Earth's Structure
Middle School
Definition
The layered interior of the Earth, made up of the thin outer crust, the thick mantle beneath it, and the dense core of mostly iron and nickel. These layers differ in composition, temperature, and state.
Worked examples
The crust is 5–70 km thick; the mantle extends to 2,900 km depth; the outer core (liquid) and inner core (solid) reach Earth's center at 6,371 km.
Each layer increases in depth, temperature, and pressure as you move toward the center.
Continental crust is thicker and less dense than oceanic crust, which is why continents sit higher than ocean floors.
Differences in crust composition and thickness explain Earth's surface elevation patterns.
Common mistakes
- The core is the hottest layer, so it must be liquid. → The inner core is solid despite extreme heat because immense pressure keeps iron atoms locked in place. Temperature alone does not determine state; pressure also plays a critical role at Earth's center.
- The mantle is liquid rock that flows freely like water. → The mantle is solid rock that flows very slowly over millions of years (plastic deformation). The mantle behaves like a viscous solid, not a liquid, due to high temperature and pressure.
- Earth's layers are mixed together with no clear boundaries. → Earth has distinct layers with sharp boundaries defined by changes in composition and physical properties. Seismic waves reveal clear discontinuities between crust, mantle, and core.
Where you'll use it next
Understanding Earth's structure is essential for studying plate tectonics, earthquakes, volcanoes, and magnetic fields. You'll apply it in geology, environmental science, and when exploring how Earth's interior drives surface processes.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026