How do scientists know how old rocks and fossils are?

Scientists read two kinds of evidence together: the order of events recorded in rocks, and natural atomic clocks that can put ages on those events. The basic methods are well established. The challenge is choosing the right method for the particular rock or fossil in front of them.

Picture a fossil in a cliff made of layers. If those layers remain in their original order, the layers below were laid down earlier and those above later. That tells us where the fossil belongs in a sequence. It does not, by itself, say how many years have passed. A geologist also checks for folding, breaks and missing layers before reading that sequence.

Now suppose volcanic ash lies above and below the fossil. Suitable minerals in that ash can contain unstable atoms that change into other atoms at a known rate. The original atoms are called parents, and the atoms they produce are daughters. A lab measures their proportions. With the right checks, those measurements provide an age for the mineral and help place the fossil between an older and a younger event.

This is why a fossil can have a well-supported age even when nobody has run a clock on the fossil itself. The surrounding evidence carries the date. Fossils shared between sites and patterns in the rocks' magnetic record can provide further comparisons.

No clock suits everything. Carbon-14 dating needs preserved organic material and works best over relatively recent spans. Other clocks can reach much further into the past. Nor does a mineral's age always mean the age of the layer containing it: a grain may have formed in an older rock before being carried into a younger deposit.

A good age report therefore names the material, the method, the event it measures and the uncertainty. Scientists gain confidence by checking whether those pieces fit the geology and agree with other evidence.