The Washington Post article Scientists discover hundreds of footprints left at the dawn of modern humanity describes the geological dating of stratified layers of mud by analyzing and dating minerals within each layer.
But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals. The youngest crystal in the footprint layer would represent the oldest possible age for the prints; the oldest crystal in the layer above it would represent the youngest they could be.
Using the argon-argon dating technique, by which scientists measure the decay of an isotope called Argon-40 into Argon-39 in order to find the age of crystals, they came up with a rough approximation of the footprints' age: 19,000 years at the oldest, 10,000 or 12,000 years at the youngest.
I can't exactly follow the logic, but I'm asking here about the dating process itself. How does argon-argon dating work?
I do not think that Argon-40 decays into Argon-39 as the article states, at least not all by itself. And when I look at the Wikipedia article, the discussion is so technical and defensive that I can't actually picture what is going on.
How does argon-argon dating work?