2. 1 Earth’s History Planet Earth is approximately 4.6 billion years old –Rocks of the crust provide clues to Earth’s past By analyzing these clues we can infer events from the past

3. 2 Earth’s History Principle of Uniformitarianism –Major assumption in geology –Events in the past occurred the same way that they are occurring today. Examples Include: Weathering/erosion Deposition Volcanism Plate tectonics

4. 3 Geologic Dating

5. 4 There are two types of Geologic Dating AbsoluteAbsolute –Determines how many years old something is RelativeRelative –Used to determine if one thing is younger or older than another

6. 5 Tools used to determine Using radiometric dating Law of superposition Use of index fossils Correlation of rock layers Absolute AgeRelative Age

7. 6 Relative Dating

8. 7 Law of Superposition In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest –Lower layers must be in place before younger rocks can be deposited on top of them –Exception: when something occurs to overturn layers

9. 8 Grand Canyon

10. GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM

11. 10 Law of Superposition Rock layers are older than folds found in them –Layers were there before they were folded

12. 11 Folds/Tilted

13. 12 Law of Superposition Rock layers are older than faults found in them This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

14. 13 Law of Superposition Fossils are generally the same age as the rock layers in which they are found –Animal remains are deposited along with the sediments that will turn into sedimentary rocks

15. 14 Law of Superposition Intrusions are younger than the rock that they cut through or flow out of

16. 15 Fossils Generally found in sedimentary rock layers  Why don’t fossils exist in most igneous or metamorphic rock?

17. 16 Fossils A study of the fossil record shows: –A great variety of plants, animals and simpler life forms have lived on Earth in the past –That life forms have evolved through time –Most life forms of the geologic past have become extinct

18. 17 Index Fossils Index Fossils--used to date rock layers –Fossils from creatures that existed for a geologically short period of time –Fossils from creatures that had a wide geographic distribution

19. Which fossil would make the best index fossil? Which fossil would make the best index fossil? Found in only 1 layer (short lived) Found in all samples (widespread)