Exploring the Geologic Time Scale: From Pre-Cambrian to Cenozoic Eras

1. Learning About Our Past The Geologic Time Scale

2. Four Major Divisions of Time • Pre-Cambrian – simple marine life • Paleozoic – invertebrates • Mesozoic – dinosaurs & reptiles • Cenozoic – mammals

3. Pre-Cambrian Era • 4.6 bya to 544 mya • Longest unit of geologic time • Not much known • Sponge fossil record • Deeply buried and changed by heat and pressure

4. Pre-Cambrian Era (cont’d.) • Cyanobacteria – produced oxygen • Ozone began forming • Soft bodied simple organisms like jellyfish existed

5. Paleozoic Era • 544 mya to 245 mya • Warm shallow seas • Marine life forms • Trilobites, Brachiopods, & Crinoids • Pangaea forms

6. Paleozoic Era (cont’d.) • Fish appeared • Appalachian Mountains formed • Plant & Animal life moved to land • Mass extinctions, tectonic activity & glaciers

7. Mesozoic Era • 245 mya to 65 mya • Dinosaurs & reptiles dominate • Birds appear • Little mammals (non-dominant) • Pine trees and flowering plants • Pangaea breaks up

8. Cenozoic Era • 65 mya to present • Dinosaurs become extinct • Climate cools; ice ages occur • THE AGE OF MAMMALS

9. swartpuntia

11. Fossil Seed Fern 330 mya

13. Crocodile Jaw Prehistoric Wasp 95 mya

14. Theropod Tracks Caturus 140 mya

15. Trex 65 mya

17. Coccolithicus pelagus 45 mya

18. Learning About Our Past Fossils: Types and Conditions

19. Paleontology • Study of fossils • Almost all fossils are found in sedimentary rocks • Usually only hard parts of organism are saved but occasionally whole organisms remain

20. Preservation of Organisms • Mummification: drying an organism so bacteria will not destroy it.

21. Preservation of Organisms • Amber: hardened tree sap that captures and preserves insects.

22. Preservation of Organisms • Tar Beds: thick petroleum • Freezing: Low temperatures protect and preserve organisms

23. Conditions to Form Fossils • Sedimentary Rock • Buried quickly so that scavengers and micro-organisms don’t disturb the remains • Hard body parts

24. 5 Types of Fossils • Mold • A cavity in a rock that has the shape of remains that were trapped there; water dissolved the remains away, leaving its imprint

25. 5 Types of Fossils • Cast • A type of fossil formed when an earlier fossil in the rock is dissolved away leaving the impression (mold) and NEW sediments or mineral crystals fill the mold

26. 5 Types of Fossils • Carbonaceous film: • A fossil impression in a rock, consisting only of a thin carbon residue that forms an outline of the original organism

27. 5 Types of Fossils • Petrified Remains • Plant or animal remains that have been petrified or “turned to rock”; this happens when minerals carried in groundwater replace the original remains.

28. Preservation of Organisms • .

29. 5 Types of Fossils • Original Remains • Bones, shells, teeth

31. Proof of the Geologic Time Scale • The Fossil Record • Determining the fossil record by • Relative Age • Absolute Age

32. Relative Age • Not an exact age; way of telling one layer of rock is older than another • Law of Superposition • each sedimentary rock layer is older than the overlying younger layer

33. Which layer is older? A N I T

34. Which layer is younger? A N I T

35. Relative Age • Sometimes due to crustal mov’t sedimentary layers aren’t always horizontal • Law of Cross-cutting Relationships • States that a fault or intrusion is younger than rock layer it cuts through

36. Relative Age • Fossils help to determine a rock layers relative age A fossil is any evidence of earlier life preserved in rock. • An index fossil is used to help determine relative age 3 overall characteristics 1. Easily recognizable 2. Must be found everywhere 3. Lived for short time period

37. Absolute Age • When round about age isn’t good enough but need the actual date of the event 1. Tree Rings & Varves 2. Radioactive Decay

38. Radioactive Decay • Most rocks contain elements that are radioactive. Since these elements give off particles and energy they will eventually form new nonradioactive elements.

39. Radioactive Decay • By comparing the amount of radioactive and nonradioactive elements scientists can predict the absolute age of a specific rock

40. Examples: • Uranium is commonly found in rocks • U-238 will decay to form U-234 (releases 2 protons & 2 neutrons) • It will eventually decay all the way to produce Pb-206

41. Examples: • U-238 = parent element • Pb-206 = daughter element

42. Half-Life • The decay of U-238 to Pb-206 is a very slow process. • Rate of decay is constant • It takes 4.5 billion yrs for half of U-238 to decay

43. Half-Life • In conclusion = the more Pb-206 in a rock the older it will be .

44. Carbon Dating • C-14 is a radioactive element found in ALL living things. • When plant/animal dies C-14 decays to form N-14 • The half-life of C-14 is about 5,730 years

45. Living Organism C-14 Organism Dies No New C-14 5,730 yrs later 50% C-14 11,600 yrs later 25% C-14

46. Let’s take some time to practice learning what is meant by an element’shalf-life