Earth's layers

0:00- [Instructor] Have you ever eaten a peach? A peach has a thin, fuzzy skin on the outside, a thick, soft, juicy layer underneath, and a hard pit at the core that you can't eat. So you nibble around it and just toss that pit away. Well, Earth is kind of like a peach.

0:19It has layers too, a thin outer skin, a thick middle layer, and a super hot core. In fact, the Earth also has an inner core, as we will see. But let's explore these layers, starting from the outside and working our way towards the center. Let's begin with the Earth's outermost layer, the crust.

0:43It's like the crust of a pie, except this is the surface we live on. Everything you can see, land, mountains, valleys, and even the ocean floor, all of that is part of Earth's crust. So what is the crust made of? Well, most of it's made of something called igneous rocks.

1:05The word igneous comes from the Latin for fire. These rocks form when hot, melted material deep inside the Earth cools and hardens. But what's interesting is that the minerals in these rocks are rich in oxygen. In fact, almost half of Earth's crust by mass is oxygen, of course, not as gas, but it's bonded with other elements like silicone, aluminum, and even traces of iron.

1:34Okay, so how thick is this crust, actually? Well, it depends upon where you are. Under the ocean, it can be as thin as five kilometers or about three miles. And under huge mountain ranges like the Himalayas, it can be as thick as 70 kilometers or about 44 miles.

1:53This diagram, though, is not drawn to scale. So let's imagine lifting a small patch of earth straight up to reveal the full thickness of the crust. Ready? Here goes, let's see how tall it really gets. This, by the way, is Mount Everest. Okay, so we have not yet reached the bottom of the crust yet.

2:14Okay, that, there we go. That's how thick the crust would be close to mountain ranges, this would be about 60 kilometers. And just for comparison, this is the deepest we have ever dug into the Earth. So nowhere close to coming to the bottom of the crust. So that means the crust is super, super thick, right?

2:41But guess what? The Earth is enormous. The crust makes up less than 1% of the Earth's total volume. In fact, if Earth were the size of a peach, the crust would be thinner than the peach's skin, and this skin holds every mountain, forest, animal, and ocean you can imagine.

3:05Okay, but is this crust one solid shell? Nope. It's broken into giant slabs called the tectonic plates. The word tectonic comes from a Greek word meaning builder. Some plates carry continents while others carry ocean floors, and together they fit like a giant jigsaw puzzle.

3:25But these plates move very slowly, just a few inches per year. But why are they moving? That brings us to the next layer, the mantle. If the crust is the skin of a peach, the mantle is the thick, juicy fruit underneath. Because the mantle is the Earth's largest layer, making up about 84% of the planet's volume.

3:49It stretches about 2,900 kilometers or 1,800 miles below the crust. How deep is that, you ask? Well, let's put it this way. If you could drive straight down at a highway speed nonstop, it would take you about a day to reach the bottom of the mantle. But good luck with that because the mantle is made of very hot rocks.

4:11These rocks are also rich in oxygen. But look, they also have a lot of magnesium now, and of course these rocks in the mantle are very hot. Now, most of it is solid, but some parts melt into thick liquid called magma. Wait, isn't that just lava? No. When magma reaches the surface during, for example, a volcanic eruptions, that's when we call it lava.

4:33But anyways, because the mantle is so warm, its rocks can slowly deform, kind of like clay or putty if you push on it long enough. This allows the mantle to move very slowly. And since the crust sits right on top of this mantle, it gets dragged along for the ride.

4:52For example, the Indian plate is slowly pushing into Asia, and that's what formed the Himalayas, the highest mountains on Earth. So the crust is literally being lifted because the mantle beneath is shifting. Anyways, that now brings us to the core. Just like the hard pit of a peach, the Earth has a core too, which makes up the remaining 15% of the Earth's volume.

5:21But of course, it's a bit more complicated. In fact, it has two layers, as you can see, the outer core and the inner core. The outer core extends down about 2,200 kilometers, about 1,400 miles, so roughly another day's drive. But more importantly, the outer core is liquid.

5:41So it's a swirling sea of molten iron and nickel. But why is it a liquid? Because it's extremely hot, about as hot as the surface of the Sun. And that brings us to the very center, the inner core. It's also pretty much made of the same stuff, iron and nickel, and it's even hotter than the outer core.

6:04But guess what? It's solid. Wait, what? If it's hotter than the outer core, then why hasn't it melted yet? The answer is pressure. All the layers about it, the outer core, the mantle, the crust, all of it, squeeze it so strongly that the metal stays solid, even at Sun-like temperatures.

6:26So how thick is this solid core? Well, it's about 1,250 kilometers. But remember, because we have now reached the deepest layer, we can also talk about its diameter. So the diameter of the solid core is twice this value, about 2,500 kilometers. And for reference, this is how big the Moon would be.

6:47So imagine a solid metallic sphere, over half the size of the Moon, but as hot as the Sun, sitting just about 2 1/2 days of drive beneath your feet. And here's what's even more amazing, the liquid core moving around the solid inner core creates Earth's magnetic field, an invisible shield that protects us from dangerous radiations from the Sun, especially the solar wind.

7:14Not every planet has a magnetic field this strong. Without it, life on Earth would struggle to survive. So the next time you use a compass to find your direction, remember, it's really liquid iron deep inside the Earth moving around the solid core that's guiding that needle and helping keep us safe.

7:34This is one peach pit you definitely don't wanna throw away.