Radioactivity had not been discovered at the time of Kelvin’s work, so he did not understand that the Earth continues to produce heat through the radioactive decay of unstable isotopes. The concept of radioactive decay was brought in to the attention of the world in 1904 by Ernest Rutherfod, when scientists discovered that rocks could be timepieces -- literally. Many chemical elements in rock exist in a number of slightly different forms, known as isotopes. Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily transforming, atom by atom, into a different isotope (giving off energy (heat) as a byproduct of this action). This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life. For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years. By measuring the ratio of lead to uranium in a rock sample, its age can be determined. Using this technique, called radiometric dating, scientists are able to "see" back in time and constrain ages within the geologic time scale.
Don Wise published an excellent article in the Journal of Geologic Education (1990, vol. 38, pp. 38-41) on how to teach the fundamental concepts of radiometric dating of rocks using the melting of ice cubes (=parent isotope) and resulting production of water (=daughter isotope). Although ice melts to water at a constant rate, and radiometric decay occurs at exponential rates, the process by which geologists can use the rate of production of daughter isotopes to determine when decay began is superbly demonstrated by this experiment. It is possible to demonstrate Wise’s melting ice analogy for an entire class or have groups of students conduct individual measurements and calculations (See link below).
Content courtesy of the University of California Museum of Paleontology: (http://evolution.berkeley.edu/)
Link to Don Wise Radioactive Decay Lesson (Click Here)
Last modified October 02 2008 03:12 PM