William E. Ferguson

1. Geologic Time William E. Ferguson

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. Amount of Time Required for Some Geologic Processes and Events

4. Some geologic processes can be documented using historical records(brown area is new land from 1887-1988)

5. Uniformitarianism The present is the key to the past. Natural laws do not change— however, rates and intensity of processes may. — James Hutton

6. Two ways to date geologic events 1 RELATIVE DATING (relative position of fossils, structure) 2 ABSOLUTE DATING (isotopic, tree rings, varves, etc.)

7. RELATIVE GEOLOGIC TIME Steno Laws (1669) developed to arrange rock units in time-order • Principle of Superposition • Principle of Original Horizontality • Law of Cross -Cutting Relationships • Law of Inclusions Laws apply to both sedimentary and volcanic rocks.

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

9. Principle of Superposition Youngest rocks Oldest rocks Jim Steinberg/Photo Researchers

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

11. Principles of original horizontality and superposition

12. Law of Cross-Cutting Relationships • A rock or feature is younger than any rock or feature it cuts across.

13. Law of Cross-cutting Relationships Fig. 9.9

14. LAW OF INCLUSIONS • Included rocks are older than surrounding rocks.

16. PRINCIPLE OF FAUNAL SUCCESSION Principle of Faunal Succession - groups of fossil plants & animals have followed one another in a definite & discernable order so certain fossil assemblages characterize a specific time. INDEX FOSSILS - fossils used to correlate a specific time period Based on distinctpreservable parts, lived a short time , in a specific environment with wide distribution - MICROFOSSILS

17. Ammonite Fossils Petrified Wood Chip Clark Tom Bean

18. Using Fossils to Correlate Rocks

19. Correlating beds using index fossils

20. Unconformity A buried surface of erosion Separates much older, eroded strata from younger ones Hiatus - the time gap or the time lost in the record

21. Unconformitites - 3 kinds • Disconformity - undeformed beds • Nonconformity - sedimentary over igneous or metamorphic rx. • Angular Unconformity - overlying tilted beds

22. Formation of a Disconformity

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

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

25. Nonconformity in the Grand Canyon

26. Nonconformity in the Grand Canyon Tapeats Sandstone (~550 million years old) Vishnu Schist (~1700 million years old)

27. Angular unconformity, Grand Canyon

28. The Great Unconformity of the Grand Canyon Geoscience Features Picture Libraryc

29. Formation of an Angular Unconformity

30. Reconstructing Relative Sequence of Events

32. CORRELATION • Process used to tie separated strata together • Based on matching physical features such as • Physical continuity - trace of rock unit • Similar rock types - marker beds, coal seams, rare minerals, odd color

34. South rim of the Grand Canyon

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

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

37. 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

38. The Geologic Column and Time Scale

39. Absolute geochronology • Adds numbers to the stratigraphic column based on fossils. • Based on the regular radioactive decay of some chemical elements.

40. 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.

41. 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

42. Naturally Occurring Isotopes of Carbon

43. Beta Decay Electron Capture Alpha Decay

44. Production and Decay of Radiocarbon

45. Radioactive Decay of Rubidium to Strontium

46. Half-life The half-life of a radioactive isotope is defined as the time required for half of it to decay.

47. Proportion of Parent Atoms Remaining as a Function of Time

48. 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

49. PROBLEMS • NEED A CLOSED SYSTEM!!! • MINERAL MAY LEAK PARENT OR DAUGHTER • MINERAL MAY BE CONTAMINATED WITH EITHER PARENT OR DAUGHTER

50. Another Clock Paleomagnetism • Earth’s magnetic field reverses every half million years • Reversals are recorded in rocks that are forming at that time - seafloor • Time scale calibrated by both relative & absolute time methods