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Chapter 8 Time and Geology

GEOL 101 Introductory Geology. Chapter 8 Time and Geology. Determining Geological Ages. Relative age dates – placing rocks and events in their proper sequence of formation Numerical (absolute age) dates – specifying the actual number of years that have passed since an event occurred.

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Chapter 8 Time and Geology

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  1. GEOL 101 Introductory Geology Chapter 8Time and Geology

  2. Determining Geological Ages • Relative age dates – placing rocks and events in their proper sequence of formation • Numerical (absolute age) dates – specifying the actual number of years that have passed since an event occurred

  3. Principles of Relative Dating • Law of superposition • Undeformed section of sedimentary or layered igneous rocks • Oldest rocks are on the bottom • Principle of original horizontality • Layers of sediment are generally deposited in a horizontal position • Rock layers that are flat have not been disturbed (deformed) • Principle of cross-cutting relationships • Younger features cut across older features

  4. Superposition Strata in the Grand Canyon

  5. Horizontality

  6. Cross-cutting Relationship

  7. Cross-cutting Relationship Which crater is youngest?

  8. Cross-cutting Relationships

  9. Principles of Relative Dating • Inclusions • A piece of rock that is enclosed within another rock • Rock containing the inclusion is younger • Unconformity • Break in rock record produced by erosion and/or nondeposition of rock • Represents period of geologic time

  10. Principles of Relative Dating • Types of unconformities • Angular unconformity – tilted rocks (disturbed) are overlain by flat-lying rocks • Disconformity – strata on either side of the unconformity are parallel • Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

  11. Angular Unconformity

  12. Angular Unconformity

  13. Angular Unconformity Animation

  14. Disconformity

  15. Disconformity

  16. Nonconformity

  17. Nonconformity

  18. Nonconformity Inclusions

  19. Grand Canyon Unconformities

  20. Animation Interpret Geologic History

  21. Correlation of Rock Layers • Matching of rocks of similar ages in different regions is known as correlation • Correlation often relies upon fossils • similar sedimentary strata in widely separated areas • identified and correlated by distinctive fossil content • Principle of fossil succession • fossil organisms succeed one another in a definite and determinable order • time period recognized by its fossil content

  22. Correlation of Rock Layers

  23. Fossils Ages of Rocks

  24. Radiometric Dating • Radioactivity: spontaneous changes (decay) in the structure of atomic nuclei • Basic atomic structure • Types of radioactive decay • Parent, daughter product, half-life • Principle of radioactive dating • Sources of error • Carbon-14 (radiocarbon) dating • Importance of radiometric dating

  25. Basic Atomic Structure • Nucleus • Protons: positively charged particles with mass • Neutrons: neutral particles with mass • Electrons: negatively charged particles, orbit nucleus • Atomic number • An element’s identifying number • Number of protons in the atom’s nucleus • Mass number • Sum of protons & neutrons in atom’s nucleus • Isotope • Variant of the same parent atom • Differs in the number of neutrons • Different mass number than the parent atom

  26. Radiometric Dating • Types of radioactive decay • Alpha emission • Emission of 2 protons & 2 neutrons (alpha particle) • Mass number is reduced by 4 and the atomic number is lowered by 2 • Beta emission • Electron (beta particle) is ejected from the nucleus • Mass number remains unchanged and the atomic number increases by 1 • Electron capture • Electron is captured by the nucleus • Electron combines with a proton to form a neutron • Mass number remains unchanged and the atomic number decreases by 1

  27. Radiometric Decay Alpha Emission Beta Emission Electron Capture

  28. Radiometric Decay • Parent – an unstable radioactive isotope • Daughter product – the isotopes resulting from the decay of a parent • Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

  29. Uranium Isotopes Parent U238 Atomic # - 92 Mass # - 238 Atomic # - 82 Mass # - 206 Daughter PB206

  30. Radiometric Dating • Principle of radioactive dating • The percentage of radioactive toms that decay during one half-life is always the same (50 percent) • However, the actual number of atoms that decay continually decreases • Comparing the ratio of parent to daughter yields the age of the sample

  31. Radioactive Decay Curve

  32. Radioactive Decay Curve

  33. Radiometric Dating

  34. Radiometric Dating • Sources of error • A closed system is required • To avoid potential problems, only fresh, unweathered rock samples should be used • Carbon-14 (radiocarbon) dating • Half-life of only 5730 years • Used to date very recent events • C14 is produced in the upper atmosphere

  35. C14 Production and Decay Neutron Capture Beta Emission

  36. Radiometric Dating • Importance of radiometric dating • Radiometric dating is a complex procedure that requires precise measurement • Rocks from several localities have been dated at more than 3 billion years • Confirms the idea that geologic time is immense

  37. Other Dating Methods Shell Growth Rings Tree Rings Rhythmic Layering in Glaciers

  38. Cross-Dating Principle in Dendrochonology Youngest Older Oldest

  39. Geologic Time Scale • Geologic time scale – Earth history calendar • Subdivides geologic history into units • Originally created using relative dates • Structure of the geologic time scale • Eon – the greatest expanse of time • Phanerozoic – “visible life”, most recent eon • Proterozoic– before life • Archean– ancient • Hadean – mythical subterraneon world of departed spirits, oldest eon • Precambrian: Proterozoic, Archean, Hadean

  40. Geologic Time Scale • Era – subdivision of an eon • Precambrian – no era subdivisions • Eras of the Phanerozoic eon • Cenozoic (“recent life”) • Mesozoic (“middle life”) • Paleozoic (“ancient life”) • Eras are subdivided into periods • Periods are subdivided into epochs

  41. Geologic Time Divisions

  42. Geologic Time Scale

  43. Geologic Time Scale • Precambrian time • Nearly 4 billion years prior to the Cambrian period • Not divided into smaller time units because the events of Precambrian history are not know in great enough detail • First abundant fossil evidence does not appear until the beginning of the Cambrian • Visualizing vast period of geologic time

  44. Evolution vs Geologic Time

  45. Visualizing Geologic Time Twice around the equator

  46. Geologic Time Scale Difficulties • Not all rocks can be dated by radiometric methods • Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed • The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed • Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages

  47. Sedimentary Ages using Radiometric Dating

  48. Stratigraphic Dating

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