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Radiometric Dating

Early attempts at establishing an absolute time scale utilized the following concepts: declining sea levels, cooling of the Earth, cooling of the Sun, Earth tidal effects, sediment accumulation, and changes in ocean salinity. Using changes in Earth's temperature or the Sun's use of energy failed because energy from nuclear reactions was unknown. Once this energy was discovered a new, successful strategy called radiometric dating would be developed. Today radiometric dating places absolute dates on the relative time scale.

Radioactive Decay Discovered

Wilhelm Konrad Rontgen (1845-1923) published a report in 1895, which described his discovery of a mysterious source of energy emitted by a cathode ray tube. This source of energy caused barium platinocyanide coated on paper to luminance. Furthermore, this energy could penetrate cardboard and even walls to fluoresce barium platinocyanide. Rontgen used the mathematical symbol for unknown to name this energy X-rays. The interest in Rontgen’s discovery would launch the second scientific revolution; the first being launched by the work of Galileo (Asimov, 1984, pp 514-515). In 1896 Rontgen demonstrated the use of X-rays to view skeletal structure by taking a picture of the German biologist Rudolf Albert von Kolliker’s (1817-1905) hand.

The French physicist Antoine Henri Becquerel (1852-1908) wondered if X rays might be among the energy given off by fluorescent materials. In 1896 Becquerel discovered that potassium uranyl sulfate (K2UO2 (SO4)2 did create energy that penetrated black paper to expose photographic film. To his surprise the compound could do this even when it was not fluorescing. This energy did not only have penetrating power it also ionized the air as shown by an electroscope.

In 1898 the Polish-French physicist team of Marie Sklodowska Curie (1867-1934) and her husband Pierre (1859-1906) discovered that thorium also gave off radiation. The phenomenon of emitting penetrating, ionizing radiation was given the name radioactivity by the Curies. The Curies found that the radiation was proportional to the Uranium content of the compound. This indicated that radioactivity was a property of atoms, not molecules. They also discovered that compounds containing U and Th emitted more radiation than either element by itself. The Curies deduced that other radioactive elements must be present, which led to their discovery of the elements polonium and radium (Dalrymple, 1991, p 69).

In 1902 the New Zealander-English physicist Ernest Rutherford (1871-1937) and the English chemist Frederick Soddy (1877-1956) published results from their experiments with radioactivity. Rutherford and Soddy made four important observations. First, the decay of what we now call the parent isotope is exponential. Furthermore, the formation of the daughter isotope is exponential and inverse to the parent decay. Second, atoms of radioactive elements are unstable and spontaneously decay to other elements by the emission of alpha and beta particles. Thus the transmutation medieval alchemist desired was in fact being done by nature continuously. Third, radioactive decay is proportional to the number of atoms present. Finally, helium might be a product of radioactive decay (Dalrymple, 1991, pp 70-71). In 1905 Rutherford suggested that radioactive decay could be used as a method to calculate absolute time.

Radioactive Decay & Chemical Ages

Early attempts at using radiometric dating are referred to as “chemical ages”. They were done without knowledge of isotopes or the decay rates and intermediate products of Uranium. Furthermore, the decay of Thorium to lead was unknown. Even so, early work done by Bertram Borden Boltwood (1870-1927), an American chemist and physicist demonstrated several important consistencies. First, the same Pb/U ratios are obtained in samples from the same geologic age. Second, Pb/U ratios are a function of geologic age. Third, altered samples have lower Pb/U ratios. Finally, as Arthur Holmes (1890-1965), a British geologist pointed out, ages obtained using these new methods were often in agreement with what many geologists had speculated (Dalrymple, 1991, pp. 72-74). The fact that this new method of dating utilized empirical evidence gathered in the field and analyzed in the lab with methods based upon experimentally determined physical principles made it promising and powerful. It soon became clear that the Earth was much older than many of the earlier methods indicated. With time, radiometric dating became more sophisticated and accurate, by 1931 radiometric dating had proven to be the credible method for determining the age of igneous rocks. For a good discussion on modern radiometric methods see The Age of the Earth by Dalrymple.

Radiometric Vocabulary Terms

1. Radiometric Dating-Measuring the passage of time by the regular rate of decay of radioactive isotopes.

2. Isotopes-Same element, but different number of neutrons. There are 350 different isotopes. Some isotopes are stable and others are radioactive.

3. Parent Isotope-Radioactive isotope incorporated during crystal formation.

4. Daughter Isotope-Stable decay product of parent isotope.

5. Radioactive isotopes decay or change into a stable element at an exponential rate that does not change. The decay rate is not affected by heat, temperature, pressure, or chemical reactions.

6. Half-life-The time it takes for half of the parent sample to decay to the stable daughter isotope.

7. Examples:

a. U-238 to Pb-206 (4.5 billion years)
b. U-235 to Pb-207 (704 million years)
c. K-40 to Ar-40 (1.3 billion years)
d. C-14 to N-14 (5,730 years)

Back to Dating
Asimov, I. (1984). The History of Physics. New York: Walker and Company.

Dalrymple, G.B. (1991). The Age of the Earth. Stanford, California: Stanford University Press.

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