How does radiometric dating work
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.
In other words there was originally 4 parts per million Parentium-123 and 0 parts per million Daughterium-123.
Since there is now only 1/4 of the original amount of Parentium-123, we know that two half-lives of Parentium-123 have elapsed.
Imagine we have an undiscovered element, Parentium, that has a radioactive isotope, Parentium-123, which decays to stable Daughterium-123.
This is the only way Parentium-123 decays, and there is no other source of Daughterium-123.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.
These were then eroded and Sedimentary Rocks B were deposited.
We could be sure that a mineral containing parentium originally had no daughterium.