Plate Tectonics

3. The pieces of the Earth's lithosphere that are broken into pieces
8. The scientist who made the theory of continental drift
9. A deep valley formed when pieces of crust diverge on land
11. Two plates slide against each other at
13. A device that uses sound waves to measure the distance to an object
15. A fernlike plant
19. The longest mountain range in North America
20. Two plates pull apart at
21. The hypothesis that the continents once were joined together in a single landmass and have since drifted apart

1. The process that continually adds new material to the ocean floor
2. Underwater canyons
4. The submarine used by scientists to explore the ocean floor
5. When two plates collide and one is pushed under the other
6. Underwater mountain ranges
7. Two plates collide at
10. Changes that affect the ocean floor
12. The supercontinent that existed 300 million years ago
14. Any trace of an ancient organism that has been preserved in rock
16. The theory that the plates are in slow constant motion driven by convection currents in the mantle
17. The longest mountain range in South America
18. A break in Earth's crust where rocks have slipped past each other