Principles of relative-age dating allow geologists to understand which rocks are older than others, but they don't reveal how old they are in years.
That changed early in the 20th century with the advent of radiometric dating techniques. Some minerals contain small amounts of unstable radioactive elements. Depending on the specific composition, the "parent" element (analogous to unstable Mr. Hyde) decays by breaking down into a unique "daughter" element (analogous to stable Dr. Jekyll) while releasing heat and various atomic particles. Each different radioactive parent decays at a different rate unique to the element. Once a mineral crystallizes (freezes) and no more radioactive elements can be incorporated into its structure, the "radioactive decay clock" begins.
Over time, the amount of unstable radioactive parent element decreases and the amount of stable non-radioactive daughter element increases. As time passes, the mineral will contain less parent and more daughter elements.
The rate at which decay occurs is measured in "half lives." No, a half life isn't the morning after an injudicious drinking binge. It's the time elapsed after half the parent is gone, then half of the remaining parent and so on. The decay rate is unaffected by temperature, pressure or chemical activity. Because none of us have the life span of a Methuselah necessary to observe even one half life of a slow-decaying radioactive element, decay rates as measured in the laboratory are extrapolated into the past. By measuring the amount of parent and daughter, and by knowing the decay rate, the age can be determined.
Different materials that decay at different rates can be used to establish the age of either young or very old materials. Rapid-decay elements are used to determine the age of archaeological materials or, by dating broken soil or rock horizons, the frequency and timing of earthquakes. Such an element is radioactive carbon-14 (parent) with a half life of 5,730 years. Materials younger than about 50,000 years are effectively dated but older materials don't have enough parent element left to measure accurately.
Using a slow-decay rate element, uranium-238 (with a half life of 4.5 million years), scientists were able to date the Mount Ashland granite at around 163 million years old.
Be comforted: there's no danger of being fried to charcoal or mutating to a different life form while skiing on Mount Ashland. Even though those images are intriguing, the amount of radioactive products generated from minerals in the granite is negligible.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at email@example.com.