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A Journey to a New Land

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Dr. Michael Wilson

Geology Department, Douglas College

Potassium Argon Dating

Potassium argon dating is another alternative that could be used. Radioactive potassium decays to an isotope of argon. Argon is a gas, like nitrogen – so it could be lost from the sample. So we can only use potassium argon dating with certain minerals that would trap the argon (an inert gas) in their mineral lattice. It’s interesting - we can use potassium argon, of course, for minerals, but we can’t use it for organic material. It’s got to have a significant amount of potassium in it. So there are certain minerals, such as the potassium feldspars, that contain enough potassium that they can actually work with this sort of dating technique.

Where potassium argon would be useful would be in the case of volcanic rocks for example. You have to remember that we are dating the formation of the mineral. So we can’t just take an artifact that’s made from volcanic rock and then get a potassium argon date on the artifact. We wouldn’t be dating the artifact – we’d be dating the rock and the rock might have formed millions of years earlier. So we have to restrict ourselves here in the case of potassium argon dating, to rocks that were formed at the time of their deposition in a sequence, and in a sequence that is of use to us. And that would of course be volcanic materials – materials that were crystallized and blown out of a volcano, and emplaced in a stratigraphic sequence in a way that is useful to us.

So potassium argon dating has been used a lot, for example at the early hominid sites in East Africa where there are volcanic ashes associated above and below some of these hominid finds, and the timing of the volcanic eruption can be determined through potassium argon dating. So here, on the West Coast for example, if we wanted to use potassium argon dating in respect to our sequence, we would have to find a volcanic ash that is relevant in our sequence. Or if we are close to a volcano, perhaps even a lava flow that might somehow give us information about local deposits and their ages. But we can’t just date the artifact, unfortunately, because then we are just dating the rock itself, not the artifact.