Lecture 14 The Age of the Earth

1. Lecture 14The Age of the Earth

2. How old is the Earth…. ….and how do we know?

3. Estimates of the Age of the Earth • Hindu chronology, ~140 BC • 2 Ga (Billion years) old • archbishop James Ussher, 1654 • October 23, 4004 B.C. • age of the Earth = length of human history • James Hutton, 1785 • Very very old • Uniformitarianism (“the present is the key to the past”) • William Thomson, Lord Kelvin, late 1800’s • 20-40 Ma • Cooling planet from molten rock • Modern estimates • 4.6 Ga • Radiometric dating

4. The time to cool the Earth Assumptions: the Earth has not gained any heat since its formation The Earth is solid rock • Heat moves from hot places to cold places • The surface of the Earth loses heat at about 100-400 W/m2 • Kelvin estimates 24- 400 Ma to cool Earth from a molten body to the present Earth temperature Cold . HOT

5. The time to cool the Earth Assumptions: • The Earth is solid rock • the Earth has not gained any heat since its formation Problem: • The inner core is still molten • The mantle convects • The surface of the Earth is heated by the Sun • Radioactivity! .

6. Why does radioactivity matter? 234U = 92Kr + 141Ba + 3 Neutrons + Energy • Radioactive decay: some elements have naturally occurring isotopes that are unstable. • Decay produces heat: the Earth is far older than Kelvin calculated.

7. Radioactive Decay Energy = Mass Loss = Mass(Starting particles) – Mass(Ending particles) E=mc Time can be determined by their rate of decay. • Alpha decay • emits an alpha particle (helium-4) • E.g., 238U = 234Th + a + energy • Beta decay • Emission or capture of electrons • E.g., energy + 40K + electron = 40Ar + neutrino • E.g., 40K = 40Ca + electron + neutrino + energy

8. Radioactive Decay occurs in half-lives Parent (Starting) Isotope Number of half-lives

9. Worksheet Time

10. K-Ar age dating • Potassium (K) is abundant in many rock-forming minerals, including: • mica (biotite and muscovite) • feldspar • amphibole • Argon (Ar) is a gas, so the initial concentration of the daughter product is 0.

11. Potassium-Argon System:half-life 1.26 x 109 years electron capture 40Ar 22 E.C. electron emission 40K 21 Neutron number N (number of neutrons) 40Ca 20 18 19 20 Atomic number Z (number of protons)

12. Radioactive Decay occurs in half-lives 40Ar Parent = 40K 40K Number of half-lives

13. If you measure 40Ar and 40K in a rock and find equal quantities, how many half lives have occurred since the rock cooled? • 1 • 2 • 3 • 4 • 5 40Ar Parent = 40K 40K Number of half-lives

14. If the half-life of the K-Ar system is 1.26 x 109 years, how old is the rock with equal quantities of 40Ar and 40K ? • 0.63 x 109 years • 1.26 x 109 years • 2.52 x 109 years • 5.04 x 109 years • 10.1 x 109 years 40Ar Parent = 40K 40K Number of half-lives

15. If a rock start with 160 moles of 40K, how many moles does it have after 3 half-lives ? • 80 • 40 • 20 • 10 • 5 40Ar Parent = 40K 40K Number of half-lives

16. Radioactive decay involves the decay of one isotope of an element into an isotope of another element. It also • Creates mass. • Produces energy. • Decreases temperatures.

17. K-Ar dating can be used to date • Any rocks. • Rocks that contain Ar. • Rocks that contain K. • Rocks that contain both.

18. The half-life of a radioactive isotope is • The time it takes all atoms of the isotope to decay. • The time it takes half the atoms of the isotope to decay. • The time it takes one quarter of the atoms of the isotope to decay. • Half the time a particular atom of the isotope will take to decay.

19. Summary: Radiometric Dating • Decay of parent element (isotope) to daughter element (isotope) at constant rate • Half-life (t1/2 ): time required for half of parent element to decay to daughter element • Assume closed system • All daughter products must be from parent only • Measure parent + daughter in sample • Parent + daughter = amount original parent • Present parent /original parent = % decayed

20. What does the date actually date?: Blocking temperature Bottom line: Radiometric dates can tell you - when an igneous rock cooled below its blocking temperature - when regional or thermal metamorphism has ceased and cooled

21. The Earth’s oldest rocks • •Imataca Complex, Venezuela: 3.77 Ga • Rhodesian Craton, Zimbabwe: 3.52 Ga • Mount Stones, Antarctica: 3.53 Ga • Northwest Territories, Canada: 3.96 Ga • Detrital zircons: 4.15 Ga • Etc....... • …..So, problem solved?

22. Not quite, we have rocks from the moon!

23. Apollo 17 (30 samples) 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Rb-Sr 40Ar/39Ar Radiometric age (Ga) Sm-Nd U-Pb

24. …And we have meteorites!

25. Irons (11 meteorites), Stony irons (1 meteorites) Achondrites (20 meteorites) Chondrites (37 meteorites) 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Rb-Sr Radiometric age (Ga) 40Ar/39Ar Sm-Nd

26. So…. Why are moon rocks and meteorites so much older than the oldest Earth rocks?

27. The Hadean Eon: The first 600 Ma (million years). -Separation of Core from Mantle. -So hot: oceans of magma, no permanent crust? -No rock record

28. With radiometric dating, we can now put “numerical” ages on the “relative” geological timescale. Relative age: a sequence of events listed ‘relative’ to each other. Ex. a ‘younger’ brother Numerical age: a specific date, given in units. Ex. a 2-year-old brother…

29. Radiometric dating – years since the parent isotope started decaying

30. Magnetic polarity – on land and in oceans – can correlate sequences – but only relative dates

31. Why isn’t magnetic polarity an absolute dating method? • Magnetic polarity is either negative or positive. • Sequences of polarity can be matched. • Polarity changes happen at irregular intervals. • Sedimentation rates vary from place to place. • Knowing the total sequence doesn’t tell you the start.

32. Tree Rings – annual growth – can yeild absolute dates if correlate with living tree!

33. Annual growth rings in a shell

34. Annual layers of ice in a glacier

35. Review of Absolute Dating Methods • Radiometric • K-Ar/Ar-Ar (~50 ka – ~4.6 Ga) • U-Pb (~10 Ma – ~4.6 Ga) • C14 (< 70 ka) • Others (including fission tracks) • Seasonally controlled growth (<100 ka) • Growth rings of trees and shells (< 10 ka) • Annual mud layers in lakes and seas • Annual snow/ice accumulations in glaciers k=thousand, M=million, G=billion, a=years ago

36. Review of Relative Dating Methods • Fossils thru faunal succession • Paleomagnetism and magnetic polarity • Principles of • Cross-cutting relations • Superposition • Baked contacts • Inclusions • Horizontality • Continuity

37. Dating of geological events… Big problem: you can’t date (absolutely) sedimentary rocks!

38. What does a radiometric date actually date?: Blocking temperature Bottom line: Radiometric dates can tell you - when an igneous rock cooled below its blocking temperature - when regional or thermal metamorphism has ceased and cooled Exception: 14C dates time of death of a living system – so works for fossils, charcoal, etc. younger than 70 ka.

39. How old is the Cretaceous sandstone? • < 50 million years old • 50 - 80 million years old • 80 – 125 million years old • > 125 million years old

40. Olduvai Gorge, Tanzania Volcanic Ash Fossils

41. Stratigraphic placement of Olduvai hominid 65 Tuff IF Ng’eju Tuff Figure 2 from Blumenschine et al., 2003

42. Review:The relativegeologic timescale

43. With dates:The geologic timescale

44. Summary • Earth is ~4.5 billion years old (Ga), but the oldest rocks are only 3-4 Ga due to the early Earth being too hot for a rock record to survive. • Relative dating places strata (or events) in an order. • Absolute dating gives the number of years ago that a fossil existed or an volcanic ash layer was deposited. • The geologic time scale has both a relative ages (based on the fossil succession and magnetic reversals) and absolute ages (based on radiometric dates).