How do magnetic stripes show the history of earth's magnetic field?

If i could heat a piece of iron so that it melted, and then held a magnet close to the molten iron and then let the iron cool to room temperature, you could use a microscope to look at the iron and tell that a magnet had been present during cooling. you could even tell if it was the north or south pole of the magnet that i held close to the molten iron.

Magma/lava contains a lot of molten iron so that when it erupts from a volcano and cools, the magnetic field of the earth leaves an imprint in it, just like the cooling iron from above. From this imprint we can tell the strength of the magnetic field and also which direction the north and south poles were at the time.