Britannica Sources used in this answer

How do magnetic stripes on the ocean floor record changes in Earth’s magnetic field?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Faux amphibolite — geology A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.

Magnetic stripes on the ocean floor act as a permanent record of Earth’s magnetic field due to the process of seafloor spreading.12

As new oceanic crust forms at mid-oceanic ridges, molten rock rises and cools. During this cooling process, iron-titanium oxide minerals within the rock undergo thermoremanent magnetization. Once the temperature drops below a specific threshold (the Curie point), these minerals lock in the orientation of Earth’s magnetic field at that time.13

Because Earth’s magnetic field periodically reverses polarity—meaning the North and South magnetic poles switch places—the cooling crust records these reversals as alternating bands of "normal" and "reversed" magnetization. As seafloor spreading pushes the crust away from the ridge, these magnetized strips are carried along, creating a symmetrical, linear pattern on both sides of the ridge axis. By mapping these anomalies, scientists can determine the age of the seafloor and reconstruct the history of tectonic plate movements over millions of years.142