# Geologists know that radioactive dating methods are valid

Assuming we start out with pure parent, as time passes, more and more daughter will be produced. A ratio of infinity (that is, all daughter and no parent) means an age of essentially infinity.By measuring the ratio of daughter to parent, we can measure how old the sample is. Each radioactive element has a half-life, which tells how long it takes for half of the element to decay.Back to top Radioactive elements decay gradually into other elements.The original element is called the parent, and the result of the decay process is called the daughter element.Potassium 40 (K40) decays to argon 40, which is an inert gas, and to calcium.

We also need to know that no parent or daughter has entered or left the system in the meantime.In two half-lives, half of the remainder will decay, meaning 3/4 in all will have decayed.In general, in n half-lives, only 1/(2^n) of the original parent material will be left.At the start, let me clarify that my main concern is not the age of the earth, the moon, or the solar system, but rather the age of life, that is, how long has life existed on earth.Many dating methods seem to give about the same ages on meteorites.Since Cambrian and later rocks are largely sedimentary and igneous (volcanic) rocks are found in Cambrian and later strata, if these rocks are really 550 million years old, then life must also be at least 550 million years old.Therefore, my main concern is with rocks of the Cambrian periods and later.For isochrons, which we will discuss later, the conditions are different.If these conditions are not satisfied, the error can be arbitrarily large.Since there doesn't seem to be any systematic error that could cause so many methods to agree with each other so often, it seems that there is no other rational conclusion than to accept these dates as accurate.However, this causes a problem for those who believe based on the Bible that life has only existed on the earth for a few thousand years, since fossils are found in rocks that are dated to be over 500 million years old by radiometric methods, and some fossils are found in rocks that are dated to be billions of years old.