Geologists generally signify time in two ways: in numbers of years before present, and by reference to blocks of time with special names. For example, we say that the Earth was formed 4.6 billion years before present, meaning that it was formed 4.6 billion years ago and is thus 4.6 billion years old. Unfortunately, determining the precise age in years of a particular rock or fossil is not always easy, or even possible. For this reason, geologists have divided time into intervals of varying lengths, and rocks and fossils can be referred to these intervals, depending upon how exactly the age of the rock or fossil can be estimated. For example, you might not know that a fossil was 92.3 million years old, but you might be able to determine that it was within the interval of time known as the Late Cretaceous, meaning that its age is somewhere between 99.6 and 65.5 million years old, dates about which you have more information.

We start our discussion with the age in years, or the geochronologic age. Later we will address the division of time into blocks of varying lengths.

Geochronology: the ages ofthe ages. Geoscientists are happiest when they can learn the "absolute" age of a rock or fossil; that is, its age in years before present. Ages in years before present are reckoned from the decay of unstable isotopes found in certain minerals. The unstable isotopes spontaneously decay from an energy configuration that is not stable (that is, that "wants" to change) to one that is more stable (that is, that will not change, but rather remain in its present form). The decay of an unstable isotope to a stable one occurs over short or long amounts of time, depending upon the isotope. The basic decay reaction runs as follows:

unstable "parent" isotope ^ stable "daughter" isotope + nuclear products + heat

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

Note that the atomic number in the decay reaction changes; it increases from 6 to 7. Now, with 7 protons and 7 electrons, the stable daughter has an atomic number of 7, which means that the element in question has become nitrogen (see Appendix 2.1 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);

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 decayed, we can estimate the amount of time that has elapsed since the rock was formed; that is the age of the rock. This is shown in Figure 2.1.

Amount of unstable parent 50% isotope

Amount of unstable parent 50% isotope

5 10* 15 Half-life

20 25 30 Time in Ma

Amount of daughter isotope with time

5 10* 15 Half-life

20 25 30 Time in Ma

Decay of parent 50 isotope with time

Figure 2.1. An isotopic decay curve. Knowing the amount of unstable isotope that was originally present, as well as the amount of unstable isotope now present and the rate of decay of the unstable isotope, it is possible to determine the age of a rock with that isotope in it. Suppose we found a rock with a ratio of 25% unstable parent : 75% stable daughter of a particular isotope. That would mean two half-lives had elapsed (half-life no. 1 = 50% of 100% parent (50% parent : 50% daughter); half-life no. 2 = 50% of 50% parent (25% parent : 75% daughter)). The amount of time represented by two half-lives can be read on the axis marked "Time;" in this case, about 25 million years. The rock would thus be about 25 million years old.

Choosing the right isotope. Since each unstable isotope has its own constant rate of decay,1 it is convenient to summarize that rate by a single number. That number is called the halflife, which is the amount of time that it takes for 50% of the atoms of an unstable isotope to decay (leaving half as much parent as was originally present). The half-life, then, is an indicator of decay rate, and provides guidance about which isotope is appropriate for which amount of time. For example, to date human remains, not likely more than several thousand years old, the rubidium/strontium isotopic system (87Rb/87Sr), with a half-life of 48.8 billion years, would hardly be the ideal isotopic system. This would be a bit like timing a 100m dash with a sundial. Likewise, dating dinosaur bones (ages that would be in the hundreds of millions of years) using 14C, which has a half-life of 5,730 years, would be like giving your own age in milliseconds. The ages that involve dinosaurs are 10s to 100s of millions years old, abbreviated Ma.2

Unstable isotopes are powerful dating tools, but they cannot be used directly on dinosaur bone. There has to be a source of unstable isotopes, which occur commonly in certain minerals, some of which form as lava cools and the minerals crystalize. The decay process begins when the unstable isotope is first formed (that is, when it crystalizes in the lava), so, age of the rock can be obtained from the moment the crystal formed as the lava cooled.

No dinosaur ever lived within hot lava - at least not for very long - so how can we get the age of the dinosaur bone when all we have is a date from some lava? This - the relationship between one body of rock (in this case, containing dinosaur bone) to another (here, the cooled lava) - is the province of lithostratigraphy.

1. In fact the rate fluctuates in the short term but is statistically constant over long periods of time.

2. We use the expression Ma, from milk annos - a million years. Thus, 65 Ma is 65 million years ago.

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