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.

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

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But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.

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.

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.

But measuring how much carbon-12 is in a formerly living thing compared to carbon-14 and doing some math with what is called a half-life you can date things to within a certain range.

Unfortunately it is not precise enough to date some really old things with much certainty, and it can only be used to date stuff that was once alive.

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.

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

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