Why are unstable isotopes useful for dating rocks

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No bones about it, fossils are important age markers.But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.Different radioactive isotopes are useful for measuring different time scales, but not all are present in any given object (ie- different minerals or rocks)..This method is not without issue, and estimates that are grossly inaccurate can occur, either by an error in execution of the methods, or because of incorrect assumptions of the original condition/quantity of component materials..You might have noticed that many of the oldest age dates come from a mineral called zircon.That’s because zircon is super tough – it resists weathering. Each radioactive isotope works best for particular applications.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.Pretty obvious that the dike came after the rocks it cuts through, right?

On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.

So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.

Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.

Take students on a neighborhood walk and see what you can observe about age dates around you.

For example, which is older, the bricks in a building or the building itself?

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