Relative dating geology methods dating members friends community


21-May-2017 08:37

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But by the same token, the other 999 times they don't, and so although any particular date produced by these methods might be called into question, it must be the case that the vast majority of dates that pass through these filters must be good; for we can hardly suppose that the confounding factors are actively conspiring to deceive us, and so these long-shot events must be as rare as statistical considerations would lead us to expect.

You might perhaps suggest that if some unknown factor, contrary to our present understanding of physics existed that sped up or slowed down radioactive decay in the past, then we would expect the radiometric dates to be concordant whether they were right or wrong.

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.

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.

In Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, La-Be, La-Ce and K-Ca dating, we check that the points we plot on the isochron diagram lie on a straight line.

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It would indeed be remarkable if this never happened, since one-in-a-thousand chances do in fact occur one time in a thousand.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 also see close agreement between dendrochronology and uncalibrated radiocarbon dates.

(I specify uncalibrated dates because as radiocarbon dating is calibrated against dendrochronology, the agreement of calibrated radiocarbon dates with dendrochronology is inevitable.) Now, each of these three methods relies on a different underlying physical process: radioactive decay, outwash from glaciers, and the growth of trees.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.There is no particular reason to suspect that this will turn out to be the case when it comes to the laws underlying absolute dating; nonetheless, an argument from principle alone can never be entirely convincing. You will recall from our discussion of sea floor spreading that the sea floor spreads out from mid-ocean rifts, and so ought to be younger nearer the rifts and progressively older further away from them.