Methods for Estimating the Age of the Earth

1. Methods for Estimating the Age of the Earth • Fossils and evolution • Rates of deposition of sediments • Cooling of the Earth from a molten state • Radiometric Dating

2. Radiometric Dating of Rocks 1896-1898 Radioactivity discovered by: Henri Becquerel and Marie and Pierre Curie.

3. Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

4. Age of the Earth

5. Geologic Time Scale

6. The Atom

7. NaCl 1 8 2 7 8 2 11 protons 17 protons Na = silvery metal Cl = poisonous gas NaCl = Salt

8. Examples of Atomic Numbers

9. Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

10. Uranium Nucleus spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

11. Alpha Particle emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

12. Uranium - Thorium Decay U He + Th 4 234 238 92 90 2 spontaneous decay “parent” “daughter product” alpha particle = 2 protons + 2 neutrons = positively charged ion of Helium Thorium: 90 protons + 144 neutrons

13. Beta Particle Emission But, Th is also unstable, and it emits a beta particle… 234 90

14. Thorium - Protactinium Decay beta particle Th + Pa 234 234 90 91 beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron Protactinium: 91 protons + 143 neutrons

15. Title beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

16. U PbSeries This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

17. U PbSeries The rate at which this process occurs is measured in terms of the “half life”.

18. Half Life Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

19. The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:   (1) As radioactive elements decay in the Earth, they heat up the surrounding rocks. (2) Radiometric Dating

20. As radioactive elements decay in the Earth, they heat up the surrounding rocks. So, there is a source of heat inside the Earth that Kelvin didn't know about. Thus: While the Earth was cooling off, it was also being heated up (due to radioactive decay in its interior), and the Earth took much longer to cool than Kelvin thought.

21. Radiometric Dating Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed. This is an example of uniformitarianism. The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

22. To estimate the age of a rock: D = amount of daughter product. P = amount of parent.   For a particular radioactive element in a rock, determine the present ratio = D/P. D P

23. Rate of decay (from theory and measurement) • Make assumptions about original ratios (from theory of geochemistry). • As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified). D D P P

24. Title Potassium - Argon Dating

25. Electron Capture

26. Title Electron Capture

27. Potassium - Argon Dating • K/Ar clock begins when K bearing minerals crystallize in a rock. • The new minerals will contain K40, but will not contain Ar40 (because Ar40 does not combine with other elements). • As time passes: • K40  Ar40 (half-life = 1.3 billion years) • K40/Ar40 ratio can be measured to determine age Uniformitarianism

28. Potassium - Argon Dating K40/Ar40 ratio can be measured to determine age. K40  Ar40 (half-life = 1.3 billion years) K40/Ar40 > 1  less than 1.3 billion years old K40/Ar40 = 1  1.3 billion years old K40/Ar40 < 1  more than 1.3 billion years old

29. Half-Lives of Radioactive Isotpes

30. Examples of Very Old Rocks Found on Earth: • 3.4 billion year old granites - South Africa. • 3.7 billion year old granites - Greenland. • Oldest terrestrial rocks > 3.8 billion years. • Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

31. Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

32. But, rocks on the Earth have been weathered, metamorphosed and recycled through a very dynamic planet. Thus, the Earth's earliest rocks have been recycled, and we need to look beyond Earth to find evidence of when the Earth solidified.

33. Plate Tectonics

34. Rock Cycle

35. Mountain Building and Erosion

36. Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

37. Similar Rock Structures

38. The Earth's earliest rocks have been weathered, metamorphosed and recycled through a very dynamic planet. We need to look beyond Earth to find evidence of when the Earth solidified.