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1,000 yr

10,400yr^

10,50CTyr v

Figure 2.1. Rocks and time. An outcrop of rock is shown, and the ages of several of the layers are given. Note that the amount of time represented is not equivalent to the thickness of the rock

1,000 yr

10,400yr^

10,50CTyr v

7,000 yr 8,500 yr

9,000 yr

2 Gould, S. J. 1987. Time's Arrow,Time's Cycle. Harvard University Press, Cambridge, MA. 222pp. (p. 3).

Obtaining an absolute age is not always possible, but it can be accomplished when rocks are found that contain minerals bearing particular radioactive elements.

Absolute age dating

Ages in years before present are reckoned from the decay of unstable isotopes. These spontaneously decay from an energy configuration that is not stable (i.e., that "wants" to change) to one that is more stable (i.e., that will not change, but rather will remain in its present form). The decay of an unstable isotope to a stable one occurs over short or long periods of time, depending upon the particular isotope. The slower the decay process, the longer the amount time that can be deduced from it. The basic decay equation goes as follows:

unstable "parent" isotope —»stable "daughter" isotope + nuclear products + heat

Carbon provides a good example. In the unstable isotope of carbon, 14C, a neutron splits into a proton and an electron in the following reaction:

l4C l4N + heat

Note that the atomic number in the decay reaction changes; it is increased from 6 to 7. Now, with seven protons and seven electrons, the stable daughter is no longer an isotope of carbon but is now nitrogen (see Appendix to this chapter for a quick review of the chemistry underlying these concepts).

The rate of the decay reaction is the key to obtaining an absolute age. If we know (1) the original amount of parent isotope at the moment that the rock was formed or the animal died (before becoming a fossil), and we know (2) how much of the parent isotope is left and (3) the rate of the decay of that isotope, we can estimate the amount of time that has elapsed. For example, suppose we know that 100% of an unstable isotope was present when a rock was new, but now only 50% remains. If we know the rate at which the element decays, we can estimate the amount of time that has elapsed since the rock was formed; that is, the age of the rock. This kind of relationship is shown in Figure 2.2. Because radioactive decay is the basis of the absolute age determination, unstable isotopic age estimations are sometimes called radiometric dating methods.

Since the rate of decay is constant (in any given stable isotope), it is convenient to summarize that rate by a single number. That number is called the half-life, which is the amount of time that it takes for 50% of a quantity of an unstable isotope to decay (leaving half as much parent as was present originally). The half-life, then, is an indicator of decay rate, and provides guidance about which isotope is appropriate for which amount of time.

Which unstable isotope is chosen for dating depends upon the probable age of the object in question. To date human remains, not more than several thousand years old, the rubidium/strontium isotopic system

Amount of unstable parent isotope

Amount of unstable parent isotope

Amount of daughter isotope with time

Decay of parent isotope with time

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