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Radiometric Dating 101


To piece together a timeline of geological events that have happened since the time Earth formed, scientists have learned to read nature's clock..The spontaneous and well-behaved transformation of one element into another through radioactive decay gives scientists a very precise means for dating Earth and its rocks. Some people try to discredit radiometric dating as flawed. Of course, some people still believe the Sun orbits the Earth. A preoponderance of evidence tells us otherwise on both accounts.



To understand the evidence upon which the arguments for models of the solar system and Earth’s formation are based requires knowledge of isotope geochemistry, the study of the chemical makeup of rocks with particular emphasis on the ratios of stable and radioactive elements. (see box). Scientists know that certain elements—and certain products of those elements—form under very specific conditions (such as stellar nucleosynthesis, supernovas and other processes which we aren’t going to touch with a ten-foot light saber) and have well-determined “lives”, meaning they exist an any particular state for a very specific amount of time from the moment they are created.

Radioisotopes that no longer exist—they were formed when the solar system formed and subsequently “emitted” all their radiation—are called extinct radioisotopes. A list of such isotopes, taken from Ernst Zinner’s 11 April 2003 Science article, will give you a very general idea of how these isotopes are used to reconcile events at the time of Earth’s formation. Take a look at the first column and read out loud their names: calcium-41, aluminum-26 and beryllium (bur-ill-eee-um)-10, an element with a most delicious name. Many of the names on this list should not be foreign to you (although their symbols might throw you without consulting a periodic table of the elements). In fact, that these elements are just extinct radioactive forms of the elements that you already know. Comprende?

A glance at the second column of this table reveals the half-lives of these elements. The half-life is the time it takes for an element to lose half its radioactive particles. In Universal time (spanning 13.7 billion years, remember?), they are quite short: calcium-41 loses half its radiation in 1000 years. As calcium-41 decays (as it loses its radioactive nuceli), it converts into its daughter product, pottassium  41 (41K). The daughter products are what’s left following the complete loss of radiation of these elements and that’s what scientists can measure in rocks.

It’s not important that you memorize the details or have a complete understanding of the chemistry of radioisotopes (although understanding radiochemistry is hugely important for things like X-rays, CAT scans, airplane flights, pottery-making, cancer treatment, nuclear power, weapons of mass destruction—you’ve heard of those?). What’s important is that you appreciate the role that radioisotopes and stable isotopes (non-radioactive ones like deuterium, heavy water) play in planetary and ocean science. Isotopes help us understand a variety of important ocean process from circulation of the oceans to the fish that you eat.

By measuring the various extinct radioisotopes, scientists have been able to piece together a rather fuzzy picture of events at the time Earth formed. Scientists know that certain elements can only be produced under certain conditions: some can only be produced at high temperatures, some can only be produced in a supernova explosion, and some can only be produced by solar flares like the ones that Chandra observed above. With that knowledge, scientists can measure the isotopes and ask: what were the conditions under which this particular rock formed? The types of isotopes and the ratios in which they occur provide clues to these conditions. But it’s never a simple story.

For example, measurements on meteorites that have fallen to the earth indicate the presence of the first four or five elements in the table support the idea that solar flares (energetic particles from the proto-sun or another stellar source) triggered the formation of the solar system (and Earth). On the other hand, the finding of the last four extinct radioisotopes in the list suggests a supernova source.

How do scientists reconcile these apparently contradictory data? They make more measurements on different types of meteorites; they carefully check their instruments and use different types of instruments to verify the accuracy of their findings; and they create computer models of the physics and chemistry of solar system and planet formation to test their ideas.


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