Atomic Clocks Measure Geologic Time (GC-02)
By measuring the present ratios of radioactive elements with sensitive instruments and knowing the rate of change (half-life), geologists can determine the time since a given rock sample was formed. Rocks that formed more than 3.8 billion years ago have been found.
The oldest rocks identified so far in Indiana are granites 1.3 to 1.5 billion years old from deep wells in Fulton and Porter Counties. Similar rocks from wells in southeastern Indiana are 1.0 to 1.2 billion years old.
Our Hoosier State Beneath Us: Geochemistry/Geophysics