Volcanic ash used dating fossils
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.
If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.
That’s because zircon is super tough – it resists weathering. Each radioactive isotope works best for particular applications.
The half-life of carbon 14, for example, is 5,730 years.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
Take students on a neighborhood walk and see what you can observe about age dates around you.
Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
You might have noticed that many of the oldest age dates come from a mineral called zircon.