Methods for Determining the Age of Rocks

Introduction

• Focus on methods to determine rock ages: relative and absolute dating.
• Understanding how these methods assist geologists in delineating geological time subdivisions.

Learning Competencies

1. Describe different methods for determining the age of stratified rocks.
2. Explain relative and absolute dating's roles in determining geological time subdivisions.

Historical Context

• Inquiry into how events thousands to billions of years ago were recorded.
• Technologies not available; humans non-existent at those times.

Methods of Dating Rocks

Relative Dating

• Definition: Determines the age of rocks by identifying their sequence relative to each other.
• Example: "The trilobite fossil is older than the dinosaur tooth fossil."

Absolute Dating

• Definition: Determines the numerical age of rocks through radioactive decay.
• Example: "The trilobite fossil is 489 million years old."

Key Differences

• Relative: Order of events (first, second, third).
• Absolute: Specific age determined through decay rates.

Principles of Relative Dating

1. Law of Superposition:
   • Younger layers on top of older layers in sedimentary rock.
2. Law of Cross-Cutting Relationships:
   • An igneous intrusion is younger than the rock it cuts through.
3. Law of Inclusion:
   • Inclusions found in sedimentary rocks are older than the rocks themselves.
4. Law of Original Horizontality:
   • Sediment layers are originally deposited horizontally.
   • Deformation indicates geological activity.
5. Fossil Succession:
   • Fossils grouped by geological periods provide timing for rock strata.

Principles of Absolute Dating

• Radiometric Dating:
  • Uses radioactive isotopes to determine ages; common isotopes include Carbon-14 and Uranium-238.
  • Half-life: Time taken for half of a radioactive substance to decay.
  • Example: Uranium-238 to Lead-206 takes 4.5 billion years.

Specific Isotope Uses

• Carbon-14: Used for dating biological remains (half-life: 5730 years).
• Potassium-40: Half-life of 1.251 billion years; used for older rocks.
• Uranium-235: Half-life of 700 million years; also used for dating older rocks.

Geologic Time Subdivision

• Both relative and absolute dating used for accurate age assessment.
• Luminescence Dating: Measures energy stored in rocks for dating.
• Early geologists noted similarities in sedimentary layers worldwide, leading to relative dating applications.
• Fossil evidence helped establish Earth's history segments (e.g., Jurassic period).

Summary

• Relative dating provides order; absolute dating provides specific years.
• Together, they create a comprehensive picture of Earth's geological history and its evolution over time.