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Geology 12

Geology 12. Presents. Methods that didn’t work. 1. Bible: 9:00 am Tuesday, Oct 26 th , 4004 BC = beginning of Earth 2. Rate of cooling: Earth from a molten ball a) George Buffon (1750): 75,000 yrs using steel balls b) Lord Kelvin (1800s): 20 – 40 ma

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Geology 12

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  1. Geology 12 Presents

  2. Methods that didn’t work • 1. Bible: 9:00 am Tuesday, Oct 26th, 4004 BC = beginning of Earth • 2. Rate of cooling: Earth from a molten ball • a) George Buffon (1750): 75,000 yrs using steel balls • b) Lord Kelvin (1800s): 20 – 40 ma • Didn’t account for radioactive heating or convection currents in the mantle.

  3. 3. Rate of sedimentation: James Walcott (1893): 75 ma • Didn’t know thickness of sediments; • didn’t account for erosion; • assumed constant sedimentation rate • 4. Salinity of oceans: John Joly (1899): 100 ma • Assumed oceans were initially fresh water; • Didn’t know of chemical sedimentation (precipitation)

  4. Bible Bible was interpreted as giving an date of 9:00 am, Tuesday, Oct 26, 4004 BC

  5. Hebrew Writing

  6. Greek Writing

  7. Old English to New English

  8. Rate of Cooling Steel balls were used and extrapolated to the size of the earth. Didn’t take into account radioactive heating (adds heat) or convection currents (removes heat).

  9. Radioactive core producing heat Convection currents

  10. Rates of Sedimentation

  11. Salinity of Oceans

  12. Salt is added to the oceans from rivers and removed through precipitation.

  13. Absolute Time • Is determining how long ago something happened (ma = millions of years)

  14. Absolute Dating Methods • 1. Tree Rings: count back/inward from cambium to centre to get age of tree when it was cut • Also gives indication of paleoclimate • Ranges up to 14,000 years; mostly in arid area

  15. Tree Rings Count back in time to centre. Range up to 14,000 yrs. Gives indication of paleoclimate.

  16. 2. Varves: thin glacial sediments 1 year Drop stone: fell out of melting iceberg.

  17. Varves

  18. 3. Fission Track Dating: minerals with or near uranium become “scratched” = etched by decay particles • Compare the number of etches to amount of uranium = age • Range: 40,000 to 1.5 ma

  19. Fission Track Dating K-Feldspar Soda-ljme Glass Lexan

  20. Apatite Muscovite

  21. Daughter nucleus “stable” • 4. Radiometric dating = unstable atomic nuclei decay at a known rate to stable ones. Parent nucleus “Unstable” Decay Particle

  22. By measuring the remaining amount of parent nuclei to the amount of produced daughter nuclei, an age can be determined • i.e.: lots of parent : little daughter = young • i.e.: little parent : lots of daughter = old • Half Life = time for half parent nuclei to decay 50 : 50 p : d

  23. Radiometric Dating

  24. Please refer to note helper WS 8.45, side 2 • Thank you.

  25. Years 0 3 6 9 12 15 Half lives 0 1 2 3 4 5 Element U T1/2 = 3 years daughter parent 12.5% 6.25% 3.125%

  26. Element V T1/2 = 5 years 12.5% 6.25% 20 5 15 10 How many years until: a) 50% gone: b) 75% gone: c) 87.5% gone: 5 10 15

  27. Element W 1 : 7 12.5 : 87.5 1 : 15 6.25:93.75 50 : 50 1 : 1 25 : 75 1 : 3 1 : 31 3.1 : 96.9 12 6 2 4 8 10 Millions of years 2.5 • What is the half life? Years • A rock sample contained a ratio of 3 stable daughter atoms for every 1 parent atom. The age of the sample is: ma. 5.0

  28. Element X 12.5% 6.25% An igneous rock specimen contains 6.25% of its original uranium 235. How old the specimen in half lives? 4

  29. Isotopes Used in Radiometric Dating Isotope T1/2 Dating Range Daughter Rocks Dated or abundant in C14 5730 100-70,000 N14 Organic Matter carbonates, CO2 U235 704 ma 80,000 – 2ba Pb207 Uraninite, zircon K40 1.3 ba 100,000 - Ar40 Muscovite,Kfs,Hbld 4.6 ba biotite, volc rx, U238 4.5 ba 10 ma – 4.6 ba Pb206 Uraninite, zircon Th 232 14 ba 0.5 ba – big bang Pb208 zircon Rb87 48.8 ba 1 ba – big bang Sr87 same as K40

  30. Sources of Error • 1. weathering can remove some of the parent or daughter isotopes distorting the ratio. • i.e. daughter elements removed ( ) makes it too young. Ar40

  31. 2. High temperatures/pressures (metamorphism) can: • a) destroy fission tracks • b) use up parent/daughter isotopes distorting the ratio • c) cause argon to escape

  32. Preventing Error: • 1. Make sure sample is frsh,unweathered, unheated, etc. • 2. cross check with several methods of dating to see if dates agree.

  33. General Rule • Radiometric dating for igneous and meta’ rx’ • Index fossils and C14 dating for sed’ rx’

  34. Do Lab 8.5Do WS 8.5Do Unit 2 (Chp 8) Review WS

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