How do we know Earth’s magnetic fields flip in intervals of 200,000-300,000 years?

Question

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?

In: 2148

Answer 1

People are forgetting to add that at places like the mid-Atlantic ocean, the make is welling up out of the earth and spreading away from a fault-line in the middle of the ocean. The magnetic field is frozen in stripes as the magma cools, kind of like a strip chart recording of the earths magnetic field as a function of time. Since the rate of spreading is known, we can tell how many years ago the magma cooled.