How do reversals of Earth’s magnetic field become recorded in oceanic crust?

Reversals of Earth’s magnetic field are recorded in the oceanic crust through the process of seafloor spreading and the permanent magnetization of cooling volcanic rock.1

As new oceanic crust forms at spreading centres, molten basaltic lava rises and cools. When these rocks cool below a specific temperature (approximately 200–300 °C), iron-titanium oxide minerals within them undergo thermoremanent magnetization, effectively "locking in" the orientation and polarity of Earth’s magnetic field at that moment in time.1

Because the magnetic field periodically reverses polarity, the crust records these changes as a series of magnetic anomalies:12

• Positive anomalies: Formed when the crust cools during a period of "normal" polarity, aligning with the current magnetic field.12
• Negative anomalies: Formed when the crust cools during a period of "reversed" polarity, creating a magnetic signature that subtracts from the present field.12

As the crust moves away from the ridge, it carries this permanent magnetic record, creating symmetrical, parallel bands of anomalies on either side of the ridge crest.12