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Methods of absolute dating of rocks
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Radiometric dating and paleomagnetism[ edit ] The polarity of rockd Earth's magnetic field is a global phenomenon: So if our methods of radiometric datihg are correct, then we would predict that rocks dated to the same age would Chester free dating the same polarity, which they do. If this does not rokcs prove that radiometric dating is correct, it does at least show that barring a datjng 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.
Comparison with historical dates[ edit ] 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. 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. D, as recorded by Roman historians at the time.
See Lanphere et al. Radiocarbon dating, varves, and dendrochronology[ edit ] Because rocsk contain organic material, it is gocks to compare the rockw from varves with the dates produced by radiocarbon datingand see that they are in good agreement. We also see close agreement between dendrochronology and uncalibrated radiocarbon dates. I specify uncalibrated dates because as radiocarbon dating is calibrated against dendrochronologythe agreement of calibrated radiocarbon dates with dendrochronology is inevitable. Now, each of these three methods relies on a different underlying physical process: We can hardly suppose Methods of absolute dating of rocks 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 Method 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 ahsolute to be actually conspiring to deceive us: Now, preposterous things do happen occasionally. But in this case there is a perfectly reasonable and straightforward explanation dting why the dates are concordantnamely that they are correct. Radiometric dating, sclerochronology absopute rhythmites[ edit ] Similar remarks may be made about the agreement between radiometric dating of rocks, sclerochronologyand dating by rhythmites. Are we to believe that one single mechanism interfered with the decay of radioactive isotopesthe secretion of calcium carbonate by molluscs, and the action of the tide?
But are we instead to believe that three separate mechanisms interfered with these processes in such a way as to leave all the dates concordant? That would be equally absurd. The straightforward explanation for the concordance of the dates is that they are in fact correct. Consider the following analogy: Skeptical of the clockmaker's claim, we subject the clocks to shock: 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? Or is it more likely that they are synchronized because nothing that's happened to them has affected their working?
Agreement with relative dating[ edit ] 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 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. Internal consistency of radiometric dates[ edit ] In our discussion of radiometric datingwe have seen that many, indeed most, radiometric methods are self-checking.
So in the U-Pb methodwe check that the two uranium isotopes produce concordant dates. In the Ar-Ar methodwe check that step heating yields the same date at every step. These precautions allow us to throw out most data that have been produced by confounding factors such as atmospheric contamination, weatheringhydrothermal events, metamorphismmetasomatismetc. Radiation levels do not remain constant over time. Fluctuating levels can skew results — for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence OSL [ edit ] Optically stimulated luminescence OSL dating constrains the time at which sediment was last exposed to light.
During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows Methods of absolute dating of rocks sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose De that the sediment has acquired since deposition, which can be used in combination with the dose rate Dr to calculate the age.
Dendrochronology The growth rings of a tree at Bristol ZooEngland. Each ring represents one year; the outside rings, near the bark, are the youngest. Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. Dendrochronology has three main areas of application: In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands.