How geologists know how many years passed since a rock formation?
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Through about 200 years of research and comparing formations from around the world, arguing, re-evaluating etc.
The actual techniques vary, with modelling accumulation (how much would be deposited in some time period according to physics and research) of rocks for younger layers, nuclear decay (radioisotope) dating of minerals (like carbon dating but for rocks) for older ones, types of rocks that could only exist at certain time periods (like when earth’s interior was hotter, also modelled based on physics) for some really old rocks, to name just a few.
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