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2. The breakdown of unstable atoms that releases heat energy.
5. (Lower_Mantle) The strong, lower part of the mantle beneath the asthenosphere.
6. The transfer of heat by the movement of fluids (liquids or gases).
9. An instrument that records vibrations from earthquakes.
11. An area on Earth’s surface where seismic waves are not detected because of how they are blocked or bent inside Earth.
12. The liquid layer of Earth’s core that produces Earth’s magnetic field.
15. (Secondary_Wave) A type of seismic wave that moves material up and down or side to side; cannot travel through liquids.
16. Heat energy that comes from the motion of particles; drives convection inside Earth.
18. The innermost layer of Earth, made of iron and nickel, divided into a solid inner core and liquid outer core.
19. The amount of mass in a given volume of a substance (how tightly matter is packed).
21. The solid, dense center of Earth, made mostly of iron and nickel.
23. The process by which a planet separates into layers based on density during formation.

1. The soft, plastic-like layer of the mantle on which the lithosphere floats.
3. Vibrations that travel through Earth, carrying energy from earthquakes and revealing Earth’s structure.
4. Circulating flows of material in the mantle caused by heat from Earth’s core.
7. Heat left over from Earth’s original formation.
8. The rigid outer layer of Earth, made of the crust and the uppermost mantle.
10. The thick layer between Earth’s crust and core, made of hot, solid rock that can flow slowly.
13. The transfer of heat through direct contact of particles.
14. A type of seismic wave that moves through solids, liquids, and gases by compressing and expanding material.
17. The bouncing back of a wave when it hits a boundary it cannot pass through.
20. The bending of a wave as it changes speed when moving from one material to another.
22. The thin, rocky outermost layer of Earth.