| A | B |
|---|
| Crust | A thin outer layer of rock above a planet’s mantle, including all dry land and ocean basins. |
| Inner core | A solid sphere of metal, mainly nickel and iron, at Earth’s center. |
| Outer core | A layer of molten metal, mainly nickel and iron, which surrounds Earth’s inner core. |
| Mantle | The layer of rock between Earth’s outer core and crust, in which most rock is hot enough to flow in convection currents; Earth’s thickest layer. |
| Lithosphere | The layer of Earth made up of the crust and the rigid rock of the upper mantle, averaging about 40 kilometers thick and broken into tectonic plates. |
| Asthenosphere | The layer in Earth’s upper mantle and directly under the lithosphere in which rock is soft and weak because it is close to melting. |
| Tectonic plate | One of the large, moving pieces into which Earth’s lithosphere is broken and which commonly carries both oceanic and continental crust. |
| Convection | A process by which energy is transferred in gases and liquids, occurring when a warmer, less dense area of gas or liquid is pushed up by a cooler, more dense area of the gas or liquid. |
| Continental drift | The hypothesis that Earth’s continents move on Earth’s surface. |
| Pangaea | A hypothetical supercontinent that included all of the landmasses on Earth. It began breaking apart about 200 million years ago. |
| Mid-ocean ridge | A long line of sea-floor mountains where new ocean crust is formed by volcanic activity along a divergent boundary. |
| Convection current | A circulation pattern in which material is heated and rises in one area, then cools and sinks in another area, flowing in a continuous loop. |
| Theory of Plate Tectonics | A theory stating that Earth’s lithosphere is broken into huge plates that move and change in size over time. |
| Divergent boundary | A boundary along which two tectonic plates move apart, characterized by either a mid-ocean ridge or a continental rift valley. |
| Convergent boundary | A boundary along which two tectonic plates push together, characterized either by subduction or a continental collision. |
| Transform boundary | A boundary along which two tectonic plates scrape past each other, and crust is neither formed nor destroyed. |
| Rift valley | A deep valley formed as tectonic plates move apart, such as along a mid-ocean ridge. |
| Magnetic reversal | A switch in the direction of Earth’s magnetic field so that the magnetic north pole becomes the magnetic south pole and the magnetic south pole becomes the magnetic north pole. |
| Hot spot | An area where a column of hot material rises from deep within a planet’s mantle and heats the lithosphere above it, often causing volcanic activity at the surface. |
