The Public Broadcasting Service (PBS) recently ran the story “1.8 million-year-old skull may revise understanding of human evolution.” Since the skull was unearthed in a medieval ninth century A. village in the former Soviet Republic of Georgia known as Dmanisi, the obvious question centers on the validity of the million year radiometric dating of the skull found within the same geological stratum as the medieval ninth century A. Without question, the theory of evolution from a single cell to a sophisticated carnivore demands millions and billions of years−not thousands of years. What’s driving the demand for millions of years dating; is it science or is it theory?
Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
For example, in a given sample of rock, after 713 million years, half of the originally-present uranium-235 will be changed to lead: this is the "half-life" of uranium-235. Origin and Evolution of Earth System: Geologic time can be estimated by observing rock sequences at various locations.
After another 713 million years, half of what was left will have decayed, etc. Current methods include using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.
Unlike people, you can’t really guess the age of a rock from looking at it.