How were scientists able to use rock on the ocean floor to determine that the earth’s magnetic field has changed direction?

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Explanation:

One important discovery that geologists made when studying such igneous rocks was that some of the rocks recorded reversals in the direction of Earth's magnetic field. ... Geologists also studied the magnetic record of the rocks of the ocean floor, particularly the igneous rocks on the sides of mid-ocean ridges.

and one more

Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized Stripes. These 'stripes' hold a record of reversals in the Earth's magnetic field, as the rock is cooled, the iron bits line up in the direction of Earth's magnetic field.