Layers of the Earth
Take a look around you. What do you see? Maybe you live in a city, and see lots of large buildings. Maybe you see a beautiful mountain view, or beaches, or rivers, or grasslands. Maybe you just see the walls of your house. All of this is on the crust, or surface, of the Earth; we will talk more about what this means. Now look up and out the window. Sky. Big, spacious, maybe cloudy, sky. In other words, the atmosphere. Compared to the Earth’s crust, the atmosphere is huge. Even still, everything beneath your feet is bigger than the whole atmosphere. In this lesson, you will learn what’s beneath your feet. In other words, you will learn what you would find if you tried to dig to China.
As we look at the different layers of the Earth, you will note one common theme: density. Just like if you were working with a test tube in the lab, heavy stuff sinks. Over the billions of years that the earth has existed, this has had time to happen. The heaviest metals sank to the center of the Earth, and the lightest stuff blows around in the atmosphere.
Atmosphere
You have probably heard the term atmosphere before. The atmosphere is the air that surrounds Earth. It is the air we breathe and our shield from the harsh environment of outer space. It is where weather comes from. It’s pretty cool, especially as you go far up. (Get it, cool, like because it’s really cold up high in the atmosphere?)
You probably even have a sense of how big the atmosphere is. It's the whole big sky. It’s in total about 300 mi (480 km) thick. This is a lot, but in comparison to the size of the earth, it’s essentially the equivalent of throwing a warm blanket over the Earth.
You can probably also recognize that the atmosphere is much less dense than the oceans and land, on account of it being made of gas. For the purposes of this class, we’ll focus on one of the many reasons why the atmosphere, the outermost layer of Earth, is the most dense: particle sorting. As Earth has been spinning over billions of years, all the heavy stuff has sunk to the middle, and all of the light stuff has floated to the top. The atmosphere gets less dense farther away from Earth, because the less dense air floats on the more dense air.
You may have already known that far-away air is less dense, if you have heard air at high altitudes, like up in the mountains, described as “thin”. This refers to the decreased density of the atmosphere as you move further away from the Earth’s surface. The thin atmosphere makes it hard for humans to get enough oxygen: that’s why airplanes are pressurized to hold in air, and why they have oxygen masks just in case that pressure starts to fall. This is also why you see stuff getting sucked out of planes when they crash in the movies: everything outside the plane is basically a vacuum by comparison. Eventually, you get far enough away from Earth that Earth’s gravity can no longer hold on to any gas, and the atmosphere ends. You are now in space.
(Other reasons that nearby air is less dense than far away air include air pressure—the force of all the air on top of nearby air compressing it closer to the Earth—and gravity—the force of Earth pulling air close to it, and not being able to hold onto far away air as well).
We will talk more about the atmosphere in a later lesson. Here’s a sneak preview:
Water
Water covers 70% of the Earth’s surface, or crust. Despite it being so important to Earth, we know more about the surface of Mars than we do about the ocean floor. What we do know is that over 98% of the Earth’s water is salt water, and that the ocean ranges from between 2 and 6 kilometers deep. It is home to many living things and plays a critical role in the Earth’s climate (for example, water is a major part of the reason that California is so much warmer than most of the East Coast in the winter time, even though it is the same distance from the equator).
Water is more dense than the air above it and less dense than the crust beneath it. This should not be a surprise to you—as you’ve learned, gas (atmosphere) is least dense, liquid (water) is medium dense, and solid (Earth) is most dense.
Beneath the air and water, the Earth is made of 4 main layers. From the outside in, it’s crust, mantle, outer core, and finally, the inner core. You can think about it like a chocolate.
Water is more dense than the air above it and less dense than the crust beneath it. This should not be a surprise to you—as you’ve learned, gas (atmosphere) is least dense, liquid (water) is medium dense, and solid (Earth) is most dense.
Beneath the air and water, the Earth is made of 4 main layers. From the outside in, it’s crust, mantle, outer core, and finally, the inner core. You can think about it like a chocolate.
The crust is like the chocolate covered crisp shell, complete with hazelnut chunks for mountains. The mantle is like the gooey hazelnut cream filling. The outer core is the lighter part of the hazelnut in the center, and the inner core is the darker part. Are you hungry yet?
Let’s go more into detail about each layer.
Let’s go more into detail about each layer.
Crust
The first solid layer of the Earth, mostly covered with water, is the crust. This is the layer that you are standing on. The name makes sense. In the chocolate analogy, the crisp outer shell is all of what you can see of the chocolate, but there is a lot more going on underneath. The crust is denser than the water surrounding it. Once again, over billions of years, denser stuff sinks, so the closer we get to the center of Earth, the denser the Earth becomes.
The crust is made up of mostly silicates, better known as sand. Strictly speaking, we only call it sand when it breaks up into tiny little pieces, but it’s rocks and dirt-like stuff nonetheless. It also has some other metals, like aluminum and magnesium, in it.
The crust is made up of mostly silicates, better known as sand. Strictly speaking, we only call it sand when it breaks up into tiny little pieces, but it’s rocks and dirt-like stuff nonetheless. It also has some other metals, like aluminum and magnesium, in it.
The Earth without water
The crust is divided into massive sections called tectonic plates (technically, each plate also includes another layer of the Earth, the upper mantle, which we will talk about next). These massive slabs of rock actually move: they’re floating on a molten river of magma. When these enormous rocks scrape by or over each other, they can cause earthquakes.
Two plates come together at a fault line, where volcanoes form
When tectonic plates crash (very slowly) into each other, they form mountains, by propelling one or both of the plates upward. Isn’t it cool to think of the solid ground beneath you as surfing on lava? The lava can even spurt out sometimes! That’s called a volcano. We will talk more about plate tectonics later.
Hot spots tend to form at the boundaries of tectonic plates, making volcanoes more likely to form.
Mantle
The mantle is the middle layer of the Earth, and it is also the thickest (as in widest; we still get more dense as we go down). Can you guess how the density of mantle compares to the crust above it?
If you said mantle is more dense, you’re getting the idea! Mantle is made of mostly molten (melted, like lava) rock. It has a lot of magnesium in it. It is denser than the crust above it and not as dense as the outer core below it, which has mainly denser metals.
Scientists do not know as much about the layers below the crust as they do about the atmosphere and the crust itself. The high temperatures and extreme pressures prevent the use of any instruments directly, so instead, scientists rely on indirect measurements. Vibrations and waves pass through different types of matter at a known rate, so, whenever an earthquake happens, scientists record exactly when and where they feel the shaking. From that information, they can make an educated guess as to what type of rock the waves moved through. Cool, huh? This is how we know what the mantle is made out of and that it is molten.
Because the core below the mantle is much hotter than the surface of Earth, the molten rock closest to the core gets very hot. When it does, its density decreases due to the expansion of liquid rock at high temperatures. But, don’t worry, we’re not breaking any rules here: that less dense material now floats back to the top of the mantle, forcing more dense stuff downward. Therefore, the hot mantle rises to the surface, and cold mantle sinks to the bottom. When the cold mantle sinks, the core heats it up again, and the circular pattern continues. (This is very similar to how wind in the atmosphere occurs; we will learn about that more later in the unit). The movement of the mantle in this way is called convection. It is responsible for carrying heat to the Earth’s surface. This movement is also responsible for plate tectonics and volcanic activity on the crust above it.
(Another reason that convection occurs is because organic material from the Earth’s crust gets recycled into the mantle and burned into CO₂. Gas bubbling back up to the surface contributes to the convection currents. More on this later.)
This video will show you an artistic rendering of what mantle looks like and what its implications are:
If you said mantle is more dense, you’re getting the idea! Mantle is made of mostly molten (melted, like lava) rock. It has a lot of magnesium in it. It is denser than the crust above it and not as dense as the outer core below it, which has mainly denser metals.
Scientists do not know as much about the layers below the crust as they do about the atmosphere and the crust itself. The high temperatures and extreme pressures prevent the use of any instruments directly, so instead, scientists rely on indirect measurements. Vibrations and waves pass through different types of matter at a known rate, so, whenever an earthquake happens, scientists record exactly when and where they feel the shaking. From that information, they can make an educated guess as to what type of rock the waves moved through. Cool, huh? This is how we know what the mantle is made out of and that it is molten.
Because the core below the mantle is much hotter than the surface of Earth, the molten rock closest to the core gets very hot. When it does, its density decreases due to the expansion of liquid rock at high temperatures. But, don’t worry, we’re not breaking any rules here: that less dense material now floats back to the top of the mantle, forcing more dense stuff downward. Therefore, the hot mantle rises to the surface, and cold mantle sinks to the bottom. When the cold mantle sinks, the core heats it up again, and the circular pattern continues. (This is very similar to how wind in the atmosphere occurs; we will learn about that more later in the unit). The movement of the mantle in this way is called convection. It is responsible for carrying heat to the Earth’s surface. This movement is also responsible for plate tectonics and volcanic activity on the crust above it.
(Another reason that convection occurs is because organic material from the Earth’s crust gets recycled into the mantle and burned into CO₂. Gas bubbling back up to the surface contributes to the convection currents. More on this later.)
This video will show you an artistic rendering of what mantle looks like and what its implications are:
Core
Continuing the trend of Earth getting denser as you go toward the middle, the core is the densest of all. It is quite literally a gigantic ball of metal. The core is divided into two sections, appropriately named the outer core and inner core. They are made of the same material, mostly molten iron and nickel. It is the rotation of this ball that provides Earth its magnetic field (what makes your compass work and shields us from harmful solar radiation).
Even though they are both made of iron, the inner core is solid while the outer core is liquid. The outer core can be liquid, even though it’s metal, because the gravity pulling it towards the center of earth basically makes a ton of friction, which makes it really hot. It’s also really hot because it was hot when the earth was made and hasn’t had time to cool off yet (think about food: the outside always cools off faster than the inside). The inner core is even hotter. But, it’s solid because of the enormous amount of pressure on top of it, from the collective weight of all of the other layers. This pressure pushes the atoms really close together, which is the definition of a solid. The dense elements sunk to the middle, and then they got even more dense because of everything pushing down on top of them. The “weight of the world on the core’s shoulders,” if you will.
If you’re interested in understanding just how hot the core of the Earth is and how it got that way, check out this video:
Even though they are both made of iron, the inner core is solid while the outer core is liquid. The outer core can be liquid, even though it’s metal, because the gravity pulling it towards the center of earth basically makes a ton of friction, which makes it really hot. It’s also really hot because it was hot when the earth was made and hasn’t had time to cool off yet (think about food: the outside always cools off faster than the inside). The inner core is even hotter. But, it’s solid because of the enormous amount of pressure on top of it, from the collective weight of all of the other layers. This pressure pushes the atoms really close together, which is the definition of a solid. The dense elements sunk to the middle, and then they got even more dense because of everything pushing down on top of them. The “weight of the world on the core’s shoulders,” if you will.
If you’re interested in understanding just how hot the core of the Earth is and how it got that way, check out this video:
Summary
The Earth is an awesome place. There is a fiery world of moving rock and molten metal starting just a few miles underneath your feet, and this entire system is kept in place largely because of some single, simple concepts that you already understand: density (heavy things sink) and gravity (heavy things are pulled in tight).
This video gives a great overview of why the Earth has layers and what this has to do with density. This is an important concept for you to understand:
You should understand:
- How particle sorting and density has led to the formation of Earth’s layers.
- How the relative densities of Earth’s layers relates to each other.
- The general composition and location of each of the Earth’s layers:
- Atmosphere: outermost; air
- Water/oceans: underneath atmosphere and above (most of the) crust; water
- Crust: the layer we stand on; dirt/sand/silicates
- Mantle: lava, underneath Earth’s surface; molten rock with a lot of magnesium
- Outer core: liquid metal, underneath mantle
- Inner core: solid metal, at the very center of the earth; mostly iron.
Learning Activity
Content contributors: Eric Klinkhammer, Grace Cheung, and Emma Moulton