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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.
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.
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.
Activity 8 is taken from Investigating Earth: A Geology Laboratory Text (1997) by Wiswall and Fletcher.
Read the pages listed below, which are available online through Library Reserves.
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.
Based on the known rates of deposition, we may therefore at least say that the depths of marine sediment found on the sea floor are consistent with the ages of the igneous rocks beneath them as produced by radiometric dating.
In this activity, you will be able to combine your knowledge of relative dating methods (learned in Activity 7) with the absolute dating method to determine more accurately the geologic history of a region.
These schematic columnar sections contain the stratigraphic sequence for the Transvaal Basin in Africa and the Nabberu Basin and Hamersley Basin in Western Australia.
Column 2(Transvaal): Granular IF, banded IF, argillite, basinal carbonates, platform carbonates, quartz arenite, and crystalline basement rock.