It’s a chronological index. Temporal patterns of the Earth’s magnetic flux influence orientations of magnetic material spewed from magma. The metallic atoms of this lava are liquid-like when they come out and align to the Earth’s current field then solidify, never changing polarity again. When analysed with magnetic devices the indication can be noted.
I’ve been looking at this topic lately, but nowhere did I find a simple statement about how we know this. Thanks for posting it