how do we date fossils and other very old rocks that are many millions if not billions of years old?
In: Planetary Science
Radiometric dating can be used, where we know the “rate of decay” of an element into a different element, such as Potassium decaying into Argon.
To make it very simple for ELI5:
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Imagine that there is a naturally occurring bowl of 60 red balls. These bowls just randomly pop up here and there, all over the place. And we discover that once a bowl of red balls appears, that each year, one of the red balls will turn green.
Now, how do we know how old any given bowl is?
We simply count how many green balls there are in the bowl. If there are 50 red balls and 10 green balls, we know the bowl is ~10 years old. If there are 19 red balls and 41 green balls, we know the bowl is ~41 years old.
^ Now, this is only accurate to within a year, since maybe one of the red balls is going to turn green tomorrow, or maybe a red ball just turned green yesterday, so if there are 5 green balls, it may have *just turned* 5 years old, or maybe it is 5.99 years old and will turn 6 tomorrow, so we don’t know the age of the bowl to the day, or to the hour, but we can get “close enough” to its age within a year, by counting green balls.
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Again, that’s a big simplification of how we measure elements, as the math for elements is based on “half life” which can be in the hundreds of thousands of years, but that’s how we can know very closely how old a fossil encased in a certain element is, for example, measuring the amount of Argon as opposed to Potassium in a given fossil, and it may be some number of years off, but “close enough.” For example, we may not know exactly how old a million-year-old fossil is by its potassium vs argon ratio, but we know it’s not only 1,000 years old.
We can also date other things around it by different methods to confirm as well.
Very few fossils are dated directly. Instead they are given dates relative to rocks that can be dated using radioactive decay.
Sedimentary rocks containing fossils rarely contain enough radioactive material for dating, but it is possible to date volcanic rocks that cooled from magma that sometimes occur in a rock sequence. So if you have a lava flow somewhere below the fossils, you can usually assume the fossil is younger than the lava because it was deposited on top. If you then have another lava flow above the fossils, you can say it is younger than the fossils and therefore the fossil’s age lies somewhere between the two dates for the lava.
If you have a pile of clothes on the floor, which have laid there the longest? It is likely the deepest ones. It is similar wit rocks. The younger rocks are usually laied down on top of younger rocks. There are some exceptions, such as when lava sulidifies in cracks, but similarly it is possible to determine which is the older rock.
But using the dating methods others have mentioned and carefully matching different places, most rock can be dated with some accuracy.
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