[raybbr]

I’m curious to see the log where the reversals were recorded previously. How on earth to they know when this happened?

[pa_dweller]

http://magneticreversal.org/

https://en.wikipedia.org/wiki/Geomagnetic_reversal

[Rebel_Ace]

"I’m curious to see the log where the reversals were recorded previously. How on earth to they know when this happened?"

An effective "log" of the earth's magnetic polarity can be found on the ocean floor, where the tectonic plates are moving apart from each other.

As the plates separate, molten magma pushes in the gap and solidifies, trapping within it iron molecules oriented in the direction of the field. Over time, these "filled in gaps" form something almost like a bar code stripe, and with a magnetometer you can see the directions change in the different "stripes".

Also, by observing how long on average it takes the current "stripes" to emerge, you can start to tell how long ago things took place, by counting the stripes outward, much like you count tree rings to judge how old the tree is.

It is not that mysterious, or even difficult to comprehend, you just have to be a serious geologist and be willing to go to really weird areas of the Earth to gather data.