Came across a (https://www.youtube.com/watch?v=g6thUIR3_rI) which describes Earth’s magnetic field having switched hundreds if not thousands of times during Earth’s 4.5 billion years.
So, how do we know thats a fact? What are scientists looking at that helped them determine this?
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1) It doesn’t flip on intervals of 200ka (ka = kilo-annum = thousands of years) to 300ka. It is not periodic at all. It flips statistically randomly over durations as short as a few thousand years to millions. One of the longest periods was the Cretaceous Quiet Zone which lasted over 30 million years. To characterize the duration the best you can do is calculate an average with a wide variability. There’s a pretty decent overview on the [wikipedia page about geomagnetic reversals](https://en.wikipedia.org/wiki/Geomagnetic_reversal#).
2) We know about reversals from rocks that preserve the orientation of the former field in magnetic minerals that formed within the rock (e.g., the most obvious example is the mineral magnetite, which is an iron oxide mineral and quite common). For igneous (formerly molten) rocks that’s normally the time the melt cooled and crystallized, and for sedimentary rocks (made up of particles deposited on the surface of the Earth) it’s usually shortly after deposition.
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