Radioactive isotope used in geological dating

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.

Pretty obvious that the dike came after the rocks it cuts through, right?

With absolute age dating, you get a real age in actual years.

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.

The narrower a range of time that an animal lived, the better it is as an index of a specific time.

No bones about it, fossils are important age markers.

I also like this simple exercise, a spin-off from an activity described on the USGS site above.

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.

It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.

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