Understanding Geologic Dating and History

Oct 15, 2024

Geologic History Lecture Notes

Introduction

  • Discussed dating of rock layers to learn about Earth's history.
  • Two main types of dating:
    • Absolute Dating
      • Involves methods like radioactive dating, carbon dating, and radioactive decay.
      • Provides numeric age (e.g., trilobite fossil is 489 million years old).
    • Relative Dating
      • Determines sequence of events (e.g., trilobite fossil is older than dinosaur tooth fossil).
      • No numeric values involved.

Absolute vs. Relative Dating

  • Absolute dating gives specific numbers.
  • Relative dating involves comparisons without specific values.
  • Examples with cars:
    • Absolute: "Red car costs $90,000, silver car costs $15,000."
    • Relative: "Red car is more expensive than the silver car."

Studying Rock Layers

  • Absolute Dating: Uses radioactive decay.
  • Relative Dating: Involves geologic sequencing.

Law of Uniformitarianism

  • Principle that current geological processes operated in the past similarly.
  • Allows interpretation of ancient rock layers.

Tools for Studying Geologic History

  • Road Cuts: Expose layers of rock, revealing historical sequences.
  • Geologic Cross-Sections: Used to analyze rock layers.

Rock Layer Identification

  • Use symbols (map symbols) from New York State Earth Science reference tables.
  • Different sedimentary and metamorphic rocks have unique symbols.

Sequence of Events

  • Determine what geological events happened first, second, etc.
  • Analyze diagrams to interpret geologic history.

Important Rules in Geological Study

Rule 1: Formation of Sedimentary Rocks

  • Form underwater from deposited sediments.
  • Example: Sand, silt, and clay settle, compress over time into sedimentary rock.

Rule 2: Weathering and Erosion

  • Occur above water.
  • Indicate a period of uplift if rock layers show signs of erosion.

Rule 3: Law of Superposition

  • Rock layers at the bottom are oldest, top layers are younger.
  • Exception is overturning (layers flipped upside down).

Rule 4: Original Horizontality

  • Sedimentary layers originally deposited horizontally.
  • Deformation (folding, faulting, tilting) disrupts horizontal layers.

Rule 5: Igneous Intrusions

  • Intrusions younger than rocks they affect/metamorphose.

Rule 6: Faults

  • Faults are younger than rocks they cut through.

Rule 7: Unconformities

  • Represent missing history due to uplift, erosion, and subsidence.
  • Analogous to missing pages in a book.

Rule 8: Law of Inclusions

  • Inclusions (fragments within a rock) are older than the rock itself.

Conclusion

  • Follow rules of relative dating to interpret sequences.
  • Aim to understand geologic history despite gaps or complexities.

These notes summarize the key concepts and examples discussed in the lecture about geologic dating and the interpretation of rock layers to understand Earth's history.

Full transcript