When rocks formed millions of years ago, they often contained magnetic minerals. These minerals act like tiny compasses, aligning themselves with the Earth's magnetic field at the time of their formation. This process is called paleomagnetism.

Here's how it works:

* Magnetic Minerals: Certain minerals, like magnetite, have a natural magnetic property. When these minerals cool down from a molten state (like in volcanic eruptions), they become magnetized and "lock in" the direction of the Earth's magnetic field at that time.

* Recording Earth's Magnetic Field: These magnetized minerals act like a time capsule, preserving a snapshot of the Earth's magnetic field at the time of the rock's formation.

* Studying Paleomagnetism: Geologists can study the magnetic orientation of these minerals in ancient rocks to determine the location of the magnetic poles at the time of their formation. This provides valuable information about:

* Earth's Magnetic Field History: Understanding how the Earth's magnetic field has shifted and reversed over millions of years.

* Continental Drift: Tracking the movement of continents over geological time.

* Plate Tectonics: Studying the movement and interaction of the Earth's tectonic plates.

Therefore, when rocks formed millions of years ago, they effectively captured a snapshot of the Earth's magnetic field at that time, allowing us to understand the location of magnetic poles in the past.