How are seafloor-spreading rates calculated from magnetic anomalies?

Seafloor-spreading rates are calculated by utilizing the Vine-Matthews hypothesis, which posits that oceanic crust records periodic reversals of Earth’s magnetic field as it forms and cools at spreading centres.

To determine the spreading rate, scientists follow these steps:

• Establish a Time Scale: Researchers use the known ages of magnetic polarity reversals, which have been documented by studying lava flows on land and confirmed through deep-sea drilling.12
• Map the Anomalies: Magnetic anomalies are mapped as linear bands parallel to the ridge crest. Positive anomalies indicate crust formed during periods of "normal" polarity, while negative anomalies indicate "reversed" polarity.132
• Compute the Rate: The spreading rate is calculated by dividing the distance of a specific magnetic anomaly from the oceanic ridge crest by the age of that anomaly.1

Because the crust splits and moves away from the center, this calculation provides the rate at which the seafloor is moving away from the ridge axis.14