Radiometric Dating

Radiometric dating is well-established science, but because it gives results creationists do not like, they will usually reject it out of hand, without good reason or even understanding what t is. This is a quick overview of what it is.

What is an isotope?

Atoms are made of relatively large protons and neutrons in the centre (the nucleus), with tiny electrons whizzing around around them. The number of electrons will be equal to the number of protons, and it is this that determines what the atom actually is, what element it is. If there are 6 of each, it is carbon, if there are 26, it is iron, if there is only 1 of each, it is hydrogen.

There are approximately the same number of neutrons, but it can vary. The number of neutrons does not change the element, but does change the isotope. An atom of hydrogen has 1 proton and 1 electron, but it can have 0, 1 or 2 neutrons (or more). Some isotopes are stable - they will last for every - some are not. The isotope of hydrogen with two neutrons, called hydrogen-3 (the number of protons plus neutrons; it is also called "tritium") has a half-life of 12.32 years. That means that if you start with 8 grams of it, after 12.32 years you will have only 4 grams left, and after 24.64 years you will have only 2 grams left.

Radioactive decay

Where has it gone? It has decayed, which means it has transformed into a different isotope, usually of another element. Different isotopes decay in different ways; hydrogen-3 decays to produce helium-3, which has two protons and one neutron - one of the neutrons has become a proton.

The half-life of an isotope is constant - it is not affected by temperature or pressure or other outside influence. Radiometric dating assumes half-lives have never changed; this seems a good assumption based on what we have observed of them, and bearing in mind we can glimpse what the laws of nature were like long ago by looking at distant stars and they appear to be identical to what we see today.

Radiometric dating

Radiometric dating is working out the age of something from the amount of the isotope that still remains. If the item has only quarter of the isotope left, then its age is twice the half-life.

In practice it is a bit more complicated...

Potassium-Argon Dating

There are several isotopes used for radiometric dating, but one of the most important is potassium-40. This has 19 protons and 21 neutrons in the nucleus. It is unusual in that it can decay to two different isotopes, but the important one for us gives argon-40, with 18 protons and 22 neutrons. Argon is an inert gas; when it is trapped in rock, we can be pretty certain it got there from the decay from potassium-40, and that that decay happened after the rock solidified - otherwise the gas would just float off into the atmosphere.

Potassium-40 has a half-life of 1250 million years, with about 10% of the atoms decaying to argon. If you measure the ratio of potassium-40 to argon-40 in your rock, you have a pretty good estimate of the age of the rock.

Suppose your sample has 10 mg of argon-40 and 100 mg of potassium-40, that would indicate about 100 mg of potassium-40 has decayed (as only 10% decays to argon-40), which indicates one half-life has passed, and the rock is therefore about 125 million years old.

The Geologic Column

The geological column is, in a sense, a hypothetical construct. It is a series of rock types laid on top of each other, but as far as I know there is no one place where the entire column is present.

However what is seen is that the same series are seen the world over in the same order. That is to say, one site may only have a dozen different layers, but those layers will always be in the same order. For example, we always see Jurassic rocks above Triassic rocks and below Cretaceous. There are a lot of layers.

An important point about radiometric dating rocks is that rocks from a particular stratum all give similar results. Why is that?

The old Earth view is that that is because the rocks were all laid down at the same time. Rocks from the Late Cretaceous all date to between 100 and 66 million years ago because that was when the rocks were laid down.

How does Flood geology explain this distribution of isotopes? How does a flood sort rocks so those with more argon-40 in them are consistently buried deeper than those with less argon-40? No creationist has ever addressed this, as far as I know - and I have asked several on various internet forums.

Carbon Dating

Another important isotope for radio-dating is carbon-14; this is used for "carbon dating" or "radiocarbon dating". Carbon-14 forms in the atmosphere, and the ratio of carbon-14 to carbon-12 (the common isotope) in the atmosphere is fairly constant; about 1.25 carbon-14 atoms to every trillion carbon-12 atoms. When the carbon is absorbed by a plant (in the form of carbon dioxide) the carbon-14 ratio starts to drop, and a comparison of the new ratio gives a good estimate of the date the plant was a live, and by inference the animal that ate the plant or the chair or sheet of paper made from it.

The half-life of carbon-14 is 5730 years, and this method of dating is not suitable for samples over about 50,000 years as the amount of carbon-14 remains is so small.

Carbon dating has been calibrated against tree rings and other methods, as it was recognised from the start that the ratio of carbon-14 in the atmosphere may not be constant. Tree ring dating can be used to go back nearly 14,000 years by the way.

Carbon dating is not used to date rocks (as far as I know) because the time frame is too short for most of geology and it is based on organisms, not rocks.