As the magma cools, grains of different minerals begin to crystalize. The left-most box in the figure above represents an initial state, with parent atoms distributed throughout molten rock (magma). A half-life is the amount of time needed for half of the parent atoms in a sample to be changed into daughter products. This rate is determined in a laboratory setting and is typically represented by its half-life. The rate at which a particular parent isotope decays into its daughter product is constant. Radioactive decay involves unstable isotopes shedding energy in the form of radiation, causing their numbers of protons and neutrons to change, in turn resulting in one element changing into another.Īs a matter of convention, we call the atomic nucleus that undergoes radioactive decay the parent and the resulting product the daughter product (or, decay product). Some isotopes are unstable, however, and undergo radioactive decay. Most isotopes are stable, meaning that they do not change. Isotopes of individual elements are defined by their mass number, which is simply the number of protons + the number of neutrons.Ĭonsider, for example, the three different isotopes of Carbon: What key discovery, then, allowed geologists to begin assigning absolute age dates to rocks and ultimately discover the age of the Earth? The answer is radioactivity.Įven though individual elements always have the same number of protons, the number of neutrons in their nuclei sometimes varies. Geologists were beginning to accept the views of Hutton that the Earth is unimaginably ancient. But, unlike Ussher's calculation, this estimate was on the order of millions of years, rather than 6,000. We now know that this estimate is far, far too young*. When they did this, they estimated that the Earth is many millions of years old. For example, some geologists observed how long it took for a given amount of sediment (say, a centimeter of sand) to accumulate in a modern habitat, then applied this rate to the total known thickness of sedimentary rocks. In the 1800's, practitioners of the young science of geology applied the uniformitarian views of Hutton and Lyell (see the introduction to this chapter) to try to determine the age of the Earth.

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