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After my three-parter on fossils, I was sure you'd be sick of them, but there was a request (seconded by a few people) to talk about one particular aspect of paleontology that I didn’t cover yet: How do you know how old a fossil is? Misconception: Paleontologists directly date fossils. Correction: Most of the time, fossils are not mean we don’t have any idea how old it is. It’s just that we can’t run some kind of neat test (involving colored water, maybe? Instead, we have to rely on two methods of dating: relative dating and radiometric dating.Relative dating—as the name suggests—is a method of determining the relative ages of rock layers and the fossils contained therein.You meet someone, there's chemistry, and then come the introductory questions: What's your name? A new smartphone app is on hand to help Icelanders avoid accidental incest. In Iceland, a country with a population of 320,000 where most everyone is distantly related, inadvertently kissing cousins is a real risk.Both the principle of original horizontality and the law of superposition seem very intuitive and obvious, but they weren’t proposed until the 1600s, both by Danish anatomist, geologist, and priest Nicolaus Steno (1636–1686).
If a magma intrusion goes through that layer and is unbroken by the fault, then you know that the magma intrusion came after the fault which came after the rock, and so forth.Conversely, the igneous rocks are younger than the sedimentary rocks.