Radiometric Age-Dating–The Assumptions


Why talk about a stuffy topic like radiometric age-dating? The answer is that people are using it to claim the earth is billions of years old and if I’m going to believe something huge like that, then I want to know where that number comes from.

Last week we defined radiometric age-dating and discovered there are two types: 1) radiocarbon dating (done only on organic material that is a few thousand years old) and 2) radioisotope age-dating (done on volcanic deposits using elements like argon, lead, and strontium). If you missed this post or want a review of it, click here. This week we’ll build on the post from last week and talk about the assumptions inherent in radiometric age-dating. I just realized how much this intro paragraph sounds like a textbook—sorry. If my geo-girl persona comes out too much in the rest of this, leave me a comment and ask me to explain in places where I might confuse you.

To get numbers in the millions or billions, scientists make some assumptions and then measure the amount of isotopes in the rock sample. Therefore, those assumptions are vital to getting a good number that you can have faith in. Reasonable assumptions might lead to a reasonable answer, but unreasonable assumptions lead to junk. I will leave it up to you to draw your own conclusions about the reasonableness of the assumptions for radiometric age-dating, but I can confidently say they are unproven assumptions. This means there is no way to know for sure if they are reasonable. Although scientists may state it like a fact that the earth is billions of years old, it really is only their opinion.

These assumptions are:

1) No daughter element was present when the rock formed (a parent radioactive element decays into a daughter element, i.e. potassium 40 decays into argon 40)

2) The decay rate of the parent element is constant

3) No alteration from groundwater or weathering has occurred in the rock

4) No daughter element has been added to the rock since formation

The first assumption has actually been proven false in the case of argon 40 (the daughter product of potassium 40). Rock samples from fresh lava show significant amounts of argon 40 already present. This explains why several rocks with known eruption dates (meaning we can say for certain they are young) have dated as millions of years old. Similar issues have occurred in rocks dated using other elements like strontium and uranium. If radiometric dating methods worked for rocks of known age, then it would validate the method. Instead, proving this assumption false, proves the entire method false. If we can’t trust the results for rocks whose dates we know, then why would we trust it for unknown rocks?

On to the decay rate. Scientists have measured the decay rates of these isotopes in the laboratory with precision. What we know is that the decay rates have measured as constant since we’ve been measuring them. Does this mean they have been constant in the distant past, even millions of years ago? Maybe, but maybe not. Some scientists think we have reason to doubt constant decay rates based on small amounts of helium which have leaked out of old uranium-lead-dated granite crystals (see reference below for more on this).

For the sake of brevity in this post, I’m going to lump 3 & 4 together because they have the same inherent problem—the earth is not a closed system. Fluids and gases are always at work in the subsurface. While scientists can look for evidence of alteration on the rock, and sometimes we can find it, the absence of alteration doesn’t mean it didn’t happen. Is it reasonable to conclude that nothing has changed in these rocks in supposedly millions of years?

All radiometric age-dating methods are based on these same four assumptions. Certainly they are unverifiable, but what do you think? Are they reasonable?


References: “Radiometric Dating: Problems with the Assumptions.” Dr. Andrew Snelling. (


2 thoughts on “Radiometric Age-Dating–The Assumptions

  1. I am bit confused that this post appearing in my readers section does not show like, but only when it is opened, then it shows like.

    I good post to open, read and like.

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