The record of the strength and direction of Earth's magnetic field (paleomagnetism, or fossil magnetism) is an important source of our knowledge about the Earth's evolution throughout the entire geological history. This record is preserved by many rocks from the time of their formation.

Why was paleomagnetism so important in discovering plate tectonics?

Paleomagnetists led the revival of the continental drift hypothesis and its transformation into plate tectonics. ... Paleomagnetic data continues to extend the history of plate tectonics back in time as it can be used to constrain the ancient position and movement of continents and continental fragments (terranes).

What can paleomagnetism tell us?

The study of this ancient magnetism is known as paleomagnetism. "Paleo" means old or ancient, so paleomagnetism means "old magnetism." By studying paleomagnetism, we can learn more about the Earth's interior, this geodynamo, and even track the moving continents (plate tectonics) throughout time.