Geology: Pressure and Heat

Okay, let’s talk about the different layers in the Earth’s structure.

But first, let’s talk about why layers happen at all.

As you go deeper into the Earth, the pressure increases. Why? Think about it. Here on the surface, what’s weighing down on our heads? About 100 km of air, most of which is in the lowest few kilometers—the air thins out pretty quickly the higher you go. The result is an air pressure of about 1.03 kilograms per square centimeter (14.7 pounds per square inch) at sea level.

Now if we go down, say, a kilometer under our feet, what’s weighing down on our heads? A kilometer of rock. That’s a heck of a lot heavier than air, so any rock down that deep is under a lot of pressure. Go down another kilometer, and now there’s two kilometers of rock weighing down on our heads: roughly double the pressure (if the rock maintains the same density).

So the pressure goes up the farther you go down, i.e. the closer you get to the center of the Earth. At certain levels, this leads to phase shifts, which are changes to the physical properties of matter.

One well-known example of a phase shift is the formation of diamonds. Diamonds are made of carbon atoms locked into a particularly tight framework of chemical bonds. You can only make such a framework by crushing the atoms close together. Normally, carbon atoms don’t like to be really near each other—the nucleus of each atom is surrounded by negatively-charged electrons, so when two atoms start getting close, the electrons on one atom repel the electrons on the other. Under the pressures we’re used to, the repulsion force is stronger than the pressure trying to push the atoms together. The atoms never get close enough to form the framework that a diamond needs.

As you go down into the Earth, however, the pressure increases. Eventually, it’s high enough to squish carbon atoms close enough together, despite the repulsion force trying to keep them apart. At that point, suddenly the atoms can link together in the required framework. The bonds established are strong enough to hold the diamond together even if you reduce the pressure again…which is why diamonds don’t explode if some geological process sends them rising to the surface. (Note however that diamonds aren’t completely stable. They do explode if they take certain types of damage.)

Another type of phase shift involves going from solid to liquid or vice versa. In this case, the cause is loss or gain of heat, but the effect is somewhat similar to the creation of diamonds.

The atoms in a solid have a fixed framework. At any temperature other than absolute zero, the atoms jiggle a bit but they pretty much stay in their position within the framework. However, if you keep adding more heat, the atoms jiggle more and more until they’re finally jumping around too much to stay in position. At that point, the framework breaks down and the solid becomes a liquid.

The temperature of the Earth increases as you go downward, just like the pressure. Why? Because the center of the Earth contains a lot of heat left over from the planet’s creation. The Earth came into existence when the remains from burnt-out stars started to cluster together due to gravity. Chunks of matter clotted together by random chance until they had enough gravity to draw in other nearby chunks. The new chunks added more mass to the whole, which increased the gravity, which dragged in more chunks, etc.

Imagine the early Earth dragging in more and more asteroids from the cosmic neighborhood. Each time a new asteroid collides with the growing planet, it adds mass and a lot of heat. The result was a stage when Earth was completely molten.

Eventually Earth had sucked in all the nearby matter, so it stopped getting a regular bombardment of random stuff. At that point, the surface started to cool, losing its heat to outer space…but the interior of the planet cooled much more slowly, because most of what the Earth is made of doesn’t conduct heat very well. The heat is trapped inside and only leaks out very slowly. Earth still contains a healthy proportion of the heat it acquired from its early components slamming together.

There’s one other source of heat inside Earth: radioactive decay. Radioactive minerals only make up a tiny percentage of the planet’s mass, but they’re constantly pumping out heat as they decay, just like a nuclear reactor. This actually makes a significant contribution to the Earth’s internal temperature.

So why does the inner Earth have layers? Because pressure and heat cause phase shifts that change the nature of how matter behaves. Matter deep down has different properties than matter near the surface, even when atoms of the various elements are present in the same proportions. You have the same stuff, but it acts differently.

And sometimes the stuff doesn’t stay the same. But we’ll talk about that next time.

[Picture of earth structure by Kelvinsong [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D, from Wikimedia Commons]

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