Step 1: Why the seafloor has stripes.
New basalt at a ridge locks in the Earth magnetic field as it cools. When the field flips now and then, the rock records bands of normal and reversed magnetism.
Step 2: Where the field comes from.
The magnetic field is made deep down, not at the surface. So we look for the layer that powers the field and its reversals.
Step 3: Pin down the outer core.
The liquid outer core churns, and this moving iron acts like a dynamo that makes the field. Its changing flow is what flips the poles.
Step 4: Reject the rest.
Mantle convection drives plates but not the field. The inner core is solid and cannot stir a dynamo. Ridge quakes are tectonic, not magnetic.
Step 5: Final choice.
So the stripes come from convection in the outer core.
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