This work gave rise to a process known as radiometric dating.
Over the decades of the 20th century, scientists documented tens of thousands of radiometric age measurements.
(Creationists do not agree with these ages of millions of years because of the assumptions they are based on.) Because of his interest in the volcanic dyke, he collects a sample, being careful to select rock that looks fresh and unaltered.
On his return, he sends his sample to the laboratory for dating, and after a few weeks receives the lab report.
Let us imagine that the date reported by the lab was 150.7 ± 2.8 million years.
Our geologist would be very happy with this result.
Likewise, scientists use radiometric dating to determine the ages of moon rocks, obtained by astronauts.