How does Earth's magnetic field form, and what evidence do we use to reconstruct past magnetism?

• A. Dynamo action in the outer core; weathering and erosion reveal past directions.
• B. Dynamo action in the outer core; paleomagnetism and magnetic anomalies on the ocean floor reveal field reversals and past directions.
• C. Solar wind generates the field; satellite data show past directions.
• D. The mantle's convection generates the field; seafloor sediment magnetism shows past directions.

Answer: (B)

The field is produced by dynamo action in the outer core, where the liquid iron moves in convection and is organized by Earth's rotation into a self-sustaining magnetic field. This motion generates and maintains a field that behaves like a dipole at the surface and can flip direction over geologic time.

To learn about past magnetism, we rely on records stored in rocks and on patterns found on the ocean floor. Minerals such as magnetite lock in their magnetic orientation as rocks cool or as sediments settle, capturing the direction and strength of the field at those moments. By dating these rocks and sediments, we piece together a timeline of how the field direction has changed.

In the oceans, new basalt at mid-ocean ridges records the field as it crystallizes. As the seafloor spreads, the recorded polarity forms stripes on either side of the ridge that mirror past reversals. Mapping these magnetic anomalies and dating them aligns with the paleomagnetic record from rocks, giving a clear picture of when reversals happened and how directions changed over time.

Choices that point to generation in the mantle or to solar wind as the field source don’t fit the physics or the observations, whereas the outer-core dynamo paired with paleomagnetic data and ocean-floor magnetic anomalies provides the best explanation for both how the field forms and how we reconstruct its history.