Relative and absolute dating lesson plan

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.

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.

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.

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

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.

As the students work on the simulation they are visualizing how stability and change in natural or designed systems can be constructed by examining the changes over time (CCC Stability and Change), as well as analyzing and interpreting data (SP4).

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

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After all, a dinosaur wouldn’t be caught dead next to a trilobite.

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