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.
If a fault line runs through a layer of rock, you know that the rock was there before the fault.
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.
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 all, a dinosaur wouldn’t be caught dead next to a trilobite.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
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…).
Perhaps the biggest impediment to a neat and orderly rock record are our old friends weathering and erosion.
With absolute age dating, you get a real age in actual years.
It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
In either case, weathering and erosion are left free to work their mischief and erase some evidence of the geologic past. We’ll never know what happened during those 1.2 billion years? The Grand Canyon is not the only location with a rock record.