The structure of our world resembles a light bulb – in layers, one after another. 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% the size of the Moon. The deeper you go, the hotter it gets – parts of the core are as hot as the surface of the sun.

Journey to the Center of the World

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.

heat is burning

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 is almost 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 of the sun that is as hot as its surface. At such a temperature, almost everything – metals, diamonds, people – evaporates into gas. But because the core is under very high pressure deep within the planet, the iron that composes it remains liquid or solid. Illustration showing the layers of Earth’s internal structure.

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 created from the solar nebula, a giant gas cloud amid endless collisions and merging of rock chips and so-called planetary debris. This process took tens of millions of years.

During these collisions, a large amount of heat is released, enough to melt the entire Earth. While some of this heat is lost in space, the rest is locked inside Earth, where much of it remains even today.

Another source of heat is the decay of radioactive isotopes commonly found on Earth.

To understand this, first think of an element as a family whose members are isotopes. Each atom of a given element has the same number of protons, but different cousin isotopes have different numbers of neutrons. Radioactive isotopes are not stable. They emit a constant stream of energy that turns into heat. Potassium-40, thorium-232, uranium-235 and uranium-238 are four radioactive isotopes that store heat inside the Earth.

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 to drive the plates.