In school, you usually learn that the three main layers of the earth are the crust, mantle, and core. Geologists call these compositional layers, because the layers are defined by the materials they are made of. However, geologists also think about how different layers of the earth move: for example, do they flow easily? Are they rigid or soft? Are they solid or liquid? Because of this, geologists also classify earth’s interior into five mechanical layers. The mechanical layers are based on movement and strength, and include the lithosphere, asthenosphere, mesosphere, inner core, and outer core.
Now, let’s learn more about both the compositional and mechanical layers of the earth:
Compositional Layers of the Earth
If you could cut the earth in half, you would find that the earth is made of three main compositional layers – crust, mantle, and core. Each layer has a unique composition – that is, what it is made of. First, the crust is a very thin layer on the surface made of rock. Beneath the crust lies the mantle, a thick layer of denser rock. Although the mantle is made of rock, it moves and flows like a liquid, only very, very slowly. Finally, the core is located in the very center of the earth and is made of metals.
You may ask – why does the earth have compositional layers? These layers are separated out based on their densities. Just as oil will settle on top of water when mixed, earth’s inner materials separate into layers based on density. The core is made of metal, making it very dense – that’s why it is at the center. In contrast, the least dense layer is the crust. In a way, the atmosphere could also be considered a compositional layer of the earth. The atmosphere is made of gases, so it is even less dense than the crust!
Mechanical Layers of the Earth
Earth has 5 mechanical layers. “Mechanical” means defined based on their movement. Together, the crust and topmost portion of the mantle create the lithosphere. The lithosphere is a rigid layer that is broken into tectonic plates. Under the lithosphere lies a more pliable portion of the mantle called the asthenosphere. The soft asthenosphere flows most easily, allowing the tectonic plates to move atop it. We call the remaining, lower part of the mantle the mesosphere. Next, the outer core is a flowing liquid layer that creates earth’s magnetic field. In contrast, the inner core is solid.
Why study the Earth’s layers?
Things happening within Earth’s layers impact our lives. For example, our planet’s metallic core generates a magnetic field. The earth’s magnetic field is pretty amazing – it allows us to navigate by compass, protects the earth from cosmic radiation, and creates the beautiful northern lights.
Additionally, movements in the lithosphere (upper mantle and crust) cause everything from the creation of certain landforms and rocks to volcanoes, earthquakes, and tsunamis! This is all tied to the theory of plate tectonics. The theory of plate tectonics is important in the field of geology, and before we can study it, we must first understand the layers of the earth.
Classroom Activity: Density Experiment & Earth’s compositional layers
As we learned, there are three compositional layers of the earth (crust, mantle, and core) that are separated based on density. In this activity, explore how differences in density lead to layering. Matter (liquids, solids, or gases) with different densities may float above or sink below one another to form layers.
- In a tall glass or beaker, stack different liquids such as dish soap, water, vegetable oil, and rubbing alcohol.
- Start by pouring in the most dense layer – the dish soap.
- Add the water next. Add liquids by pouring gently: hold the beaker at a slight angle and pour slowly to minimize mixing and bubbles forming.
- Add the oil next, pouring gently.
- Add the alcohol, pouring gently.
- As a bonus, you can drop small solid objects into the jar to see what level they sink to – for example, a piece of candle wax, a marble, and a small piece of Styrofoam.
Try out my Plate Tectonics Curriculum:
If you enjoyed this post, I know you will love the Plate Tectonics Unit that I’ve put together for you. It includes all the activities, worksheets, and diagrams that you will need for a great Plate Tectonics unit with your class or homeschool.
Explore curriculum from Wild Earth Lab:
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References and Further Reading
- C.C. Plummer, D.H. Carlson, and L. Hammersley (2019). Physical Geology. McGraw-Hill Education. (Chapter 1)
- Dobrijevic, D (2023). Earth’s layers: Exploring our planet inside and out. Spage.com. Available: https://www.space.com/17777-what-is-earth-made-of.html
- PBS Digital Studios (2016). Why does the earth have layers? Available: https://www.pbs.org/video/its-okay-be-smart-layers/
- ScienceBuddies.org (n.d.). Stacking Liquids. Available: https://www.sciencebuddies.org/stem-activities/liquid-stacking-density
- Super Experiment (2017). Amazing 7 Layer Liquid Stacking! (How To) Denser Than You Think. YouTube. Available: https://www.youtube.com/watch?v=qKHUNvMsF6U
- U.S. Geological Survey (1999). Inside the Earth. Available: https://pubs.usgs.gov/gip/dynamic/inside.html .