We've all eaten boiled eggs. After removing a freshly boiled egg from the pot and soaking it in cold water, we peel off its shell and wait a while before eating it. Otherwise, if you take a bite too soon, you'll find that the inside of the egg is still very hot. If we could peel off the crust of the Earth, a "super-large egg," we would find that its interior is also extremely hot.
How high is the temperature inside the Earth? Although we can't directly measure the temperature deep inside the Earth, we can estimate it through indirect methods such as heat conduction and convection. The Earth's outermost lithosphere is mainly solid rock, with heat transfer primarily through conduction; while the mantle below the lithosphere is a fluid state, with heat transfer via convection. Geoscientists have calculated that the temperature rises from about 10°C at the Earth's surface to 4,500°C at its core. Specifically, near the Earth's surface, due to the influence of solar radiation, the temperature fluctuates daily, seasonally, and over multi-year cycles. This layer is called the thermocline. At its lower boundary, about 20-30 meters below the Earth's surface, the temperature remains constant year-round, equal to or slightly higher than the local annual average temperature, known as the isothermal layer. From the isothermal layer to the lower boundary of the lithosphere, the temperature increases by 1°C every 30 meters, reaching over 1,000°C at the lower boundary of the lithosphere, near 200 kilometers deep. At the boundary between the Earth's core and mantle, the temperature is about 3,700°C. At the interface between the Earth's inner and outer cores, the temperature is about 4,300°C.
In winter, when we sunbathe outdoors, we feel warm all over. This heat comes from solar radiation. For the Earth, solar radiation can only make the Earth's surface "warm," but it cannot affect the Earth's interior. To explore the sources of high temperature inside the Earth, we need to trace back to the long history of the Earth's formation. Geoscientists have found that during the early stages of the Earth's formation, when various small celestial bodies collided and merged to form the primitive Earth, a large amount of heat was generated. Therefore, the temperature from the surface to the interior of the early Earth was very high, and the entire Earth was in a molten state. Later, the Earth gradually cooled, forming the layered structure of the crust, mantle, and core. Although it has cooled for 4.6 billion years, there is still a lot of heat stored inside the Earth. In addition to the residual heat sources from the early Earth, there are also a large number of radioactive isotopes inside the Earth, which continue to decay and release heat. During World War II, the United States destroyed two cities in Japan with just two atomic bombs, and the core material of the atomic bombs was radioactive isotopes. Throughout the history of the Earth's formation, radioactive isotopes inside the Earth have also released enormous amounts of heat. In addition, the movement and phase transformation of substances inside the Earth are also sources of heat.
Since the underground is so hot, why not put it to good use for human beings? Geoscientists have long thought of this. The heat inside the Earth can be transferred to the shallow part of the surface through convection and conduction, or released into the atmosphere through volcanic eruptions, or appear on the surface as hot springs by heating underground water. At present, the development and utilization of geothermal energy mainly focus on underground hot water, and many countries, including China, have begun to use geothermal energy for power generation. For example, Yangbajing in Tibet is famous for its abundant geothermal resources, and the Yangbajing Geothermal Power Station is also the largest geothermal power station in China.