The potassium argon clock is only one of many clocks that are available to geologists, all using the same principle on their different timescales. Above is a table of clocks, ranging from slow to fast. Notice, yet again, the astonishing range of half-lives, from 49 billion years at the slow end to less than 6,000 years at the fast end. The faster clocks, such as carbon-14, work in a somewhat different way. This is because the 'zeroing' of these higher-speed clocks is necessarily different. For isotopes with a short half-life, all the atoms that were present when the Earth was originally formed have long since disappeared. Before I turn to how carbon dating works, it is worth pausing to consider another piece of evidence in favour of an old Earth, a planet whose age is measured in billions of years.
Among all the elements that occur on Earth are 150 stable isotopes and 158 unstable ones, making 308 in all. Of the 158 unstable ones, 121 are either extinct or exist only because they are constantly renewed, like carbon-14 (as we shall see). Now, if we consider the 37 that have not gone extinct, we notice something significant. Every single one of them has a half-life greater than 700 million years. And if we look at the 121 that have gone extinct, every single one of them has a half-life less than 200 million years. Don't be misled, by the way. Remember we are talking half-life here, not life! Think of the fate of an isotope with a half-life of 100 million years. Isotopes whose half-life is less than a tenth or so of the age of the Earth are, for practical purposes, extinct, and don't exist except under special circumstances. With exceptions that are there for a special reason that we understand, the only isotopes that we find on Earth are those that have a half-life long enough to have survived on a very old planet. Carbon-14 is one of these exceptions, and it is exceptional for an interesting reason, namely that it is being continuously replenished. Carbon-14's role as a clock therefore needs to be understood in a different way from that of longer-lived isotopes. In particular, what does it mean to zero the clock?
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