Earth's Interior Layers
Grade 7 · Science · Study Guide
Journey to the center of the Earth and learn about the crust, mantle, outer core, and inner core.
Earth looks like a solid blue-and-green ball from space, but inside it is actually made up of distinct layers with very different compositions, temperatures, and states of matter. Scientists have never been able to drill more than a few kilometers into Earth, yet they know a remarkable amount about its interior — thanks to the way seismic waves (vibrations from earthquakes) travel through different materials.
The outermost layer is the crust — the thin rocky shell we live on. There are two types: oceanic crust (thinner, about 5–10 km, made mostly of dense basalt rock, forms the ocean floors) and continental crust (thicker, 30–70 km, made of less dense granite-like rock, forms the continents). Below the crust is the mantle, Earth's thickest layer at about 2,900 km deep. The upper mantle (including the asthenosphere) is partially molten and flows very slowly. The lower mantle is solid due to immense pressure despite very high temperatures.
Deep in Earth's center is the core, made mostly of iron and nickel. The outer core (2,200 km thick) is liquid metal that flows and generates Earth's magnetic field — which protects us from harmful solar radiation. The inner core is a solid ball of iron and nickel about 1,200 km in radius. Even though the inner core is hotter than the outer core, extreme pressure keeps it solid.
The temperature increases as you go deeper into Earth — a property called geothermal gradient. The inner core reaches about 5,200°C — nearly as hot as the surface of the Sun. This internal heat drives plate tectonics, volcanic activity, and is left over from Earth's formation 4.5 billion years ago.
| Layer | Depth | State | Composition |
|---|---|---|---|
| Crust | 0–70 km | Solid | Basalt (oceanic) / Granite (continental) |
| Mantle | 70–2,900 km | Solid (plastic-like in upper) | Silicate minerals (peridotite) |
| Outer core | 2,900–5,100 km | Liquid | Iron and nickel |
| Inner core | 5,100–6,371 km | Solid | Iron and nickel |
Key terms
- Crust
- Earth's thin, solid outermost layer; includes oceanic (basalt) and continental (granite) types.
- Mantle
- The thick layer of rock between the crust and core; makes up about 84% of Earth's volume.
- Outer core
- The liquid iron-nickel layer surrounding the inner core; its motion generates Earth's magnetic field.
- Inner core
- Earth's solid iron-nickel center; solid due to extreme pressure despite very high temperature.
- Seismic waves
- Vibrations produced by earthquakes that travel through Earth and reveal information about its interior.
- Geothermal gradient
- The increase in temperature with depth inside Earth.
Flashcards
- Crust
- Thin, solid outermost layer; oceanic crust is denser and thinner; continental crust is thicker and less dense.
- Mantle
- Thickest layer (~2,900 km); mostly solid silicate rock; upper portion flows slowly.
- Outer core
- Liquid iron-nickel layer; generates Earth's magnetic field by convection.
- Inner core
- Solid iron-nickel sphere at Earth's center; extreme pressure keeps it solid despite high temperature.
- Seismic waves
- Earthquake vibrations used to study Earth's interior layers by how they speed up, slow down, or stop.
- Lithosphere
- The rigid outer layer of Earth including the crust and uppermost mantle; broken into tectonic plates.
Practice questions
Which layer of Earth generates its magnetic field?
- Crust
- Mantle
- Outer core
- Inner core
Show answer
Answer: Outer core
The liquid outer core's convecting iron-nickel generates electric currents that produce Earth's magnetic field — the magnetosphere — which shields us from solar radiation.
Earth's inner core is liquid because of its extreme temperature.
Show answer
Answer: False
The inner core is actually SOLID, even though it is hotter than the outer core. The enormous pressure at Earth's center (3.6 million atmospheres) forces the iron and nickel to remain solid.
Seismic waves produced by earthquakes help scientists study Earth's ___.
Show answer
Answer: interior
P-waves and S-waves from earthquakes travel differently through solid, liquid, and partially molten materials, letting scientists map Earth's interior layers without ever drilling there.
Which layer makes up approximately 84% of Earth's volume?
- Crust
- Mantle
- Outer core
- Inner core
Show answer
Answer: Mantle
The mantle extends from about 70 km to 2,900 km depth, making it Earth's largest layer by volume — about 84% of Earth's total volume.
Oceanic crust is thicker than continental crust.
Show answer
Answer: False
Oceanic crust is THINNER (5–10 km) and DENSER (made of basalt). Continental crust is THICKER (30–70 km) but less dense (made of granite-like rock).
Fun facts
- The deepest hole ever drilled by humans is the Kola Superdeep Borehole in Russia — just 12 km deep, barely scratching the crust.
- Earth's magnetic field, generated by the liquid outer core, extends tens of thousands of kilometers into space and deflects solar wind particles that would strip away our atmosphere.
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