Archaeologists and biologists are also sometimes able to use potassium-argon dating to measure the age of artifacts and fossils, when these have become trapped in or buried under volcanic rock.The mathematical formula that is used to figure the age of the rock depends on the half-life of potassium-40 (the time it takes for half the potassium-40 in a given sample to decay).“I felt a strong subconscious urge to go with Tom,” he later wrote.“I felt it was one of those days…when something terrific might happen.” Ignoring the already scorching heat and the mountain of paperwork on his worktable, Johanson hopped in a Land Rover with Gray and made the four-mile journey to a gully on an ancient, dried out lakebed.His international field team had already found leg bones and several jaws that were among the oldest examples of hominids—the family of bipedal primates that includes humans and their ancestors—and Johanson was convinced that an even bigger discovery was in the offing.When an American graduate student named Tom Gray announced he was leaving to scout out a nearby fossil site, Johanson had a hunch he should tag along. This is an informational tour in which students gain a basic understanding of geologic time, the evidence for events in Earth’s history, relative and absolute dating techniques, and the significance of the Geologic Time Scale.
Feel good.” Johanson and Gray’s search turned up very little at first.If the rate of radioactive decay has changed over time, the formula will not give correct dates.Most scientists believe that the rate of potassium-argon decay has not changed over the history of the earth.But after the rock solidifies, any potassium-40 that is present continues to decay, and the argon-40 that is produced cannot escape from the rock.
Thus, geologists use potassium-argon dating to measure the age of volcanic rocks.
All told, the pieces amounted to about 40 percent of what appeared to be at least a three million-year-old hominid skeleton.