Because the decay rate of potassium is well known, geochronologists use the amounts of potassium and argon present in rocks today to determine the age of the rock.
The Toroweap Dam (Figure 1) is a lava flow exposed on river right just upstream from Lava Falls Rapid (rm 179). W., 1990, Tectonic geomorphology of the Toroweap fault, western Grand Canyon, Arizona: Implications for transgression of faulting on the Colorado Plateau. K-Ar ages of western Grand Canyon lava flows range from 10,000 years (Vulcan’s Throne) to 1.8 million years (Prospect Dam). Argon-argon dating, a newer technique also known as 40Ar/39Ar dating, is very similar to K-Ar dating, but 40Ar/39Ar dating is more accurate and can account for most—but not always all—of this extra gas brought up from the earth’s interior. The 500,000 year-old age is likely most accurate, whereas the 1.8 million year age is likely affected by extra argon gas, and the 395,000 year age affected by erosion of the rock’s surface. So, as you can see, all of these dating techniques have their own sources of error. Dating and Methods of Low-Level Counting, Monaco 2-10 March, 1967: International Atomic Energy Agency: 24 p. Both the Toroweap and Prospect lava dams have approximately 145 feet of displacement along the fault, but interestingly enough, the flows have K-Ar ages that differ by 600,000 years.