The polarity of the Earth's magnetic field is a global phenomenon: at any given time it will either be normal everywhere or reversed everywhere.So if our methods of radiometric dating are correct, then we would predict that rocks dated to the same age would have the same polarity, which they do.Throughout this process, they all go on showing exactly the same time.Is it plausible that we have damaged their very different internal mechanisms in such a way that they are all running fast or slow but still in perfect synchrony?It has also been possible to test Ar-Ar dating against the historical record, since it is sufficiently sensitive to date rocks formed since the inception of the historical record.For example, Ar-Ar dating has been used to give an accurate date for the eruption of Vesuvius in 79 A. (See Lanphere et al., Ar ages of the AD 79 eruption of Vesuvius, Italy, Bulletin of Volcanology, 69, 259–263.) Because varves contain organic material, it is possible to compare the dates from varves with the dates produced by radiocarbon dating, and see that they are in good agreement.It is hard to think that this is a coincidence; it is also hard to think of any mechanism that could produce this agreement other than that the rocks are as old as radiometric methods tell us.We began our discussion of absolute dating by saying that sedimentation rates could not be relied on for absolute dating.
In the Ar-Ar method, we check that step heating yields the same date at every step.Or is it more likely that they are synchronized because nothing that's happened to them has affected their working?Relative dating by definition does not produce actual dates, but it does allow us to put an order on the rocks, and so if absolute dating is to be trusted, it should agree with this order, telling us, for example, that Ordovician rocks are older than Triassic rocks; and it does.If this does not completely prove that radiometric dating is correct, it does at least show that (barring a wildly improbable coincidence) there is at least a one-to-one relationship between the dates produced by radiometric methods and the true dates, and so it must be taken as an argument in favor of these methods.It is possible to test radiocarbon dating by using it to put a date on historical artifacts of known date, and to show that it is usually very accurate.
Consider the following analogy: a clockmaker sells us an electric clock, a pendulum clock, and a spring-driven clock, and guarantees that they are shockproof.