Re: Are radioactive dating methods really as accurate as they appear to be?

Radioactive dating methods are fairly accurate - certainly much more so than your story would indicate. But there are a few qualifications.

1. Each dating method is based on several assumptions - what species are naturally present in the rock at the time of its formation; what species have been fixed and what species might have been somewhat mobile during the history of the rock; and so on. For certain special rock samples these assumptions might not be secure.

2. Each dating method has an optimum age where the fractional error is least. A rough calculation shows that it is best to use a method on samples with age between 0.5 and 3.5 half-lives of the crucial radioactive decay. An isotope measurement accuracy of +/- 1% will give an age measurement accuracy of below +/- 4% only in this range.

With potassium/argon dating, the crucial decay has a half-life of 1.1*10^9 years (1.1 American billion). The method is really good for most rocks - age between 550 million years and 3.8 American billion years. For a modern rock from an eruption 200 years ago, the dating method will only tell you that the age is, say, between zero and 30 million years, which may not prove entirely satisfactory. It will not even tell you this, if it comes from a part of the eruption where the lava solidified before all of the argon already present in the liquid magma had an opportunity to outgas.

Lava from modern eruptions is more appropriately dated by carbon 14 dating of vegetation remains found in or immediately under the lava flow. The half-life of carbon 14 is 5730 years, so this method is best between 3000 and 20000 years old, and should be capable of letting you know that your lava is between 140 and 300 years old.

I will take up a bit more space with two other points. Firstly, there is an important distinction drawn in scientific work between the precision of a determination, and the accuracy of a determination. How precise a result is is determined by how closely it could be reproduced by an independent determination using exactly the same method; how reproducible it is. How accurate a determination is depends on how close the measured result comes to the true value of the quantity. Systematic errors, or the failure of one of the assumptions made in interpreting the measurement can cause a result to be precise, but inaccurate. A lucky fluke in a carelessly done experiment could lead to a result that is accurate, but imprecise! Radioactive dating methods are, as I understand it, fairly precise if they are done carefully (and the experimental techniques are usually difficult and demanding), and quite accurate on most samples, but there are special cases where one of the underlying assumptions might break down for a specific sample.

Secondly, there is a group of people with a vested interest in putting down the scientific validity of radioactive dating as much as possible. They usually call themselves 'creation scientists' or something similar. Their starting point is a belief that the first part of the book of Genesis in the Bible is a scientific/historical record to be interpreted as a literal account of actual events. Radioactive dating is inconsistent with this view, and they therefore find themselves needing to work at refuting the results of the technique rather than genuinely weighing the evidence. They believe passionately that they already know the answers without having to weigh this sort of evidence. They may or may not be right. But their method is not science, and their contributions are not genuine contributions to a scientific debate. If this has anything to do with your question, I will be happy to discuss further with you in private email. My personal view, for which I claim no special authority, is (i) that they are wrong. (ii) that it is possible to be a good Christian, and even to accept the Bible as the authoritative Word of God, without going down this track.

John.