1. Geologic time
Review

2. Age
Relative age is the age of a rock compared to the ages of other rocks
Absolute age is the age of a rock given as the number of years since it has formed

3. Radioactive decay
Radioactive decay is the process in which the nuclei of radioactive elements break down, releasing fast- moving particles and decay

4. Types of fossils
Petrified fossil: minerals replace all or part of an organism
Trace fossil: provides evidence of the activities of ancient organisms

5. Types of fossils
Mold fossil: hollow area in sediment in the shape of an organism or part of an organism
Preserved fossil: preserves entire organism
Cast fossil: solid copy of an organism’s shape, formed when minerals seep into a mold

6. Types of fossils
Carbon film: type of fossil consisting of an extremely thin coating of carbon on rock
Index fossil: fossil that lived during a geologically short period

7. animals Vertebrate: animal with a backbone
Invertebrate: animal without a backbone

8. paleontologist
A paleontologist is a scientist who studies fossils to learn about organisms that lived long ago

9. unconformity
An unconformity is a gap in the geologic record that shows where rock layers have been lost due to erosion

10. review

11. Law of superposition
In horizontal layers of rock, each layer is older than the layer above it

12. Eons, Eras, periods, and epochs
Divisions of the geologic time scale

13. Intrusive rock
Igneous rock layer formed from when magma hardens beneath Earth’s surface

14. fossil
Preserved remains or traces of an organism that lived in the past

15. Extrusive rock
Igneous rock later formed when lava flows onto Earth’s surface and hardens

16. Half-life
Time it takes for half of the atoms of a radioactive element to decay

17. Petrified fossil
Minerals replace all or part of an organism

18. Mold fossil
Hollow area in sediment in the shape of an organism or part of an organism

19. Carbon film
Type of fossil consisting of an extremely thin coating of carbon on rock

20. Trace fossil
Provides evidence of the activities of ancient organisms

21. Preserved fossil
Preserves entire organism

22. Cast fossil
Solid copy of an organism’s shape, formed when minerals seep into a mold

23. Index fossil
Fossil that lived during a geologically short period

24. unconformity
A gap in the geologic record that shows where rock layers have been lost due to erosion