Geological Dating Methods and Principles

Sep 7, 2024

Geology Lecture Notes: Dating Rocks and Geological Principles

Overview of Earth's Geological Timeline

  • Earth's history spans 4.5 billion years, from the Hadean Eon to the Phanerozoic Eon.
  • Geologists determine Earth's age and rock formations using two main methods:
    • Relative Dating
    • Absolute Dating (Radiometric Dating)

Methods of Dating Rocks

Relative Dating

  • Compares the age of formations based on stratigraphic sequence.
  • Used when radiometric dating is not applicable, especially for sedimentary rocks.

Absolute Dating

  • Also known as radiometric dating.
  • Utilizes decay of radioisotopes to calculate precise ages.
  • Developed in the early 1900s.

Principles of Relative Dating

  1. Principle of Original Horizontality

    • Sediments accumulate in horizontal layers (beds).
    • Crossbedding occurs when sediments are deposited at an angle, typical of sand dunes.
    • Indicates time gaps when horizontal and vertical layers are observed.
  2. Principle of Superposition

    • Older beds are typically at the bottom, younger at the top, unless disturbed by tectonic activity.
  3. Principle of Cross-Cutting Relationships

    • Features that cut through rocks (e.g., igneous intrusions) are younger than the rocks they disrupt.
  4. Principle of Inclusions

    • Fragments within a rock (e.g., schist in sandstone) are older than the rock containing them.
  5. Principle of Faunal Succession

    • Fossils follow a historical sequence that helps date rocks.
    • Index fossils are particularly useful for dating specific periods.

Unconformities: Gaps in the Rock Record

  • Represent missing time in geological layers.
  • Can be caused by various geological processes like erosion and non-deposition.

Types of Unconformities

  1. Disconformity

    • Erosional boundary between sedimentary layers.
  2. Paraconformity

    • A boundary marking non-deposition periods, not erosional.
  3. Nonconformity

    • Boundary between older igneous/metamorphic and younger sedimentary rocks.
  4. Angular Unconformity

    • Boundaries between tilted sedimentary layers and horizontal strata above.

Challenges and Advancements

  • Relative dating has limitations but was the primary method before radiometric dating.
  • Radiometric dating provides more precise dating and is a significant advancement in understanding geological timeframes.