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Dynamic Earth

Dynamic Earth. Class 2 12 January 2006. Any Questions?. Uniformitarianism. Geologic processes that we see in operation today have worked much the same way over geologic time — however, rates and intensity of processes may have changed. The present is the key to the past — James Hutton.

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Dynamic Earth

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  1. Dynamic Earth Class 2 12 January 2006

  2. Any Questions?

  3. Uniformitarianism Geologic processes that we see in operation today have worked much the same way over geologic time — however, rates and intensity of processes may have changed. The present is the key to the past — James Hutton

  4. Early estimates of Earth’s age • 1650: James Ussher -- Age: 6000 years(From reconstruction of the Bible) • By the mid-19th Century, the age of the Earth was still only an educated guess, but the general feeling was that the Earth was at least 500 million years old, and probably much older.

  5. Radiometric Dating • Use of radioactive decay to determine the age of a rock • First proposed ~1896-1902

  6. Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons i.e.: 235U, 238U 87Sr, 86Sr 14C, 12C

  7. Naturally Occurring Isotopes of Carbon

  8. Production and Decay of Radiocarbon

  9. Beta Decay Electron Capture Alpha Decay

  10. Radioactive Decay of Rubidium to Strontium

  11. Radiometric Dating • Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay

  12. Radioactivity and Absolute Time Decay of parent atoms Growth of daughter atoms

  13. Radioactive Decay of Uranium 238to Lead 206

  14. Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters) • The rate at which this decay occurs is constant and known • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding

  15. Major Radioactive Elements Used in Isotopic Dating

  16. Geologically useful decay schemes Parent Daughter Half-life (years) 235U 207Pb 4.5 x 109 238U 206Pb 0.71 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 14C 14N 5730

  17. Radiometric Dating • Shows that the earth is much older than people had previously suspected • Earth formed about 4.6 billion years ago

  18. Relative vs Absolute Age • Usually geologists first establish relative ages then try to get absolute age dates • Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries

  19. Principle of Superposition • In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom

  20. Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion: Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below

  21. Principle of Superposition Youngest rocks Oldest rocks

  22. Principle of Superposition • Although this is really obvious, it was not stated until 1669 • This principle generally applies to volcanic rocks as well as sedimentary rocks

  23. Principle of Original Horizontality • Layers of sediment are deposited in a horizontal or nearly horizontal position parallel to the Earth’s surface

  24. Principles of original horizontality and superposition

  25. Principle of Original Horizontality • Note that original horizontality is not strictly applicable to volcanic rocks because they are often deposited on slopes

  26. Principle of Cross-cutting Relationships • Something (such as a dike or fault) that cuts across a layer must be younger than the layer

  27. Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption

  28. Unconformity • Results from interruption of deposition • Represents a long period of time during which there either was no deposition, or earlier deposited material was eroded away

  29. Sedimentation of Beds A-D Beneath the Sea

  30. Uplift and Exposure of D to Erosion

  31. Continued Erosion Removes D and Exposes C to Erosion

  32. Subsidence and Sedimentation of E over C Unconformity: a buried surface of erosion

  33. Formation of a Unconformity

  34. Unconformity • Often not easy to recognize if the layers are all parallel • Much easier to recognize when there is a period of folding of the rocks before a period of erosion and renewed deposition

  35. South rim of the Grand Canyon

  36. South rim of the Grand Canyon 250 million years old Paleozoic Strata 550 million years old 1.7 billion years old Precambrian

  37. South rim of the Grand Canyon 250 million years old 550 million years old 1.7 billion years old Unconformity

  38. The Great Unconformity of the Grand Canyon

  39. Siccar Point, Scotland: Hutton’s Classic Unconformity -- Old Red Sandstone (~345 my) overlies rocks that are ~425 million years old

  40. Siccar Point, Scotland Buried and tilted erosional surface

  41. Summary of Geologic Eventsin a small area

  42. Relative Geologic Dating

  43. These methods work well in small areas where we can see the relationships between rock layers. • What happens when we want to tell the relative ages of the strata on Oahu with respect to strata on Maui? • We have to figure out some way to correlate the layers of interest.

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