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Relative and Absolute Dating

Relative and Absolute Dating. Relative dating - the age of a rock, fossil, or other feature measured compared to another. Absolute dating - (numerical dating) the age of a rock in years. Relative Dating Laws:.

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Relative and Absolute Dating

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  1. Relative and Absolute Dating

  2. Relative dating - the age of a rock, fossil, or other feature measured compared to another.

  3. Absolute dating - (numerical dating)the age of a rock in years.

  4. Relative Dating Laws:

  5. #1 Original HorizontalitySedimentary rocks are formed in layers which were originally horizontal

  6. Observe flat layers - rocks undisturbedObserve tilted layers - tectonics at work

  7. #2 Lateral ContinuityThe horizontal layers continue in all directions at the time that they form

  8. Lateral Continuity image

  9. #3 SuperpositionOldest rocks are on the bottom,Youngest rocks are on the top

  10. Superpositionimage Younger

  11. #4 Cross-cutting relationshipsIgneous intrusions and faults are younger than the sedimentary rocks that they cut across

  12. Cross-cutting relationships image

  13. Unconformities • Represent intervals of time when layers are missing from the geologic record • Caused by Uplift and Erosion

  14. Grand Canyon Example Grand CanyonExample

  15. Principles of Fossil Succession • Fossil organisms succeed one another in a definite order • Any time period can be recognized by its fossil content • Examples: age of humans, ages of fishes, age of dinosaurs, etc. • This is application of the Principle of Superposition • Important for Correlation

  16. Index Fossils • Fossils useful as time indicators • Short age range • Easily recognized • Widespread distribution

  17. Index Fossils image

  18. Relative dating tells us what order things happened, but not how many years ago they happened For that, we need Absolute dating

  19. Absolute dating:

  20. Radiometric DatingProcess used to determine the absolute age of a rock or a fossil by determining the ratio of parent nuclei to daughter nuclei within a given sample.

  21. Radioactive atoms are like clocksAll minerals contain some radioactive atomsOnly works for IGNEOUS rocks

  22. Radiometric Dating Procedures that count the products of radioactivity to date rocks

  23. Radioactivity • When atomic nuclei spontaneously break apart • Types: • Alpha emission • Beta emission • Gamma rays

  24. Alpha Emission

  25. Beta Emission

  26. Radiometric dating keys: • Knowing the HALF LIFE • Having the parent and the daughter together in your mineral

  27. Half-life Example #1 • You start with $1000 in your bank account • Every week your spouse or boyfriend/girlfriend removes half of the money • The half-life of your account = 1 week • Questions to think about: • When is the most money taken out? • Will the amount in your bank account ever go to zero?

  28. Start: $1000 Week 1: $500 Week 2: $250 Week 3: $125 Week 4: $62.50 Week 5: $31.25 Week 6: $15.63 Week 7: $7.81 Week 8: $3.90 Week 9: $1.95 Week 10: $0.98 Week 11: $0.49 Week 12: $0.24 Week 13: $0.12 Week 14: $0.06 Week 15: $0.03 Week: 16: One and a half cents left in your bank account Your Bank Account

  29. Radioactive Decay Curve image

  30. Radiometric Dating Limitations • Sedimentary Rocks • Weathered material of other rocks • Radiometric clock has been altered • Metamorphic Rocks • Radiometric clock has been altered? • Igneous Rocks • Minerals form when rock forms • No Problem – best source for radiometric dating

  31. Radiometric Decay DataFront Page of ESRT’s

  32. Carbon 14Only useful in finding the age of ORGANIC materials up to 75,000 years old.

  33. Carbon 14 Decay Curve

  34. Other types of Absolute Dating

  35. Tree Rings • Annual growth rings are a function of their environment, i.e. temperature, humidity, precipitation, insects, fires, etc. • Limited to recent geologic past (furthest back 8000 years)

  36. Ice Layers • Ice Sheets in Greenland and Antarctica record annual changes in accumulation and snowmelt • Records up to 65,000 years • Ice also traps volcanic ash which can be radiometrically dated • Record climatic conditions of the Earth

  37. Ash Layers • Ash Layers from major volcanic eruptions create KEY BEDS – a distinct layers of known age

  38. GeologicTime Scale • Eon/Era/Period names - based on fossil assemblages, superposition • Absolute ages, from radiometric dating

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