## What is radiometric dating simple

We can calculate the half-lives of all of these elements. All the intermediate isotopes between U and Pb are highly unstable, with short half-lives.

That means they don't stay around very long, so we can take it as given that these isotopes don't appear on Earth today except as the result of uranium decay. We can find out the normal distribution of lead isotopes by looking at a lead ore that doesn't contain any uranium, but that formed under the same conditions and from the same source as our uranium-bearing sample.

Then any excess of Pb must be the result of the decay of U When we know how much excess Pb there is, and we know the current quantity of U, we can calculate how long the U in our sample has been decaying, and therefore how long ago the rock formed. Th and U also give rise to radioactive series -- different series from that of U, containing different isotopes and ending in different isotopes of lead.

Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample. Uranium and thorium have similar chemical behavior, so all three of these isotopes frequently occur in the same ores. If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt.

Since all three of these isotopes have substantially different half-lives, for all three to agree indicates the technique being used is sound. But even so, radioactive-series dating could be open to question. It's always possible that migration of isotopes or chemical changes in the rock could yield incorrect results. The rock being dated must remain a closed system with respect to uranium, thorium, and their daughter isotopes for the method to work properly.

Both the uranium and thorium series include isotopes of radon, an inert gas that can migrate through rock fairly easily even in the few days it lasts. To have a radiometric dating method that is unquestionably accurate, we need a radioactive isotope for which we can get absolutely reliable measurements of the original quantity and the current quantity.

Is there any such isotope to be found in nature? The answer is yes. Which brings us to the third method of radiometric dating. Only K40 is radioactive; the other two are stable. K40 is unusual among radioactive isotopes in that it can break down two different ways. It can emit a beta particle to become Ca40 calcium , or it can absorb an electron to become Ar40 argon Argon is a very special element.

It's one of the group of elements called "noble gases" or "inert gases". Argon is a gas at Earth-normal temperatures, and in any state it exists only as single atoms. It doesn't form chemical compounds with any other element, not even the most active ones.

It's a fairly large atom, so it would have trouble slipping into a dense crystal's molecular structure. By contrast, potassium and calcium are two of the most active elements in nature. They both form compounds readily and hold onto other atoms tenaciously.

What does this mean? It means that potassium can get into minerals quite easily, but argon can't. It means that before a mineral crystallizes, argon can escape from it easily.

It also means that when an atom of argon forms from an atom of potassium inside the mineral, the argon is trapped in the mineral. So any Ar40 we find deep inside a rock sample must be there as a result of K40 decay.

We know K40's half-life, and we know the probability of K40 decaying to Ar40 instead of Ca That and some simple calculations produce a figure for how long the K40 has been decaying in our rock sample. However, again it's important to remember that we're dealing with assumptions, and we always have to keep in mind that our assumptions may be wrong. What happens if our mineral sample has not remained a closed system?

What if argon has escaped from the mineral? What if argon has found its way into the mineral from some other source? If some of the radiogenic argon has escaped, then more K40 must have decayed than we think -- enough to produce what we did find plus what escaped.

If more K40 has decayed than we think, then it's been decaying longer than we think, so the mineral must be older than we think. In other words, a mineral that has lost argon will be older than the result we get says it is.

In the other direction, if excess argon has gotten into the mineral, it will be younger than the result we get says it is. An isochron dating method isochron dating is described in the next section can also be applied to potassium-argon dating under certain very specific circumstances. When isochron dating can be used, the result is a much more accurate date.

Rubidium-Strontium Dating Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The isotope rubidium Rb87 decays to strontium Sr87 with a half-life of 47 billion years. Strontium occurs naturally as a mixture of several isotopes. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium isotopes. Remember, chemical processes can't differentiate between isotopes.

The total amount of strontium might be different in the different minerals, but the ratios will be the same. Now, suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-between amount.

That means that when the minerals crystallize there is a fixed ratio of Rb As time goes on, atoms of Rb87 decay to Sr, resulting in a change in the Rb Sr87 ratio, and also in a change in the ratio of Sr87 to other isotopes of strontium. The decrease in the Rb Sr87 ratio is exactly matched by the gain of Sr87 in the strontium-isotope ratio. It has to be -- the two sides of the equation must balance.

If we plot the change in the two ratios for these three minerals, the resulting graph comes out as a straight line with an ascending slope. This line is called an isochron. The line's slope then translates directly into a figure for the age of the rock that contains the different minerals. When every one of four or five different minerals from the same igneous formation matches the isochron perfectly, it can safely be said that the isochron is correct beyond a reasonable doubt.

Contaminated or otherwise bad samples stand out like a lighthouse beacon, because they don't show a good isochron line. There are numerous other radiometric dating methods: However, I simply haven't time or room to deal with all of them.

A full cite for this book is given in the bibliography. Possible Sources of Error Now, why is all this relevant to the creation-vs.

Every method of radiometric dating ever used points to an ancient age for the Earth. For creationists to destroy the old-Earth theory, they must destroy the credibility of radiometric dating. They have two ways to do this. They can criticize the science that radiometric dating is based on, or they can claim sloppy technique and experimental error in the laboratory analyses of radioactivity levels and isotope ratios.

Criticize the Theory Is there any way to criticize the theory of radiometric dating? Well, look back at the axioms of radiometric dating methods. Are any of those open to question. Or at least, they seem to be. Do we know, for a fact, that half-lives are constant axiom 1? Do we know for a fact that isotope ratios are constant axiom 2? Regarding the first question: There are sound theoretical reasons for accept-ing the constancy of isotope half-lives, but the reasons are based in the remote and esoteric reaches of quantum mechanics, and I don't intend to get into that in this article.

However, if all we had were theoretical reasons for believing axiom 1, we would be right to be suspicious of it. Do we have observational evidence? On several occasions, astronomers have been able to analyze the radiation produced by supernovas. In a supernova, the vast amount of energy released creates every known isotope via atomic fusion and fission. Some of these isotopes are radioactive. We can detect the presence of the various isotopes by spectrographic analysis of the supernova's radiation.

We can also detect the characteristic radiation signatures of radioactive decay in those isotopes. We can use that information to calculate the half-lives of those isotopes. In every case where this has been done, the measured radiation intensity and the calculated half-life of the isotope from the supernova matches extremely well with measurements of that isotope made here on Earth.

Now, because light travels at a fixed rate a bit under , kilometers per second , and because stars are so far away, when we look at a distant star we're seeing it as it was when that light left it and headed this way. When we look at a star in the Andromeda Galaxy, 2,, light-years away, we're seeing that star as it was 2,, years ago.

And when we look at a supernova in the Andromeda Galaxy, 2,, years old, we see isotopes with the exact same half-lives as we see here on Earth. Not just one or two isotopes, but many. For these measurements to all be consistently wrong in exactly the same way, most scientists feel, is beyond the realm of possibility. What about isotope ratios? Are they indeed constant? Well, let's think about it: Minerals form by recognized chemical processes that depend on the chemical activity of the elements involved.

The chemical behavior of an element depends on its size and the number of electrons in its outer shell.