Relative dating of fossil
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).In a nice, simple world, we would have a complete and undisturbed record of sedimentary rocks to help us plot out Earth’s past. All kinds of things happen to rock layers after they form.They get uplifted, tilted, faulted, squashed and squeezed, and magma gets injected into them.It’s all a bit of a mess—but a mess that can be put in chronological order using relative dating principles.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.
Unlike relative time, absolute time assigns specific ages to events or formations and is typically recorded in years before present.
This feature is produced by changes in deposition over time.
With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?
In the Grand Canyon, for example, there is a layer of 1.7-billion-year-old igneous rock right under a layer of 550-million-year-old sedimentary rock.
What happened in the intervening 1.2 billion years? We just don’t, at the Grand Canyon, have a record of it.
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The geologic record is like a big puzzle that (frankly) is really fun to sort out, especially if you are a little OCD and like things to be in order (not that I’d know anything about that…).