| A | B |
|---|
| crust | The thin outer layer of Earth that varies in thickness from 5km to 48 km. |
| magnetic field | The space around a magnet within which the force of the magnet is exerted. |
| magnetic reversal | The switching or changing of Earth's magnetic poles such that the north magnetic pole becomes located at the south magnetic pole's position and vice versa. |
| mantle | The middle, thick, rocky layer of earth. |
| mid-ocean ridge | A chain of mountains under the ocean. |
| Pangea | A supercontinent that existed about 200 million years ago. |
| plate boundaries | Places where the plates that make up Earth's crust and upper mantle either move together, apart, or past each other. |
| plates | A large continuous piece of land mass. |
| sea-floor spreading | The process by which new ocean floor is made as magma rises to the surface and hardens into rock between separating plates. |
| tectonic plates | One of the slabs that make up Earth's crust and upper mantle. Some carry Earth's continents. |
| Theory of continental drift | An idea, originally stated by Alfred Wagener, that says that the continents formed a single land mass in the past and drifted over time to their present positions. |
| Theory of plate tectonics | The idea that Earth's lithosphere is broken into enormous plates that are in motion. |
