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13 geologictime forstudents

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  • 1. Geologic Time Chapter 12
  • 2. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12 Chapter 12
  • 3. Geologic Time Chapter 12
  • 4. Geologic Time• Understanding time permits assigning ages to… Chapter 12
  • 5. Geologic Time• Prior to late 1600s, geologic time was thought to = historical time. • Archbishop James Ussher, Ireland, 1654. Chapter 12
  • 6. Geologic Time• Scientists began to find clues to an ancient Earth. • Nicolaus Steno (1638–1686) – Danish physician. fossil shark tooth Chapter 12
  • 7. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12 Chapter 12
  • 8. Geologic Time2 ways to date geological materials: 1. Relative age – 2. Numerical (absolute) age – Chapter 12
  • 9. Relative vs. Absolute Chapter 12
  • 10. Relative Age• Logical tools are useful for defining relative age.• Principles of: 1. Uniformitarianism 2. Superposition 3. Original horizontality 4. Original continuity 5. Cross-cutting relations 6. Inclusions 7. Baked contacts. Chapter 12
  • 11. Geologic Time1. Uniformitarianism – “The present is key to the past”. • Chapter 12
  • 12. Defining Relative Age2. Superposition. Chapter 12
  • 13. Relative Age3 & 4. Horizontality and continuity. Chapter 12
  • 14. Relative Age5. Cross-cutting relations. Chapter 12
  • 15. Relative Age6. Inclusions – a rock fragment within another. Chapter 12
  • 16. Relative Age7. Baked contacts. Chapter 12
  • 17. Relative Age• Determining relative ages empowers geologists to unravel complicated geologic histories. Chapter 12
  • 18. Geologic History• Deposition of horizontal strata below sea level in order 1-> 8 (old to young). *Horizontality & continuity* Chapter 12
  • 19. Geologic History• Igneous intrusion of a sill. *baked contact* Chapter 12
  • 20. Geologic History• Intrusion solidified into sill• Tectonic compression Chapter 12
  • 21. Geologic History• Compression results: • Folding (inference: layers had to exist to be folded). • Uplift (above sea level) & erosion.• Intrusion of a pluton. *baked contact/cross-cutting* Chapter 12
  • 22. Geologic History• Extension -> normal faulting. • Faulting cross-cuts pluton & rock layers. Chapter 12
  • 23. Geologic History• Dike intrusion. • Dike cross-cuts everything (even normal fault). Chapter 12
  • 24. Geologic History• Erosion to present landscape. • Removed volcano and cuts down the dike top. Chapter 12
  • 25. Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram! Chapter 12
  • 26. Geologic History• Test yourself at home: Chapter 12
  • 27. Fossil Succession• Fossils (organism traces) can be preserved in sedimentary rocks. Chapter 12
  • 28. Fossil Succession• Species evolve, exist, and then go extinct. Chapter 12
  • 29. Fossil Succession• Fossil range – first to last appearance.• Permit correlation of strata. Chapter 12
  • 30. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12 Chapter 12
  • 31. Unconformity• An unconformity is • Causes: Chapter 12
  • 32. Unconformities3 Types: 1. Disconformity – Chapter 12
  • 33. Disconformities Chapter 12
  • 34. Unconformities3 Types: 2. Nonconformity – Chapter 12
  • 35. Nonconformity Chapter 12
  • 36. Unconformities3 Types: 3. Angular unconformity – represents a big gap in time Horizontal rocks deposited, then deformed Then eroded Then sediments horizontally deposited on erosion surface Chapter 12
  • 37. Types of Unconformity Types of UnconformityThis animation shows the stages in the development ofthree main types of unconformity in cross-section, andexplains how an incomplete succession of strata provides arecord of Earth history. View 1 shows a disconformity, View2 shows a nonconformity and View 3 shows an angularunconformity. For more information, see Section 12.5Unconformities: Gaps in the Record starting on p.423 andFigure 12.9 in your textbook. Chapter 12
  • 38. Unconformities• Earth history is in strata.• Missing strata = missing history Chapter 12
  • 39. Stratigraphic Correlation• In 1793, William “Strata” Smith noted strata could be matched across distances.• Similar rock types in a similar order• Rock layers contained same distinctive fossils• He made the 1st geologic map of the UK Chapter 12
  • 40. Stratigraphic Correlation• Stratigraphic columns depict strata in a region. • Drawn to portray relative thickness • Rock types depicted by fill patterns • Divided into formations (mapable rock units) • Formations separated by contacts Chapter 12
  • 41. Stratigraphic Correlation• National Parks of Arizona & Utah. • Formations can be traced long distances • Overlap in rock type sequences • Overlapping rock columns are used to build a composite Chapter 12
  • 42. The Geologic Column• A composite global stratigraphic column exists.• Constructed from incomplete sections across the globe• It brackets almost all earth history Chapter 12
  • 43. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12 Chapter 12
  • 44. Numerical (Absolute) Dating• Based on radioactive decay of atoms in minerals.• Radioactive decay proceeds at a known fixed rate• Radioactive elements act as internal clocks• Numerical dating is called geochronology Chapter 12
  • 45. Radioactive DecayIsotopesAtoms with same # of protons, different # of neutronsHave similar but different mass numbersSome are Stable – never changeSome are Unstable (radioactive) –spontaneously change to something else (decay) at a fixed rate Chapter 12
  • 46. Radioactive Decay• Decay process has 2 main components: • Parent – isotope that decays • Daughter – decay product isotope• Decay process can:• Have 1 step (parent>daughter)• Have many steps (parent>daughter>etc) • Decay product is unstable and hence also decays • Eventually proceeds to a stable endpoint Chapter 12
  • 47. Radioactive Decay Time• Half-life (t½) – time for ½ unstable parent to decay. • t½ is unique for each isotope • After one t½ -1/2 original parent remains • After three t½ -1/8 original parent remains • Parent disappears (nonlinear) daughter accumulates Chapter 12
  • 48. Radiometric Dating• Mineral age can be determined by:• Measuring parent/daughter isotope ratio• Calculating time Chapter 12
  • 49. Chapter 12
  • 50. What Is a Radiometric Date?• Time since a mineral began to retain all parent & daughter isotopes. • Requires cooling below “closure (blocking) temperature.” • Daugther retained only below closure T • Daughter leaks out above closure T • Thus, if rock is reheated above closure T, the radiometric clock can be reset to zero Chapter 12
  • 51. Other Numerical Ages• Numerical ages are possible without isotopes. • Growth rings – annual layers from trees or shells • Rhythmic layering – annual layers in seds or ice Chapter 12
  • 52. Other Numerical Ages• Magnetostratigraphy –magnetic signatures in strata are compared to global reference column Chapter 12
  • 53. Other Numerical Ages• Decay process can cause scars (tracks) in minerals. • Decay by fission (explosion) produces scar (track) • Daughter isn’t another isotope, it’s a damage zone Chapter 12
  • 54. Dating the Geologic Column• Use geochronology to:• Date specific strata OR• Bracket those that cant be dated directly Chapter 12
  • 55. The Geologic Time Scale Chapter 12
  • 56. Age of the Earth• Oldest rocks are 3.96 Ga.• Zircon minerals in some sandstones are 4.1-4.2 Ga.• Earth is ~4.57 Ga based on correlation with… • Meteorites, moon rocks. Chapter 12