Geologic Time

1. Geologic Time

2. Geologic Time • A major difference between geologists and most other scientists is their attitude about time. • A "long" time may not be important unless it is > 1 million years.

3. Two ways to date to “date” geologic events • relative dating (fossils, cross cutting relationships, structural relationships) (2) absolute dating (isotopic, tree rings, etc.)

4. Amount of Time Required for Some Geologic Processes and Events Fig. 9.1

5. Some geologic processes can be documented using historical records(Brown is new land from 1887-1988) Fig. 9.2

6. Steno's Laws Nicolaus Steno (1669) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity Laws apply to both sedimentary and volcanic rocks.

7. Principle of Superposition • In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.

8. Principle of Superposition Youngest rocks Oldest rocks Fig. 9.3b Jim Steinberg/Photo Researchers

9. Principle of Original Horizontality • Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.

10. Principle of Lateral Continuity Layered rocks are deposited in continuous contact.

11. Principle of Lateral Continuity Zero Thickness Map view

12. Paleontology The study of life in the past based on fossilized plants and animals. Fossil: Evidence of past life Fossils preserved in sedimentary rocks are used to determine: 1) Relative age 2) Environment of deposition

13. Using Fossils to Correlate Rocks • Index Fossil – A fossil known to be restricted to a specific period of geologic time • Faunal Succession – Groups of different fossils occur in a specific stratigraphic order Fig. 9.5 Fig. 9.5

14. Unconformity • A buried surface of erosion or nondepositon • Types of unconformity • Disconformity (Simple unconformity) • Angular unconformity • Nonconformity

15. Sedimentation of Beds A-D Beneath the Sea Fig. 9.6

16. Uplift and Exposure of D to Erosion Fig. 9.6

17. Continued Erosion Removes D and Exposes C to Erosion Fig. 9.6

18. Subsidence and Sedimentation of E over C Unconformity: a buried surface of erosion Fig. 9.6

19. Formation of a Disconformity • Lack of deposition of sedimentary units at a specific time interval Fig. 9.6

20. South Rim of the Grand Canyon

21. South rim of the Grand Canyon 250 million years old Paleozoic Strata 550 million years old 1.7 billion years old Precambrian

22. South rim of the Grand Canyon 250 million years old 550 million years old 1.7 billion years old Nonconformity

23. The Nonconformity of the Grand Canyon Fig. 9.7

24. The Great (Angular) Unconformity of the Grand Canyon Geoscience Features Picture Libraryc Fig. 9.7

25. Angular Unconformity, Grand Canyon

26. Generalized Stratigraphic Section of Rocks Exposed in the Grand Canyon after: Beus & Moral (1990)

27. Some of the Geologic Units Exposed in the Grand Canyon Michael Collier

28. Sedimentation of Beds A-D Beneath the Sea Fig. 9.8

29. Deformation and Erosion During Mountain Building Fig. 9.8

30. Erosional Surface Cuts Across Deformed Rocks Fig. 9.8

31. Subsidence and Subsequent Deposition Buries Erosional Surface Angular Unconformity Fig. 9.8

33. Cross-cutting Relationships Fig. 9.9

34. The Geologic time scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

35. The Geologic Time Scale • Phanerozoic • Cenozoic • Mesozoic • Paleozoic ---- 540 Ma --- Precambrian 4.5 Ga (Age of the Earth) Fig. 9.13

36. Absolute geochronology • Add numbers to the stratigraphic column based on fossils. • Based on the regular radioactive decay of some chemical elements present in minerals.

37. Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e.: 235U, 238U 87Sr, 86Sr 14C, 12C

38. Radioactive Decay of Rubidium to Strontium 87Rb 87Sr Fig. 9.14

39. Half-life The half-life of a radioactive isotope is defined as the time required for half of the atoms of the isotope present in a geologic material it to decay to the daughter isotope.

40. Proportion of Parent Atoms Remaining as a Function of Time Fig. 9.15

41. Proportion of Parent and Daughter as a Function of Time Daughter Atoms

42. Parent Daughter Ratios and the Number of Half Lives

43. Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and knowable. • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

44. Example of Radiometric Age Calculation • Isotope X has an half life of 1 billion years, it decays to Isotope Y of a different element • In a mineral the proportion of X to Y is 1:7 (D/P = 7) • This equals 3 half lives (3 billion years)

45. Parent Daughter Ratios and the Number of Half Lives

46. Geologically Useful Decay Schemes Parent Daughter Half-life (years) 235U 207Pb 4.5 x 109 238U 206Pb 0.71 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 14C 14N 5730

47. Potassium-Argon Dating Laboratory

48. The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions

49. The big assumption The half-lives of radioactive isotopes are the same as they were billions of years ago.

50. Test of the assumption Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data.