Radiometric dating and deep time

If you haven’t yet read the Radiocarbon dating post, and if you’re not already au fait with the elements of half-lives and radiometric dating, it’s probably worth clicking on the link and reading that post first, then coming back to this one.

Radiocarbon dating can only take us back on the order of a few tens of thousands of years. Humans and proto-humans are believed to have been around for a couple of million years, the last dinosaurs to have become extinct about 65 million years ago, and the Cambrian to be about half a billion years ago. Earth itself is believed to be about 4.5 billion years old.

That means we need some other dating methods, and some of those also rely on radioactive decay. Carbon-14 has a half-life of 5730 years, but other radioactive elements have much, much shorter half-lives – on the order of nanoseconds or even femtoseconds – and some have much, much longer half-lives.

A few different methods and decays are used:

DecayHalf-life (years)
Postassium-Argon1.3 billion
Uranium-Lead*4.5 billion
Rubidium-Strontium50 billion
Samarium-Neodymium106 billion

*This is the U-238 to Pb-206 decay, but there is also a U-235 to Pb-207 decay with a half-life of 700 million years that runs in parallel and can be used as an extra check.

The method is as for radiocarbon dating, but in most of these cases the ‘daughter nuclide’ – the second name in each of the pairs in the table, the thing that the first-name element decays into – is solid and stays around, unlike the nitrogen that is the product of radiocarbon dating. This means that the age is usually calculated in terms of the ratio of the parent and the daughter nuclide in the sample.

There are similar ‘gotcha’ examples sometimes used for these dates, but since many creationists are also proponents of a very short age of the Earth – some 6000 years in accordance with Bishop Ussher’s chronology based on the Biblical genealogies, some a little longer but not much – most would suggest that the planet1 is younger than even the 80,000 years of a single half-life of the Uranium-Thorium decay.

That means that a common theme is “But you assume that the rates of radioactive decay has always been constant. Maybe it was different in the past.” Some suggest that at the time of Noah’s flood there was also a dramatic increase in the rate of radioactive decay.

The thing is, every radioactive decay reaction also releases heat. If there had been sufficient acceleration to fit 4.5 billion years worth of decay into 40 days and 40 nights (while it was raining (in the account)) or even a year (before the floodwaters subsided)), the heat released would have been sufficient to melt the entire planet, many times over.

The response I’ve sometimes received when raising that issue is “God has infinite power and could shield the Earth from the heat”. Well, I guess so, but that just piles ad hoc intervention on top of ad hoc intervention. If God wanted to do that, perhaps it would have been simpler just to specifically set the isotope ratios… and also do things like create the polonium halos in granite that certainly suggest radioactive decay occurred over a very long period.

My goal in these posts is really not to pick fights, but to enhance understanding of the relevant science. Occasionally, though, enhancing understanding involves addressing common misunderstandings.

  1. Or life: beliefs differ, but even those who believe the planet has been around longer believe it was ‘without form and void’, so there were not features capable of being dated.

For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.

Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Evolution and entropy
Facts, Theories and Laws
Radiocarbon dating
Four Forces of the Universe
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Mitochondrial Eve and Y-chromosomal Adam
Transitional fossils
Complexity – irreducible and otherwise

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