2. Optional Field Trip Exercise Changed from March 18 to March 4 Due March 9 at 1:10 pm Zuhl Museum Alumni Center, 775 College Av.

3. Geologic Time Scale Precambrian (4.6 Ga to 540 Ma) Paleozoic (540 Ma to 250 Ma) Mesozoic (250-65 Ma) –Triassic –Jurassic –Cretaceous Cenozoic (65 Ma to the present)

4. LIFE ON EARTH 3.5 billion year old bacteria and algae

5. Cambrian explosion of life: 540 million years ago

6. Trilobites-Paleozoic arthropods Today: 80% of species are arthropods

7. another Trilobite

8. Crinoids

9. First land animals: 365 Ma

10. Dimetrodon-Permian reptile

11. End of the Paleozoic Major extinction event: 96% of marine life was killed 70% land vertebrates killed

12. End of the Paleozoic

13. Mesozoic-Ammonites

15. Mesozoic-Triceratops

16. Cretaceous- Tyrannosaurus Rex

17. End of the Mesozoic Major extinction event: 85% of all species died.

19. Cenozoic-Mammoth

20. Cenozoic-Sabre tooth tiger

21. Summary Principle of superposition Principle of original horizontality Principle of faunal succession Unconformities Time scale Evolution of life

23. ISOTOPES Elements with SAME number of protons and different number of neutrons

24. ISOTOPES SOME isotopes are radioactive... Example: 12 C is stable 14 C is radioactive, and decays into 14 N

25. Radioactivity Radioactive decay--constant rate Gives off energy Constant rate of decay gives us a geologic clock

26. Half-lives Different elements decay at different rates. The time it takes for HALF of the atoms to decay is the “half-life” Parents decay into daughters… Need both parents and daughters

28. 0 half lives: 100 parents 0 daugh- ters Ratio: 100:0

29. 1 half lives: 50 parents 50 daugh- ters Ratio: 1:1

30. 1 half lives: 50 parents 50 daugh- ters Ratio: 1:1

31. 2 half lives: 25 parents 75 daugh- ters Ratio: 1:3

32. 3 half lives: 13 parents 87 daugh- ters Ratio: 1:7

33. Half-lives of elements ParentDaughterHalf Life 14 C 14 N5730 yrs 40 K 40 Ar1.3 Ga 238 U 206 Pb4.4 Ga

34. Pick the best isotope for the job C: Young, organic material –Ex.: Charcoal, peat K: Young or old minerals with K –Ex.: Feldspar, biotite U: Old minerals containing U –Ex.: Zircon

35. To get the age of a mineral... Need to know: 1)Ratio of parent to daughter atoms (measured with Mass-Spectrometer) 2) Half-life of the element (known by experiment or estimated)

36. Example You have 1000 parent atoms to start. After one 1/2 life, you have how many left? If the half life is 700 million years, the rock is 700 Ma. If you have 250 parent atoms left, how old is the rock?

37. ZIRCONS ARE FOREVER Zircons have Uranium and make excellent geologic clocks 4.364 Ga zircon: Jack Hills Quartzite, Australia

43. This zircon is about 0.1 mm long

44. Igneous Zircons

45. Metamorphic zircons

46. Zircons in sedimentary rock

48. Zircons Igneous: age of cooling of magma Metamorphic: age of heating Sedimentary: age of source rocks

49. Carbon dating Short half life: 5730 yrs. Why is it still here? Cosmic rays convert N to C Photosynthesis and animal food: C exchange with environment After death, C exchange stops, “clock starts” 14 C decays and forms 14 N

50. Applications Useful for archeology Shroud of Turin: 3 labs determined an age of 1260-1390 A.D.