Radioactive Dating and Earth's History

Radioactive Decay

  • Radioactive elements are everywhere: rocks, plants, animals, and even inside ourselves.
  • These elements help determine the absolute age of rocks and fossils.
  • Key question: What can radioactive dating reveal about Earth's history?

Key Vocabulary

  • Atom
  • Carbon-14 dating
  • Daughter element
  • Element
  • Half-life
  • Parent element
  • Potassium-argon dating
  • Radioactive decay

Relative Age vs. Absolute Age

  • Relative age: Age compared to something else (older than, younger than).
  • Absolute age: Number of years since something came into being (actual age).

Determining Absolute Age

  • Geologists use special tests to find the absolute age of rocks and minerals.
  • All matter, including rock, is made of atoms.
  • If all atoms in a matter are the same type, it's called an element.

Radioactive Decay Explained

  • Some elements are unstable and decay by releasing particles and energy.
  • This process is called radioactive decay.
  • Elements that release particles over time are radioactive elements.
  • Geologists use the decay rate to calculate a rock's age.

Half-Life

  • An unstable element loses particles at a constant rate (half-life).
  • After each half-life, half of the radioactive material remains.
  • Example: If it takes 2,000 years for half of an element to decay, and a sample has only 1/4 of the original element, it's 4,000 years old.

Radioactive Dating Methods

  • Radioactive dating is like an hourglass; elements are trapped in igneous rock.
  • Unstable elements (parent elements) decay at a constant rate (half-life) into stable elements (daughter elements).
  • Geologists compare the amount of parent and daughter elements to determine a rock's age.

Potassium-Argon Dating

  • Potassium (K) decays into Argon (Ar).
  • The half-life of potassium-40 is 1.3 billion years.
  • Useful for dating ancient volcanic rocks.
  • Works only for igneous rock.
  • To figure out the age of a sample, you multiply the number of half life cycles by the length of the half life\text{age} = \text{# of half lives} \cdot \text{length of half life}

Carbon-14 Dating

  • All plants and animals contain carbon-12 and carbon-14.
  • When an organism dies, carbon-14 decays into stable nitrogen-14.
  • Scientists compare carbon-14 and nitrogen-14 to determine age.
  • The half-life of carbon-14 is 5,730 years.
  • Useful for dating fossils up to 50,000 years old.

Radiometric Dating Formula

  • Age = n \times t_{\frac{1}{2}}
  • Where:
    • n = number of half-lives elapsed
    • t_{\frac{1}{2}}= half-life of the radioactive element

Sample Calculation Use Case example

  • Meteorite with potassium-40 decaying from 20 grams to 5 grams.
  • Half-life of potassium-40 is 1.3 billion years.
  • Two half lives have elapsed to get to 5 grams as follows:
    • Half life 1 = 10 grams
    • Half life 2 = 5 grams
  • 2 \times 1,300,000,000 = 2,600,000,000
  • The meteorite is 2.6 billion years old.