How are scientists able to tell how old a rock is?
In: Chemistry
Well you have the absolute dating of a rock with the radiometric method described by the others here. But what us also important is relative dating. If you dig into the ground you get layers with specific characteristics. How deeper you go how older the (sedimentary) rock grnerally. You can try to describe this sequence of layers and correlate it with the sequence of layers in the region with similar conditions. And you don’t have to dig hundreds of meters deep either, generally these layers are not flat and level like they used to be but they are deformed and not level with the ground. Because most peaks from geological history rock from different geological periods will be found on the surfafe at different locations, snd so you can kinda try to puzzle together this sequence of characteristic layers. Although you have to be careful with this as well. This layer profile cannot be extended too far because conditions naturally are different in different regions now, also in the geological past. The further two locations are apart the harder it gets. Also depositions can also vary gradually in space and not in time, eg. when a river deposits a sandy layer in the nearby sea and clay somewhat further for a long period of time while the coast moves, then these sand and clay will not correspobd with the same period.
Also nice is to use fossils to correlate layers, because species appear, die out, become less and more common over time.
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