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The work of geologists is to tell the true story of Earth's history—more precisely, a story of Earth's history that is ever more true.
A hundred years ago, we had little idea of the story's length—we had no good yardstick for time.
No one knew even how much of Earth history was unknown!
We needed a more precise tool, some sort of clock, to begin to measure it.
A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. Judging from the amount of rocks there are, plus the imperceptible rates of the processes forming them—erosion, burial, fossilization, uplift—the geologic record must represent untold millions of years of time.
It is that insight, first expressed in 1785, that made James Hutton the father of geology.
Counting a few million atoms (easy with just micrograms of rock) yields dates that are quite good.
Thus the older a sample gets, the smaller the percentage of potassium-40, and conversely the greater the percentage of argon-40 relative to argon-36 and argon-38.Consider this analogy: a barbecue grill full of burning charcoal.The charcoal burns at a known rate, and if you measure how much charcoal is left and how much ash has formed, you can tell how long ago the grill was lit.Even so, it can take weeks of patient counting to get accurate results, especially in old samples in which very few radiocarbon atoms remain.This method is still in use for scarce, highly radioactive isotopes like carbon-14 and tritium (hydrogen-3).