How did the interior of the Earth stay as hot as the surface of the Sun for billions of years?

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How does the Earth’s interior stay boiling for billions of years? Henry, 11, Somerville, Mass.


Our Earth is structured much like an onion – it’s layer after layer.

Starting from top to bottom, there is the crust, which includes the surface you walk on; then lower down, the mantle, mainly made up of solid rock; then even deeper, the outer core, made of liquid iron; and finally, the inner core, made of solid iron, and whose radius corresponds to 70% of the size of that of the Moon. The deeper you dive, the hotter it gets – parts of the core are as hot as the Sun’s surface.

An illustration that shows the structure of the Earth: its crust, mantle, inner core and outer core.

This illustration depicts the four sections beneath the Earth’s surface.
eliflamra/iStock via Getty Images Plus

Journey to the Center of the Earth

Like a teacher of earth and planetary sciences, I study the interior of our world. Just as a doctor can use a technique called ultrasound To image structures inside your body with ultrasound waves, scientists use a similar technique to image the Earth’s internal structures. But instead of ultrasound, geoscientists use seismic waves – sound waves produced by earthquakes.

On the Earth’s surface, you see dirt, sand, grass, and pavement, of course. Seismic vibrations reveal what’s underneath: rocks, big and small. It’s all part of the earth’s crust, which can go down as far as 30 kilometres; it floats above the layer called the mantle.

The upper part of the mantle generally moves with the crust. Together they are called the lithospherewhich is about 60 miles (100 kilometers) thick on average, although it may be thicker in some places.

The lithosphere is divided into several large blocks called plates. For example, the Pacific Plate lies beneath the entire Pacific Ocean, and the North American Plate covers most of North America. The plates are a bit like jigsaw pieces that roughly fit together and cover the surface of the Earth.

Plates are not static; instead, they move. Sometimes it’s the smallest fraction of an inch over a period of years. Other times there’s more movement, and it’s more sudden. This type of movement is what triggers earthquakes and volcanic eruptions.

Moreover, plate motion is a critical, and probably essential, factor in the evolution of life on Earth, because moving plates change the environment and force life to adapt to new conditions.

You will be amazed to see all the life happening under your feet.

The heat is on

Plate movement requires a warm coat. And indeed, the deeper you go into the Earth, the more the temperature increases.

At the bottom of the plates, about 100 kilometers deep, the temperature is about 2,400 degrees Fahrenheit (1,300 degrees Celsius).

By the time you get to the boundary between the mantle and the outer core, which is 1,800 miles (2,900 kilometers) below, the temperature is nearly 5,000 F (2,700 C).

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

Without plate tectonics, human beings would probably not exist.

Collisions in space

Where does all this heat come from?

It does not come from the Sun. Although it warms us and all plants and animals on the Earth’s surface, sunlight cannot penetrate miles into the planet.

Instead, there are two sources. One is the heat the Earth inherited when it was formed 4.5 billion years ago. the earth was made solar nebulaa gigantic gaseous cloud, in the midst of endless collisions and mergers between pieces of rock and debris called planetesimals. This process took tens of millions of years.

An enormous amount of heat was produced in these collisions, enough to melt the entire Earth. Although some of this heat was lost to space, the rest was locked inside the Earth, where much of it still remains today.

The other source of heat: the disintegration of radioactive isotopes, distributed everywhere on the Earth.

To understand this, first imagine an element like a family with isotopes as members. Each atom of a given element has the same number of protons, but different cousin isotopes have varying numbers of neutrons.

Radioactive isotopes are not stable. They release a constant flow of energy which turns into heat. Potassium-40, thorium-232, uranium-235 and uranium-238 are four of the radioactive isotopes keeping the Earth’s interior warm.

Some of these names may sound familiar to you. Uranium 235, for example, is used as fuel in nuclear power plants. The Earth is not likely to run out of these heat sources: Although most the original uranium-235 and potassium-40 have disappearedthere is enough thorium-232 and uranium-238 to last for billions of years.

Along with the hot core and mantle, these energy-releasing isotopes provide the heat necessary for plate motion.

No heat, no plate movement, no life

Even now, moving plates continue to change the Earth’s surface, constantly making new lands and new oceans over millions and billions of years. Plates also affect the atmosphere on equally long timescales.

But without the internal heat of the Earth, the plates would not have moved. The Earth would have cooled. Our world would probably have been uninhabitable. You wouldn’t be here.

Think about it the next time you feel the Earth beneath your feet.


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