Abosolute 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.

What is more, we can measure the rate of spreading directly by GPS, SLR, and VLBI.

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.

These precautions allow us to throw out most data that have been produced by confounding factors such as atmospheric contamination, weathering, hydrothermal events, metamorphism, metasomatism, etc.

It is, as we have explained, possible for the occasional incorrect date to slip through this filter, since it is possible for some of these confounding factors to accidentally change the isotope ratios in such a way as to produce something that looks like a good date: apparently concordant dates for Ar-Ar or U-Pb, or a false isochron for the various isochron methods.

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.Science, since it concerns just one universe with one set of laws, constitutes a seamless whole; we cannot unpick the single thread of absolute dating without the whole thing beginning to unravel.Still, it has happened in the past that scientists have thought they'd got hold of a law of nature and then found out it was false.If there is one possible exception to this, it would be the deposition of marine sediment, since it is not subject to erosion, and since we would expect the rates of deposition of the various sediments to be, if not actually constant, then not subject to such a degree of variation as (for example) glacial till.

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