Frozen compasses

When lava cools, iron-rich minerals inside it lock in the direction of Earth’s magnetic field at that moment — a fossil compass. Read enough of these frozen compasses and you can constrain its past latitude and orientation; magnetism alone does not determine its ancient longitude.

Earth’s magnetic field also reverses every so often — north and south swap. On the ocean floor, these reversals are recorded as symmetric magnetic stripes on either side of a mid-ocean ridge, like a tape recording of seafloor being made.