Absolute radiometric dating

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.

absolute radiometric dating-59

It is hard to see this as a coincidence; it is equally hard to think of some alternate explanation of why we can correlate isotope ratios or sclerochronological data with the relative order of rocks as deduced from stratigraphic methods — other than the straightforward explanation that absolute dating is producing the right dates.

In our discussion of radiometric dating, we have seen that many, indeed most, radiometric methods are self-checking.

So in the U-Pb method, we check that the two uranium isotopes produce concordant dates.

In the Ar-Ar method, we check that step heating yields the same date at every step.

We can hardly suppose that there is some single mechanism which would interfere with all three of these very different processes in such a way as to leave the dates derived from them still concordant.

But it is equally far-fetched to imagine that three different mechanisms interfered with the three processes in such a way as to leave the dates concordant; that would require either a preposterous coincidence, or for natural processes to be actually conspiring to deceive us: an idea which is, if anything, even more preposterous. But in this case there is a perfectly reasonable and straightforward explanation for why the dates are concordant, namely that they are correct.

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.

Throughout this process, they all go on showing exactly the same time.

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