The way it really is: little-known facts about radiometric dating

Long-age geologists will not accept a radiometric date unless it matches their pre-existing expectations.

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. Even the way dates are reported (e.g. 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.

Before we can calculate the age of a rock from its measured chemical composition, we must assume what radioactive elements were in the rock when it formed.1 And then, depending on the assumptions we make, we can obtain any date we like.

It may be surprising to learn that evolutionary geologists themselves will not accept a radiometric date unless they think it is correct—i.e. it matches what they already believe on other grounds. It is one thing to calculate a date. It is another thing to understand what it means.

So, how do geologists know how to interpret their radiometric dates and what the ‘correct’ date should be?

Field relationships

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….

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