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Geologic Time                Chapter 12
Outline• Geologic time: perspective & a bit of history• Dating geologic materials   -General: relative & absolute dating  ...
Geologic Time                Chapter 12
Geologic Time•   Understanding time permits assigning ages to…                                                    Chapter 12
Geologic Time•   Prior to late 1600s, geologic time was thought to =    historical time.     • Archbishop James Ussher, Ir...
Geologic Time•   Scientists began to find clues to an ancient Earth.    •   Nicolaus Steno (1638–1686) – Danish physician....
Outline• Geologic time: perspective & a bit of history• Dating geologic materials   -General: relative & absolute dating  ...
Geologic Time2 ways to date geological materials:   1. Relative age –   2. Numerical (absolute) age –                     ...
Relative vs. Absolute                        Chapter 12
Relative Age•   Logical tools are useful for defining relative age.•   Principles of:     1. Uniformitarianism     2. Supe...
Geologic Time1. Uniformitarianism – “The present is key to the past”.   •                                                 ...
Defining Relative Age2. Superposition.                                     Chapter 12
Relative Age3 & 4. Horizontality and continuity.                                       Chapter 12
Relative Age5. Cross-cutting relations.                                    Chapter 12
Relative Age6. Inclusions – a rock fragment within another.                                                  Chapter 12
Relative Age7. Baked contacts.                                    Chapter 12
Relative Age•   Determining relative ages empowers geologists to    unravel complicated geologic histories.               ...
Geologic History•   Deposition of horizontal strata below sea level in order    1-> 8 (old to young). *Horizontality & con...
Geologic History•   Igneous intrusion of a sill. *baked contact*                                                   Chapter...
Geologic History•   Intrusion solidified into sill•   Tectonic compression                                     Chapter 12
Geologic History•   Compression results:    •   Folding (inference: layers had to exist to be folded).    •   Uplift (abov...
Geologic History•   Extension -> normal faulting.     • Faulting cross-cuts pluton & rock layers.                         ...
Geologic History•   Dike intrusion.    • Dike cross-cuts everything (even normal fault).                                  ...
Geologic History•   Erosion to present landscape.     • Removed volcano and cuts down the dike top.                       ...
Geologic History•   Relative ages help to unravel a complicated history.•   Those rules permit one to decipher this diagra...
Geologic History•   Test yourself at home:                                        Chapter 12
Fossil Succession•   Fossils (organism traces) can be preserved in    sedimentary rocks.                                  ...
Fossil Succession•   Species evolve, exist, and then go extinct.                                                  Chapter 12
Fossil Succession•   Fossil range – first to last appearance.•   Permit correlation of strata.                            ...
Outline• Geologic time: perspective & a bit of history• Dating geologic materials   -General: relative & absolute dating  ...
Unconformity•   An unconformity is     • Causes:                                   Chapter 12
Unconformities3 Types:  1. Disconformity –                                  Chapter 12
Disconformities                  Chapter 12
Unconformities3 Types:  2. Nonconformity –                                  Chapter 12
Nonconformity                Chapter 12
Unconformities3 Types:  3. Angular unconformity – represents a big gap in time  Horizontal rocks deposited, then deformed ...
Types of Unconformity            Types of UnconformityThis animation shows the stages in the development ofthree main type...
Unconformities•   Earth history is in    strata.•   Missing strata =    missing history                                   ...
Stratigraphic Correlation•   In 1793, William “Strata” Smith noted strata could be    matched across distances.•   Similar...
Stratigraphic Correlation•   Stratigraphic columns depict strata in a region.     • Drawn to portray relative thickness   ...
Stratigraphic Correlation•   National Parks of Arizona & Utah.    •   Formations can be traced long distances    •   Overl...
The Geologic Column•   A composite global stratigraphic column exists.•   Constructed from incomplete sections across the ...
Outline• Geologic time: perspective & a bit of history• Dating geologic materials   -General: relative & absolute dating  ...
Numerical (Absolute) Dating•   Based on radioactive decay of atoms in minerals.•   Radioactive decay proceeds at a known f...
Radioactive DecayIsotopesAtoms with same # of protons, different # of neutronsHave similar but different mass numbersSome ...
Radioactive Decay•   Decay process has 2 main components:    •   Parent – isotope that decays    •   Daughter – decay prod...
Radioactive Decay Time•   Half-life (t½) – time for ½ unstable parent to decay.    • t½ is unique for each isotope    • Af...
Radiometric Dating•   Mineral age can be determined by:•   Measuring parent/daughter isotope ratio•   Calculating time    ...
Chapter 12
What Is a Radiometric Date?•   Time since a mineral began to retain all parent &    daughter isotopes.    •   Requires coo...
Other Numerical Ages•   Numerical ages are possible without isotopes.    • Growth rings – annual layers from trees or shel...
Other Numerical Ages•   Magnetostratigraphy –magnetic signatures in strata are    compared to global reference column     ...
Other Numerical Ages•   Decay process can cause scars (tracks) in minerals.    • Decay by fission (explosion) produces sca...
Dating the Geologic Column•   Use geochronology to:•   Date specific strata OR•   Bracket those that cant be dated directl...
The Geologic Time Scale                     Chapter 12
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 b...
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13 geologictime forstudents

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Transcript of "13 geologictime forstudents"

  1. 1. Geologic Time Chapter 12
  2. 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. 3. Geologic Time Chapter 12
  4. 4. Geologic Time• Understanding time permits assigning ages to… Chapter 12
  5. 5. Geologic Time• Prior to late 1600s, geologic time was thought to = historical time. • Archbishop James Ussher, Ireland, 1654. Chapter 12
  6. 6. Geologic Time• Scientists began to find clues to an ancient Earth. • Nicolaus Steno (1638–1686) – Danish physician. fossil shark tooth Chapter 12
  7. 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. 8. Geologic Time2 ways to date geological materials: 1. Relative age – 2. Numerical (absolute) age – Chapter 12
  9. 9. Relative vs. Absolute Chapter 12
  10. 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. 11. Geologic Time1. Uniformitarianism – “The present is key to the past”. • Chapter 12
  12. 12. Defining Relative Age2. Superposition. Chapter 12
  13. 13. Relative Age3 & 4. Horizontality and continuity. Chapter 12
  14. 14. Relative Age5. Cross-cutting relations. Chapter 12
  15. 15. Relative Age6. Inclusions – a rock fragment within another. Chapter 12
  16. 16. Relative Age7. Baked contacts. Chapter 12
  17. 17. Relative Age• Determining relative ages empowers geologists to unravel complicated geologic histories. Chapter 12
  18. 18. Geologic History• Deposition of horizontal strata below sea level in order 1-> 8 (old to young). *Horizontality & continuity* Chapter 12
  19. 19. Geologic History• Igneous intrusion of a sill. *baked contact* Chapter 12
  20. 20. Geologic History• Intrusion solidified into sill• Tectonic compression Chapter 12
  21. 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. 22. Geologic History• Extension -> normal faulting. • Faulting cross-cuts pluton & rock layers. Chapter 12
  23. 23. Geologic History• Dike intrusion. • Dike cross-cuts everything (even normal fault). Chapter 12
  24. 24. Geologic History• Erosion to present landscape. • Removed volcano and cuts down the dike top. Chapter 12
  25. 25. Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram! Chapter 12
  26. 26. Geologic History• Test yourself at home: Chapter 12
  27. 27. Fossil Succession• Fossils (organism traces) can be preserved in sedimentary rocks. Chapter 12
  28. 28. Fossil Succession• Species evolve, exist, and then go extinct. Chapter 12
  29. 29. Fossil Succession• Fossil range – first to last appearance.• Permit correlation of strata. Chapter 12
  30. 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. 31. Unconformity• An unconformity is • Causes: Chapter 12
  32. 32. Unconformities3 Types: 1. Disconformity – Chapter 12
  33. 33. Disconformities Chapter 12
  34. 34. Unconformities3 Types: 2. Nonconformity – Chapter 12
  35. 35. Nonconformity Chapter 12
  36. 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. 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. 38. Unconformities• Earth history is in strata.• Missing strata = missing history Chapter 12
  39. 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. 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. 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. 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. 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. 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. 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. 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. 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. 48. Radiometric Dating• Mineral age can be determined by:• Measuring parent/daughter isotope ratio• Calculating time Chapter 12
  49. 49. Chapter 12
  50. 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. 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. 52. Other Numerical Ages• Magnetostratigraphy –magnetic signatures in strata are compared to global reference column Chapter 12
  53. 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. 54. Dating the Geologic Column• Use geochronology to:• Date specific strata OR• Bracket those that cant be dated directly Chapter 12
  55. 55. The Geologic Time Scale Chapter 12
  56. 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
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