So I’ve been scrolling through a book that shows ancient rocks/ artifacts archeologists have discovered over the years and it’s always been really confusing to me just how they’re able to figure out how many millions or thousand of years these things have been around for. Just how do they know, what information do they need to know the age of an artifact and is there a certain method used universally?
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For really old rocks and fossils (millions of years old), age is usually determined by carefully keeping track of which layer of dirt/rock the fossil was in. The deeper through layers of rock you go, the older they get. If you can figure out the age of a particular layer of rock, you can then figure out the approximate age of things embedded in that layer. Layer ages can be determined in any number of ways, including looking for traces of known events. For instance if you know when a major volcano erupted, then you can date things embedded in the lava flow. Some rocks look different depending on the climate when they formed, and you can also look for trace elements like iridium that indicate a major meteorite strike.
For newer artifacts (thousands or tens of thousands of years old) you can use techniques like radiocarbon dating. This measures the amount of radioactive Carbon 14 in the artifact. The amount of Carbon 14 in the atmosphere stays relatively constant, so when things are alive, they have about the same amount of C14 as the atmosphere. When they die, they stop absorbing C14, and the amount slowly decreases.
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