Isotope preferred dating very old rocks
Soon after radioactivity was discovered, experimenters published some trial dates of rocks.
Realizing that the decay of uranium produces helium, Ernest Rutherford in 1905 determined an age for a piece of uranium ore by measuring the amount of helium trapped in it.
That only worked for sedimentary rocks and only some of those.
The principle of using radioactive decay as a dating method is simple.In 1896, Henri Becquerel's accidental discovery of radioactivity showed what might be possible.We learned that some elements undergo radioactive decay, spontaneously changing to another type of atom while giving off a burst of energy and particles.The geologic equivalent of lighting the grill is the time at which a mineral grain solidified, whether that is long ago in an ancient granite or just today in a fresh lava flow.The solid mineral grain traps the radioactive atoms and their decay products, helping to ensure accurate results.
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No one knew even how much of Earth history was unknown!