When a geologist says that a particular rock is one hundred million years old, how do you know he isn’t just picking a number out of his hat? Well, you don’t, unless you decide to study the methods by which scientists date rocks and minerals. I can’t think of a better place to start than Doug Macdougal’s Nature’s Clocks: How Scientists Measure the Age of Almost Everything. In this book, Macdougal covers the basics of the various sorts of radioactive dating that scientists use to date samples. He begins, appropriately enough at the beginning with the realization by early geologists like James Hutton that the Earth must be far older than Archbishop Usser’s 6000 years.
By studying the layers of rocks laid down and the fossils associated with each layer, geologists were able to get a good idea of the relative ages of these rocks, a layer on top of another layer is usually younger, but they had no way to measure the absolute age of these rocks or of the Earth until the discovery of radioactivity at the beginning of the twentieth century. It didn’t take long for scientists to realize that the unvarying rate of decay of a sample of a radioactive isotope into its daughter elements provided them with a clock they could use to measure the age of rocks and the Earth. All they had to do was to compare the amount of the daughter element to the amount of the mother element, and by knowing the half-life of the mother element, they could know the age of the sample.
It seems straightforward, but the process is much more complicated and took decades to develop. Scientists needed to find ways of knowing how much of the daughter elements were in the sample when it first formed. They needed to learn ways of minimizing any contamination of the sample. Macdougal describes the development of the three most common methods used for radioactive dating, radiocarbon, potassium-argon, and uranium-lead. This last was developed by Clair Patterson, who realized that the unusually high content of lead in the atmosphere that threatened to contaminate his samples was due to the use of leaded gasoline. This discovery drew attention to the health threat of such widespread usage of lead and his work ultimately led to the banning of leaded gasoline.
Macdougal moves on from the development of radioactive dating techniques to more of the more exciting recent uses, including the accurate dating of small grains of zircon to 4.1 billion years, making them the oldest material ever discovered on Earth. Macdougal explains these processes and discoveries in language that is accessible to the non-scientist, without sacrificing clarity or a basic understanding of just how they know a rock is that old. His enthusiasm for his work is contagious and makes Nature’s Clock actually fun to read. I can recommend it highly for anyone who wants to know more about radioactive dating.