How did the inside of the Earth stay as hot as the surface of the Sun for billions of years? Earth’s layered structure, which includes moving plates, is heated by the remnants of the planet’s formation and the decay of radioactive isotopes. Geologists use seismic waves to study these internal structures and movements critical to environmental changes and the development of life on Earth. The internal heat causes the plates to move, making the Earth habitable by contributing to events such as earthquakes, volcanic eruptions, and the formation of new land and oceans.

From top to bottom is the cortex, which contains the surface you walk on; later the mantle, mostly hard rock; then, at a deeper level, an outer core made of liquid iron; and finally, an inner core made of solid iron with a radius of 70% of that of the Moon. The deeper you go, the hotter it gets – parts of the core are just as hot as the surface of the Sun.

This drawing shows four areas below the Earth’s surface.

As a professor of earth and planetary sciences, I examine the interior of our world. Just as a doctor can use a technique called sonography to take pictures of structures inside your body using ultrasound waves, scientists use a similar technique to take pictures of Earth’s interior structures. But geophysicists use seismic waves instead of ultrasound—sound waves created during earthquakes.

On the surface of the earth, of course, you see dirt, sand, grass and pavement. Seismic vibrations reveal what lies beneath: rocks big and small. All are part of the Earth’s crust that can descend as much as 20 miles (30 kilometers); It floats above a layer called the mantle. The upper part of the mantle usually moves with the crust. Together they are referred to as the lithosphere, which averages about 60 miles (100 kilometers) thick, although it can be thicker in some places.

The lithosphere is divided into several large blocks called plates. For example, the Pacific Plate lies under the entire Pacific Ocean, while the North American Plate covers most of North America. The plates are like pieces of a puzzle that fit together roughly and cover the Earth’s surface. The plates are not static; instead they act. Sometimes it is the smallest fraction of an inch for several years. Other times there is more movement and they are more abrupt. This movement is the cause of earthquakes and volcanic eruptions.

At most, plate motion is a critical and possibly fundamental factor in the evolution of life on Earth, as moving plates change the environment and force life to adapt to new conditions. A warm mantle is required for plate movement. And indeed, as you go deeper into the Earth, the temperature increases. At the bottom of the plates, at a depth of about 60 miles (100 kilometers), the temperature is about 2,400 degrees Fahrenheit (1,300 degrees Celsius). When you reach the boundary between the mantle and the outer core, which is 2,900 kilometers away, the temperature will reach about 2,700 °C.

Then, at the boundary between the outer and inner cores, the temperature nearly doubles to 10,800°F (over 6,000°C). This is the part that is as hot as the surface of the Sun. At such a temperature, almost everything – metals, diamonds, people – evaporates into gas. But because the core is under such high pressure deep within the planet, the iron that formed it remains liquid or solid.

collision in space

Where is all this heat coming from?

Not from the sun. Although it warms us and all plants and animals on the Earth’s surface, sunlight cannot penetrate many kilometers into the interior of the planet. Instead, there are two sources. One of these is the heat that the Earth inherited during its formation 4.5 billion years ago. Earth was formed by the solar nebula, a giant gas cloud between endless collisions and fusions of rock and debris called planetary. This process took tens of millions of years.

Another source of heat: the decay of radioactive isotopes common on Earth. To understand this, first think of an element as a family of isotopes. Each atom of a given element has the same number of protons, but different relatives of the isotope have different numbers of neutrons. Radioactive isotopes are unstable. They emit a constant stream of energy that turns into heat. Potassium-40, thorium-232, uranium-235, and uranium-238 are the four radioactive isotopes that keep the Earth warm.

Some of these names may sound familiar to you. For example, uranium-235 is used as fuel in nuclear power plants. These Earth’s heat sources are not in danger of running out: with most of the original uranium-235 and potassium-40 gone, there is enough thorium-232 and uranium-238 to last billions of years. Along with the hot core and mantle, these energy-releasing isotopes provide heat for plate motion.

No heat, no plate movement, no life

Even now, moving plates continue to reshape the Earth’s surface, constantly creating new lands and new oceans over millions and billions of years. Plates similarly affect the atmosphere on long time scales. But without the Earth’s internal heat, the plates could not move. The earth would cool. Most likely, our world would be uninhabitable. You wouldn’t be here. Source