Plate Tectonics

3. ridge – an underwater mountain chain where new oceanic crust is formed.
6. boundary – a plate boundary where two tectonic plates move toward each other.
9. – the rigid outer layer of Earth consisting of the crust and uppermost mantle.
10. tectonics – the theory that Earth's outer shell is divided into moving plates that interact with one another.
11. – a fracture in Earth's crust along which movement has occurred.
15. – the soft, partially molten layer of the upper mantle beneath the lithosphere.
16. boundary – a plate boundary where two tectonic plates move apart.
18. crust – the dense outer layer beneath the oceans, composed mainly of basalt.
20. – the movement of magma to Earth's surface, producing volcanic activity.

1. plate – a large, rigid section of Earth's lithosphere that moves over the mantle.
2. – the process in which one tectonic plate sinks beneath another into the mantle.
4. crust – the thicker, less dense outer layer that forms the continents.
5. convection – the slow circulation of material within the mantle that helps drive plate movement.
7. spreading – the formation of new oceanic crust as magma rises at a mid-ocean ridge.
8. – the sudden shaking of the ground caused by the movement of rocks along a fault.
12. drift – the gradual movement of continents across Earth's surface over geological time.
13. trench – a long, narrow depression in the ocean floor formed at a subduction zone.
14. boundary – a plate boundary where two tectonic plates slide past one another horizontally.
17. valley – a long, narrow valley formed where Earth's crust is pulled apart.
19. – an area where hot mantle material rises and forms volcanoes away from plate boundaries.