Themes of the Day:

• Even More Cross Section and Block Diagram Relative Age Interpretation
• Absolute Age Dating
• Geologic Time

Interpreting Relative Ages in Geologic Cross-Sections

• Minor Canyon: Minor Canyon Today - Time Sequence: 1 2 3 4 5 6 7 8 9 10 11
• Cross Section #1: Figure 18.4
• Cross Section #2: Home on the Range
• Cross Section #3: Grand Canyon
• Cross Section #4: Chapter Review (Colorado Plateau region)

Absolute Age Dating

• Measures Quantifiable Spans of Time
• Calendars, Tree Rings, Ice Cores, Varved Sediments
• Radiometric Dating
  • radioactive isotope (parent) undergoes one or more nuclear decays to form stable (daughter) isotope
  • Decay rate is exponential - not linear
  • Half-life - time that it takes for half of the parent to decay to daughter
  • Visualize exponential decay
  • Accurate dating requires:
    • material that forms with parent isotope, but no daughter isotope
    • closed system - no gain or loss of parent or daughter
    • known decay rate
    • accurate and precise measurements of parent and daughter isotopes
  • Examples:
    • radiocarbon dating - ¹⁴C --> ¹⁴N - Half-life of 5730 years - useful for archeology
    • Potassium-Argon dating - ⁴⁰K --> ⁴⁰Ar - argon is a noble gas, does not bond to mineral structures
    • Rubidium-Strontium dating - ⁸⁷Rb --> ⁸⁷Sr - useful in many geologic contexts
    • Uranium-Lead - ²³⁸U --> ²⁰⁶Pb and ²³⁵U --> ²⁰⁷Pb - twice as nice, two independent clocks in the same mineral (zircon)
  • Read more about using the U/Pb decay system in zircons to date very old geologic events.

Geologic Time

• Correlation - linking distant outcrops - Try it yourself.
• The use of Fossils - index fossils and fossil assemblages
• The use of Fossils and Correlation
• The Geologic Time Scale