Geologic Time and Climates Notes

Module Overview
  • Topic: Geologic Time and Climates
  • Assignment Due: Learning Journal 17 by 4/2 @ 11:59 pm
Geologic Dating Methods

  • Relative Dating:

    • Sequences events and places rock units in chronological order from oldest to youngest.

  • Absolute or Numeric Dating:

    • Assigns a specific numeric age or range to rock units or events.
Principles of Stratigraphic (Relative) Dating

  1. Principle of Original Horizontality:

    • Layers of sediment are originally deposited horizontally under the action of gravity.

  2. Law of Superposition:

    • In any undisturbed sequence of rocks, the oldest layer is at the bottom.
    • Sequence: Bottom stratum = oldest, top stratum = youngest.

  3. Principle of Cross-Cutting Relationships:

    • If one geological feature cuts across another, the feature that has been cut is older.
Radioactive Decay

  • Definitions:

    • Parent Isotope: The original unstable radioactive isotope.
    • Daughter Isotope: The product of the decay of the parent isotope.
    • Half-Life: Time it takes for half of the parent isotopes to decay into daughter isotopes.

  • Each element has a unique atomic number and can have multiple isotopes. For example:

    • Potassium (K) with atomic number 19 has isotopes K-39, K-40, and K-41.
Types of Radioactive Decay

  1. Alpha Decay:

    • Loses 2 protons and 2 neutrons; atomic number decreases by 2, atomic mass decreases by 4.

  2. Beta Decay:

    • A neutron is converted to a proton, resulting in an atomic number increase by 1.
Using Radioactive Decay to Determine Age
  • The decay process is random for a single atom, but can be measured across large sample sizes to determine the age of rocks.
    • As time passes, the amount of parent isotopes decreases while daughter isotopes increase.
Practical Application: Half-Life Calculations
  • Example Calculation:
    • If starting with 400,000 parent isotopes, after 3 half-lives:
      • 1st half-life: 200,000 parent
      • 2nd half-life: 100,000 parent
      • 3rd half-life: 50,000 parent
Commonly Used Isotopes
  • Half-lives for different isotopes, important in dating rocks:
    1. Uranium-238 to Lead-206: 4.5 billion years
    2. Uranium-235 to Lead-207: 704 million years
    3. Potassium-40 to Argon-40: 1.25 billion years
    4. Carbon-14 to Nitrogen-14: 5730 years
Determining the Age of the Earth
  • Earth formed approximately 4.5 billion years ago supported by:
    1. Dating of continental rocks and meteorite samples.
    2. Studies of solar mass and luminosity in comparison to other stars.
Geologic Time Scale
  • Divisions based on major geological and biological events:
    • Eon: Hadean, Archean, Proterozoic, Phanerozoic.
    • Era: Paleozoic, Mesozoic, Cenozoic.
Practice Question on Radioactive Isotopes
  • Problem: Rock with isotope X decays to stable isotope Y; half-life = 400 million years.
    • If 7 times more Y than X is present, how old is the rock?
    • Choices: A. 2800 million years, B. 1200 million years, C. 800 million years, D. 57 million years
    • Answer Calculation: Each half-life results in the ratio changing, with additional calculations requiring attention to the decay process.
Conclusion
  • Understanding geologic dating methods is crucial for placing geological events and formations within a historical framework, assisting in the broader understanding of Earth's history and processes.