The radiocarbon clock's limit
The famous multi-colored logs of Petrified Forest National Park are a geological puzzle. They are composed of ancient organic material, yet they cannot be dated using the familiar radiocarbon method. The reason lies in the physics of radioactive decay. Radiocarbon dating relies on the unstable isotope Carbon-14 (C-14), which all living things absorb from the atmosphere. When an organism dies, it stops taking in C-14, and the existing C-14 begins to decay into stable Nitrogen-14. This decay happens at a known rate, defined by a half-life of about 5,730 years. After about 10 half-lives, or roughly 50,000 to 60,000 years, so little C-14 remains that it becomes impossible to measure accurately against background radiation.
The trees of the Petrified Forest are vastly older than this limit. They lived during the Late Triassic Period, a time when the first dinosaurs were just beginning to appear. The organic wood has long since been replaced by minerals, primarily silica, in a process called permineralization. Even if the original carbon were present, it would be far too ancient for C-14 analysis. The radioactive clock of carbon has simply wound down completely, requiring scientists to use different clocks that tick much, much slower.
Dating with uranium and lead
To determine the age of the petrified logs, geologists look to the surrounding rock layers of the Chinle Formation. This formation contains volcanic ash beds with tiny, durable crystals called zircons. When zircon crystals form in magma, their structure readily incorporates uranium atoms but rejects lead. This pristine state creates a perfect radiometric clock. Over immense spans of time, the uranium isotopes trapped in the zircon decay into stable lead isotopes at a very slow, fixed rate. Specifically, Uranium-238 decays to Lead-206, and Uranium-235 decays to Lead-207.
By measuring the ratio of parent uranium isotopes to daughter lead isotopes within the zircon crystals, scientists can calculate how long ago the crystals formed and, by extension, the age of the volcanic ash that buried the ancient forest. This uranium-lead (U-Pb) dating method places the age of the Chinle Formation and the trees within it—at approximately 205 to 225 million years old. The most common tree species found, Araucarioxylon arizonicum, grew to heights of nearly 200 feet in a tropical environment located much closer to the equator than modern-day Arizona.