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Radiometric dating attempts to use the constant decay of radioactive substances as a "clock" to measure the age of rocks and minerals. Unlike earth clocks such as erosion rates and saline accumulation, radiometric clocks do give dates that are compatible with the standard geological ages (a 4.5 billion year old earth, and 570 million years of phanerozoic fossil layers). In this complex field of study, scientists attempt to deduce time for events in the past, using the presently determined rate of disintegration of unstable forms (isotopes) of atoms. The techniques are varied and complex, and the methods involve a number of assumptions with varying degrees of certainty. The most common dating methods are radiocarbon (often called carbon-14), used for dating materials of biological origin, and Potassium-Argon, useful principally for dating rocks of igneous origin. Other commonly used dating techniques include Rubidium-Strontium, Lead-Uranium, fission track, thermoluminescence and a variety of others.

______________________________________________________ Ó 2010 Arthur V. Chadwick, Ph.D.