Janice Boekhoff


Many of you have probably heard the assertion that radiometric age-dating has proven the earth is old, somewhere around 4.6 billion years old. A bunch of scientists believe this, but there are those who don’t and I happen to be one of them. Some will say my belief on the age of the earth is based on the Bible and this is partly true. I believe the Bible and what it says about the origin of the earth (which also gives clues to its age), but if you read the A Little More About Me page on my blog, you’ll see that I also believe you can be a saved Christian without believing in a young earth.

After I became a Christian, I believed in an old earth for many years. I trusted the scientists who told me the earth was old. Surely, they wouldn’t base these dates on faulty, unproven methods. I had faith in them—until I actually investigated the evidence.

Most of the claims for an old earth are based on radiometric age-dating. When I looked into the science behind it, I was surprised at the assumptions and lack of accuracy involved in this technique. Because of this, and also because the technique itself is loaded with complex terms, any one of which could intimidate a non-scientist from investigating it themselves, I wanted to do a blog series on radiometric age-dating. These next few posts will be somewhat technical, but I encourage you to stick with me. I have faith that you are smart enough to understand this subject and can make a decision for yourself about whether these methods work.

First, before we can pick radiometric age-dating apart, you need to know what it is. Simply put, radiometric age-dating calculates an age by measuring the amount of decay of a parent radioactive isotope into its daughter element (as a ratio) using the known decay rate. In other words, the parent isotope turns into the daughter isotope at a certain rate, so how much daughter element is in a rock tells us how long the parent isotope has been decaying, if certain assumptions are met.

Radiometric age-dating uses four large assumptions that may or may not be valid: 1) no daughter element was present when the rock formed, 2) the decay rate of the parent element is constant, 3) no alteration from groundwater or weathering has occurred in the rock, and 4) no daughter element has been added to the rock since formation. Rock units are not a closed system, so these are some pretty major assumptions, which I will deal with more in the next several posts.

For now, I want to go over the difference between radiocarbon dating (not used for dating the earth) and other types of radiometric age-dating because many people believe they are the same. Radiocarbon dating is used to date once living things, not rocks, and is pretty accurate when used for the recent past. Living things are constantly absorbing Carbon-14 from the atmosphere. When they die, they no longer absorb this radioactive isotope and it begins to decay into Nitrogen-14. If you know the ratio of C-14 to the stable form Carbon-12 in the atmosphere, then you can get an age based on the decay rate and how the ratio has changed. The reason it’s not used to date the earth is because C-14 has a half-life of 5,720 years (which means half of it decays in this amount of time). With this half-life, the amount of C-14 becomes too small to measure at around a 100,000 years. Therefore, radiocarbon dating is used to date artifacts made out of wood or the bones of animals, but not rocks. If anyone says to you that radiocarbon dating has proven the earth is billions of years old, they don’t know what they’re talking about.

Okay, on to the other types of radiometric age-dating, the ones that are put forth as a claim for an old earth. These methods also use ratios of parent material to daughter material to determine an age and are subject to the same assumptions detailed above. The radioactive elements used in these methods have half-lives in the billions of years, such as Rubidium-87 (which decays into Strontium-87), Uranium-238 (which decays into Lead-206), and Potassium-40 (which decays into Argon-40). Most of these dating methods are used on igneous rocks (like basalt or ash), although some metamorphic rocks are used, as well. These are the dating methods I’ll discuss in more detail next week.

Whew! I hope you made it through this post without a headache (if not, then it’s a good thing we’re stopping here for the week). I know this information can be technical, but I think it’s important to understand the methods before trying to make a decision about whether they work. To that end, we’ll talk more about the assumptions that go along with these methods and why they might cause problems in the data, in the post next Wednesday.

Any questions? Please leave your questions as comments or even leave just a comment.

For more information on radiometric age-dating, see: https://answersingenesis.org/geology/radiometric-dating/does-radiometric-dating-prove-the-earth-is-old/

Photo Credit: ID 35649535 © Tomert | Dreamstime.com


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