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.

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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.

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

Are there repairs or cracks in the sidewalk that came after the sidewalk was built?

Unlike people, you can’t really guess the age of a rock from looking at it.

Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.

Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.

Absolute age dating: Have students work alone or in pairs to find an article or paper that uses radiometric age dating.

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