How do magnetic stripes record seafloor spreading?

The ocean floor can preserve a magnetic history even though its rocks do not look striped to our eyes. Near a spreading ridge, molten material rises and cools into new crust. As suitable magnetic minerals cool, their magnetization can become locked in with a direction related to Earth's magnetic field. The rock can retain that record after the field changes.

Earth's magnetic field has reversed many times. Successive bands of newly cooled rock can therefore record different polarity intervals as the seafloor moves away from the ridge. Corresponding sequences on opposite sides are evidence of a shared formation history. The diagram's colors identify polarity; they are not the rocks' visible colors. Matching the sequence to an independently dated reversal record connects spatial bands to time.

Once a boundary is correctly identified and dated, distance provides an average spreading rate. The fictional boundary lies 20 kilometers from the ridge and formed 1 million years ago. That is 20 kilometers per million years, or 2 centimeters per year, from ridge to boundary. This half-spreading rate describes one side rather than the entire separation between two sides.

If the two sides spread equally, their separation increases at twice that rate, 4 centimeters per year. Unequal sides require separate estimates. The result is also an average across the dated interval; it does not show that the speed stayed identical in every year. Boundary correlation, independent ages and an undisturbed enough magnetic record are essential assumptions.

The ocean floor becomes a record of both a changing magnetic field and moving plates. A polarity sequence supplies recognizable events, dating supplies their times, and distance reveals how far the crust moved. Stripe width alone supplies none of that history without the connections that make the pattern interpretable.