Lecture Notes: Age of the Earth

Introduction

• Earth is approximately 4.54 billion years old.
• Formed almost 10 billion years after the Big Bang.
• Human understanding of Earth's age resulted from persistence and scientific inquiry.

Historical Theories

Aristotle's Theory

• Greek philosopher Aristotle believed Earth was eternal.

Theological Estimates

• 16th-17th centuries: Biblical references suggested Earth was 6,000 years old.
  • James Ussher (17th-century Archbishop) claimed Earth was formed on October 22, 4004 BC.
  • Based on genealogical tables in Genesis.

Geological and Scientific Approaches

Early Geologists

• Nicholas Steno: Proposed superposition principle (older layers are deeper).
• William Smith: Added that layers with identical fossils are likely the same age.
• John Phillips: Estimated Earth's age at 100 million years based on Steno and Smith's work.
• Mikhail Lomonosov: Suggested Earth created separately from the universe.

Experimental Estimates

• George Louis Leclerc (1779): Estimated Earth was 75,000 years old using cooling experiments.

Advances in Geology

Uniformitarianism

• Charles Lyell & James Hutton: Proposed ongoing changes in Earth's geology.
  • Rocks continuously form, erode, and redeposit.
  • Past geological processes key to understanding current ones.

19th-century Calculations

• William Thompson (1862): Estimated 20-40 million years based on cooling rates.
  • Did not consider radioactive decay or mantle convection.
• John Jolley: Estimated ocean age at 80-100 million years based on salt accumulation.

Radioactivity and Modern Understanding

Discovery of Radioactivity

• Early 1900s: Radioactivity discovered; changed understanding of Earth's cooling.
  • Radioactive isotopes decay at a predictable rate (radioactive decay).

Development of Radioactive Dating

• Ernest Rutherford, Frederick Soddy, Bertram B. Boltwood: Key figures in early radioactive dating.
• Arthur Holmes: Used radioactive lead dating; estimated Earth at over 1.6 billion years.

Establishing Current Age

• Clair Cameron Patterson (1956): Determined age of 4.55 billion years using uranium lead isotope dating on meteorites.

Importance of Meteorites

• Meteorites offer pure and unaltered samples for dating.
• Earth and solar system formed simultaneously; dating helps determine Earth's age.

Conclusion

• Current accepted age: 4.54 billion years.
• Scientific understanding of Earth's age continues to evolve.
• Future research will refine age estimates and pose new questions.

[Music] [End of Lecture]