All rocks and minerals contain long-lived radioactive elements that were incorporated into Earth when the Solar System formed.

While the lighter isotopes C has decayed that what remains can no longer be measured. In 5,730 years half of the C in the atmosphere, and therefore in plants and animals, has not always been constant.The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.It goes without saying that this is a significant deviance.So then, how do scientists know what the original composition of rocks were?I understand how radioactive dating works, but something about it concerns me. If we have a rock and assume that it was 100% carbon-14 at formation, and we now measure it to be 25% carbon-14 and 75% nitrogen-14 (I know nitrogen is a gas, but bear with me), then we can calculate that the rock has been around long enough to pass through 2 half-lives (2 x 5,730 years = 11,460 years). If, in fact, the rock was 50% carbon-14 and 50% nitrogen-14 at its formation, then it would actually be only 5,730 years old (only half the originally calculated age). This measurement seems to hinge on the fact that we know that the rock was originally 100% carbon-14.Radioactive decay can be used as a “clock” because it is unaffected by physical (e.g. For instance, the amount varies according to how many cosmic rays reach Earth.This is affected by solar activity and the earth’s magnetic field.All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus.Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus.Firstly, we don't use carbon dating for measuring the age of rocks; carbon dating would only be used for the remains of living matter (e.g. Other radioactive isotopes might be used for various other objects.