Radiometric Dating - A Brief Explanation
Radiometric dating is the primary dating scheme employed by scientists to determine the age of the earth. Radiometric dating techniques take advantage of the natural decay of radioisotopes. An isotope is one of two or more atoms which have the same number of protons in their nuclei, but a different number of neutrons. Radioisotopes are unstable isotopes: they spontaneously decay (emitting radiation in the process -- thus making them radioactive). They continue to decay going through various transitional states until they finally reach stability. For example, Uranium-238 (U238) is a radioisotope. It will spontaneously decay until it transitions into Lead-206 (Pb206). The numbers 238 and 206 represent these isotopes' atomic mass. The Uranium-238 radioisotope goes through 13 transitional stages before stabilizing into Lead-206 (U238 > Th234 > Pa234 > U234 > Th230 > Ra226 > Rn222 > Po218 > Pb214 > Bi214 > Po214 > Pb210 > Bi210 > Po210 > Pb206). In this instance, Uranium-238 is called the "parent" and Lead-206 is called the "daughter". By measuring how long it takes for an unstable element to decay into a stable element and by measuring how much daughter element has been produced by the parent element within a specimen of rock, scientists believe they are able to determine the age of the rock. This belief is based upon three significant assumptions.
Radiometric Dating - The AssumptionsThe rate of decay remains constant.There has been no contamination (that is, no daughter or intermediate elements have been introduced or leeched from the specimen of rock).We can determine how much daughter there was to begin with (if we assume there was no daughter to begin with, yet there was daughter at the formation of the rock, the rock would have a superficial appearance of age).
Many of the ages derived by radiometric dating techniques are highly publicized. Nevertheless, the fundamental assumptions employed are not. Here are the three major assumptions for your consideration:
Are these foundational assumptions reasonable? Recent findings seem to indicate that though we ourselves have not been able to vary the decay rates by much in the laboratory, the decay rates may have been accelerated in the unobservable past . If this were the case, the first assumption would be deemed unreasonable. This would completely upset our current standardized view of earth's history. Dr Carl Wieland summarizes the recent findings: "When uranium decays to lead, a by-product of this process is the formation of helium, a very light, inert gas which readily escapes from rock. Certain crystals called zircons, obtained from drilling into very deep granites, contain uranium which has partly decayed into lead. By measuring the amount of uranium and 'radiogenic lead' in these crystals, one can calculate that, if the decay rate has been constant, about 1.5 billion years must have passed. (This is consistent with the geologic 'age' assigned to the granites in which these zircons are found.) There is a significant amount of helium from that '1.5 billion years of decay' still inside the zircons. This is at first glance surprising, because of the ease with which one would expect helium (with its tiny, light, unreactive atoms) to escape from the spaces within the crystal structure. There should hardly be any left, because with such a slow buildup, it should be seeping out continually and not accumulating. Drawing any conclusions from the above depends, of course, on actually measuring the rate at which helium leaks out of zircons. This is what one of the recent RATE  papers reports on. The samples were sent… to a world-class expert to measure these rates. The consistent answer: the helium does indeed seep out quickly over a wide range of temperatures. In fact, the results show that because of all the helium still in the zircons, these crystals (and since this is Precambrian basement granite, by implication the whole earth) could not be older than between 4,000 and 14,000 years. In other words, in only a few thousand years, 1.5 billion years' worth (at today's rates) of radioactive decay has taken place. Interestingly, the data has since been refined and updated to give a date of 5680 (+/- 2000) years." 
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- D. Russel Humphreys, Steven A. Austin, John R. Baumgardner, Andrew A. Snelling, Helium Diffusion Rates Support Accelerated Nuclear Decay; Article available online at http://www.icr.org/research/icc03/pdf/helium_ICC_7-22-03.pdf.
- The "RATE" project stands for, "Radioisotopes and the Age of The Earth"
- Carl Wieland, RATE Group Reveal Exciting Breakthroughs, 2003