Radiometric Dating:
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)