Atomic Clocks Measure Geologic Time (GC-02)
Radioactive elements, such as uranium and certain isotopes of potassium, rubidium, thorium and carbon, act as atomic clocks for measuring geologic time. These elements decay spontaneously but slowly and change into daughter elements while altering their isotope ratios at a constant rate throughout geologic time. Uranium 238, for example, decays and leaves Lead 206 as the stable daughter product with a half-life of 4.5 billion years.

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