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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.

If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.

Like the other kind of dating, geologic dating isn’t always simple.

Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.

Relative age dating also means paying attention to crosscutting relationships.

In a way this field, called geochronology, is some of the purest detective work earth scientists do.

This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.

What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.

There are two basic approaches: relative age dating, and absolute age dating.

Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.

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