Method for relative dating
After all, a dinosaur wouldn’t be caught dead next to a trilobite.The narrower a range of time that an animal lived, the better it is as an index of a specific time.The half-life of carbon 14, for example, is 5,730 years.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.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.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.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.
If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.