Earth’s history: time scale, layers and half-life

Some atoms in rocks are radioactive: they change into other atoms by themselves, at a steady rate that heat, cold or pressure do not change. The half-life is the time in which half of the atoms in a sample have changed. After one half-life, 1/2 is left; after two, 1/4; after three, 1/8. If a rock sample began with 800 radioactive atoms and a half-life of 10 000 years, after 10 000 years 400 are left, after 20 000 years 200, and after 30 000 years 100. So if scientists find that only 1/8 is left, the sample is about 3 × 10 000 = 30 000 years old. This is radiometric dating. The result is a good estimate, not an exact age, because the measurements have small errors. Different elements have different half-lives, from seconds to billions of years, so scientists choose one that fits the age of what they study. Carbon-14 (half-life about 5 730 years) is used for the remains of once-living things up to roughly 50 000 years old. Very old rocks are dated with long-lived elements such as uranium, measured in minerals and ash layers, not in the sedimentary rock around a fossil; carbon-14 cannot date rocks that old.