This article was originally published on The conversation. (opens in new tab) The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.
Shichun Huang (opens in new tab)Associate Professor of Earth and Planetary Sciences, University of Tennessee.
Our Earth is structured much like an onion – it’s one layer after another.
Starting from top to bottom, there’s the crust, including the surface you walk on; then further down, the mantle, usually solid rock; then even deeper, the outer core, made of liquid iron; and finally the inner core, made of solid iron, and with a radius 70% that of the moon. The deeper you dive, the hotter it gets – parts of the core are as hot as the surface of the sun.
Related: The Layers of the Earth: Exploring Our Planet Inside and Out
Journey to the Center of the Earth
As a professor of earth and planetary sciences (opens in new tab), I study the inside of our world. Just like a doctor can use a technique called ultrasound (opens in new tab) To take pictures of the structures in 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.
At the surface of the earth, you will of course see dirt, sand, grass and pavement. Seismic tremors reveal what lies beneath (opens in new tab): rocks, large and small. This is all part of the crust, which can go down as much as 30 kilometers; it floats on top of the layer called the mantle.
The upper part of the mantle typically moves along with the crust. Together they are called the lithosphere (opens in new tab)which averages about 100 kilometers thick, although it can be thicker in some places.
The lithosphere is divided into several large blocks called plates (opens in new tab). For example, the Pacific Plate is under all of the Pacific Ocean, and the North American Plate covers most of North America. Plates are like puzzle pieces that roughly fit together and cover the Earth’s surface.
The plates are not static; instead they move. Sometimes it is the tiniest fraction of centimeters over a period of years. Other times there is more movement and it is more sudden. This kind of movement causes earthquakes and volcanic eruptions.
In addition, plate motion is a critical and probably essential factor driving the evolution of life on Earth, as the moving plates change the environment and force life to adapt to new conditions. (opens in new tab).
The heating is on
Plate movement requires a hot mantle. And indeed, as you go deeper into the earth, the temperature rises.
At the bottom of the plates, about 100 kilometers deep, the temperature is about 1300 degrees Celsius.
By the time you reach the boundary between the mantle and the outer core, which is 2,900 kilometers lower, the temperature is almost 2,700 degrees Celsius.
Then, at the boundary between the outer and inner cores, the temperature doubles to nearly 10,800 F (over 6,000 C). That is the part that is as hot as the surface of the sun. At that temperature, almost everything – metals, diamonds, people – evaporates to gas. But because the core is under such high pressure deep inside the planet, the iron that makes it up remains liquid or solid.
Collisions in space
Where does all that heat come from?
It’s not from the sun. While it warms us and all the plants and animals on the Earth’s surface, sunlight cannot penetrate miles of the planet’s interior.
Instead, there are two sources. One is the heat that the Earth inherited during its formation 4.5 billion years ago. The Earth is made of the solar nebula (opens in new tab), a giant gaseous cloud, amid endless collisions and fusions between bits of rock and debris called planetesimals. This process took tens of millions of years.
An enormous amount of heat was produced during those collisions, enough to melt the entire Earth. While some of that heat was lost to space, the rest was trapped in the Earth, where much of it remains even today.
The other source of heat: the decay of radioactive isotopes, scattered all over the Earth.
To understand this, first imagine an element as a family with isotopes as members (opens in new tab). Each atom of a given element has the same number of protons, but different isotope cousins have different numbers of neutrons.
Radioactive isotopes (opens in new tab) are not stable. They release a steady stream of energy which is converted into heat. Potassium-40, thorium-232, uranium-235 and uranium-238 are four of the radioactive isotopes that keep the Earth’s interior warm.
Some of those names may sound familiar to you. For example, uranium-235 is used as a fuel in nuclear power plants. Earth is in no danger of running out of these heat sources: Although most of the original uranium-235 and potassium-40 is gone, there is enough thorium-232 and uranium-238 to last for billions of years.
Together with the hot core and mantle, these energy-releasing isotopes provide the heat to drive the movement of the plates.
No heat, no plate movement, no life
Even now, the moving plates continue to change the surface of the earth, constantly creating new lands and new oceans over millions and billions of years (opens in new tab). The plates also affect the atmosphere over equally long timescales.
But without the Earth’s internal heat, the plates wouldn’t have moved. The earth would have cooled down. Our world would probably have been uninhabitable. You wouldn’t be here.
Think about that the next time you feel the earth beneath your feet.
This article has been republished from The conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).
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