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About Isotopic Dating: Yardsticks for Geologic Time

The Century in Review

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The work of geologists is to tell the true story of Earth's history—more precisely, a story of Earth's history that is ever more true. A hundred years ago, we had little idea of the story's length—we had no good yardstick for time. Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves. For that we can thank radioactivity, discovered at the turn of the last century.

The Need for a Geologic Clock

A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. But obviously rocks are very old things. Judging from the amount of rocks there are, plus the imperceptible rates of the processes forming them—erosion, burial, fossilization, uplift—the geologic record must represent untold millions of years of time. It is that insight, first expressed in 1785, that made James Hutton the father of geology.

So we knew about "deep time," but exploring it was frustrating. For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy. That only worked for sedimentary rocks, and only some of those. Rocks of Precambrian age had only the rarest wisps of fossils. No one knew even how much of Earth history was unknown! We needed a more precise tool, some sort of clock, to begin to measure it.

The Rise of Isotopic Dating

In 1896, Henri Becquerel's accidental discovery of radioactivity showed what might be possible. We learned that some elements undergo radioactive decay, spontaneously changing to another type of atom while giving off a burst of energy and particles. This process happens at a uniform rate, as steady as a clock, unaffected by ordinary temperatures or ordinary chemistry.

The principle of using radioactive decay as a dating method is simple. Consider this analogy: a barbecue grill full of burning charcoal. The charcoal burns at a known rate, and if you measure how much charcoal is left and how much ash has formed, you can tell how long ago the grill was lit.

The geologic equivalent of lighting the grill is the time at which a mineral grain solidified, whether that is long ago in an ancient granite or just today in a fresh lava flow. The solid mineral grain traps the radioactive atoms and their decay products, helping to ensure accurate results.

Soon after radioactivity was discovered, experimenters published some trial dates of rocks. Realizing that the decay of uranium produces helium, Ernest Rutherford in 1905 determined an age for a piece of uranium ore by measuring the amount of helium trapped in it. Bertram Boltwood in 1907 used lead, the end-product of uranium decay, as a method to assess the age of the mineral uraninite in some ancient rocks.

The results were spectacular but premature. The rocks appeared to be astonishingly old, ranging in age from 400 million to more than 2 billion years. But at the time, no one knew about isotopes. Once isotopes were explicated, during the 1910s, it became clear that radiometric dating methods were not ready for prime time. (For an introduction to atoms and isotopes see this article by About Chemistry Guide Anne Marie Helmenstine.)

With the discovery of isotopes, the dating problem went back to square one. For instance, the uranium-to-lead decay cascade is really two—uranium-235 decays to lead-207 and uranium-238 decays to lead-206, but the second process is nearly seven times slower. (That makes uranium-lead dating especially useful.) Some 200 other isotopes were discovered in the next decades; those that are radioactive then had their decay rates determined in painstaking lab experiments.

By the 1940s, this fundamental knowledge and advances in instruments made it possible to start determining dates that mean something to geologists. But techniques are still advancing today because with every step forward, a host of new scientific questions can be asked and answered.

Methods of Isotopic Dating

There are two main methods of isotopic dating. One detects and counts radioactive atoms through their radiation. The pioneers of radiocarbon dating used this method because carbon-14, the radioactive isotope of carbon, is very active, decaying with a half-life of just 5730 years. The first radiocarbon laboratories were built underground, using antique materials from before the 1940s era of radioactive contamination, with the aim of keeping background radiation low. Even so, it can take weeks of patient counting to get accurate results, especially in old samples in which very few radiocarbon atoms remain. This method is still in use for scarce, highly radioactive isotopes like carbon-14 and tritium (hydrogen-3). (Anne Marie has also prepared this worked-out example of radiocarbon dating.)

Most decay processes of geologic interest are too slow for decay-counting methods. The other method relies on actually counting the atoms of each isotope, not waiting for some of them to decay. This method is harder, but more promising. It involves preparing samples and running them through a mass spectrometer, which sifts them atom by atom according to weight as neatly as one of those coin-sorting machines.

For an example, consider the potassium-argon dating method. Atoms of potassium come in three isotopes. Potassium-39 and potassium-41 are stable, but potassium-40 undergoes a form of decay that turns it to argon-40 with a half-life of 1,277 million years. Thus the older a sample gets, the smaller the percentage of potassium-40, and conversely the greater the percentage of argon-40 relative to argon-36 and argon-38. Counting a few million atoms (easy with just micrograms of rock) yields dates that are quite good.

Isotopic dating has underlain the whole century of progress we have made on Earth's true history. And what happened in those billions of years? That's enough time to fit all the geologic events we ever heard of, with billions left over. But with these dating tools we've been busy mapping deep time, and the story is getting more accurate every year.

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