Geology: The Core and Mantle

All right, let’s do this for real: describing the various layers inside the Earth. Remember that these layers are dictated by the combined effects of pressure and heat.

At the Earth’s very center is a solid inner core. The core is very very hot—above 5000 degrees Celsius, about the same temperature as the surface of the sun. That’s so hot that the major elements in the core would be gases on Earth’s surface. So why is the inner core solid? Because the pressure is so high that atoms are squeezed in on each other and can’t flow around as they do in a liquid or gas. The core’s atoms are basically locked into a tight crystal structure even though they’ve got so much heat, they’re jiggling like mad and would jiggle free of the structure if they could.

The inner core is mostly made of iron. That’s because iron is the heaviest element produced during the normal burning of a sun, and our solar system (including Earth) consists of the leftovers from old stars that burned themselves out long ago in this region of the galaxy. These leftovers also included small amounts of elements heavier than iron; such elements can’t be created by a sun’s normal fusion, but they can be created by supernovas smashing smaller atoms together. Earth contains elements heavier than iron, all the way up to uranium, and all such elements were created by supernovas going off in this vicinity long before Earth was born.

Earth was created when leftover star junk started to clump together in a molten mass. At that point, a good percentage of the heaviest elements sank to the middle of the mass. The most abundant of these elements was iron. Other elements sank too—the core probably contains a significant amount of nickel—and it’s not like all the heavy elements sank, because there’s a significant amount of iron and heavy elements in other layers of the Earth. But the core is predominantly iron with some other stuff thrown in.

Surrounding the solid inner core is the liquid outer core. It has a similar composition to the inner core; the major difference is that the pressure is lower (because the outer core isn’t as deep down) so the pressure isn’t high enough to squeeze everything into a solid. Apart from that, the outer core and inner core aren’t very different.

Above the outer core is the mantle. One major difference between the mantle and the core is that the mantle doesn’t have nearly as much iron (since most of the iron sank down to the core when the Earth was molten). The major elements of the mantle are oxygen, silicon and magnesium, which together make up almost 90% of the mantle by weight.

The mantle is solid, but mobile. I like to picture it as very very thick peanut butter: solid, but it has some give. Over long periods of time, the contents of the mantle can and do move. Some very hot bits down near the core slowly rise, in the same way that hot air rises. Cooler bits that start near the top of the mantle slowly sink…so you get very slow circulation and convection currents.

We’ll leave it there for now. Next time, I’ll talk about the crust which sits on top of the mantle, including how the crust and the mantle interact with each other.

Geology

So the good news is that two weeks ago, I finished the first draft of Project Tech-Bro. I’ve set that aside to marinate, i.e. to clear my head and get a little distance from the novel. In a few weeks, I’ll go back and start Draft 2.

In the meantime, I’ve been getting a ton of working done on Project Moon (see here for a completely uninformative mention of these projects).

The bad news is that with all the writing that I’m charging through, I haven’t had time to write blog entries. I haven’t even had time to think of blog topic ideas. But recently I chatted with someone who had no idea of even the most basic principles of geology. And since I’m a fan of geology (as perhaps revealed in All Those Explosions Were Someone Else’s Fault), I’ve decided to write some posts about the basics of geology.

We’ll start with the picture at the top of this post. It’s a cartoon of what the Earth is like inside. (One of my first geology profs insisted on using the word “cartoon” for such pictures to emphasize that they’re huge oversimplifications. Real geology is messy, messy, messy; the Earth has been around for 4.6 billion years, and in that time, it’s developed all kinds of anomalies and glitches.)

So how do we know what the inside of the Earth looks like? A lot comes from measurements taken around the world after an earthquake occurs. Earthquakes cause four different types of vibrations, which then travel outward as waves. Two of these waves go through the planet, while the other two mostly stay on the surface. Each of the four waves has different properties, including the speed with which they travel and what they will or won’t go through.

For example, so-called secondary waves (S-waves) can’t pass through liquid, but primary waves (P-waves) can. So let’s say there’s an earthquake somewhere. Monitoring stations all over the world detect the quake’s vibrations as they travel outward. Some stations pick up both the P-waves and S-waves, while others only pick up the P-waves. This indicates that the S-waves must have hit a liquid layer inside the earth and couldn’t keep going.

By taking measurements from many earthquakes at many monitoring stations, scientists gradually built up a picture of the layers that make up the inside of the planet. That’s what you see in the picture above.

Next time, I’ll talk about what these layers are and why we might care.

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