0
Lithosphere:  solid Earth
Geologic time: dating <ul><li>2 types </li></ul><ul><li>Relative Dating = sequential age </li></ul><ul><ul><li>based on po...
Radiometric dating <ul><li>= dating based on the steady decay of unstable isotopes </li></ul><ul><li>Atoms   = nucleus of ...
Radiometric Dating Half-life   = time required for ½ of the unstable isotopes in a sample to decay into stable isotopes De...
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
Radiometric Dating P = P 0 e - kt P = # unstable isotopes (parent) at time t P 0  = # unstable isotopes (parent) at time 0...
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 isot...
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...
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 ∙ ...
How old are the oldest rocks on Earth? <ul><li>Given: Sample from Acasta Gneiss in Canada </li></ul><ul><li>It has crystal...
How old are the oldest rocks on Earth? <ul><li>Given: Sample from Acasta Gneiss in Canada </li></ul><ul><li>H of  238 U in...
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....
t = 3.96 Ga + 1 ln ∙ t = 1 k D P How old are the oldest rocks on Earth?
How old is the Earth? <ul><li>oldest rocks = 3.96 billion years old </li></ul><ul><li>difficult to find older rocks becaus...
Radiometric dating <ul><li>= dating based on the steady decay of unstable isotopes </li></ul><ul><li>This is where we get ...
Geologic Time <ul><li>Earth ≈ 4.6 billion years old </li></ul><ul><li>Geological Time Scale </li></ul><ul><li>calendar of ...
This is just for your own edification;  I don't expect you to memorize this!!
Geologic Time <ul><li>Earth ≈ 4.6 billion years old </li></ul><ul><li>Geological Time Scale </li></ul><ul><li>calendar of ...
Geologic Time Millions of years End of the last ‘Ice Age’: ~12,000 yrs ago End of the dinosaurs: ~65 million yrs ago
If the Earth were the age of the USA… <ul><li>Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of...
Scaling problem <ul><li>Revisiting the concept of scale… </li></ul><ul><li>99% of our atmosphere: 50 km thick </li></ul><u...
Scaling problem <ul><li>Given </li></ul><ul><li>Earth’s radius = 6400 km </li></ul><ul><li>Thickness of the atmosphere = 5...
Scaling problem calculation <ul><li>1. </li></ul><ul><li>Thickness of mini atm   = Radius of mini Earth </li></ul><ul><li>...
Scaling problem calculation <ul><li>4.  X cm  =  (10 cm)(50 km) </li></ul><ul><li>  6400 km </li></ul><ul><li>5.  X = 0.07...
If the Earth were the age of the USA… <ul><li>Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of...
If the Earth were the age of… <ul><li>Pick some familiar time interval </li></ul><ul><ul><li>Possibilities: a day, a week,...
Upcoming SlideShare
Loading in...5
×

Lecture6 radiometricdating

250

Published on

0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
250
On Slideshare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
Downloads
14
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Transcript of "Lecture6 radiometricdating"

  1. 1. Lithosphere: solid Earth
  2. 2. Geologic time: dating <ul><li>2 types </li></ul><ul><li>Relative Dating = sequential age </li></ul><ul><ul><li>based on positions of rocks relative to each other </li></ul></ul><ul><ul><li>e.g., rock layer B is older than A and younger than C </li></ul></ul><ul><li>2. Absolute Dating = actual age </li></ul><ul><ul><li>based on radiometric dating </li></ul></ul><ul><ul><li>e.g., rock layer B is 50 million years old </li></ul></ul>
  3. 3. Radiometric dating <ul><li>= dating based on the steady decay of unstable isotopes </li></ul><ul><li>Atoms = nucleus of protons & neutrons orbited by electrons </li></ul><ul><li>Isotopes = different forms of element w/ different numbers of neutrons </li></ul><ul><li>unstable (Mother or Parent) isotopes </li></ul><ul><li> stable (Daughter) isotopes </li></ul>decay
  4. 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. 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. 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. 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. 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. 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. 10. How old are the oldest rocks on Earth? <ul><li>Given: Sample from Acasta Gneiss in Canada </li></ul><ul><li>It has crystals of zircon that </li></ul><ul><li>contain 238 U. </li></ul><ul><li>238 U is unstable. </li></ul><ul><li>It decays into 206 Pb. </li></ul><ul><li>The half-life (H) of decay </li></ul><ul><li>of 238 U into 206 Pb = 4.47 billion years </li></ul><ul><li>We can measure how much 206 Pb (D) and 238 U (P) is in the rock and report it as a ratio… </li></ul>
  11. 11. How old are the oldest rocks on Earth? <ul><li>Given: Sample from Acasta Gneiss in Canada </li></ul><ul><li>H of 238 U into 206 Pb </li></ul><ul><li>= 4.47 billion years </li></ul><ul><li>D/P = 0.85 </li></ul>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. 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. 13. t = 3.96 Ga + 1 ln ∙ t = 1 k D P How old are the oldest rocks on Earth?
  14. 14. How old is the Earth? <ul><li>oldest rocks = 3.96 billion years old </li></ul><ul><li>difficult to find older rocks because early crust destroyed </li></ul><ul><li>most meteorites = 4.5 - 4.6 Ga </li></ul><ul><li>  we presume that the </li></ul><ul><li>age of the Earth </li></ul><ul><li>≈ 4.6 billion years </li></ul>
  15. 15. Radiometric dating <ul><li>= dating based on the steady decay of unstable isotopes </li></ul><ul><li>This is where we get the numbers on the following geologic time scale </li></ul>
  16. 16. Geologic Time <ul><li>Earth ≈ 4.6 billion years old </li></ul><ul><li>Geological Time Scale </li></ul><ul><li>calendar of Earth history </li></ul><ul><li>divided into eons, eras, and periods </li></ul>Millions of years
  17. 17. This is just for your own edification; I don't expect you to memorize this!!
  18. 18. Geologic Time <ul><li>Earth ≈ 4.6 billion years old </li></ul><ul><li>Geological Time Scale </li></ul><ul><li>calendar of Earth history </li></ul><ul><li>divided into eons, eras, and periods </li></ul>Millions of years
  19. 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. 20. If the Earth were the age of the USA… <ul><li>Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of the Republic </li></ul><ul><li>How long ago (in Republic time) did the dinosaurs die out? </li></ul><ul><li>How long ago did the last ice age end? </li></ul>
  21. 21. Scaling problem <ul><li>Revisiting the concept of scale… </li></ul><ul><li>99% of our atmosphere: 50 km thick </li></ul><ul><li>If you shrunk the Earth down to the size of a basketball, how thick would the atmosphere be? </li></ul>≡
  22. 22. Scaling problem <ul><li>Given </li></ul><ul><li>Earth’s radius = 6400 km </li></ul><ul><li>Thickness of the atmosphere = 50 km </li></ul><ul><li>Need to figure out </li></ul><ul><li>Basketball radius? </li></ul><ul><li>How thick the atmosphere would be if the Earth was the size of a basketball </li></ul>
  23. 23. Scaling problem calculation <ul><li>1. </li></ul><ul><li>Thickness of mini atm = Radius of mini Earth </li></ul><ul><li>Thickness of atm Radius of Earth </li></ul><ul><li>2. X cm = 10 cm </li></ul><ul><li>50 km 6400 km </li></ul><ul><li>3. (6400 km)(X cm) = (50 km)(10 cm) </li></ul>
  24. 24. Scaling problem calculation <ul><li>4. X cm = (10 cm)(50 km) </li></ul><ul><li> 6400 km </li></ul><ul><li>5. X = 0.078 cm </li></ul><ul><li>Unit conversion? </li></ul><ul><li>Significant figures? </li></ul>
  25. 25. If the Earth were the age of the USA… <ul><li>Let’s condense the ~4.6 billion years of the Earth down to the ~230 years of the Republic. </li></ul><ul><li>How long ago (in Republic time) did the dinosaurs die out (65 Ma)? </li></ul><ul><li>How long ago did the </li></ul><ul><li>last ice age end (12 ka)? </li></ul>
  26. 26. If the Earth were the age of… <ul><li>Pick some familiar time interval </li></ul><ul><ul><li>Possibilities: a day, a week, your life, your time at W&M, etc. </li></ul></ul><ul><li>Scale down Earth’s age to that time interval. How long is recorded history on your new time scale? </li></ul><ul><li>The Sumerians developed cuneiform writing in ~4000 B.C. (~6000 years ago) </li></ul><ul><li>Earth is ~4.6 billion years old </li></ul>
  1. A particular slide catching your eye?

    Clipping is a handy way to collect important slides you want to go back to later.

×