Relative age dating and radiometric dating
From his research, our evolutionary geologist may have discovered that other geologists believe that Sedimentary Rocks A are 200 million years old and Sedimentary Rocks B are 30 million years old.Thus, he already ‘knows’ that the igneous dyke must be younger than 200 million years and older than 30 million years.No matter what the radiometric date turned out to be, our geologist would always be able to ‘interpret’ it.He would simply change his assumptions about the history of the rock to explain the result in a plausible way. Wasserburg, who received the 1986 Crafoord Prize in Geosciences, said, ‘There are no bad chronometers, only bad interpretations of them!’ In fact, there is a whole range of standard explanations that geologists use to ‘interpret’ radiometric dating results.Someone may ask, ‘Why do geologists still use radiometric dating?
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.In other words, the age should lie between 197.2 million years and 203.6 million years.
However, this error is not the real error on the date.Would he have concluded that the fossil date for the sediments was wrong? Would he have thought that the radiometric dating method was flawed? Instead of questioning the method, he would say that the radiometric date was not recording the time that the rock solidified.He may suggest that the rock contained crystals (called xenocrysts) that formed long before the rock solidified and that these crystals gave an older date.It relates only to the accuracy of the measuring equipment in the laboratory.