So if you pick up a rock, it's already been emiting radiation a long time. How do you know when it started?
That's a very difficult question to answer briefly.
The radiation started when the atoms were created, in the heart of some supernova. That's not when the
radiometric clock started.
There are many different radiometric methods. All of them depend on the premise that radioactive decay rates have been constant. There are many, many excellent theoretical and experimental reasons to believe that this is true. Sylas summarized some in
Message 12, and there's some discussion and references at
Claim CF210.
Simple accumulation methods are based on the additional premises that, when the rock solidified, there was none of the daughter product (that is, the element that is produced when the parent isotope decays) present and there was no gain or loss of daugher product other than by radioactive decay (that is, "closed system"). These turn out to be true most of the time in the case when the daughter product is a gas (like argon) that easily escapes liquids, and samples are chosen with reasonable common sense (e.g. don't test the weathered outer surface of a rock). We know that these are true most of the time because so many simple accumulation measurements agree with the more sophisticated methods. Potassium-argon is about the only simple accumulation method used, because it's well understood, low cost, and almost always accurate. It's common to cross-check potassium-argon results with another method. Creationist criticism of radiometric dating is almost exclusively confined to simple accumulation methods.
By far the most commonly used methods are
age-diagnostic methods. These
do not depend on the closed-system premise listed above, and most do not depend on the zero-initial-daughter premise listed above. Age-diagnostic methods produce an age (or fail to do so)
and an indication of how reliable that age is. All of them indicate when the system has not been closed, and some of them can produce a valid age even when the system has not been closed. All of them work this "magic" by making use of extra information that is not used in simple accumulation methods.
Isochron methods use the fact that, in suitable systems, there is an isotope of the daughter element that is not produced by radioactive decay. When several sample points fail to plot as a straight line on an appropriate graph, the system has been opened and no date is produced. The
argon-argon method uses incremental heating to release differently-bound samples from one rock, and compares the results to a rock of known age. When the results do not plot as a horizontal line on an appropriate graph, the system has been open; but if a large portion of the results are on a horizontal line, a valid age is produced. The most widely used method,
U-Pb concordia-discordia, uses two similar but different radioactive decay systems (
238U and
235U) and compares the results of the two methods. The results are presented on a graph called a "concordia diagram"; if the results do not fall on a predetermined concordia curve, the system has been opened; but if the results fall on a straight line, the upper intersection of this straight line with the concordia curve is a valid age.
Of the above-mentioned methods, only the concordia-discordia method presumes that none of the daughter product (lead) is present when the rock solidifies. Lucky for us there are several minerals that so strongly reject lead at solidification that it's physically impossible to get one that had a noticeable amount of lead at solidification, and one of these minerals (zircon) is found in all sorts of different rocks and has several other properties that make it convenient for radiometric dating. Even the creationist RATE group admits that, when lead is found in zircons, it is the result of radioactive decay:
quote:
Samples 1 through 3 had helium retentions of 58, 27, and 17 percent. The fact that these percentages are high confirms that a large amount of nuclear decay did indeed occur in the zircons. Other evidence strongly supports much nuclear decay having occurred in the past [14, pp. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotopic ages are merely an artifact of analysis, not really indicating the occurrence of large amounts of nuclear decay. But according to the measured amount of lead physically present in the zircons, approximately 1.5 billion years worth ” at today’s rates ” of nuclear decay occurred. Supporting that, sample 1 still retains 58% of all the alpha particles (the helium) that would have been emitted during this decay of uranium and thorium to lead.
(Humphreys, D.R.; S.A. Austin; J.R. Baumgardner and A.A. Snelling, "Helium Diffusion Rates Support Accelerated Nuclear Decay," Proceedings of the Fifth International Conference on Creationism, R. Ivey (ed.), Creation Science Fellowship, Pittsburgh, PA.
Error | The Institute for Creation Research)
All of these age-diagnostic systems can be fooled by low probability occurences, so some of our dates may be wrong (although the extensive cross-checking makes even this unlikely). But there's no possibility that all or even many of our dates are wrong.
Why and how these methods work as they do is more complex than can be gone into in this medium.
Radiometric Dating: A Christian Perspective is an excellent overview with more detail than I can present here.
Isochron Dating is a moderately technical exposition of, duh, isochron dating. There's a seriously technical exposition of concordia-discordia dating at
THE U-TH-PB SYSTEM: ZIRCON DATING.
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