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Radioactive dating is a method for calculating the age of rocks and fossils through the concentrations of certain radioactive elements in close proximity to such objects or as a part of their chemical structure. There are various radioactive dating methods used depending on whether what is being analyzed is organic or inorganic, and each process is built upon assumptions about the original state of the material being dated and accepted geological time scales. While the nature of radioactive decay is based on established scientific principles for radioactive elements that are well-proven, the assumptions used to calculate the actual age of an object from these principles is subject to some debate and controversy.
Radioactive carbon dating is the most common method used to date fossils of human origin or artifacts from ancient human civilizations. The isotope of carbon 14 (14C) is used, as it has an effective short half-life of decay of 5,725 years where it decays to nitrogen 14 (14N), and it is found in minute concentrations in virtually all organic compounds on Earth. Carbon 14 is present in known concentrations in the atmosphere and in all plants and animals involved in the exchange of CO2 gas through processes of respiration. After a plant or animal has died and is sealed off from further exposure to the air, the amount of carbon 14 slowly diminishes in the remains, as well as in the surrounding soil. This variation can be compared to atmospheric concentrations to determine a rough age for when the creature died, or when an inorganic artifact was buried in the soil near organic remains.
Radioactive dating methods for older periods of time or fossils believed to be millions of years old involve the use of elements with much slower decay rates than carbon 14. Commonly, uranium 238 (238U) is used, as it slowly decays to a stable form of lead (206Pb) over the course of 4,500,000,000 years. Another isotope with a long decay rate that is used to date geological formations is potassium 40 (40K), which decays to argon 40 (40Ar) in 1,250,000,000 years. While radioactive elements like carbon or uranium isotopes decay, they remain unaffected by other processes going on around them, such as changes in heat, pressure, and chemical reactions. This makes them predictable in terms of their rate of change, and their decay rates are the foundational assumption upon which the science of radioactive dating is built.
The primary argument concerning the accuracy of radioactive dating is centered around the geological age science assumes for the Earth, as of 2011. Since it is impossible for humans to know the exact state of a rock or fossil deposit when it was originally created thousands or millions of years ago, it is possible that elements in the deposit accounted for in present time were not a byproduct of decay of other elements in the sample. Elements that appear to be decay byproducts may have been deposited in the sample over time through other methods, or always there in higher-than-expected concentrations along with the decaying elements, throwing off the calculations as to an object's true age. Tests of the age of recently-formed rock samples from volcanic eruptions, by multiple independent laboratories, have also yielded wildly varying ages of several million years, when the rocks themselves were formed through processes that occurred less than 100 years ago, casting some doubt on the methodology used in conventional dating practices.