Earth Parts Lecture by Dr. Johnson

Introduction

• Topic: Earth Parts
• Focus on Coastal Redwoods and Giant Sequoia Trees
• Largest trees on Earth, but not the oldest

Dendrochronology

• Study of tree rings to date events
• Trees lay down thinner rings during harsh years, thicker during optimal times
• Crossdating: Aligning growth rings from different trees to extend climatic records
• Can date back approximately 14,000 years

Ice Cores

• Extracted from Antarctica and Greenland
• Layers similar to tree rings; provide year-by-year records
• Contain bubbles of ancient atmosphere, pollen, ash from volcanic eruptions
• Ice cores can date back about 800,000 years

Geological Time and Age of Earth

• Early geologists like James Hutton and William Smith realized the Earth was millions of years old
• Charles Lyell popularized geology with "Principles of Geology"
• Relative dating provides sequence but not absolute dates

Estimates of Earth's Age in the 19th Century

• Various methods attempted to estimate Earth's age
• William Thompson (Lord Kelvin) estimated Earth’s age using thermal physics
• Estimates ranged from millions to billions of years

Discovery of Radioactivity

• Radioactive decay: Atoms decay into another element over time
• Radioactive isotopes decay at a predictable rate (half-life)

Radiometric Dating

• Arthur Holmes used radioactive decay to estimate Earth’s age
• Established the science of geochronology for dating rocks

Demonstration of Radioactive Decay (Penny Example)

• 100 pennies represent radioactive atoms
• Over time, about half decay (tails up)
• Demonstrates statistical nature of radioactive decay
• Plot of decay shows curve of declining parent isotopes

Conclusion

• Geochronology allows for dating of rocks by measuring parent-daughter isotope ratios
• Radioactive decay provides a reliable method to assess the age of rocks and Earth's history