How are hot springs formed？

　　First, the formation reason:

　　One is formed by magmatism inside the earth’s crust, or accompanied by volcanic eruption. In the extinct volcanic terrain area where volcanic activity has occurred, the surface uplifted by crustal plate movement, and there is still uncooled magma under the ground, all of which will continuously release a lot of heat energy. Because of the heat concentration of such heat sources, as long as there are pores in the water-bearing rocks nearby, they will not only be heated into high-temperature hot water, but most of them will boil into steam, mostly sulfate springs.

　　Second, it is formed by surface water infiltration and circulation, that is to say, when the rain drops to the surface and penetrates downwards, the aquifer deep into the earth’s crust forms groundwater (sandstone, conglomerate, volcanic rock, these good aquifers). Groundwater is heated by geothermal energy below to become hot water, and most of the deep hot water contains gas, which is mainly carbon dioxide. When the temperature of hot water rises, if there is dense and impermeable rock on it to block the way, the pressure will get higher and higher, so that hot water and steam will be in a high pressure state. When there is a crack, they will surge up, and the pressure will gradually decrease as the hot water rises closer to the surface. As the pressure gradually decreases, the gas contained will gradually expand, reducing the density of hot water, which is more conducive to the rise of hot water. The rising hot water circulates repeatedly with the pressure (hydrostatic pressure difference) generated by the density difference of the cold water heated later in sinking to generate convection. When the resistance of the open fracture is small, the hot water rises along the fracture and gushes out of the surface, so that the hot water can rise endlessly and finally flow out of the surface to form a hot spring. With the cooperation of the topography of high mountains and deep valleys, the surface water at the valley bottom may be higher than that of high mountains, and the water level in the middle and lower groundwater levels. Therefore, the valley bottom of deep valleys may be the place with the largest hydrostatic pressure difference, and the possibility of hot water upwelling from the valley bottom is the greatest. Most hot springs occur in the river bed in the valley.　

　　Second, the formation conditions:

　　1. There must be hot water underground (there is a heat source underground);

　　2. There must be a hydrostatic pressure difference leading to hot water upwelling (there are cracks in the rock stratum to make hot springs gush out);

　　3. There must be deep and long cracks in the rock for hot water to reach the ground (there is space for storing hot water in the stratum).