How to Teach the Layers of the Earth

Mar 12, 2021

8 min read

Teaching Earth’s compositional layers is always a good way to start your middle school earth science unit. First, it is a pretty simple concept for students to understand. Second, you can tie the compositional layers of the Earth to how the Earth formed. When everything was hot, elements settled according to density, so the inner layers are more dense than the outer layers. Speaking of density, this is a great opportunity to show your students how scientists use topics such as density to explain the world around them.

I like to teach Earth’s compositional layers before teaching the mechanical layers of the Earth because it is easier to understand, and it gives students a firm foundation to learn about the mechanical layers. How will your students understand that the asthenosphere is a part of the mantle if they don’t know what the mantle is?

Using Pictures to Teach the Earth’s Compositional Layers

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Whenever I can, I teach using pictures. Our brains understand pictures faster and easier than words. Maybe that is why a picture is worth 1,000 words. So, I would start my lesson on the Earth’s compositional layers with a picture. It doesn’t have to be fancy, you can even draw it on the board. I would also have students draw the picture in their science notebooks. Then, they can add information to the picture as they learn about the compositional layers of the Earth.

Using Videos to Teach the Earth’s Compositional Layers

I also like using videos to teach scientific concepts. If a picture is worth 1,000 words, how many words is a video worth?! I especially like videos that model how the Earth formed in space. I find this fascinating, and I always try to find ways to bring a sense of wonder to my science lessons.

Here are some of my favorite videos for teaching the compositional layers of the Earth:

Birth of the Earth – This video explains how the Earth formed.

Layers of the Earth. What is Beneath Us? – This is a simple introduction to the compositional layers of the Earth.

The Diagram of the Earth is a Lie – This video explains what the Earth’s compositional layers really look like.

Layers of the Earth Based on Chemical Composition and Physical Properties – This video is good to bridge the gap between compositional layers and mechanical layers.

The Compositional Layers of the Earth

Scientists have only understood the composition of the Earth for about the past 100 years. It wasn't until the modern seismograph was invented that they had the tools to explore the different layers of the Earth.

The Crust

The outermost layer of the Earth is called the crust. Scientists have known all about the crust for much longer than the other layers because it is where humans live. The crust is made up of rocks covered by soil. Igneous rocks such as granite and basalt make up most of the crust, but there are also sedimentary rocks and metamorphic rocks. The crust is by far the thinnest layer of the Earth. Under the oceans, it is about seven kilometers deep. Continental crust is between thirty and seventy kilometers deep.

The crust is not one complete layer like the skin of an apple. Instead, the crust is made up of about twelve tectonic plates. These plates float on the mantle below and are always moving.

Millions of years ago, all of the land on Earth combined into one supercontinent called Pangaea. Over time, the crustal plates moved apart to form the seven continents known today. Pangaea wasn't the first supercontinent. Scientists estimate there have been at least three supercontinents formed and broken apart in the Earth's history. Another supercontinent will form someday far in the future. Currently, North America and Asia are moving closer together at a rate of about 2.5 centimeters a year.

The Mantle

The mantle is the layer below the crust. It is the thickest layer on Earth, spanning 3,000 kilometers. Like the crust, the mantle is solid, but its silicate rocks are softer than the rocks of the crust. Their consistency is similar to Silly Putty, and over millions of years, the solid rock flows in convection currents that push the tectonic plates of the crust around the Earth. Rock heated by the high temperatures of the outer core below the mantle slowly rises to the top of the mantle. When the rock cools, it falls back down to the bottom of the mantle.

The heat from the mantle melts rocks within the crust above it. The melted rock forms the magma that fills volcanoes around subduction zones and hot spots. Magma in the crust also creates new oceanic crust at mid-ocean ridges. While the mantle provides the heat to melt rocks, it is not liquid. The mantle is made up of soft, solid rock that flows in convection currents about as fast as your fingernails grow.

Surprisingly, the mantle contains water. A lot of water. The mantle is too hot to hold liquid water, but the components of water, hydrogen and oxygen, are trapped as hydroxide ions within the rocks of the mantle. Scientists believe the mantle holds more water than all of the Earth's oceans. Unfortunately, we can't get to the water because we don't have the technology to drill into the mantle yet.

The Outer Core

Below the mantle is the outer core. This layer is about 2,250 kilometers thick and is filled with liquid iron and nickel. It is the only liquid layer within Earth. The iron and nickel are liquid because they have melted in the intense heat of the outer core.

Inge Lehmann, a seismologist from Denmark, discovered the outer core. A seismologist is a person who studies earthquakes. By looking at the way earthquake waves moved through the Earth, Lehmann was able to determine that the outer core was liquid. Seismic waves move much more slowly through liquids than solid rocks. They are also reflected back to the source of the wave instead of moving through in a straight line as they do through solid materials.

The Inner Core

The inner core lies within the outer core. Around 1,200 kilometers thick, the inner core is even hotter than the outer core. In fact, at about 7,000 degrees Celsius, the inner core is even hotter than the surface of the Sun. However, the iron and nickel within it are solid because of the high pressure exerted on them by the outer layers of the Earth.

Lehmann was able to predict this based on the fast, straight seismic waves she observed passing through the inner core. How did Lehmann track the speed of seismic waves in the center of the Earth without being there? By measuring the amount of time it took waves to move from one seismic station to another, she could determine how fast the waves were traveling. Comparing results from stations all around the world created a picture of the center of the Earth.

Scientists continue to study and debate the different layers of the Earth today. The more technology improves, the more information scientists can gather about the layers of the Earth. Someday, scientists may even be able to travel to the outer and inner core.

Teaching the Mechanical Layers of the Earth

Once students know about the compositional layers of the Earth, they are ready to learn about the mechanical layers of the Earth. In my opinion, the mechanical layers are a little bit more difficult to understand. That’s why I like to teach them second.

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The Mechanical Layers of the Earth

The layers of the Earth can be defined either by their composition or their mechanical properties. Compositionally, oceanic crust is mainly made up of basalt, and continental crust is mostly granite. These rocks float on the denser silicate rocks found in the mantle below the crust. Iron and nickel make up both the inner and outer cores.

When the layers of the Earth are defined by their mechanical properties, scientists analyze how the parts of the Earth move. The outermost layer of the Earth is the lithosphere. The lithosphere is made up of the crust and the transition between the crust and the mantle called the Moho Discontinuity. Both the crust and the Moho Discontinuity are solids. Around twelve tectonic plates come together to form the lithosphere. The lithosphere is between 50 and 140 km thick. Continental crust is thicker than oceanic crust, so the thickness of the lithosphere varies.

Within the mantle, beneath the lithosphere, lies the asthenosphere. The asthenosphere is solid but soft, like Silly Putty. The silicate rocks flow at about the same rate your fingernails grow. Convection currents within the asthenosphere send hot magma up through volcanic vents and spreading centers in the middle of the ocean to form new rocks on the surface of the Earth. Colder rocks fall to the transition zone between the asthenosphere and the mesosphere. The asthenosphere is about 180 km thick.

The mesosphere stretches from 350 to 2,900 km below the surface of the Earth. It is the thickest part of the mantle. Inside the mesosphere, the silicate rocks flow even more slowly than the rocks of the asthenosphere.

The mesosphere rests on the outer core, which reaches from 2,900 to 5,150 km below the Earth's surface. The outer core makes up about 30% of the Earth's mass. The mantle makes up about 67%, the crust 1%, and the inner core nearly 2%.

The outer core is the only liquid layer within the Earth. Hot liquid iron and nickel swirl around the outer core as the Earth rotates on its axis. The spinning metals create Earth's magnetic field. The magnetic field surrounds Earth and reaches thousands of kilometers into space. It protects Earth by deflecting solar winds carrying charged particles that could destroy our ozone layer and expose us to harmful radiation from the sun.

Inside the outer core lies the inner core, which is around 1,200 km thick. Like the outer core, the inner core is made up of iron and nickel. However, even though the temperature within the inner core is the highest on Earth, the iron and nickel are solid because of the intense pressure at the center of the planet.

The temperature within the inner core is a scorching 7,000 degrees Celsius, hotter than the surface of the sun. The heat in the inner core comes from residual heat from the formation of the planet and the radioactive decay of atoms in the inner part of Earth.

We didn't know the center of the Earth was solid until 1936, when Inge Lehmann, a mathematician and seismologist, studied data on earthquake waves from around the world and realized the center of the Earth must be solid.

In fact, because scientists can't visit the layers within the Earth, studying earthquake waves is a primary tool they use for studying the mechanical and compositional layers of the Earth.

Layers of the Earth Flashcards

Flashcards are such a powerful tool for helping students remember what they learn about the layers of the Earth. You can get the flashcards at Teachers Pay Teachers, or you can use them here for free!

Moving on from the Layers of the Earth

Now that your students know about the Earth's layers, they have the background knowledge they need to understand plate tectonics and the rock cycle. Again, how could students understand the tectonic plates if they didn’t know about the lithosphere or asthenosphere?

Before moving on, you will want to assess your students’ knowledge of the layers of the Earth. That means it is time for a test. A quick test will tell you if your students need more instruction before moving on from the Earth’s layers. Using assessments to drive your instruction will ensure that your students have the background knowledge they need to be successful with the next topic.

Feeling overwhelmed? I got your back! I have reading passage sets on both the Earth’s compositional layers and the Earth’s mechanical layers. The reading passage sets have everything your students need to learn about the layers of the Earth. They are on Google Slides, so you can share them on Google Classroom or print them for your students. These reading passage sets are designed to be used independently by students so that you can focus on supporting the students who need it while others learn at their own pace.

Are You Teaching Another Science Topic?

I am working on creating more science units so that every science teacher can get exactly what he or she needs for her students. You can also read about how I use brain science to teach other science topics on my blog.