1. Dynamic Earth Class 3 17 January 2006

2. Any Questions?

3. Radiometric Dating Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay Earth formed about 4.6 billion years ago Earth formed about 4.6 billion years ago

4. Oldest rocks Youngest rocks Principle of Superposition

5. Principle of Original Horizontality

6. Principle of Cross-cutting Relationships

7. CORRELATION Process used to tie separated strata together Process used to tie separated strata together Based on matching physical features such as Based on matching physical features such as Physical continuity - trace of rock unit Physical continuity - trace of rock unit Similar rock types - marker beds, coal seams, rare minerals, odd color Similar rock types - marker beds, coal seams, rare minerals, odd color

8. Within sedimentary layers there are often the remains of small animals (fossils) Within sedimentary layers there are often the remains of small animals (fossils) Fossils are quite useful for correlating between two sections that are not laterally continuous Fossils are quite useful for correlating between two sections that are not laterally continuous CORRELATION

9. Paleontology The study of life in the past based on fossilized plants and animals. Fossil: Evidence of past life Fossils preserved in sedimentary rocks are used to determine: 1) Relative age 2) Environment of deposition

10. Ammonite Fossils Petrified Wood Chip Clark Tom Bean

11. Using Fossils to Correlate Rocks

12. Fossils have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age Fossils have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age CORRELATION

13. T his technique is not very useful in Hawaii -- Why? T his technique is not very useful in Hawaii -- Why? Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocks Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocks CORRELATION

14. Radiometric dates provide absolute ages to the local stratigraphy

15. Radiometric dating of lava flows provides absolute dating of glacial events in Hawaii

16. Bracketing ages

17. Magnetostratigraphy An alternate method for absolute age dating that works well with volcanic rocks An alternate method for absolute age dating that works well with volcanic rocks

18. Magnetostratigraphy Technique that works best in volcanic rocks Technique that works best in volcanic rocks Time scale based on polarity reversal of Earth's magnetic field Time scale based on polarity reversal of Earth's magnetic field Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks

19. Earth’s Magnetic Field

20. Magnetization of Magnetite

21. Lavas record magnetic reversals

22. Magnetic reversals over the past 20 million years

23. Magnetic time scale 0-700,000 -- Normal 0-700,000 -- Normal 700,000 - 2.5 my -- Reversed 700,000 - 2.5 my -- Reversed > 2.5 my -- Normal > 2.5 my -- Normal Ko`olau lavas mostly reversed in polarity, so they must be older than 700,000 yrs, but younger than 2.5 my Ko`olau lavas mostly reversed in polarity, so they must be older than 700,000 yrs, but younger than 2.5 my Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they must be older than 2.5 my Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they must be older than 2.5 my

24. The Geologic time scale Divisions in the worldwide stratigraphic column based on variations in preserved fossils Divisions in the worldwide stratigraphic column based on variations in preserved fossils Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages

25. The Geologic Column and Time Scale

26. Hominoid fossils can be dated using both radiometric and paleomagnetitic techniques.

27. Generalized Stratigraphic Section of Rocks Exposed in the Grand Canyon

28. Some of the Geologic Units Exposed in the Grand Canyon Michael Collier

29. The first ~4 billion years of Earth’s history was nearly devoid of life – this time is known as the Precambrian.

30. After the Precambrian, life began to develop very rapidly. The Paleozoic lasted for 300 my. Dinosaurs dominated the Mesozoic, which lasted for almost 200 my. After the dinosaurs died out, mammals began to dominate in the Cenozoic. These are the main subdivisions that you should know.

31. The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions

32. Bracketing ages

33. The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions

34. What are these subdivisions based on? Boundaries are marked by mass extinctions Boundaries are marked by mass extinctions Cretaceous-Tertiary boundary (death of the dinosaurs): 65 million years Cretaceous-Tertiary boundary (death of the dinosaurs): 65 million years Permian-Triassic boundary (the biggest mass extinction of them all): 250 million years Permian-Triassic boundary (the biggest mass extinction of them all): 250 million years Start of the Cambrian (first hard animal parts--nice fossils): 550 million years Start of the Cambrian (first hard animal parts--nice fossils): 550 million years

35. The Geologic Column and Time Scale

36. How do we know when it started? What is the age of the Earth?

37. Many methods have been used to determine the age of the Earth 1) Bible: In 1664, Archbishop Usher of Dublin used chronology of the Book of Genesis to calculate that the world began on Oct. 26, 4004 B.C. 2) Salt in the Ocean: (ca. 1899) Assuming the oceans began as fresh water, the rate at which rivers are transporting salts to the oceans would lead to present salinity in ~100 m.y.

38. Many methods have been used to determine the age of the Earth 3) Sediment Thickness: Assuming the rate of deposition is the same today as in the past, the thickest sedimentary sequences (e.g., Grand Canyon) would have been deposited in ~ 100 m.y. 4) Kelvin’s Calculation: (1870): Lord Kelvin calculated that the present geothermal gradient of ~30°C/km would result in an initially molten earth cooled for 30 – 100 m.y.

39. Flawed assumptions Bible is not a science text or history book Bible is not a science text or history book Salt is precipitated in sedimentary formations Salt is precipitated in sedimentary formations Both erosion and non-deposition are major parts of the sedimentary record Both erosion and non-deposition are major parts of the sedimentary record Radioactivity provides another heat source Radioactivity provides another heat source

40. The heat inside the Earth The discovery of radioactivity at the turn of the century by Bequerel, Curie, and Rutherford not only provided the source of the heat to override Kelvin’s calculations but provided the basis for all later quantitative estimates of the ages of rocks.

41. Oldest rocks on Earth Slave Province, Northern Canada Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb method Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb method Yilgarn block, Western Australia Detrital zircons in a sandstone dated at 4.10 Ga by U-Pb method. Detrital zircons in a sandstone dated at 4.10 Ga by U-Pb method. Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.

42. Age of the Earth Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1), we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

43. Three types of rocks IgneousSedimentaryMetamorphic Igneous rocks make up most of the earth sedimentary rocks make up most of the surface.

44. Major Rock Groups IGNEOUSSEDIMENTARYMETAMORPHIC

45. High Relief of Andes

46. Low Relief of Western Australia

47. The Rock Cycle

48. Age of the Earth Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1), we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

49. Age of the Earth This is based on the age of rocks brought back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to be good representatives of the early solar system as well as more complicated geochemical modeling. This data suggests that the present chemical composition of the crust must have evolved for more than 4.5 Ga.

50. The big assumption The half-lives of radioactive isotopes are the same as they were billions of years ago.

51. Test of the assumption Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data.

52. Age of the Earth We have not found a way to determine the exact age of the Earth directly from Earth rocks We have not found a way to determine the exact age of the Earth directly from Earth rocks Earth's oldest rocks have been recycled and destroyed (rock cycle) Earth's oldest rocks have been recycled and destroyed (rock cycle) We have been able to determine the probable age of the Solar System and to calculate an age for the Earth We have been able to determine the probable age of the Solar System and to calculate an age for the Earth Assume the Earth and the rest of the solid bodies in the Solar System formed at the same time and are, therefore, of the same age Assume the Earth and the rest of the solid bodies in the Solar System formed at the same time and are, therefore, of the same age

53. Age of the Earth Oldest rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent Oldest rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent Interestingly, these rocks are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water Interestingly, these rocks are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water Indicates that Earth history began well before these rocks were deposited Indicates that Earth history began well before these rocks were deposited

54. Age of the Earth Moon rocks returned to Earth on the Apollo missions are generally 4.4-4.5 billion years old Moon rocks returned to Earth on the Apollo missions are generally 4.4-4.5 billion years old No rock cycle on the Moon No rock cycle on the Moon Minimum age of our nearest planetary neighbor Minimum age of our nearest planetary neighbor More than 70 meteorites of many different types date 4.53 to 4.59 billion years old More than 70 meteorites of many different types date 4.53 to 4.59 billion years old Primitive objects provide best ages the formation of the Solar System Primitive objects provide best ages the formation of the Solar System So, the best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system So, the best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system

55. Any Questions?

56. Thursday Homework #1 is due – check web page Homework #1 is due – check web page First video: Dating the Earth First video: Dating the Earth

57. Big Island Field Trip Need deposit of $50 on February 17 th Need deposit of $50 on February 17 th