Have you ever eaten a jawbreaker? As you eat it, different colors and layers appear until you get to the white center that the jawbreaker company didn’t bother to color. Maybe they figure that, by the time you get to the middle, you don’t care about the colors any more. Anyway, it’s easy to see the different layers in a jawbreaker, but it’s not as easy to see them in Earth.
That’s because the Earth is big. Real big. If you tried to dig a hole through the center of the Earth to the other side, it would be a 8,000 mile long hole. Think about it: if you tried to drive that distance at 80 miles per hour, without stopping, it would still take you over four days. Along the way, you’d get a little chilly, then extremely hot, then boiled and crushed. And that’s just the first day!
The Earth is made up of four major layers: the crust, the mantle, the outer core and the inner core. The crust averages about 25 miles in thickness, or about 20 minutes of your 80 mph journey to the center of the Earth. Not long at all. But the crust is the furthest down that humans have ever been in the Earth, and it’s where everything else is: water, land, plants, animals, our stuff, etc. In other words, we know a tremendous amount about the crust but not nearly as much about the other layers.
That’s too bad, because the other layers are pretty interesting, too. The mantle is about 1800 miles thick (almost as long as the Mississippi River) and makes up most of the mass of the Earth. But it’s hot. The mantle is 1000F near the core and 4000F near the crust, so watch out! Despite the fact that it’s that hot, it’s mostly made up of solid rocks, not liquids. Those solid rocks move around quite a bit, the warmer materials moving toward the crust and the cooler materials moving towards the core. This is called a convection current, just like the hot air in your house moves towards the attic and the cool air moves towards the basement.
Convection currents work because the outer core is super hot. We’re talking about 6000F. In fact, it’s so hot that almost any rock found in the outer core is completely melted. Because the outer core touches the mantle, it uses conduction (heat transfer by contact) to heat up that cool, 1000F rock, and sends it rising up to the top of the mantle. This keeps the mantle in motion and makes the tectonic plates riding on top of the mantle move. The bad news? We live on those plates, and so do the oceans. The good news? They only move at about an inch a year, or the same rate as your fingernails.
OK, so really there’s actually some more bad news. Because the mantle can’t just stay where it is, tectonic plates end up crashing into each other, causing earthquakes, volcanoes and mountain building. Earthquakes happen when plates crash into each other (normal or thrust) or when they slide past each other (strike-slip). Volcanoes happen when the mantle finds an opening in the crust and pushes its way through, sending hot magma through the crust as lava. And mountain building happens when plates push together, but they’re so evenly matched that the only place to go for the plates is up, resulting in nice, pretty mountains. In fact, the pointier and taller the mountain, the younger it is: the Rockies in the west are much younger than the Appalachians in the east of the U.S.
Finally, you may be wondering how all of this heat came about in the first place – how is it that the core and mantle are so hot? Well, remember that the Earth bubbled off the sun, so some of that same heat still remains deep inside the Earth. The inner core is thought to be as warm as the surface of the sun, and is made up of solid metals (mostly nickel and iron). It’s part of the reason that, even when it’s cold outside, it’s still a lot warmer than it is in space. It’s about -450F in space, so a 15F night doesn’t seem so cold anymore, does it? Moreover, there’s something else going on that keeps things warm on the surface of Earth. It’s called radiation. During the day, electromagnetic radiation, otherwise known as light, strikes the surface of the Earth, warming it up. At night, some of that heat is released into the atmosphere. Cloudy days tend to lead to cooler nights due to this phenomenon!