Earth's magnetic field is trapped in rocks as lava cools down. what do the rocks tell us?

The magnetic field changes every time it erupts along the mid-oceanic ridge and also these rocks helps in determining the age of the rocks.

Explanation:

Due to the constant plate tectonic activity, the rocks are constantly affected and are erupted along the mid-oceanic ridge where seafloor spreading takes place. These rocks forms here are the igneous basaltic rocks, and when cools after eruptions, it records the polarity of the magnetic field of the earth. The successive polarity is oriented in opposite directions, in the form of stripes which are very prominent. This data also helped in determining the relative age of the rocks of oceanic crust. The mid-oceanic rocks are younger whereas those that are formed further away from it are the oldest.