Geologic Time

3. Fossils of living things that lived for a relatively short period of time.
5. Rock that forms when layers of sediment on Earth's surface harden.
7. Finding the exact age of an object by analyzing its chemical makeup.
10. Large bodies of ice that move very slowly down a sloped surface of land.
13. A timeline that lists and describes the timing and relationships of events in Earth's history.
16. A geologic period of time when much of Earth was covered by ice.
19. Bits of rock, sand, and dead organic materials that settle at the bottom of a mixture and can be pressed together to form sedimentary rock.
20. Divisions of geologic time that are shorter than a period.
21. Rock that forms when molten rock cools and hardens.
22. The idea that the same geological processes that occur today have for all of Earth's history.
24. A law stating that in undisturbed layers of rock, the layers at the bottom are the oldest, and the layers at the top are the youngest.

1. The idea that Earth's history has been affected by sudden, violent worldwide events, such as an asteroid strike.
2. The longest units of geologic time.
3. Long cylinders of ice that are drilled out of an ice sheet and are studied in order to learn about Earth's past.
4. All the fossils that have been discovered.
6. No longer existing.
8. An event in which a large number of species become extinct in a short period of time.
9. The average weather conditions of an area over a long period of time.
11. The history of Earth as recorded in the rocks that make up its crust.
12. Estimating the age of an object by comparing it with something else.
14. A collection of related rocks.
15. A technique used to find the ages of rocks and fossils. It is based on the decay rates of radioactive isotopes.
17. Divisions of geologic time that are longer than an epoch and shorter than an era.
18. The preserved remains or traces of an organism that lived in the geologic past.
23. Large divisions of geologic time, usually shorter than an eon.