How are scientists able to tell how old a rock is?
In: Chemistry
There are already several very fine answers to this question so I want to address some issues that non-scientists sometimes have trouble with, The first is the idea of accuracy vs precision. Accuracy is the closeness to the correct answer, precision is a measure of the smallest unit that can be determined. It is thus possible to have a very precise answer that is absurdly inaccurate. Getting an accurate answer requires one set of conditions, getting a precise answer requires a different set. Both sets must be met to have a good answer.
Another issue that comes up is the age of the rock vs the age of its constituent grains or minerals. In the case of volcanics getting an age date on the time of crystallization of some of the minerals is not the same as getting the time of eruption. And in sedimentary rocks there is the time of deposition of the grains, the time of solidification of the cementing minerals, the possibility that some of the cementing minerals may be at least partially dissolved and then new ones precipitated in the voids. All three will give different ‘ages’ for the rock.
A third issue that affects sedimentary rocks is that of erosion, transportation, and redeposition. This one can be particularly tricky if one or a very few fossils are used to give a date. In a particular instance I encounterred a few years ago the law of superposition indicated one age but the included fossils indicated a much older (like about 100 my older) date. We resolved the issue by comparing the fossils in questionable bed to those in other beds and noting that the fossils in one were heavily eroded as would be expected in the stream deposited sands and pebbles of the younger formation while they were vey shap and well defined in the older formation from which they had been eroded and subsequently transported.
Geochronology is a wondrous thing but it is neither the worthless joke some maintain nor the infallible ‘truth’ some non-geologists think it is when they read a tag on a specimen in a museum.
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