1. Unit 6 Geologic Time Chapter 21 Fossils and the Rock Record

2. 21.1 The Rock Record The geologic time scale - the Earth is 4.6 billion years old. - in order to break up this vast amount of time, scientists divide it into Eons

3. Eons The three earliest eons are grouped into the “Precambrian” Hadean – The formation of the earth 542 Hadean

4. Eons Archaen – Continental plates begin to form and earliest fossils 542 Hadean

5. Eons Proterozoic – multicellular life cyanobacteria increases oxygen levels 542 Hadean

6. Eons Phanerozoic– “visible life” Most of the fossils with hard parts 542 Hadean

7. Eons are divided into Eras • Phanerozoic is divided into three Eras • Paleozoic – ancient life • Mesozoic – middle life • Cenozoic – recent life 542 Hadean

8. Eras are divided into Periods • Mesozoic is divided into three Periods • Triassic • Jurassic • Cretaceous 542 Hadean

9. Periods are divided into Epochs Eons Eras Periods Epochs

10. If 1 Second was 1 year

11. You were born 15 seconds ago

12. If 1 Second was 1 year The Revolutionary War was 4 minutes ago

13. The First Christmas was 33 minutes ago

14. The pyramids were built 1.5 hours ago

15. The Last Ice Age Was 2 hours and 12 minutes ago

16. Dinosaurs were wiped out 2 years ago

17. Pangaea broke apart • 6 years and 4 months ago

18. Dinosaurs showed up (Triassic) • 8 years ago

19. Cambrian began 17 years ago

20. Earth formed • 145 years ago

21. 21.2 Relative-Age Dating • What happened first? • Oldest and newest?

22. James Hutton • Geologist and Physician • Proposed that the earth was millions of years old based on geologic evidence

23. Sir Charles Lyell • Geologist • Proposed that geologic changes occur slowly over long periods of time.

24. Uniformitarianism • The idea that the same processes occurring today have been the same throughout time • This idea replaced the idea of catastophism which suggested major changes by sudden catastrophic events

25. Principles for Determining Relative Age • Original horizontality – sedimentary rocks are deposited in horizontal layers

26. Superposition The deeper strata (layers) were deposited first. Fossils found in lower layers are older than in higher layers.

30. Cross cutting relationships Plutons are younger than the rocks they are found in

31. Principle of Inclusions Fragments of a rock included in a rock layer are older than the rock layer

32. Unconformities • Signs of surface erosion that are buried show a gap in the rock record

33. Disconformities • Signs of surface erosion that are buried between sedimentary layers

34. Nonconformities • Signs of erosion that are found between a sedimentary layer and metamorphic or igneous layers

35. Angular Unconformity Nonhorizontal rock layers indicate uplifting

38. Correlation • Matching of a unique rock layer or fossil in one area to similar layers in another area.

39. Key Bed A rock layer used as a marker

40. Fossil Correlation Using index fossils to compare ages of rock layers

41. 21.3 Absolute-Age Dating So how old is it? Absolute-age dating gives the numerical age of rocks.

42. Radioactive Isostopes Isotopes are atoms of an element with more or less neutrons Carbon – has 6 protons Carbon-12 has 6 protons and 6 neutrons

43. Nucleus C-12

44. Radioactive Isostopes Isotopes are atoms of an element with more or less neutrons Carbon – has 6 protons Carbon-12 has 6 protons and 6 neutrons Carbon-14 has 6 protons and 8 neutrons

45. Nucleus C-14

46. Radioactive Isostopes Isotopes are atoms of an element with more or less neutrons Carbon – has 6 protons Carbon-12 has 6 protons and 6 neutrons Carbon-14 has 6 protons and 8 neutrons Carbon-14 decays into Nitrogen-14

47. Nucleus N-14

48. Radioactive Decay The decay of radioactive isotopes into the daughter product

50. Half life The time required for ½ of the isotopes to decay