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.