Clocks in the rocks radioactive dating method
Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.
For that we can thank radioactivity, discovered at the turn of the last century.
This technique is generally used to date igneous and metamorphic rock, which are rocks that were once melted due to extreme heat and pressure.
Radiometric dating determines how long ago the liquid rock solidified into solid rock.
For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy.
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.
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.