Earth's Layers
Grade 8 · Science · Study Guide
Explore the four main layers of Earth, their composition, and the evidence scientists use to study them.
Earth is divided into four main layers: the crust (outermost, solid rock), the mantle (largest layer, hot semi-solid rock that flows slowly), the outer core (liquid iron and nickel), and the inner core (solid iron and nickel, despite extreme temperature, due to immense pressure).
Scientists cannot drill to Earth's core — the deepest borehole only reaches about 12 km into the 6,371 km radius Earth. Instead, they study Earth's interior using seismic waves from earthquakes, which travel through Earth and change speed and direction based on the material they pass through.
The lithosphere includes the crust and upper mantle and is broken into tectonic plates. The asthenosphere is the semi-fluid layer of the upper mantle on which the lithosphere floats and moves. Heat from Earth's core drives convection currents in the mantle that move tectonic plates.
| Layer | State | Composition | Thickness |
|---|---|---|---|
| Crust | Solid | Rock (granite/basalt) | 5–70 km |
| Mantle | Semi-solid (plastic) | Silicate rock | ~2,900 km |
| Outer Core | Liquid | Iron and nickel | ~2,200 km |
| Inner Core | Solid | Iron and nickel | ~1,200 km radius |
| Feature | Oceanic Crust | Continental Crust |
|---|---|---|
| Density | Denser (basalt) | Less dense (granite) |
| Thickness | Thinner (~5–10 km) | Thicker (~30–70 km) |
| Age | Younger | Older |
Key terms
- Crust
- Earth's thin, solid outermost layer; oceanic crust is denser than continental crust.
- Mantle
- The thick layer of hot, semi-solid rock beneath the crust; makes up most of Earth's volume.
- Core
- Earth's innermost region, made of iron and nickel; the outer core is liquid, the inner core is solid.
- Seismic Wave
- Vibrations from earthquakes that travel through Earth and reveal information about its interior.
Practice questions
What is the correct order of Earth's layers from the surface to the center?
- Core, mantle, crust
- Crust, outer core, mantle, inner core
- Crust, mantle, outer core, inner core
- Mantle, crust, outer core, inner core
Show answer
From Earth's surface to its center: crust → mantle → outer core → inner core.
How do scientists know what Earth's interior looks like if they can't drill there?
- They use special telescopes
- They analyze seismic waves from earthquakes
- They collect samples from volcanoes
- They use X-ray satellites
Show answer
Seismic waves change speed and direction as they pass through different materials, allowing scientists to map Earth's interior.
Earth's inner core is liquid because of its extremely high temperature.
Show answer
Despite being extremely hot (~5,400°C), the inner core is solid due to the immense pressure from the weight of all layers above it.
The layer of Earth that makes up most of its volume is the ___.
Show answer
The mantle extends from below the crust to the outer core and makes up about 84% of Earth's volume.
Why does the oceanic crust sink beneath the continental crust when the two plates collide?
Show answer
Density differences between the two crust types determine which plate goes under in a collision.
Fun facts
- The temperature at Earth's inner core reaches about 5,400°C — roughly as hot as the surface of the Sun.
- Earth's liquid outer core generates our planet's magnetic field, which protects us from harmful solar radiation.
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