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Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
Lecture6 radiometricdating
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Lecture6 radiometricdating

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  • 1. Lithosphere: solid Earth
  • 2. Geologic time: dating
    • 2 types
    • Relative Dating = sequential age
      • based on positions of rocks relative to each other
      • e.g., rock layer B is older than A and younger than C
    • 2. Absolute Dating = actual age
      • based on radiometric dating
      • e.g., rock layer B is 50 million years old
  • 3. Radiometric dating
    • = dating based on the steady decay of unstable isotopes
    • Atoms = nucleus of protons & neutrons orbited by electrons
    • Isotopes = different forms of element w/ different numbers of neutrons
    • unstable (Mother or Parent) isotopes
    • stable (Daughter) isotopes
    decay
  • 4. Radiometric Dating Half-life = time required for ½ of the unstable isotopes in a sample to decay into stable isotopes Decay is a perfectly random process; every atom has a 50/50 chance of decaying in a given time period (i.e., the half-life)
  • 5. time (My) Proportion of unstable isotopes 1.0 Half-life = 1 My Radiometric Dating 1 2 3 4 0 0 5 0.5 0.25 0.125 0.0625
  • 6. Radiometric Dating P = P 0 e - kt P = # unstable isotopes (parent) at time t P 0 = # unstable isotopes (parent) at time 0 (the initial # of isotopes) e = 2.718… k = decay constant = 0.693/H (to be precise, -k = ln(½)/H = -0.693/H) H = half-life t = time
  • 7. Radiometric Dating P = P 0 e - kt To find the age of a rock, solve this eq. for t: But we don’t know how many parent isotopes we started with!! ln ∙ t = 1 k P 0 P
  • 8. Radiometric Dating P = P 0 e - kt To find the age of a rock, solve this eq. for t: ln ∙ t = 1 k P 0 P P 0 - P = # stable (daughter) isotopes at time t! Let’s call this D. + 1 ln ∙ t = 1 k P 0 - P P ln ∙ t = 1 k P 0 - P + P P
  • 9. Radiometric Dating P = P 0 e - kt To find the age of a rock, solve this eq. for t: This is the equations we’ll use. ln ∙ t = 1 k P 0 P + 1 ln ∙ t = 1 k D P
  • 10. How old are the oldest rocks on Earth?
    • Given: Sample from Acasta Gneiss in Canada
    • It has crystals of zircon that
    • contain 238 U.
    • 238 U is unstable.
    • It decays into 206 Pb.
    • The half-life (H) of decay
    • of 238 U into 206 Pb = 4.47 billion years
    • We can measure how much 206 Pb (D) and 238 U (P) is in the rock and report it as a ratio…
  • 11. How old are the oldest rocks on Earth?
    • Given: Sample from Acasta Gneiss in Canada
    • H of 238 U into 206 Pb
    • = 4.47 billion years
    • D/P = 0.85
    where t = age D = amount of daughter (stable) isotope P = amount of mother (unstable) isotope k = 0.693/H H = half-life + 1 ln ∙ t = 1 k D P
  • 12. How old are the oldest rocks on Earth? t = 6.45 Ga · ln(1.85) t = 6.45 Ga · 0.615 + 1 ln ∙ t = 1 k D P + 1 ln ∙ t = H 0.693 D P 0.85 + 1 ln ∙ t = 4.47 Ga 0.693 G = giga = billion a = annum = years
  • 13. t = 3.96 Ga + 1 ln ∙ t = 1 k D P How old are the oldest rocks on Earth?
  • 14. How old is the Earth?
    • oldest rocks = 3.96 billion years old
    • difficult to find older rocks because early crust destroyed
    • most meteorites = 4.5 - 4.6 Ga
    •  we presume that the
    • age of the Earth
    • ≈ 4.6 billion years
  • 15. Radiometric dating
    • = dating based on the steady decay of unstable isotopes
    • This is where we get the numbers on the following geologic time scale
  • 16. Geologic Time
    • Earth ≈ 4.6 billion years old
    • Geological Time Scale
    • calendar of Earth history
    • divided into eons, eras, and periods
    Millions of years
  • 17. This is just for your own edification; I don't expect you to memorize this!!
  • 18. Geologic Time
    • Earth ≈ 4.6 billion years old
    • Geological Time Scale
    • calendar of Earth history
    • divided into eons, eras, and periods
    Millions of years
  • 19. Geologic Time Millions of years End of the last ‘Ice Age’: ~12,000 yrs ago End of the dinosaurs: ~65 million yrs ago
  • 20. If the Earth were the age of the USA…
    • Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of the Republic
    • How long ago (in Republic time) did the dinosaurs die out?
    • How long ago did the last ice age end?
  • 21. Scaling problem
    • Revisiting the concept of scale…
    • 99% of our atmosphere: 50 km thick
    • If you shrunk the Earth down to the size of a basketball, how thick would the atmosphere be?
  • 22. Scaling problem
    • Given
    • Earth’s radius = 6400 km
    • Thickness of the atmosphere = 50 km
    • Need to figure out
    • Basketball radius?
    • How thick the atmosphere would be if the Earth was the size of a basketball
  • 23. Scaling problem calculation
    • 1.
    • Thickness of mini atm = Radius of mini Earth
    • Thickness of atm Radius of Earth
    • 2. X cm = 10 cm
    • 50 km 6400 km
    • 3. (6400 km)(X cm) = (50 km)(10 cm)
  • 24. Scaling problem calculation
    • 4. X cm = (10 cm)(50 km)
    • 6400 km
    • 5. X = 0.078 cm
    • Unit conversion?
    • Significant figures?
  • 25. If the Earth were the age of the USA…
    • Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of the Republic.
    • How long ago (in Republic time) did the dinosaurs die out (65 Ma)?
    • How long ago did the
    • last ice age end (12 ka)?
  • 26. If the Earth were the age of…
    • Pick some familiar time interval
      • Possibilities: a day, a week, your life, your time at W&M, etc.
    • Scale down Earth’s age to that time interval. How long is recorded history on your new time scale?
    • The Sumerians developed cuneiform writing in ~4000 B.C. (~6000 years ago)
    • Earth is ~4.6 billion years old

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