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Unformatted text preview: Lab 10: Absolute Time and Radiometric Dating Introduction In order to determine the history of a planet, most geological techniques rely on relative dating techniques, which determine the relative ages of two or more features. Indications of absolute age may be given by these techniques, but these ages are very difficult to quantify accurately. Following the discovery of radioactivity at the beginning of this century, absolute dating techniques have been developed which use the very regular rate at which naturally unstable radioactive isotopes decay to determine time. This branch of geology is generally known as geochronology. Geochronology - Radiometric Dating One of the early discoveries about radioactivity was that spontaneous radioactive decay occurs at a very predictable rate proportional to the amount of the undecayed material left at any time. This decay rate is independent of temperature, pressure and other external conditions, which makes it very useful and quite unusual. Most unstable conditions in nature may be accelerated by external conditions. For example, the oxidation of wood in the atmosphere becomes very rapid if the temperature is raised; the solubility of calcium carbonate (limestone) decreases with increasing temperature but increases with increasing pressure; weathering and soil formation increases with increasing temperature and moisture content. Radioactive Decay Radioactive decay is independent of all of the factors mentioned above (the physical and chemical history of the material of a rock does not affect its radioactive decay rate), making it a useful measure of absolute time. The reason that radioactive decay is independent of external conditions is that it is a quantum process, an everyday example of the strange world of quantum physics. A radioactive isotope is in an unstable condition rather like a ball in a small hole on the side of a hill. If the ball is moved a small distance it comes out of the hole and rolls down the hill releasing its gravitational energy. For most of the examples given above, the energy associated by external changes is large enough to increase the change that the ball will pop out of its hole. For radioactive isotopes, no external change is large enough to release the ball from the hole. However, as we are working here on the atomic scale, quantum effects must be taken into account, one of which is the statistical uncertainty associated with position. In quantum mechanics the position of a particle can be defined only in terms of probability: for example, at any instant in time there is a 90% probability that the ball is in the hole and a 10% probability that the ball is out of the hole. As soon as the ball is out of the hole, however, it rolls down the hill and radioactive decay occurs. The probability function that describes this position is independent of external conditions, so the decay rate is independent of external conditions. In the case of induced nuclear reactions the ball is essentially knocked out of the hole by the impact of an external particle. Normal nuclear decay is simply a matter of random probabilities as the particle....
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This note was uploaded on 01/01/2011 for the course GLG 190 taught by Professor C.v. during the Spring '09 term at University of Arizona- Tucson.
- Spring '09