it matches what they already believe on other grounds. A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.
The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.
It is clear that the sedimentary rock was deposited and folded before the dyke was squeezed into place.
By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.
From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
Many people think that radiometric dating has proved the Earth is millions of years old.
That’s understandable, given the image that surrounds the method. 200.4 ± 3.2 million years) gives the impression that the method is precise and reliable (box below).
However, although we can measure many things about a rock, we cannot directly measure its age.
For example, we can measure its mass, its volume, its colour, the minerals in it, their size and the way they are arranged.
We can crush the rock and measure its chemical composition and the radioactive elements it contains.
But we do not have an instrument that directly measures age.