second, radioactive substances in rocks, third, heat generated by fault activity, and fourth, heat derived from magma, are considered to be the most influential among them.
Strictly speaking, a hot spring is when the temperature is higher than the average annual temperature at the source. Therefore, the temperature of hot springs can vary depending on the latitude and climate of the spring’s location, which is why different countries have different standards for hot springs. In Korea, hot springs are generally referred to as hot springs when the temperature is higher than the body temperature, and cold springs when the temperature is lower.
In Germany, all mineral springs with a temperature of 20°C or higher are called hot springs. Korea and Japan, on the other hand, use a 25°C threshold to categorize springs below 25°C as cold springs, 25-34°C as lukewarm springs, 34-42°C as hot springs, and above 42°C as hot springs. The classification of hot springs varies by scholar and country, but in general, it is based on temperature, the form of elution, the concentration of liquid ions, the solubility of minerals, and the development status of the hot spring.
Classifications based on temperature include cold springs (below 25℃), microthermal springs (25-34℃), hot springs (34-42℃), and hyperthermal springs (above 42℃). The classification according to the type of eruption is based on Suess (E.) and includes springs (湧泉) and geysers (間歇泉). A hot spring is a continuous eruption of hot water, while a geyser is an eruption at regular intervals.
The formation of geysers is a phenomenon in which hot water or superheated water vapor rises from deep underground to the shallow part of the groundwater, and this is explained by the cavity theory and the vertical tube theory.
The cavity theory was proposed by Mackenzie (G.) in England. It is a theory that a large cavity is formed in the ground, and when a narrow and long underground channel along this cavity is connected to the surface, water vapor and heat water heated in the underground cavity are filled in the underground channel, and as the temperature rises and the heat water increases, the air in the upper part of the cavity is gradually compressed, and at some point, the water is pushed up to the surface by the force of steam pressure.
On the other hand, Bunsen’s theory is based on the cone-shaped hydrothermal vents. This means that under normal atmospheric pressure, the temperature of the water surface boils at 100℃, but under the pressure of the hydrothermal vents, it does not boil at 100℃, and over time, when the lower water and water vapor exceed the upper water pressure, hot water erupts with water vapor and becomes a boiling spring.
The theory is that between the time this happens and the time it happens again, there is no eruption, so it becomes a geyser. Some of the world’s most famous geysers are the Great Geyser in Iceland, Yelldw Stone in the United States, and Waymang in New Zealand.
Depending on the value of the ion concentration, they are acidic (pH 3 or less), slightly acidic (pH 3 to 6), neutral (pH 6 to 7.5), slightly alkaline (pH 7.5 to 8.5), and alkaline (pH 8.5 or more). Depending on the solubility of minerals, mineral springs are called mineral springs if they contain more than 1,000 mg of solids per kilogram, and simple springs if they contain less. Even among simple springs, if the pH is above 8.5, it is called alkaline simple spring.
Among mineral springs, alkaline springs, which are mainly composed of calcium carbonate, and iron springs, which contain more than 0. 001 mg or more of iron carbonate, sulfate springs (黃酸鹽泉, 苦味泉) with SO4 as the main component, sulfur springs with more than 1 mg of sulfur per kilogram of hot spring water, acid springs with more than 1 mg of H+ ions, radioactive springs with more than 1 curie unit of radium (Ra) per liter of hot spring water, and other alum springs.
Depending on the development status of the hot spring area, there are three types of hot springs: natural hot springs that bubble up naturally, recreational hot springs with convenient transportation and accommodation facilities, and tourist hot springs that are more aimed at tourism than recreation or health.